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Stable Isotope Biogeochemistry of Seabird Guano Fertilization: Results from Growth Chamber Studies with Maize (Zea Mays)


16 Aug 2012: Szpak P, Longstaffe FJ, Millaire JF, White CD (2012) Correction: Stable Isotope Biogeochemistry of Seabird Guano Fertilization: Results from Growth Chamber Studies with Maize (Zea Mays). PLOS ONE 7(8): 10.1371/annotation/8e51b001-d65f-4331-a5df-9aa2c7d5454b. View correction



Stable isotope analysis is being utilized with increasing regularity to examine a wide range of issues (diet, habitat use, migration) in ecology, geology, archaeology, and related disciplines. A crucial component to these studies is a thorough understanding of the range and causes of baseline isotopic variation, which is relatively poorly understood for nitrogen (δ15N). Animal excrement is known to impact plant δ15N values, but the effects of seabird guano have not been systematically studied from an agricultural or horticultural standpoint.

Methodology/Principal Findings

This paper presents isotopic (δ13C and δ15N) and vital data for maize (Zea mays) fertilized with Peruvian seabird guano under controlled conditions. The level of 15N enrichment in fertilized plants is very large, with δ15N values ranging between 25.5 and 44.7‰ depending on the tissue and amount of fertilizer applied; comparatively, control plant δ15N values ranged between −0.3 and 5.7‰. Intraplant and temporal variability in δ15N values were large, particularly for the guano-fertilized plants, which can be attributed to changes in the availability of guano-derived N over time, and the reliance of stored vs. absorbed N. Plant δ13C values were not significantly impacted by guano fertilization. High concentrations of seabird guano inhibited maize germination and maize growth. Moreover, high levels of seabird guano greatly impacted the N metabolism of the plants, resulting in significantly higher tissue N content, particularly in the stalk.


The results presented in this study demonstrate the very large impact of seabird guano on maize δ15N values. The use of seabird guano as a fertilizer can thus be traced using stable isotope analysis in food chemistry applications (certification of organic inputs). Furthermore, the fertilization of maize with seabird guano creates an isotopic signature very similar to a high-trophic level marine resource, which must be considered when interpreting isotopic data from archaeological material.


Seabird excrement (guano) was arguably the most economically significant organic fertilizer in the world prior to the twentieth century. The guano was mined from small, nearshore islands off the arid western coast of South America in the Peru-Humboldt upwelling region. The guano islands of Peru and Chile are typically composed of rocky cliffs essentially devoid of vascular plants, with a relatively small number of fauna (ants, spiders, scorpions, lizards) that are supported by allocthonous inputs from the guano birds (guano, carcasses, feathers, eggshells) [1]. Because the region receives virtually no precipitation, the guano accumulates in sedimentary layers. The once thick deposits of seabird guano (>50 m in some cases) were mined extensively during the guano boom of the 1800 s, and today the islands rarely have more than several years worth of droppings accumulated [1]. The trade in guano peaked during the middle of the nineteenth century, with 20 million tons being exported to Europe and North America between 1848 and 1875 [2]. The popularity of guano with European and North American farmers waned in the latter part of the nineteenth century for a number of reasons, including: increasing guano prices, irregular availability, unsuitability for particular crops (especially turnips), a dwindling supply, and the development of the chemical fertilizer industry [3]. In recent years, however, there has been a resurgence in its popularity (particularly in horticulture) as worldwide demand for organically grown produce has increased [4], [5]. The importance of guano as a fertilizer prior to the nineteenth century is less well known, but is mentioned by Spanish chroniclers and in colonial administrative documents [6], [7]. On this basis, some have suggested that it may have been of some importance in prehispanic agriculture [8], [9].

From an ecological perspective, the importance of ornithogenic nitrogen to marine and terrestrial ecosystems has long been recognized [10][12]. A number of studies conducted in tropical, temperate, subpolar, and polar regions have shown that seabird guano alters the concentration of soil nutrients (particularly NH4+, NO3, PO43−, K+, Mg2+), plant tissue nutrients (N, P, K), and plant productivity [13][21]. Seabird guano may also affect the diversity of plant species present, though results from such studies are inconsistent [21]. Numerous factors other than the presence of guano may also affect the chemistry, physiology, and ecology of plants growing within or near seabird colonies. In field studies it is often difficult, or impossible, to rule out the effects of these factors, which include: physical disturbance caused by birds such as plant clipping or trampling [22], [23], deposition of seabird carcasses, feathers and eggshells [24][26], and avian-aided seed dispersal [27].

Particularly large 15N enrichments in soils, plants, and animals (5–40‰) have been recorded in and around seabird nesting sites, allowing for the relative contribution of avian-derived nutrients to be assessed (Table 1). Despite this large body of literature, there have been no investigations that examine the biogeochemical effects of seabird guano on the western coast South America, with the majority of studies focusing on Oceania, Japan, California, and Antarctica [21]. Furthermore, no studies have addressed the isotopic biogeochemistry of seabird guano from an agricultural or horticultural standpoint. The purpose of this study, therefore, is to assess the isotopic and vital effects of Peruvian seabird guano fertilization on maize (Zea mays) under controlled conditions. In particular we examine the extent of the enrichment in plant 15N resulting from guano fertilization.

Table 1. Summary of studies examining the effects of seabird guano on the isotopic composition (δ15N) of plants and soils.

Plants are capable of utilizing several different soil N sources, both organic (amino acids) and inorganic (NH4+, NO3, N2). From a biogeochemical perspective, the uptake, assimilation, and allocation/reallocation of N compounds are all significant. Uptake of NO3 in plant root cells occurs through at least three different NO3 transport systems [28]. Once inside the root, NO3 can be assimilated into organic N, or translocated to the shoot for assimilation by nitrate reductase (NR), nitrite reductase (NiR), and glutamine synthetase (GS) [29]. Little or no fractionation of 15N is reported to be associated with the uptake of NO3 [30][32]; fractionation of 15N does not appear to vary with respect to source [NO3] [33][35]. Some variability in fractionation is associated with NR activity, and it has been difficult in some cases to differentiate between isotopic fractionation associated with N uptake and assimilation, respectively [36]. Ledgard et al. [37] report the fractionation for the entire process to be −15‰, while a range of 0 to −19‰ is reported by Robinson [38].

NH4+ is taken up by plants via high or low affinity transporters depending on extracellular [NH4+] [39]. NH4+ is assimilated into organic N only in the roots via GS and most estimated Δ15N values for NH4+ uptake and assimilation fall between −5 and −20‰ [38], [40]. Unlike NO3, however, there are substantial differences in Δ15N with source [NH4+]. For example, in two different rice cultivars, Yoneyama et al. [33] found Δ15N for NH4+ uptake to be −6.1 to −12‰ at low source [NH4+], and −13.4 to −28.9‰ at high source [NH4+].

Materials and Methods


All plants were grown in a walk-in growth chamber at the Biotron Centre for Experimental Climate Change Research at the University of Western Ontario. The substrate utilized for all treatments was Pro-mix® for containers (75–85% sphagnum moss, 15–25% perlite and limestone). Peruvian seabird guano (Guano Company International, Cleveland, Ohio, United States) was obtained from an organic gardening outlet. The nitrogen content of the guano was reported to be 10% and determined to be 11.2±0.2% based on five analyses of dried, powdered guano as described for plant samples below. The ‘Early Sunglow’ maize cultivar was used (Zea mays cv. Early Sunglow, Lot E1, 2010, Ferry Morse, Fulton, Kentucky, United States) for all experiments because it is a relatively small variety of maize that accommodated physical restrictions on plant height imposed by the growth chamber.

Growth Chamber Conditions

Growth chamber temperature was 25/18°C (day/night), with a photoperiod of 13 h provided by 185 W fluorescent bulbs. Relative humidity was set at 80% for the first four daylight hours, and 60% for the remainder of the day. These conditions were monitored electronically, and did not deviate from these parameters for the duration of the experiment.

Maize Germination Experiment

Guano (well-mixed with soil) was applied to 1.2 L plastic containers (1.0 L of soil) in the following amounts: 0 g, 1.0 g, 2.5 g, 5.0 g, 7.5 g, 10.0 g and 15.0 g. Six replicates of each treatment were prepared. One hour after addition of the guano, maize seeds were planted ∼2.5 cm below the surface in the containers. Emergence and growth of the plants were recorded every 2–3 days for 35 days.

Maize Fertilization Experiment

Fifteen maize seeds were planted ∼2.5 cm below the surface in 1.2 L plastic containers (1.0 L of soil). At this time, guano was mixed with soil in free-draining (perforated at the base) 18.9 L plastic buckets containing 16 L of soil in the following amounts: 0 (C0), 80 g (G1, 5 g guano/L), 160 g (G2, 10 g guano/L). Five replicates of each treatment were prepared. Maize is typically fertilized prior to planting, and sometimes again approximately three weeks after emergence, although this second application is uncommon [41]. To avoid complications associated with additional fertilizer applications, only one fertilizer application was employed. After germination (7 days after sowing) maize plants were moved into the 18.9 L plastic buckets. Plants were watered every 2–3 days and the height and general growth of the plants was monitored. Distal leaf samples (∼3 cm×6 cm) were taken at 30 and 75 days after planting (d). Plants at 30 d were characterized by only vegetative growth, while plants sampled at 75 d had begun reproductive growth (tassels fully emerged, silks beginning to appear). Anthers were sampled at 75 d. At completion of the experiment (115 d), the following tissues were sampled: leaves, grains, roots, and stalks. All buckets were relocated randomly within the growth chamber five times (30, 45, 60, 75, 100 d) during the course of the experiments to account for any micro-variations in light, temperature or humidity, although such changes were not expected.

Stable Isotope Analysis

All plant materials were stored at −25°C following sampling until needed for analysis. Samples were then dried at 90°C under normal atmosphere for 72 hours, ground using a Wig-L-Bug (Crescent, Lyons, Illinois, United States) and the resulting powders stored at room temperature in sealed glass vials. Isotopic compositions (δ13C and δ15N values determined separately) and relative percentages of carbon and nitrogen were determined using a Delta V isotope ratio mass spectrometer (Thermo Scientific, Bremen, Germany) coupled to an elemental analyzer (Costech Analytical Technologies, Valencia, California, United States). For the analysis of δ15N, excess CO2 was removed using a Carbo-Sorb trap (Elemental Microanalysis, Okehampton, Devon, United Kingdom). Sample reproducibility was ±0.09‰ for δ13C and ±0.90% for %C (6 replicates), and ±0.12‰ for δ15N and ±0.10% for %N (24 replicates). A δ15N value of 20.31±0.18‰ was obtained for 37 analyses of IAEA-N2, which compared well with its accepted value of 20.30‰. A δ13C value of −29.87±0.29‰ was obtained for 11 analyses of NBS-22, which compared well with its accepted value of −30.00‰.

Statistical Analyses

Comparisons between treatments and between organs were completed using one-way analysis of variance (ANOVA). Levene's test was used to assess homogeneity of variance; if variance was homoscedastic, a post hoc Tukey's honestly significant difference (HSD) test was applied and if variance was not homoscedastic, a post hoc Dunnett's T3 test was applied. All statistical analyses were conducted at a significance level of 5% (p<0.05). All statistical analyses were performed in SPSS 16 for Windows.

Results and Discussion

Maize Germination and Seedling Establishment

All unfertilized plants germinated and commenced normal growth (Figure 1). There was a clear trend towards the inhibition of germination and seedling emergence with increasing rate of guano applied (Figure 1). It is apparent that the presence of seabird guano in the soil has the potential to inhibit germination and that this effect is concentration dependent. Ishida [42] found lower germination rates in oak and pine trees within, compared to outside of, cormorant colonies but did not offer a detailed explanation for this pattern. Mulder and Keall [43] also found that seabird guano negatively affected seed germination and seedling survival. Germination inhibition with increasing concentrations of guano probably results from a number of factors, including reduced soil pH and the presence of a high concentration of soluble salts, both of which are characteristic of ornithogenic soils [20]. Very high concentrations of NO3 and especially NH4+ are also characteristic of ornithogenic soils and these characteristics can inhibit maize germination [44], with the early stages of growth being the most detrimental for plants under NH4+ stress [45], [46].

Figure 1. Relative percentages of seedlings that germinated and emerged with differing amounts of seabird guano applied.

Vital Effects of Guano Fertilization

Plant growth was strongly inhibited in the heavy guano treatment (G2). Maximum plant heights were significantly lower in G2 compared to C0 (p = 0.02) and G1 (p = 0.008) (Figure 2). While the G1 plants did not attain greater maximum heights than the C0 plants (p = 0.83), they yielded significantly more grain (p = 0.004). The G2 plants yielded less grain than the G1 plants (p = 0.03) and more grain than the C0 plants, although this difference was not statistically significant (p = 0.42) (Figure 2).

Figure 2. Maximum heights of maize plants throughout experiment.

Harvest occurred at 115 d. Inset: grain yield for each experiment.

In this study, we observed a positive influence of guano on maize growth at moderate concentrations (G1), but a negative influence at high concentrations (G2). A number of studies have found that plant abundance and/or species richness tends to be lower within seabird colonies, but is often higher in areas in relatively close proximity to the colonies [14], [22], [47], [48].

Very high levels of soil P can have deleterious effects on plant growth [49]. Ornithogenic soils may contain fifty times more available phosphorous than normal, but the P salts in bird excrement tend to be immobile in soil because of their limited solubility, making them generally unavailable for uptake by plants [4], [50], [51]. It is thus unlikely that the reduced growth observed in the G2 plants is the result of P toxicity. The most likely cause for the reduced growth of the G2 plants is NH4+ toxicity.

Very high [NH4+] is a ubiquitous trait of ornithogenic soils [52][56]. High soil NH4+ can negatively impact plants in several ways: (1) soil acidification, particularly of the rhizosphere [57], ‘scorching’ of root hairs [46]; (2) accumulation of free NH4+ in plant tissues, which has the capacity to uncouple plastid energy gradients [46]; (3) assimilation of NH4+ in the roots and associated translocation of carbon skeletons from the shoot, which is metabolically expensive and places ‘carbon stress’ on roots [58]; (4) suppression of the expression of certain proteins (aquaporins), which can have detrimental effects on the uptake of water [59]; and (5) the influx and efflux of NH4+ through root cells, which is associated with a very high metabolic cost when source [NH4+] is high [60].

Both the G1 and G2 plants exhibited significantly reduced growth compared to the control plants for the first 45 days of the experiment (Figure 2; p = 0.01), but this trend did not continue as the G1 plants produced the greatest yields, and had similar maximum heights to the control plants. This is likely the result of initially very high soil [NH4+], which negatively impacted the growth of the fertilized plants, followed by increased soil NO3 resulting from nitrification of guano-derived NH4+. When plants largely supplied with NH4+ as an N source are supplemented with NO3, NH4+ uptake is suppressed and plants are able to resume normal growth [58]. The fact that the G2 plants still produced grain even though they were characterized by reduced heights and less above–ground biomass than either the control or G1 plants suggests that there was some acclimatization of these plants to the high [NH4+], and/or nitrification was substantially delayed and [NH4+] remained high in the soil for a much longer period of time. Schortemeyer et al. [46] observed a similar result in maize plants grown with NH4+ as the sole N source.

The effects of guano on plants are difficult to generalize. There is considerable variability at the community level and also within a community in accordance with plant physiology (nutrient demands, salt tolerance) at the species level [47], [52]. Even within maize there are differences in NH4+ tolerance, with some varieties being able to survive higher concentrations than others [46]. Therefore, it cannot be assumed that the results of this study are directly applicable to all maize varieties.

Nitrogen Isotope Composition of Seabird Guano

Most inorganic N fertilizers have δ15N values close to 0‰, with organic fertilizers generally having highly variable but positive δ15N values (Table 2). The δ15N value of the seabird guano used in this experiment was 26.7±0.6‰ (5 analyses), which is much higher than any other organic fertilizer analyzed to date. This is the product of avian nitrogen metabolism and excretion, which is quite different than in mammals, combined with the high trophic position of the guano-producing birds. Guano contains 9–21% nitrogen, which is composed primarily of uric acid (∼80%), with smaller amounts of protein (∼10%), ammonia (∼7%), and nitrate (∼0.5%) [23], [61][67]. In addition, guano contains ∼4% phosphorous (∼50% of which is PO43−) and 2% potassium [62], [67], [68].

Table 2. δ15N values of organic and inorganic fertilizers.

A simplified pathway for guano nitrogen, with associated nitrogen-isotope fractionation factors, is shown in Figure 3. The principal producers of guano on the western coast of South America are the Peruvian booby (Sula variegata), brown pelican (Pelecanus occidentalis thagus), and guanay cormorant (Phalacrocorax bougainvilli) [1], [12]. These birds, and similar species, feed at high trophic levels, and typically have tissue δ15N values in the range of 17 to 20‰ [69][71], suggesting a δ15Ndiet of 14 to 18‰ assuming a diet–tissue fractionation of 3–4‰ for δ15N [72]. Thus, the high trophic level of the birds only partially explains the very high δ15Nbulk guano of 26.7‰.

Figure 3. Simplified schematic of fractionation factors associated with decomposition and uptake of seabird guano N.

A) Simplified pathway for guano-derived nitrogen. (a) Incorporation of dietary N into consumer tissue N. Tissue–diet fractionation for birds has been calculated to be ∼3‰ for most tissues [160], [161]. (b) Excretion of dietary N as uric acid. Wainright et al. [162] found bulk guano to be depleted of 15N by 2.5‰ relative to seabird blood. Moreover, Mizutani et al. [75], [163] and Bird et al. [164] found δ15N of uric acid to be very similar to bulk guano δ15N. (c) Conversion of uric acid to NH4+, according to the experiment performed by Mizutani et al. [163]. (d) Ammonia volatilization. Many studies have found this process to be associated with a large equilibrium fractionation that concentrates 15N in the remaining substrate (*NH4+ in the diagram) [54], [75], [165]. (e) Nitrification. The fractionation factor for the entire process of nitrification in the soil (NH4+→NO2→NO3) is estimated to be between −12 and −35‰ [38], [166], [167]. (f) Uptake of NH4+ is associated with a nitrogen isotope fractionation ranging from −6 to −30‰ and appears to depend on the concentration of the source NH4+ [33], [168]. (g) Uptake of NO3 by the plant does not appear to be associated with any fractionation [33], [169], [170]. Both NO3 and NH4+ may be effluxed from the plant, passively and in some cases actively [171]. B) (h) NO3 assimilation into organic N occurs in the root by the NR-NiR (nitrate reductase-nitrite reductase) and GS-GOGAT (glutamine synthetase–glutamine:oxoglutarate aminotransferase) pathways (see Figure 4C). The reduction of NO3 to NH4+ is associated with a fractionation factor of −15‰ [37], [172]. (i) NH4+ assimilation occurs in the root via the GS-GOGAT pathway and is associated with a fractionation factor of −10 to −15‰ [40], [94]. (j, k) NO3 may also be mobilized to the shoot for assimilation. In this case, this NO3 pool has already been exposed to NO3 assimilation in the root and is enriched in 15N [95]. Therefore, organic N formed from NO3 in the shoot (*NO3) will have a higher δ15N value than organic N formed from NO3 in the root. (l) Organics may be moved between the root and shoot. C) Simplified schematic for the assimilation of N by plants. For a more detailed description see Miller and Cramer [171]. All fractionation factors are approximate values representing medians of ranges, which may be large (see text for discussion).

After deposition in the soil, the uric acid in guano is rapidly mineralized to NH4+, and this process occurs much more rapidly in the presence of water [68], [73], [74]. Based on results presented by Mizutani and Wada [65], uric acid quickly decomposed (75% in ten days) in soil, but the δ15N value of the remaining uric acid was unchanged. A very large isotopic fractionation (−40 to −60‰) occurs during NH3 volatilization, leaving the remaining soil NH4+ highly enriched in 15N [38], [75]. Ammonia volatilization is largely responsible for the high δ15N values in ornithogenic soils and in some cases, seabird guano (Table 1). The relatively high δ15N value of the guano utilized in this study suggests that some of the NH4+ in the guano had been subject to volatilization prior to deposition in the soil during the experiment; similar observations have been made concerning other avian manures [76].

15N Enrichment in Guano Fertilized Plants

Plant isotopic compositions are summarized in Table 3; raw data are presented in Table S1. Plant organs of fertilized plants (G1, G2) sampled at 115 d were significantly enriched in 15N compared to control plants in every case (Tables 3, 4; Figure 4). Also, the δ15N values of plant tissues were significantly higher for heavily fertilized (G2) versus more lightly fertilized (G1) plants (Tables 3, 4). The difference in mean δ15N values between the G1 and G2 plant organs was fairly consistent: 6.2‰ for stalks and roots, 6.4‰ for leaves (at 115 d), 7.6‰ for grain, and 7.8‰ for anthers.

Figure 4. Mean nitrogen isotope composition of maize organs; horizontal bars represent means, vertical bars represent standard deviations.

Values above G1 and G2 maize represent differences in nitrogen isotopic composition relative to C0 maize.

Table 3. Isotopic and elemental compositions of plant samples (mean±1σ).

Table 4. Results of ANOVA for differences in isotopic and elemental tissue compositions between treatments.

A growing body of literature has emerged in recent years demonstrating that organic fertilizers, specifically those derived from animal waste, can cause large 15N enrichments of plant tissues (Table 5). The δ15N values reported here for plants grown in guano-fertilized soils are significantly higher than any published δ15N values for plants grown on other organic fertilizers to date (Table 5), but comparable to δ15N values for plants growing in ornithogenic soils (Table 2). The higher δ15N values in the G1 and G2 compared to the C0 plants is the result of the uptake of 15N-enriched guano-derived nitrogen. Moreover, the significantly higher tissue δ15N values in the G2 compared to G1 plants reflects, at least in part, the greater availability of guano-derived nitrogen throughout the course of the experiment. This does not imply that guano-derived N was absent in the G1 treatment towards the end of the experiment, but it is possible that N immobilization had overtaken N mineralization, reducing the amount of guano-derived N available to the plants.

Table 5. Summary of studies examining the influence of organic fertilization on plant δ15N values.

Elemental Concentration in Plant Parts

There were significant differences in N content between fertilized and control plants, with fertilized plants tending to have significantly higher N (Tables 3, 4). There were no significant differences in C content between control and fertilized plants for all organs, with the exception of the stalks, which had significantly lower %C in the fertilized plants compared to the control, and in G2 compared to G1 plants.

In general, the differences in C and N content between fertilized and unfertilized plants can be attributed to the accumulation of proteins, particularly those related to the GS-GOGAT pathway, that assimilate NH4+ and amino acids. Free amino acids tend to accumulate unabated in plant tissues with increasing supply of N irrespective of source, although different amino acids may accumulate at different rates depending on plant species and N source [45], [77][80]. Moreover, many studies have noted an increase in proteins, such as GS, in plant tissue in accordance with increasing NH4+ supply [80], [81]. Thus, the relatively high N content of the organs of fertilized plants likely reflects the accumulation of these N compounds.

The two amino acids that dominate the free amino acid pool when plants are supplied with excess N are glutamine and arginine [77], [82]. Arginine, which has a very low C∶N ratio (6∶4), has been implicated as an important product for the accumulation of excess N, possibly as a buffering mechanism against NH4+ toxicity [45], [83], [84]. Again, the accumulation of high levels of arginine in NH4+-fed plants fits with the pattern observed in the G1, and particularly the G2 plants. The very high levels of N and low levels of C in the stalks of the fertilized plants (compared to the control) suggests that the stalk was the most important accumulator for metabolites produced from excess N.

A notable exception to the pattern of increased N with fertilization is the grain, for which there was no significant difference in N content between treatments (Table 4). Our results suggest that at different levels of N supply and plant N content, there was no preferential allocation of accumulated N to the grain, and N that was absorbed post-silking was probably not allocated to the grain. A similar pattern was observed by Ma and Dwyer [85], although it is important to bear in mind the variability among maize hybrids in N metabolism during grain filling [86].

As plants progress through various stages of growth, their uptake, metabolism and partitioning of N may change dramatically. In maize, a significant portion (45–65%) of the grain N is obtained from endogenous N reallocated primarily from the stalk and leaves, while the remaining grain N is obtained from uptake of exogenous soil N [87][90]. Leaf N content at 75 d and 115 d varied as a function of the amount of guano applied (ie. C<G1≤G2), although this was not the case for leaves sampled at 30 d, where there was no clear relationship between quantity of fertilizer applied and leaf N content (Figure 5a). This likely reflects both a reliance on stored seed N early in growth, and the short period of growth prior to transplanting (7 d) during which no fertilizer N was available.

Figure 5. Temporal patterns in isotopic and element composition.

(A) Leaf N content, (B) Leaf δ15N, and (C) standard deviation for Leaf δ15N.

We observed decreases in leaf N content over time, with leaf N content decreasing by 77.9% (C0), 46.9% (G1) and 47.1% (G2) between 30 and 115 d (Figure 5a). The maintenance of very high levels of N in G1 and G2 relative to C0 plants suggests the accumulation of plant N as a result of excess source N [91].

Based on the results of this study, seabird guano fertilization has the potential to significantly alter the C and N economy of maize plants. Specifically, fertilization results in increased N and decreased C∶N ratio in plant tissues, which likely arises because of increased accumulation of N–rich metabolites such as arginine, glutamine, and proteins related to NH4+ metabolism.

Intraplant Variation in δ15N

Intraplant variability in nitrogen isotopic composition for all treatments was large, with maximum differences between mean organ δ15N being 3.0‰ for C0, 12.9‰ for G1 and 11.4‰ for G2 (Figure 4). We found significant differences in the δ15N values between maize plant organs for both control (F4,20 = 7.41, p<0.001) and fertilized (F4,20 = 18.60, p<0.001 for G1; F4,20 = 28.73, p<0.001 for G2) treatments (Figure 4). In all treatments, the grain possessed the lowest δ15N value, while anthers had the highest δ15N values in the control treatment and the second-highest δ15N values in the fertilized treatments, following stalks (Figure 4).

Significant variability in δ15N within plants has been recorded in several studies [30], [92][98]. Evans [99] suggests that, in general, plants with NO3 as the primary N source are characterized by significant intraplant variability, while this is not true for plants with NH4+ as their primary N source. This general pattern results largely from the fact that NH4+ is assimilated into organic N only in the root, while NO3 assimilation occurs both in roots and shoots (Figure 3B) [57], [100], [101]. Therefore, organics derived from NH4+ are assimilated from the same N pool in the roots, while NO3 that has been translocated to the shoot prior to assimilation has already undergone some fractionation (in the roots) and is thus enriched in 15N [30], [95], [99].

The δ15N values of the roots were intermediate compared to other above-ground tissues, which does not fit with the scenario described above for NO3 fed plants in which shoot tissues have higher δ15N values than roots. In the C0 and G1 plants, the roots did not differ significantly from stalks, grains, or leaves in terms of δ15N (Table 6). In the G2 plants, root δ15N was significantly lower relative to the stalk, but significantly higher than the leaf or grain (Table 6). The lack of a consistent pattern of root vs. shoot δ15N observed in this study likely reflects complex N metabolism, with relative reliance on NH4+ and NO3, as well as guano-derived N changing over time.

Table 6. Results of ANOVA for differences in nitrogen isotopic composition between plant parts.

The relatively low grain δ15N values observed in this study are indicative of the reallocation of stored N. Choi et al. [102] also observed that grain tended to be depleted of 15N compared to stalks and leaves. This can be attributed to a kinetic isotope effect associated with catabolism and remobilization of stored plant N, which discriminates against 15N [103]. The high δ15N values of stalks suggest that this organ is an important source of accumulated N that is remobilized during grain filling. This supports the findings of Ta [104], who found that maize stalks functioned as a significant temporary storage reservoir for N-compounds. It is surprising that the leaves at 115 d are not characterized by higher δ15N values in comparison to the grain, as they are thought to be a significant contributor to grain N [105], [106]; this is discussed in more detail below. The importance of stalk, compared to leaf, N during grain filling may be specific to this variety of maize. Further study of the nitrogen metabolism of different maize hybrids is needed to clarify this issue.

Temporal Variation in Plant δ15N Values

There was significant variability in maize leaves over the course of the experiment (Figure 5B). Maize leaves sampled at 115 d had lower δ15N values than those sampled at 75 d for all treatments; these differences were statistically significant for the fertilized groups, but not for the control group (Table 4). For all treatments, leaf δ15N values were significantly lower at 30 d compared to 75 d (Table 4).

Several studies have attempted to document changes in plant δ15N values over time and/or arising from natural leaf senescence. Kolb and Evans [97] and Garten [107] found no significant differences in the δ15N values of living and abscised leaves, which suggested a lack of 15N discrimination with N remobilization. Conversely, several other studies have found older or senescent plant leaves to be characterized by higher δ15N values, which has been attributed to a kinetic isotopic fractionation associated with N catabolism and reallocation [108][110]. We observed no significant difference between leaf δ15N at 75 d and 115 d for the control group, suggesting that under normal circumstances, there is no significant fractionation associated with N remobilization from leaves for this variety of maize. That there was a concurrent decrease in N content and δ15N for leaves between 75 and 115 d in the fertilized plants is counterintuitive, as the reallocation of leaf N to the grain should result in a 15N-enriched leaf. As was previously suggested for the stalk, we suspect that a significant portion of the leaf N pool consisted of accumulated N in the form of free amino acids (especially arginine and glutamine) as a result of high N supply and, in particular, high source [NH4+]. The reason that older or senescent plant parts are characterized by higher δ15N values is because the metabolic processes involved (e.g. deamination, transamination) are associated with large kinetic fractionations that concentrate the remaining substrate in 15N [111]. Therefore, if the majority of the decrease in leaf N between 75 and 115 d is the result of the transfer of organic N products (amino acids) to another part in the plant (e.g. the stalk), which is not associated with any known 15N fractionation [112], this would help to explain why the leaves are not relatively enriched in 15N at 115 compared to 75 d.

Leaf δ15N values were more variable at 30 d than at either 75 or 115 d (Figure 5c). This is likely a result of variable reliance on stored and absorbed N sources. Kolb and Evans [97] found that young leaves (Quercus and Encelia) had an isotopic composition (δ15N) that reflected both stored and absorbed N, while mature leaf δ15N values reflected primarily absorbed N. Very low leaf δ15N values (−12.4, −12.4, −10.2‰) were observed at 30 d for three of the guano-fertilized maize plants. These compositions probably arise from physiological responses to high soil [NH4+]. At high extracellular [NH4+], influx of NH4+ occurs only via a low-affinity transport system, with high-affinity transport system proteins being down-regulated; this process occurs in concert with the active efflux of NH4+ from the roots [34]. Yoneyama et al. [33] suggest that when NH4+ assimilation is slow (because extracellular [NH4+] is high), NH4+-N isotopic fractionation is larger, with relatively more 15N-enriched NH4+ being effluxed from the cell. Ariz et al. [34] found plants that were most sensitive to NH4+ toxicity also had the lowest tissue δ15N values. The fact that not all plants in the present study were characterized by low leaf δ15N values is difficult to explain, but may be the result of heterogeneous distribution of the guano throughout the soil or genotypic variability in resilience to NH4+ toxicity.

Temporal patterns in plant δ15N values are complicated and are determined by a number of factors. We suspect that significant changes in the N source occurred over time as a result of soil nitrification, and there were also significant changes in [source N] over time. This complication, however, is a reality of working with animal fertilizers, rather than hydroponic solutions, and must be taken into account when interpreting data from field settings.

Guano Fertilization and Plant Carbon Isotopic Composition

We observed no difference in plant δ13C values resulting from guano fertilization for any of the organs analyzed (Tables 3, 4). In earlier studies, variable plant N sources have been associated with small, but significant variations in δ13C values [113]. It is thought that this association arises because different N sources (and different N source concentrations) may alter plant water-use efficiency and thus change the carbon isotope composition of plant tissues [114].

Previous studies have found plant δ13C values to be distinct in organic vs. inorganic fertilization regiments, an outcome ascribed to higher rates of soil microbiological activity [115], [116]. Specifically, Georgi et al. [116] suggest that CO2 released during decomposition is depleted of 13C. Because control and fertilized plants were grown in the same growth chamber, there would be no differences in the δ13C of CO2 utilized by either group of plants, although this may not be true for an agricultural field fertilized with guano. In general, the influence on nitrogenous fertilizers (both organic and inorganic) on plant δ13C is unclear. Experimental results have been conflicting, with studies finding δ13C values to increase [117][122], decrease [123], or be unaffected [120], [124] in response to N fertilization. The relationship between N fertilizer application and plant δ13C is likely mediated by several factors and warrants further study. We likely did not detect any difference in plant δ13C values resulting from fertilization because the magnitude of difference would be quite small [113] and our sample size was also quite small (n = 5 per treatment).

Implications for Food Chemistry

Seabird guano is becoming increasingly popular as an organic alternative among farmers in the United States and Europe [5]. Moreover, as the demand for organically grown produce soars worldwide [125], there is an increased incentive for farmers in areas in close proximity to guano deposits (e.g. Peru, Ecuador, Chile, and Namibia) to use this fertilizer and market their produce as organic [5]. In recent years, there has been a surge in isotopic research directed at demonstrating isotopic distinctions between conventional and organically grown produce [126][136]. The reason that this technique may sometimes be effective is primarily that inorganic fertilizers tend to have δ15N values close to 0‰, while organic fertilizers tend to have higher δ15N values, although there is great variability (Table 2). Based on the results of this study, the application of seabird guano in an organic fertilization regime would result in a very large 15N enrichment of all plant tissues in comparison to unfertilized plants, or to plants treated with chemical fertilizers. The magnitude of this difference is much greater than what has been observed for other organic fertilizers (Table 5), and thus isotopic data would be useful in verifying use of seabird guano. Moreover, the very high δ15N value of the guano itself suggests that its presence in mixed organic fertilizers should also be detectable via isotope ratio mass spectrometry.

Implications for Archaeology

Stable isotope analysis (δ13C and δ15N in particular) plays an increasingly important role in the reconstruction of prehistoric diet. Dietary reconstruction requires a thorough understanding of the sources of isotopic variation in the foods that were consumed [137]. Recently, the notion that animal manure may have influenced the δ15N values of plants grown in prehistoric Europe has been proposed [138][140] and integrated into regional paleodietary studies. In the Andean region, several fertilizers are thought to have been of some importance in prehispanic agriculture including llama dung [141] and seabird guano [9], [142], [143]. Based on the large settlements that developed on the coast of Peru (e.g. Moche, Chimú) and the relative infertility of local soils, Nordt et al. [8] have suggested that the application of some kind of nitrogenous fertilizer, possibly seabird guano, would have been necessary to maintain agricultural productivity in at least some parts of the region. Direct evidence for fertilization, however, is very difficult to come by. One of the primary goals of this study was to determine whether or not the enrichment in 15N resulting from guano fertilization would be sufficient to detect this agricultural practice in the isotopic composition of a human or animal consuming the fertilized plant. Based on the results of this study and others that have examined the biogeochemistry of seabird-associated sites (summarized in Table 2), the application of seabird guano to agricultural fields would have caused a significant increase in the δ15N value of plants and of animals consuming these plants. In archaeological bone collagen from western South America, high δ15N values are usually accompanied by high δ13C values. This pattern applies to both humans [144][146] and domestic animals [147], and has generally been attributed to the consumption of high trophic-level marine resources (e.g. predatory fish, marine mammals). Conversely, this pattern may also be caused by the consumption of maize (a C4 plant) fertilized with seabird guano, which appears (isotopically) very much like a high-trophic level marine organism. As such, it is important to be mindful of the possibility of guano-fertilization when interpreting diet, not just on the coast, but in the interior highland region as well. According to ethnohistoric documents, guano was moved great distances and prized by groups living in the highlands as an essential component in maize agriculture [9].

The Andes were certainly not the only region in which seabird guano was used extensively as a fertilizer. Millions of tonnes of guano were exported to Europe and North America during the nineteenth century and Peruvian seabird guano was the most highly prized fertilizer at that time [148][150]. Isotopic analysis is being employed with increased frequency within the context of historical archaeology [151][159], a period during which the possible influence of seabird guano must also be considered.

Supporting Information

Table S1.

Raw isotopic and elemental data for all samples analyzed.



Steve Bartlett (Biotron), Kim Law and Li Huang (LSIS) provided technical assistance. Sharon Buck assisted with sample preparation. This is Laboratory for Stable Isotope Science Contribution #279.

Author Contributions

Conceived and designed the experiments: PS FJL JFM CDW. Performed the experiments: PS. Analyzed the data: PS FJL CDW. Wrote the paper: PS.


  1. 1. Duffy DC (1994) The guano islands of Peru: the once and future management of a renewable resource. In: Nettleship DN, Burger J, Gochfeld M, editors. Seabirds on Islands, Threats, Case Studies and Action Plans. Cambridge: BirdLife Conservation Series, No. 1. pp. 68–76.
  2. 2. Murphy RC (1981) The guano and the anchoveta fishery. In: Glantz MH, Thompson JD, editors. Resource Management and Environmental Uncertainty: Lessons from Coastal Upwelling Fisheries. New York: Wiley. pp. 81–106.
  3. 3. Mathew WM (1970) Peru and the British Guano Market, 1840–1870. The Economic History Review 23: 112–128.
  4. 4. Zapata F, Arrillaga JL (2002) Agronomic evaluation of guano sources by means of isotope techniques. In: Sikora F, editor. Assessment of soil phosphorus status and management of phosphatic fertilisers to optimise crop production. Vienna: International Atomic Energy Agency. pp. 83–89.
  5. 5. Romero S (2008) Peru guards its guano as demand soars again. New York Times. May 30 ed. New York.
  6. 6. Garcilaso de la Vega I (1966) Royal Commentaries of the Incas and General History of Peru. Livermore HL, translator. Austin: University of Texas Press. 1530 p.
  7. 7. Cieza de León Pd (1964) The Travels of Pedro de Cieza de León: A.D. 1532–50, contained in the first part of his Chronicle of Peru. Markham CR, translator. New York: Franklin. 438 p.
  8. 8. Nordt L, Hayashida F, Hallmark T, Crawford C (2004) Late prehistoric soil fertility, irrigation management, and agricultural production in northwest coastal Peru. Geoarchaeology 19: 21–46.
  9. 9. Julien CJ (1985) Guano and resource control in sixteenth-century Arequipa. In: Masuda S, Shimada I, Morris C, editors. Andean Ecology and Civilization: An Interdisciplinary Perspective on Andean Ecological Complementarity. Tokyo: University of Tokyo Press. pp. 185–231.
  10. 10. Powell GVN, Fourqurean JW, Kenworthy WJ, Zieman JC (1991) Bird colonies cause seagrass enrichment in a subtropical estuary: Observational and experimental evidence. Estuarine, Coastal and Shelf Science 32: 567–579.
  11. 11. Ishizuka K (1966) Ecology of the ornithocoprophilous plant communities on breeding places of the black-tailed gull, Larus crassirostris, along the coast of Japan: I. Vegetation analysis. Ecological Review 16: 229–244.
  12. 12. Hutchinson GE (1950) Survey of Existing Knowledge of Biogeochemistry: 3. The Biogeochemistry of Vertebrate Excretion. Bulletin of the American Museum of Natural History 96: 1–554.
  13. 13. Anderson WB, Polis GA (1999) Nutrient fluxes from water to land: seabirds affect plant nutrient status on Gulf of California islands. Oecologia 118: 324–332.
  14. 14. Ryan PG, Watkins BP (1989) The influence of physical factors and ornithogenic products on plant and arthropod abundance at an Inland Nunatak group in Antarctica. Polar Biology 10: 151–160.
  15. 15. Burger AE, Lindeboom HJ, Williams AJ (1978) The mineral and energy contributions of guano of selected species of birds to the Marion Island terrestrial ecosystem. South African Journal of Antarctic Research 8: 59–70.
  16. 16. Engelskjøn T (1986) Botany of two Antarctic mountain ranges: Gjelsvikfjella and Mühlig-Hofmannfjella, Dronning Maud Land. Polar Research 4: 205–224.
  17. 17. Leentvaar P (1967) Observations in guanotrophic environments. Hydrobiologia 29: 441–489.
  18. 18. McColl JG, Burger J (1976) Chemical Inputs by a Colony of Franklin's Gulls Nesting in Cattails. American Midland Naturalist 96: 270–280.
  19. 19. Smith VR (1978) Animal-plant-soil nutrient relationships on Marion Island (Subantarctic). Oecologia 32: 239–253.
  20. 20. Speir TW, Cowling JC (1984) Ornithogenic soils of the Cape Bird Adelie penguin rookeries, Antarctica: 1. Chemical Properties. Polar Biology 2: 199–205.
  21. 21. Ellis JC (2005) Marine Birds on Land: A Review of Plant Biomass, Species Richness, and Community Composition in Seabird Colonies. Plant Ecology 181: 227–241.
  22. 22. Vidal E, Jouventin P, Frenot Y (2003) Contribution of alien and indigenous species to plant-community assemblages near penguin rookeries at Crozet archipelago. Polar Biology 26: 432–437.
  23. 23. Lindeboom HJ (1984) The Nitrogen Pathway in a Penguin Rookery. Ecology 65: 269–277.
  24. 24. Williams AJ, Berruti A (1978) Mineral and energy contributions of feathers moulted by penguins, gulls and cormorants to the Marion Island terrestrial ecosystem. South African Journal of Antarctic Research 8: 71–74.
  25. 25. Williams AJ, Burger AE, Berruti A (1978) Mineral and energy contributions of carcasses of selected species of seabirds to the Marion Island terrestrial ecosystem. South African Journal of Antarctic Research 8: 53–58.
  26. 26. Siegfried WR, Williams AJ, Burger AE, Berruti A (1978) Mineral and energy contributions of eggs of selected species of seabirds to the Marion Island terrestrial ecosystem. South African Journal of Antarctic Research 8: 75–87.
  27. 27. Gillham ME (1956) Ecology of the Pembrokeshire Islands: V. Manuring by the Colonial Seabirds and Mammals, with a Note on Seed Distribution by Gulls. Journal of Ecology 44: 429–454.
  28. 28. Crawford NM, Glass ADM (1998) Molecular and physiological aspects of nitrate uptake in plants. Trends in Plant Science 3: 389–395.
  29. 29. Andrews M (1986) The partitioning of nitrate assimilation between root and shoot of higher plants. Plant, Cell & Environment 9: 511–519.
  30. 30. Yoneyama T, Kaneko A (1989) Variations in the Natural Abundance of 15N in Nitrogenous Fractions of Komatsuna Plants Supplied with Nitrate. Plant and Cell Physiology 30: 957–962.
  31. 31. Mariotti A, Mariotti F, Amargar N, Pizelle G, Ngambi JM, et al. (1980) Fractionnements isotopiques de l'azote lors des processus d'absorption des nitrates et de fixation de l'azote atmosphérique par les plantes. Physiologie Végétale 18: 163–181.
  32. 32. Yoneyama T, Ito O, Engelaar WMHG (2003) Uptake, metabolism and distribution of nitrogen in crop plants traced by enriched and natural 15N: Progress over the last 30 years. Phytochemistry Reviews 2: 121–132.
  33. 33. Yoneyama T, Matsumaru T, Usui K, Engelaar WMHG (2001) Discrimination of nitrogen isotopes during absorption of ammonium and nitrate at different nitrogen concentrations by rice (Oryza sativa L.) plants. Plant, Cell & Environment 24: 133–139.
  34. 34. Ariz I, Cruz C, Moran J, González-Moro M, García-Olaverri C, et al. (2011) Depletion of the heaviest stable N isotope is associated with NH4+/NH3 toxicity in NH4+-fed plants. BMC Plant Biology 11: 83.
  35. 35. Flores-Delgadillo L, Fedick SL, Solleiro-Rebolledo E, Palacios-Mayorga S, Ortega-Larrocea P, et al. (2011) A sustainable system of a traditional precision agriculture in a Maya homegarden: Soil quality aspects. Soil and Tillage Research 113: 112–120.
  36. 36. Pritchard ES, Guy RD (2005) Nitrogen isotope discrimination in white spruce fed with low concentrations of ammonium and nitrate. Trees - Structure and Function 19: 89–98.
  37. 37. Ledgard SF, Woo KC, Bergersen FJ (1985) Isotopic fractionation during reduction of nitrate and nitrite by extracts of spinach leaves. Australian Journal of Plant Physiology 12: 631–640.
  38. 38. Robinson D (2001) δ15N as an integrator of the nitrogen cycle. Trends in Ecology & Evolution 16: 153–162.
  39. 39. Glass ADM, Brito DT, Kaiser BN, Kronzucker HJ, Kumar A, et al. (2001) Nitrogen transport in plants, with an emphasis on the regulation of fluxes to match plant demand. Journal of Plant Nutrition and Soil Science 164: 199–207.
  40. 40. Yoneyama T, Kamachi K, Yamaya T, Mae T (1993) Fractionation of Nitrogen Isotopes by Glutamine Synthetase Isolated from Spinach Leaves. Plant and Cell Physiology 34: 489–491.
  41. 41. Subedi KD, Ma BL (2005) Nitrogen Uptake and Partitioning in Stay-Green and Leafy Maize Hybrids. Crop Science 45: 740–747.
  42. 42. Ishida A (1997) Seed germination and seedling survival in a colony of the common cormorant, Phalacrocorax carbo. Ecological Research 12: 249–256.
  43. 43. Mulder C, Keall S (2001) Burrowing seabirds and reptiles: impacts on seeds, seedlings and soils in an island forest in New Zealand. Oecologia 127: 350–360.
  44. 44. Bremner JM, Krogmeier MJ (1989) Evidence that the adverse effect of urea fertilizer on seed germination in soil is due to ammonia formed through hydrolysis of urea by soil urease. Proceedings of the National Academy of Sciences 86: 8185–8188.
  45. 45. Roosta HR, Schjoerring JK (2007) Effects of Ammonium Toxicity on Nitrogen Metabolism and Elemental Profile of Cucumber Plants. Journal of Plant Nutrition 30: 1933–1951.
  46. 46. Schortemeyer M, Stamp P, Feil B (1997) Ammonium Tolerance and Carbohydrate Status in Maize Cultivars. Annals of Botany 79: 25–30.
  47. 47. Wootton JT (1991) Direct and indirect effects of nutrients on intertidal community structure: variable consequences of seabird guano. Journal of Experimental Marine Biology and Ecology 151: 139–153.
  48. 48. Weseloh DV, Brown RT (1971) Plant Distribution within a Heron Rookery. American Midland Naturalist 86: 57–64.
  49. 49. Clarkson DT, Scattergood CB (1982) Growth and Phosphate Transport in Barley and Tomato Plants During the Development of, and Recovery from, Phosphate-stress. Journal of Experimental Botany 33: 865–875.
  50. 50. Ligeza S, Smal H (2003) Accumulation of nutrients in soils affected by perennial colonies of piscivorous birds with reference to biogeochemical cycles of elements. Chemosphere 52: 595–602.
  51. 51. García LV, Marañón T, Ojeda F, Clemente L, Redondo R (2002) Seagull influence on soil properties, chenopod shrub distribution, and leaf nutrient status in semi-arid Mediterranean islands. Oikos 98: 75–86.
  52. 52. Young HS, McCauley DJ, Dirzo R (2011) Differential responses to guano fertilization among tropical tree species with varying functional traits. American Journal of Botany 98: 207–214.
  53. 53. Mizota C (2009) Temporal variations in the concentration and isotopic signature of ammonium- and nitrate-nitrogen in soils under a breeding colony of Black-tailed Gulls (Larus crassirostris) on Kabushima Island, northeastern Japan. Applied Geochemistry 24: 328–332.
  54. 54. Mizutani H, Hasegawa H, Wada E (1986) High nitrogen isotope ratio for soils of seabird rookeries. Biogeochemistry 2: 221–247–247.
  55. 55. Schmidt S, Dennison WC, Moss GJ, Stewart GR (2004) Nitrogen ecophysiology of Heron Island, a subtropical coral cay of the Great Barrier Reef, Australia. Functional Plant Biology 31: 517–528.
  56. 56. Wait DA, Aubrey DP, Anderson WB (2005) Seabird guano influences on desert islands: soil chemistry and herbaceous species richness and productivity. Journal of Arid Environments 60: 681–695.
  57. 57. Raven JA, Smith FA (1976) Nitrogen assimilation and transport in vascular land plants in relation to intracellular pH regulation. New Phytologist 76: 415–431.
  58. 58. Deignan MT, Lewis OAM (1988) The inhibition of ammonium uptake by nitrate in wheat. New Phytologist 110: 1–3.
  59. 59. Guo S, Kaldenhoff R, Uehlein N, Sattelmacher B, Brueck H (2007) Relationship between water and nitrogen uptake in nitrate- and ammonium-supplied Phaseolus vulgaris L. plants. Journal of Plant Nutrition and Soil Science 170: 73–80.
  60. 60. Britto DT, Siddiqi MY, Glass ADM, Kronzucker HJ (2001) Futile transmembrane NH4+ cycling: A cellular hypothesis to explain ammonium toxicity in plants. Proceedings of the National Academy of Sciences 98: 4255–4258.
  61. 61. Gillham ME (1960) Destruction of indigenous heath vegetation in Victorian sea-bird colonies. Australian Journal of Botany 8: 277–317.
  62. 62. Hartz TK, Johnstone PR (2006) Nitrogen availability from high-nitrogen-containing organic fertilizers. HortTechnology 16: 39–42.
  63. 63. Mizutani H, Kabaya Y, Wada E (1991) Linear correlation between latitude and soil15N enrichment at seabird rookeries. Naturwissenschaften 78: 34–36.
  64. 64. McNabb EMA, McNabb RA, Prather ID, Conner RN, Adkisson CS (1980) Nitrogen Excretion by Turkey Vultures. The Condor 82: 219–223.
  65. 65. Mizutani H, Wada E (1985) High-performance liquid chromatographic determination of uric acid in soil. Journal of Chromatography A 331: 359–369.
  66. 66. Gaskell M, Smith R (2007) Nitrogen sources for organic vegetable crops. HortTechnology 17: 431–441.
  67. 67. Staunton Smith J, Johnson CR (1995) Nutrient inputs from seabirds and humans on a populated coral cay. Marine Ecology Progress Series 124: 189–200.
  68. 68. Hadas A, Rosenberg R (1992) Guano as a nitrogen source for fertigation in organic farming. Nutrient Cycling in Agroecosystems 31: 209–214.
  69. 69. Forero MG, Bortolotti GR, Hobson KA, Donazar JA, Bertelloti M, et al. (2004) High trophic overlap within the seabird community of Argentinean Patagonia: a multiscale approach. Journal of Animal Ecology 73: 789–801.
  70. 70. Schoeninger MJ, DeNiro MJ (1984) Nitrogen and carbon isotopic composition of bone collagen from marine and terrestrial animals. Geochimica et Cosmochimica Acta 48: 625–639.
  71. 71. Szpak P, Orchard TJ, Gröcke DR (2009) A Late Holocene vertebrate food web from southern Haida Gwaii (Queen Charlotte Islands, British Columbia). Journal of Archaeological Science 36: 2734–2741.
  72. 72. Szpak P, Orchard TJ, McKechnie I, Gröcke DR (2012) Historical ecology of late Holocene sea otters (Enhydra lutris) from northern British Columbia: isotopic and zooarchaeological perspectives. Journal of Archaeological Science 39: 1553–1571.
  73. 73. Loder TC, Ganning B, Love JA (1996) Ammonia nitrogen dynamics in coastal rockpools affected by gull guano. Journal of Experimental Marine Biology and Ecology 196: 113–129.
  74. 74. Kirchmann H (1991) Carbon and nitrogen mineralization of fresh, aerobic and anaerobic animal manures during incubation with soil. Swedish Journal of Agricultural Research 21: 165–173.
  75. 75. Mizutani H, Kabaya Y, Wada E (1985) Ammonia volatilization and high 15N/14N ratio in a penguin rookery in Antarctica. Geochemical Journal 19: 323–327.
  76. 76. Burger M, Venterea RT (2008) Nitrogen Immobilization and Mineralization Kinetics of Cattle, Hog, and Turkey Manure Applied to Soil. Soil Science Society of America Journal 72: 1570–1579.
  77. 77. Okano K, Chutani K, Matsuo K (1997) Suitable level of nitrogen fertilizer for tea (Cameliia sinensis L.) plants in relation to growth, photosynthesis, nitrogen uptake and accumulation of free amino acids. Japanese Journal of Crop Science 66: 279–287.
  78. 78. Näsholm T, Ericsson A (1990) Seasonal changes in amino acids, protein and total nitrogen in needles of fertilized Scots pine trees. Tree Physiology 6: 267–281.
  79. 79. Warren CR, Adams MA (2000) Capillary electrophoresis for the determination of major amino acids and sugars in foliage: application to the nitrogen nutrition of sclerophyllous species. Journal of Experimental Botany 51: 1147–1157.
  80. 80. Ruan J, Gerendás J, Härdter R, Sattelmacher B (2007) Effect of root zone pH and form and concentration of nitrogen on accumulation of quality-related components in green tea. Journal of the Science of Food and Agriculture 87: 1505–1516.
  81. 81. Garnica M, Houdusse F, Zamarreño AM, Garcia-Mina JM (2010) Nitrate modifies the assimilation pattern of ammonium and urea in wheat seedlings. Journal of the Science of Food and Agriculture 90: 357–369.
  82. 82. Ruan J, Haerdter R, Gerendás J (2010) Impact of nitrogen supply on carbon/nitrogen allocation: a case study on amino acids and catechins in green tea [Camellia sinensis (L.) O. Kuntze] plants. Plant Biology 12: 724–734.
  83. 83. Smolders AJP, den Hartog C, van Gestel CBL, Roelofs JGM (1996) The effects of ammonium on growth, accumulation of free amino acids and nutritional status of young phosphorus deficient Stratiotes aloides plants. Aquatic Botany 53: 85–96.
  84. 84. Potel F, Valadier M-H, Ferrario-Méry S, Grandjean O, Morin H, et al. (2009) Assimilation of excess ammonium into amino acids and nitrogen translocation in Arabidopsis thaliana– roles of glutamate synthases and carbamoylphosphate synthetase in leaves. FEBS Journal 276: 4061–4076.
  85. 85. Ma BL, Dwyer LM (1998) Nitrogen uptake and use of two contrasting maize hybrids differing in leaf senescence. Plant and Soil 199: 283–291.
  86. 86. Rajcan I, Tollenaar M (1999) Source: sink ratio and leaf senescence in maize: II. Nitrogen metabolism during grain filling. Field Crops Research 60: 255–265.
  87. 87. Gallais A, Coque M, Quilléré I, Prioul J-L, Hirel B (2006) Modelling postsilking nitrogen fluxes in maize (Zea mays) using 15N-labelling field experiments. New Phytologist 172: 696–707.
  88. 88. Gallais A, Coque M (2005) Genetic variation and selection for nitrogen use efficiency in maize: A synthesis. Maydica 50: 531–547.
  89. 89. Tsai CY, Huber DM, Warren HL (1980) A Proposed Role of Zein and Glutelin as N Sinks in Maize. Plant Physiology 66: 330–333.
  90. 90. Below FE, Christensen LE, Reed AJ, Hageman RH (1981) Availability of Reduced N and Carbohydrates for Ear Development of Maize. Plant Physiology 68: 1186–1190.
  91. 91. Binford GD, Blackmer AM, El-Hout NM (1990) Tissue test for excess nitrogen during corn production. Agronomy Journal 82: 124–129.
  92. 92. Dijkstra P, Williamson C, Menyailo O, Doucett R, Koch G, et al. (2003) Nitrogen stable isotope composition of leaves and roots of plants growing in a forest and a meadow. Isotopes in Environmental and Health Studies 39: 29–39.
  93. 93. Hobbie EA, Macko SA, Williams M (2000) Correlations between foliar δ15N and nitrogen concentrations may indicate plant-mycorrhizal interactions. Oecologia 122: 273–283.
  94. 94. Yoneyama T, Omata T, Nakata S, Yazaki J (1991) Fractionation of Nitrogen Isotopes during the Uptake and Assimilation of Ammonia by Plants. Plant and Cell Physiology 32: 1211–1217.
  95. 95. Evans RD, Bloom AJ, Sukrapanna SS, Ehleringer JR (1996) Nitrogen isotope composition of tomato (Lycopersicon esculentum Mill. cv. T-5) grown under ammonium or nitrate nutrition. Plant, Cell & Environment 19: 1317–1323.
  96. 96. Högberg P, Högberg MN, Quist ME, Ekblad ALF, Näsholm T (1999) Nitrogen isotope fractionation during nitrogen uptake by ectomycorrhizal and non-mycorrhizal Pinus sylvestris. New Phytologist 142: 569–576.
  97. 97. Kolb KJ, Evans RD (2002) Implications of leaf nitrogen recycling on the nitrogen isotope composition of deciduous plant tissues. New Phytologist 156: 57–64.
  98. 98. Högberg P, Högbom L, Schinkel H, Högberg M, Johannisson C, et al. (1996) 15N abundance of surface soils, roots and mycorrhizas in profiles of European forest soils. Oecologia 108: 207–214.
  99. 99. Evans RD (2001) Physiological mechanisms influencing plant nitrogen isotope composition. Trends in Plant Science 6: 121–126.
  100. 100. Lewis OAM, Leidi EO, Lips SH (1989) Effect of nitrogen source on growth response to salinity stress in maize and wheat. New Phytologist 111: 155–160.
  101. 101. Murphy AT, Lewis OAM (1987) Effect of nitrogen feeding source on the supply of nitrogen from root to shoot and the site of nitrogen assimilation in maize (Zea mays L. CV. R201). New Phytologist 107: 327–333.
  102. 102. Choi W-J, Lee S-M, Ro H-M, Kim K-C, Yoo S-H (2002) Natural 15N abundances of maize and soil amended with urea and composted pig manure. Plant and Soil 245: 223–232.
  103. 103. Tcherkez G (2011) Natural 15N/14N isotope composition in C3 leaves: are enzymatic isotope effects informative for predicting the 15N-abundance in key metabolites?. Functional Plant Biology 38: 1–12.
  104. 104. Ta CT (1991) Nitrogen Metabolism in the Stalk Tissue of Maize. Plant Physiology 97: 1375–1380.
  105. 105. Beauchamp EG, Kannenberg LW, Hunter RB (1976) Nitrogen Accumulation and Translocation in Corn Genotypes Following Silking. Agronomy Journal 68: 418–422.
  106. 106. Donnison IS, Gay AP, Thomas H, Edwards KJ, Edwards D, et al. (2007) Modification of nitrogen remobilization, grain fill and leaf senescence in maize (Zea mays) by transposon insertional mutagenesis in a protease gene. New Phytologist 173: 481–494.
  107. 107. Garten CT (1993) Variation in Foliar 15N Abundance and the Availability of Soil Nitrogen on Walker Branch Watershed. Ecology 74: 2098–2113.
  108. 108. Gebauer G, Giesemann A, Schulze E, Jäger H (1994) Isotope ratios and concentrations of sulfur and nitrogen in needles and soils of Picea abies stands as influenced by atmospheric deposition of sulfur and nitrogen compounds. Plant and Soil 164: 267–281.
  109. 109. Näsholm T (1994) Removal of nitrogen during needle senescence in Scots pine (Pinus sylvestris L.). Oecologia 99: 290–296.
  110. 110. Choi W-J, Chang SX, Ro H-M (2005) Seasonal Changes of Shoot Nitrogen Concentrations and 15N/14N Ratios in Common Reed in a Constructed Wetland. Communications in Soil Science and Plant Analysis 36: 2719–2731.
  111. 111. Högberg P (1997) Tansley Review No. 95 15N natural abundance in soil-plant systems. New Phytologist 137: 179–203.
  112. 112. Robinson D, Handley LL, Scrimgeour CM (1998) A theory for 15N/14N fractionation in nitrate-grown vascular plants. Planta 205: 397–406.
  113. 113. Yin Z-H, Raven JA (1998) Influences of different nitrogen sources on nitrogen- and water-use efficiency, and carbon isotope discrimination in C3 Triticum aestivum L. and C4 Zea mays L. plants. Planta 205: 574–580.
  114. 114. Raven JA, Wollenweber B, Handley LL (1992) A comparison of ammonium and nitrate as nitrogen sources for photolithotrophs. New Phytologist 121: 19–32.
  115. 115. Camin F, Perini M, Bontempo L, Fabroni S, Faedi W, et al. (2011) Potential isotopic and chemical markers for characterising organic fruits. Food Chemistry 125: 1072–1082.
  116. 116. Georgi M, Voerkelius S, Rossmann A, Graßmann J, Schnitzler W (2005) Multielement Isotope Ratios of Vegetables from Integrated and Organic Production. Plant and Soil 275: 93–100.
  117. 117. Serret M, Ortiz-Monasterio I, Pardo A, Araus J (2008) The effects of urea fertilisation and genotype on yield, nitrogen use efficiency, δ15N and δ13C in wheat. Annals of Applied Biology 153: 243–257.
  118. 118. Kondo M, Pablico P, Aragones D, Agbisit R (2004) Genotypic variations in carbon isotope discrimination, transpiration efficiency, and biomass production in rice as affected by soil water conditions and N. Plant and Soil 267: 165–177.
  119. 119. Cabrera-Bosquet L, Molero G, Bort J, Nogués S, Araus JL (2007) The combined effect of constant water deficit and nitrogen supply on WUE, NUE and Δ13C in durum wheat potted plants. Annals of Applied Biology 151: 277–289.
  120. 120. Jenkinson D, Coleman K, Harkness D (1995) The influence of fertilizer nitrogen and season on the carbon-13 abundance of wheat straw. Plant and Soil 171: 365–367.
  121. 121. Zhao LJ, Xiao HL, Liu XH (2007) Relationships Between Carbon Isotope Discrimination and Yield of Spring Wheat Under Different Water and Nitrogen Levels. Journal of Plant Nutrition 30: 947–963.
  122. 122. Iqbal MM, Akhter J, Mohammad W, Shah SM, Nawaz H, et al. (2005) Effect of tillage and fertilizer levels on wheat yield, nitrogen uptake and their correlation with carbon isotope discrimination under rainfed conditions in north-west Pakistan. Soil and Tillage Research 80: 47–57.
  123. 123. Shangguan ZP, Shao MA, Dyckmans J (2000) Nitrogen nutrition and water stress effects on leaf photosynthetic gas exchange and water use efficiency in winter wheat. Environmental and Experimental Botany 44: 141–149.
  124. 124. Clay DE, Engel RE, Long DS, Liu Z (2001) Nitrogen and Water Stress Interact to Influence Carbon-13 Discrimination in Wheat. Soil Science Society of America Journal 65: 1823–1828.
  125. 125. Winter CK, Davis SF (2006) Organic Foods. Journal of Food Science 71: R117–R124.
  126. 126. Schmidt H-l, Roβmann A, Voerkelius S, Schnitzler WH, Georgi M, et al. (2005) Isotope characteristics of vegetables and wheat from conventional and organic production. Isotopes in Environmental and Health Studies 41: 223–228.
  127. 127. Šturm M, Lojen S (2011) Nitrogen isotopic signature of vegetables from the Slovenian market and its suitability as an indicator of organic production. Isotopes in Environmental and Health Studies 47: 214–220.
  128. 128. Bateman AS, Kelly SD, Jickells TD (2005) Nitrogen Isotope Relationships between Crops and Fertilizer: Implications for Using Nitrogen Isotope Analysis as an Indicator of Agricultural Regime. Journal of Agricultural and Food Chemistry 53: 5760–5765.
  129. 129. Bateman AS, Kelly SD, Woolfe M (2007) Nitrogen Isotope Composition of Organically and Conventionally Grown Crops. Journal of Agricultural and Food Chemistry 55: 2664–2670.
  130. 130. Flores P, Fenoll J, Hellín P (2007) The Feasibility of Using δ15N and δ13C Values for Discriminating between Conventionally and Organically Fertilized Pepper (Capsicum annuum L.). Journal of Agricultural and Food Chemistry 55: 5740–5745.
  131. 131. Rapisarda P, Calabretta ML, Romano G, Intrigliolo F (2005) Nitrogen Metabolism Components as a Tool To Discriminate between Organic and Conventional Citrus Fruits. Journal of Agricultural and Food Chemistry 53: 2664–2669.
  132. 132. Rapisarda P, Camin F, Fabroni S, Perini M, Torrisi B, et al. (2010) Influence of Different Organic Fertilizers on Quality Parameters and the δ15N, δ13C, δ2H, δ34S, and δ18O Values of Orange Fruit (Citrus sinensis L. Osbeck). Journal of Agricultural and Food Chemistry 58: 3502–3506.
  133. 133. Rogers KM (2008) Nitrogen Isotopes as a Screening Tool To Determine the Growing Regimen of Some Organic and Nonorganic Supermarket Produce from New Zealand. Journal of Agricultural and Food Chemistry 56: 4078–4083.
  134. 134. Camin F, Moschella A, Miselli F, Parisi B, Versini G, et al. (2007) Evaluation of markers for the traceability of potato tubers grown in an organic versus conventional regime. Journal of the Science of Food and Agriculture 87: 1330–1336.
  135. 135. Šturm M, Kacjan-Maršić N, Lojen S (2011) Can δ15N in lettuce tissues reveal the use of synthetic nitrogen fertiliser in organic production? Journal of the Science of Food and Agriculture 91: 262–267.
  136. 136. Flores P, Murray PJ, Hellín P, Fenoll J (2011) Influence of N doses and form on 15N natural abundance of pepper plants: considerations for using δ15N values as indicator of N source. Journal of the Science of Food and Agriculture 91: 2255–2258.
  137. 137. Schwarcz HP (1991) Some theoretical aspects of isotope paleodiet studies. Journal of Archaeological Science 18: 261–275.
  138. 138. Bogaard A, Heaton THE, Poulton P, Merbach I (2007) The impact of manuring on nitrogen isotope ratios in cereals: archaeological implications for reconstruction of diet and crop management practices. Journal of Archaeological Science 34: 335–343.
  139. 139. Commisso RG, Nelson DE (2007) Patterns of plant δ15N values on a Greenland Norse farm. Journal of Archaeological Science 34: 440–450.
  140. 140. Fraser RA, Bogaard A, Heaton T, Charles M, Jones G, et al. (2011) Manuring and stable nitrogen isotope ratios in cereals and pulses: towards a new archaeobotanical approach to the inference of land use and dietary practices. Journal of Archaeological Science 38: 2790–2804.
  141. 141. Chepstow-Lusty AJ (2011) Agro-pastoral and social change in the Cuzco heartland of Peru: a brief history using environmental proxies. Antiquity 85: 570–582.
  142. 142. Kubler G (1948) Towards Absolute Time: Guano Archaeology. Memoirs of the Society for American Archaeology 4: 29–50.
  143. 143. Netherly PJ (1977) Local Level Lords on the North Coast of Peru [Unpublished Ph.D. Dissertation]. Ithaca: Cornell University. 366 p.
  144. 144. Slovak NM, Paytan A (2011) Fisherfolk and farmers: Carbon and nitrogen isotope evidence from Middle Horizon Ancón, Peru. International Journal of Osteoarchaeology 21: 253–267.
  145. 145. Knudson KJ, Aufderheide AE, Buikstra JE (2007) Seasonality and paleodiet in the Chiribaya polity of southern Peru. Journal of Archaeological Science 34: 451–462.
  146. 146. Tomczak PD (2003) Prehistoric diet and socioeconomic relationships within the Osmore Valley of southern Peru. Journal of Anthropological Archaeology 22: 262–278.
  147. 147. DeNiro MJ (1988) Marine food sources for prehistoric coastal Peruvian camelids: isotopic evidence and implications. In: Wing ES, Wheeler JC, editors. Economic Prehistory of the Central Andes. Oxford: British Archaeological Reports International Series 427. pp. 119–128.
  148. 148. Cushman GT (2003) 762 p. The Lords of Guano: Science and the Management of Peru's Marine Environment, 1800–1973 [Unpublished Ph.D. Dissertation]: University of Texas, Austin.
  149. 149. Cordle C (2007) The Guano Voyages. Rural History 18: 119–133.
  150. 150. Simmons D (2006) Waste Not, Want Not: Excrement and Economy in Nineteenth-Century France. Representations 96: 73–98.
  151. 151. Cox G, Sealy J (1997) Investigating Identity and Life Histories: Isotopic Analysis and Historical Documentation of Slave Skeletons Found on the Cape Town Foreshore, South Africa. International Journal of Historical Archaeology 1: 207–224.
  152. 152. Katzenberg MA (1995) Nitrogen isotope evidence for weaning age in a nineteenth century Canadian skeletal sample. In: Grauer AL, editor. Bodies of Evidence: Reconstructing History through Skeletal Analysis. Cambridge: Wiley-Liss. pp. 221–235.
  153. 153. Cox G, Sealy J, Schrire C, Morris A (2001) Stable Carbon and Nitrogen Isotopic Analyses of the Underclass at the Colonial Cape of Good Hope in the Eighteenth and Nineteenth Centuries. World Archaeology 33: 73–97.
  154. 154. Klippel WE (2001) Sugar Monoculture, Bovid Skeletal Part Frequencies, and Stable Carbon Isotopes: Interpreting Enslaved African Diet at Brimstone Hill, St Kitts, West Indies. Journal of Archaeological Science 28: 1191–1198.
  155. 155. Valentin F, Bocherens H, Gratuze B, Sand C (2006) Dietary patterns during the late prehistoric/historic period in Cikobia island (Fiji): insights from stable isotopes and dental pathologies. Journal of Archaeological Science 33: 1396–1410.
  156. 156. Katzenberg M, Saunders S, Abonyi S (2000) Bone Chemistry, Food and History: A Case Study from 19th Century Upper Canada. In: Ambrose SH, Katzenberg MA, editors. Biogeochemical Approaches to Paleodietary Analysis. New York: Kluwer Academic. pp. 1–22.
  157. 157. Roy DM, Hall R, Mix AC, Bonnichsen R (2005) Using stable isotope analysis to obtain dietary profiles from old hair: A case study from Plains Indians. American Journal of Physical Anthropology 128: 444–452.
  158. 158. Schroeder H, O'Connell TC, Evans JA, Shuler KA, Hedges REM (2009) Trans-Atlantic slavery: Isotopic evidence for forced migration to Barbados. American Journal of Physical Anthropology 139: 547–557.
  159. 159. Sealy J, Armstrong R, Schrire C (1995) Beyond lifetime averages: tracing life histories through isotopic analysis of different calcified tissues from archaeological human skeletons. Antiquity 69: 290–300.
  160. 160. Hobson KA, Clark RG (1992) Assessing avian diets using stable isotopes II: factors influencing diet-tissue fractionation. The Condor 94: 189–197.
  161. 161. Hobson KA (1995) Reconstructing Avian Diets Using Stable-Carbon and Nitrogen Isotope Analysis of Egg Components: Patterns of Isotopic Fractionation and Turnover. The Condor 97: 752–762.
  162. 162. Wainright SC, Haney JC, Kerr C, Golovkin AN, Flint MV (1998) Utilization of nitrogen derived from seabird guano by terrestrial and marine plants at St. Paul, Pribilof Islands, Bering Sea, Alaska. Marine Biology 131: 63–71.
  163. 163. Mizutani H, Kabaya Y, Wada E (1985) High-performance liquid chromatographic isolation of uric acid from soil for isotopic determination. Journal of Chromatography A 331: 371–381.
  164. 164. Bird MI, Tait E, Wurster CM, Furness RW (2008) Stable carbon and nitrogen isotope analysis of avian uric acid. Rapid Communications in Mass Spectrometry 22: 3393–3400.
  165. 165. Kirshenbaum I, Smith JS, Crowell T, Graff J, McKee R (1947) Separation of the Nitrogen Isotopes by the Exchange Reaction between Ammonia and Solutions of Ammonium Nitrate. Journal of Chemical Physics 15: 440–446.
  166. 166. Shearer G, Kohl DH (1986) N2-Fixation in Field Settings: Estimations Based on Natural 15N Abundance. Australian Journal of Plant Physiology 13: 699–756.
  167. 167. Feigin A, Shearer G, Kohl DH, Commoner B (1974) The Amount and Nitrogen-15 Content of Nitrate in Soil Profiles from two Central Illinois Fields in a Corn-Soybean Rotation. Soil Science Society of America Journal 38: 465–471.
  168. 168. Hoch MP, Fogel ML, Kirchman DL (1992) Isotope Fractionation Associated with Ammonium Uptake by a Marine Bacterium. Limnology and Oceanography 37: 1447–1459.
  169. 169. Yoneyama T, Fujiwara H, Wilson JW (1998) Variations in fractionation of carbon and nitrogen isotopes in higher plants: N-metabolism and partitioning in phloem and xylem. In: Griffiths H, editor. Stables Isotopes, Integration of Biological, Ecological and Geochemical Processes. Oxford: BIOS Scientific Publishers. pp. 99–109.
  170. 170. Mariotti A, Mariotti F, Champigny M-L, Amarger N, Moyse A (1982) Nitrogen Isotope Fractionation Associated with Nitrate Reductase Activity and Uptake of NO3 by Pearl Millet. Plant Physiology 69: 880–884.
  171. 171. Miller A, Cramer M (2005) Root Nitrogen Acquisition and Assimilation. Plant and Soil 274: 1–36.
  172. 172. Tcherkez G, Farquhar GD (2006) Isotopic fractionation by plant nitrate reductase, twenty years later. Functional Plant Biology 33: 531–537.
  173. 173. Barrett K, Anderson WB, Wait DA, Grismer LL, Polis GA, et al. (2005) Marine subsidies alter the diet and abundance of insular and coastal lizard populations. Oikos 109: 145–153.
  174. 174. Bokhorst S, Huiskes A, Convey P, Aerts R (2007) External nutrient inputs into terrestrial ecosystems of the Falkland Islands and the Maritime Antarctic region. Polar Biology 30: 1315–1321.
  175. 175. Cocks MP, Balfour DA, Stock WD (1998) On the uptake of ornithogenic products by plants on the inland mountains of Dronning Maud Land, Antarctica, using stable isotopes. Polar Biology 20: 107–111.
  176. 176. Erskine PD, Bergstrom DM, Schmidt S, Stewart GR, Tweedie CE, et al. (1998) Subantarctic Macquarie Island – a model ecosystem for studying animal-derived nitrogen sources using 15N natural abundance. Oecologia 117: 187–193.
  177. 177. Harding JS, Hawke DJ, Holdaway RN, Winterbourn MJ (2004) Incorporation of marine-derived nutrients from petrel breeding colonies into stream food webs. Freshwater Biology 49: 576–586.
  178. 178. Hawke DJ, Holdaway RN (2005) Avian assimilation and dispersal of carbon and nitrogen brought ashore by breeding Westland petrels (Procellaria westlandica): a stable isotope study. Journal of Zoology 266: 419–426.
  179. 179. Hawke DJ, Newman J (2007) Carbon-13 and nitrogen-15 enrichment in coastal forest foliage from nutrient-poor and seabird-enriched sites in southern New Zealand. New Zealand Journal of Botany 45: 309–315.
  180. 180. Hobara S, Koba K, Osono T, Tokuchi N, Ishida A, et al. (2005) Nitrogen and phosphorus enrichment and balance in forests colonized by cormorants: Implications of the influence of soil adsorption. Plant and Soil 268: 89–101.
  181. 181. Kameda K, Koba K, Hobara S, Osono T, Terai M (2006) Pattern of natural 15N abundance in lakeside forest ecosystem affected by cormorant-derived nitrogen. Hydrobiologia 567: 69–86.
  182. 182. Kolb G, Jerling L, Hambäck P (2010) The Impact of Cormorants on Plant–Arthropod Food Webs on Their Nesting Islands. Ecosystems 13: 353–366.
  183. 183. Markwell TJ, Daugherty CH (2002) Invertebrate and lizard abundance is greater on seabird-inhabited islands than on seabird-free islands in the Marlborough Sounds, New Zealand. Ecoscience 9: 293–299.
  184. 184. Mizota C, Naikatin A (2007) Nitrogen isotope composition of inorganic soil nitrogen and associated vegetation under a sea bird colony on the Hatana islands, Rotuma Group, Fiji. Geochemical Journal 41: 297–301.
  185. 185. Mizota C (2009) Nitrogen isotopic patterns of vegetation as affected by breeding activity of Black-tailed Gull (Larus crassiostris): A coupled analysis of feces, inorganic soil nitrogen and flora. Applied Geochemistry 24: 2027–2033.
  186. 186. Mizutani H, Wada E (1988) Nitrogen and Carbon Isotope Ratios in Seabird Rookeries and their Ecological Implications. Ecology 69: 340–349.
  187. 187. Mizutani H, Kabaya Y, Moors PJ, Speir TW, Lyon GL (1991) Nitrogen Isotope Ratios Identify Deserted Seabird Colonies. The Auk 108: 960–964.
  188. 188. Stapp P, Polis GA, Sanchez Pinero F (1999) Stable isotopes reveal strong marine and El Nino effects on island food webs. Nature 401: 467–469.
  189. 189. Young HS, McCauley DJ, Dunbar RB, Dirzo R (2010) Plants cause ecosystem nutrient depletion via the interruption of bird-derived spatial subsidies. Proceedings of the National Academy of Sciences 107: 2072–2077.
  190. 190. Zhu R, Liu Y, Ma E, Sun J, Xu H, et al. (2009) Nutrient compositions and potential greenhouse gas production in penguin guano, ornithogenic soils and seal colony soils in coastal Antarctica. Antarctic Science 21: 427–438.
  191. 191. Bateman AS, Kelly SD (2007) Fertilizer nitrogen isotope signatures. Isotopes in Environmental and Health Studies 43: 237–247.
  192. 192. Dijkstra P, Menyailo OV, Doucett RR, Hart SC, Schwartz E, et al. (2006) C and N availability affects the 15N natural abundance of the soil microbial biomass across a cattle manure gradient. European Journal of Soil Science 57: 468–475.
  193. 193. Kerley SJ, Jarvis SC (1996) Preliminary studies of the impact of excreted N on cycling and uptake of N in pasture systems using natural abundance stable isotopic discrimination. Plant and Soil 178: 287–294.
  194. 194. Lim S-S, Choi W-J, Kwak J-H, Jung J-W, Chang S, et al. (2007) Nitrogen and carbon isotope responses of Chinese cabbage and chrysanthemum to the application of liquid pig manure. Plant and Soil 295: 67–77.
  195. 195. Yun S-I, Ro H-M, Choi W-J, Chang SX (2006) Interactive effects of N fertilizer source and timing of fertilization leave specific N isotopic signatures in Chinese cabbage and soil. Soil Biology and Biochemistry 38: 1682–1689.
  196. 196. Choi W-J, Ro H-M, Hobbie EA (2003) Patterns of natural 15N in soils and plants from chemically and organically fertilized uplands. Soil Biology and Biochemistry 35: 1493–1500.
  197. 197. Choi W-J, Ro H-M (2003) Differences in isotopic fractionation of nitrogen in water-saturated and unsaturated soils. Soil Biology and Biochemistry 35: 483–486.
  198. 198. del Amor FM, Navarro J, Aparicio PM (2008) Isotopic Discrimination as a Tool for Organic Farming Certification in Sweet Pepper. Journal of Environmental Quality 37: 182–185.
  199. 199. Nakano A, Uehara Y (2007) Effects of different kinds of fertilizer and application methods on δ15N values of tomato. Japan Agricultural Research Quarterly 41: 219–226.