Figures
Abstract
Nitrogen (N) deposition and its ecological effects on forest ecosystems have received global attention. Plantations play an important role in mitigating climate change through assimilating atmospheric CO2. However, the mechanisms by which increasing N additions affect net ecosystem production (NEP) of plantations remain poorly understood. A field experiment was initialized in May 2009, which incorporated additions of four rates of N (control (no N addition), low-N (5 g N m−2 yr−1), medium-N (10 g N m−2 yr−1), and high-N (15 g N m−2 yr−1)) at the Saihanba Forestry Center, Hebei Province, northern China, a locality that contains the largest area of plantations in China. Net primary production (NPP), soil respiration, and its autotrophic and heterotrophic components were measured. Plant tissue carbon (C) and N concentrations (including foliage, litter, and fine roots), microbial biomass, microbial community composition, extracellular enzyme activities, and soil pH were also measured. N addition significantly increased NPP, which was associated with increased litter N concentrations. Autotrophic respiration (AR) increased but heterotrophic respiration (HR) decreased in the high N compared with the medium N plots, although the HR in high and medium N plots did not significantly differ from that in the control. The increased AR may derive from mycorrhizal respiration and rhizospheric microbial respiration, not live root respiration, because fine root biomass and N concentrations showed no significant differences. Although the HR was significantly suppressed in the high-N plots, soil microbial biomass, composition, or activity of extracellular enzymes were not significantly changed. Reduced pH with fertilization also could not explain the pattern of HR. The reduction of HR may be related to altered microbial C use efficiency. NEP was significantly enhanced by N addition, from 149 to 426.6 g C m−2 yr−1. Short-term N addition may significantly enhance the role of plantations as an important C sink.
Citation: Du Z, Wang W, Zeng W, Zeng H (2014) Nitrogen Deposition Enhances Carbon Sequestration by Plantations in Northern China. PLoS ONE 9(2): e87975. https://doi.org/10.1371/journal.pone.0087975
Editor: Shuijin Hu, North Carolina State University, United States of America
Received: September 27, 2013; Accepted: January 2, 2014; Published: February 3, 2014
Copyright: © 2014 Du et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This research was supported by the National Basic Research Program of China (No. 2010CB950600 and 2013CB956303), Projects of the National Natural Science Foundation of China (No. 31222011, 31270363, and 31070428), Projects of Innovative Research Groups of the National Natural Science Foundation of China (No. 31021001) and University Construction Projects from Central Authorities in Beijing. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Introduction
Terrestrial ecosystems sequester nearly 30% of anthropogenic carbon (C) emissions, offering the most effective, yet natural, means to mitigate climate change [1]. Nitrogen (N) is a major limiting nutrient to plant growth in most terrestrial ecosystems [2] and thus affects C sequestration in terrestrial ecosystems [3]. Human activity has led to a significant increase in N deposition owing to industrialization, agricultural practices, and the combustion of fossil fuels [4]–[6]. Numerous studies have shown that N deposition can increase net ecosystem production (NEP), as an indicator of ecosystem C sequestration [7]–[9]. However, the magnitude of the increased NEP following N addition varied greatly from 24.5 to 225 kg C per kg N [10]–[12]. Therefore, there is an urgent need to explore the mechanisms underlying this effect.
NEP is determined by the difference between net primary production (NPP) and soil heterotrophic respiration (HR). One important issue that needs addressing is how additional N affects the process of plant growth and thus enhances NPP. Many studies have attributed the increased tree growth to significantly higher foliar N concentrations in fertilized plots [8], [13]–[15]. The increased foliar N concentrations could improve biomass production through the following three pathways: by increasing the uptake of CO2 [16]–[18], by increasing water-use efficiency of foliage via altering CO2 assimilation and stomatal conductance [19], and by reducing the thermally dissipated light [20]. At the same time, N addition may also decrease leaf N resorption [21], and thus increase litter N concentrations. Consequently, more available N is released via decomposition to supply plant growth [22]. However, little research has been conducted to comprehensively analyze the mechanism of plant biomass growth caused by N addition.
How soil respiration (SR) responds to N addition is also relevant. SR consists of autotrophic respiration (AR, respiration by live roots, rhizospheric microorganism, and mycorrhizal fungi) and HR, which mainly originates from microbial decomposition of soil organic matter. With N addition, AR was either inhibited by decreasing the below-ground C allocation and fine root biomass of trees [23] or promoted by increasing the N concentration in fine roots [24]–[28]. At the same time, the enhanced tree growth caused by N addition is also likely to lead to more plant photosynthate being transported from above ground to below ground, thus increasing AR. HR is also commonly considered to be related to microbial biomass and activity [29]–[31]. For instance, decreased HR was observed along with a consistent decrease in microbial biomass and extracellular enzyme activity [32]. Soil acidification caused by N deposition [33] is also a potential factor that could contribute to decreased HR. However, the inherent reasons concerning the responses of AR and HR to N addition are poorly understood.
Although there have been numerous studies investigating the effects of N deposition on ecosystem C sequestration [27], [34], [35], most of them focused on natural forests. Plantations are becoming a key component of world forest resources and play important roles in the context of overall sustainable forest management. Well-designed, multi-purpose plantations can reduce pressure on natural forests, restore some ecological services provided by natural forests, and mitigate climate changes through direct C sequestration [36]. However, there remain great uncertainties in the potential of plantations to sequestrate C [37]. Compared with natural forests, plantations appear to have lower NPP, root biomass, and soil microbial biomass [37]. Whether plantations have the same ecosystem C sequestration capacity as natural forests remains to be confirmed [38]–[40]. Among a few studies, increased ecosystem C sequestration with N deposition has been observed [41], [42]. However the underlying mechanisms by which N increases the plant C accumulation and affects SR and its autotrophic and heterotrophic components are still poorly understood.
In China, the total plantation area reached 5.33×107 ha in 1998, accounting for 30% of the total forest area of China and 29% of the world's total plantation area [43]. C accumulation in China is mainly ascribable to its extensive afforestation efforts, as 80% of the observed increase in tree C stocks in China occurred on its 213,106 ha of plantations [43]. These reforestation and afforestation programs are considered to influence C storage in China. Thus, to assess the C sequestration capacity of plantations and optimize their role as C sinks, it is necessary to systematically explore ways in which N deposition affects C sequestration. Consequently, a 3-year field N addition experiment was conducted in the Saihanba Forestry Center, Hebei Province, northern China, which contains the largest area of plantations in China, with the dominant species being Pinus sylvestris var. mongolica (Mongolia pine). NPP, SR, and its autotrophic and heterotrophic components were measured. Relevant influential factors were also measured, including plant tissue C and N concentrations (foliage, litter, and fine roots), microbial biomass C, microbial community composition, potential extracellular enzyme activities (EEAs), and soil pH values. The study aimed to address three questions: (1) how does the N addition affect NPP? (2) What are the responses of SR and its autotrophic and heterotrophic components to N addition? (3) What is the effect of N addition on NEP? We hypothesized that: (1) N addition would increase NPP via increasing foliage or litter N concentrations; (2) AR would remain stable because of the contrasting effects from decreasing below-ground C allocation and fine root biomass and increased fine root N concentrations and photosynthate transport from above ground to below ground; HR would be reduced because of decreased microbial biomass, inhibited microbial activity, and reduced pH values; and (3) NEP would be enhanced because of the increasing NPP and decreased HR.
Materials and Methods
Ethics Statement
The administration of the Saihanba Forestry Center gave permission for the use of their plantation for our study site. We confirm that the field studies did not involve endangered or protected species.
Site description
The study was conducted at the Saihanba Forestry Center in Hebei Province, northern China (117°12′–117°30′ E, 42°10′–42°50′ N, 1400 m a.s.l.). The study area belongs to a typical forest-steppe ecotone of the temperate area. The climate is semi-arid and semi-humid, with a long and cold winter (November to March), and a short spring and summer. Annual mean air temperature and precipitation over the period from 1964 to 2004 were −1.4°C and 450.1 mm, respectively. The soils are predominantly sandy. The study site is located in the largest area of plantations in China, with the dominant species being Pinus sylvestris var. mongolica. The herbaceous layer is dominated by Carex rigescens, Thalictrum aquilegifolium, Galium verum, Geum aleppicum, Artemisia tanacetifolia, and Agrimonia pilosa.
Experimental treatments
The N addition experiment was initiated in May 2009. Urea solution was evenly sprayed once a month from May to September with the same dose each year. Four N addition treatments (in three replicates) were established, including a control (without N added), low N (5 g N m−2 yr−1), medium N (10 g N m−2 yr−1), and high N (15 g N m−2 yr−1). Twelve plots, each of 20 m × 20 m dimensions were established, each surrounded by a 10-m wide buffer strip. All plots and treatments were randomly laid out. During each application, the fertilizer was weighed, mixed with 10 L of water, and applied to each plot below the canopy using a backpack sprayer. The control plot received 10 L of water without N.
Field measurements
Biomass production and accumulation.
An allometric method was used to estimate biomass production through establishing the relationship between component biomass (foliage, branches, stem, and roots) and diameter at breast height (DBH) and tree height (H) [44]. In July 2010, stems were cut at the soil surface in the area near our experimental plots. Total tree heights, length of live crown, DBH, and diameter at the base of the live crown were measured and recorded. All foliage on each live branch was collected and weighed. All live and dead branches from each canopy position were cut and weighed separately. The stems were cut into 1-m sections and weighed. Litter from each deforested tree was carefully collected and weighed. The entire root system of the sample trees was excavated using a combination of a pulley device and manual digging, and cleaned of adhering soil. The fresh mass of each component was determined to the nearest 1 g using an electronic balance. All of these procedures were conducted in the field immediately after the tree was felled. The total biomass was calculated as the sum of foliage, branch, litter, stem, and root biomass.
An allometric equation was established as: where H is the height of trees (m), D is DBH (cm), and a and b are regression constants (b = 0.70, a = 107.01). DBH was recorded on all living stems in each plot in July 2010 and July 2012. The height of each living tree was measured using a DME (Haglöf Vertex IV, Sweden). Because biomass production (BP) constitutes the largest fraction of NPP, BP is commonly used as a proxy for NPP [45]–[48]. It is important to note that NPP includes numerous C-consuming processes such as plant growth, root exudation, and C allocation to symbionts [49]. The NPP in our study was thus underestimated. We used 50% as the C concentration in plant tissue [28]. The net primary production was calculated by the following equation:where NPP is the net primary productivity (g C m−2 yr−1) and BP is the estimated biomass production (g C m−2 yr−1) in 2012 and 2010. The annual net ecosystem production (NEP, g C m−1 yr−1) was calculated as the annual NPP minus annual soil HR.
Fine root biomass.
In July 2011 and 2012, five soil core samples were taken randomly using a 5.8-cm-diameter soil corer around the trees in each replicate plot, causing as little disturbance as possible to the surrounding soil. The roots were transported to the laboratory where they were washed free of soil, dried at 70°C, and weighed.
SR and its autotrophic and heterotrophic components.
SR was measured using a Li-8100 soil CO2 flux system (LI-COR Inc. Lincoln, NE, USA). Measurements were conducted at least once per month from May to October in 2011 and 2012. There were five subsamples (i.e., SR collars) in each plot. We used two kinds of soil collars in each plot to measure total SR and HR. A shallow surface collar (10 cm inside diameter, 6 cm height) that penetrated 3 cm into the soil was used to measure SR. The other kind of collar (10 cm inside diameter, 35 cm height), which was used for HR measurement, was inserted 30 cm into the soil with a 5-cm height above the soil surface. Because the majority of roots are found within the upper 30 cm of the soil profile in this forest (data not shown), these deeper collars should eliminate the majority of live roots and their contributions to respiration. All the polyvinyl chloride (PVC) collars were installed 6 months prior to the first measurements to minimize any disturbance of the soil environment. SR in the growing season was obtained from the monthly data directly measured in the field experiment using linear extrapolation methods. Winter SR was obtained from the data of Wang et al. (2010) [50] from the same study site.
Laboratory analyses
Plant chemical analyses.
Five subsamples were collected in each plot for chemical analyses. Green foliage was sampled from vigorously growing trees in late July 2012 using a pole pruner and a steel ladder. Foliar litter was collected from litter traps. Fine root samples were selected after the soil was passed through a 2-mm sieve. All the green foliage, foliar litters, and fine roots were dried at 60°C to constant mass, and ground using an intermediate mill (0.5-mm mesh screen) to generate homogeneous samples for chemical analysis. The C and N concentrations were measured using an element analyzer (Vario EL III, Elementar, Hanau, Germany).
Soil chemical analyses.
In July 2012, mineral soils were sampled at 0–10 cm depth from five random locations per plot using 5.8-cm-diameter soil corers. Once collected, the soils were immediately placed in a cooler and transported to the nearby laboratory (less than 30 min travel time per site). The cores from each plot were then combined and frozen for later processing. Within 24 h, frozen soils were allowed to thaw at room temperature. Plant litter in the upper layer, as well as all the coarse and fine roots, was carefully removed. The soils were then separated into four sub-samples for laboratory analysis, including pH, microbial biomass C and N, microbial community composition (PLFAs), and potential EEAs.
Air-dried soil had any roots removed, and was passed through a 2-mm sieve. Soil pH was determined using a 1∶5 soil:water ratio with a pH meter (Model PHS-2; INESA Instrument, Shanghai, China).
MBC and MBN were measured using the chloroform fumigation extraction technique [51], [52]. Two replicate samples, one unfumigated and one fumigated with alcohol-free CHCl3 for 24 h, were pre-incubated at 25°C for 7 days and then extracted with 0.5 mol/L K2SO4 (1∶2.5 w/v). The extracts were analyzed for total dissolved C and N using a total C analyzer (TOC-500; Shimadzu, Kyoto, Japan). The microbial biomass was calculated as the difference in extractable C and N between the fumigated and unfumigated soils. The efficiency factors used to calculate the respective MBC and MBN were KC = 0.379 [52] and KN = 0.54 [51].
Phospholipid fatty acids (PLFAs) analysis was used to assess microbial community composition. PLFAs were extracted and analyzed using a procedure described by [53]. Briefly, the soil was extracted in a single-phase mixture of chloroform: methanol: citrate buffer (1: 2: 0.8) [54]. After extraction, the lipids were separated into neutral lipids, glycolipids, and polar lipids (phospholipids) on a silicic acid column. The phospholipids were methylated and separated on a gas chromatograph equipped with a flame ionization detector. Peak areas were quantified by adding methyl nonadecanoate fatty acid (19:0) as the internal standard before the methylation step. Peaks were identified by chromatographic retention time and a standard qualitative mix in the range of C9–C30 using a microbial identification system (Microbial ID Inc., Newark, DE, USA). The fatty acid 18: 2ω6, 9 was recognized as the fungal biomarker [55]. The sum of the following PLFAs was used a measure of the bacterial biomass: i14:0, i15:0, a15:0, 15:0, i16:0, 10Me16:0, i17:0, a17:0, cy17:0, 17:0, br18, 10Me17:0, 18:1ω7, 10Me18:0, and cy19:0 [56].
Seven EEAs were measured, including five enzymes involving C metabolism (α-glucosidase (AG), β-1,4-glucosidase (BG), leucine aminopeptidase (LAP), β-D-cellobiosidase (CB), and xylosidase (XS)), one involving N metabolism (N-acetyl-glucosaminidase (NAG), and one involving phosphorus (P) metabolism (acid phosphatase (AP)). The measurements were conducted following the method of Saiya-Cork et al. (2002) [57]. Briefly, sample suspensions were prepared by adding 2 g of fresh soil to 90 ml of 50 mmol/L, pH 6.0 acetate buffer and homogenizing for 1 min. Continuously, 200-µl suspensions were combined with the corresponding substrate in a 96-well microplate. There were six replicate wells per sample per assay. The micro-plates were incubated at 25°C for up to 3 h. Fluorescence was then measured using a microplate reader with 365-nm excitation and 450-nm emission filters (Tecan Infinite M200, Salzburg, Austria). Finally, the concentration was divided by incubation time and dry weight soil to estimate potential enzyme activity.
Statistical analysis
All statistical analyses were performed using SPSS statistical software (SPSS 18.0 for Windows; SPSS Inc., Chicago, IL, USA). One-way analysis of variance with Duncan's test was used to test the differences among the different N addition treatments in NPP, SR, and its AR and HR components, and in NEP, as well as plant and soil chemical parameters. Significant effects were determined at P<0.05 unless otherwise stated. Data was expressed as mean values ± S.E. (standard error).
Results
Biomass production and accumulation
No significant differences were observed in DBH for both 2010 and 2012 among the different N addition treatments (Fig. S1a). However, in 2012, the tree height significantly increased with fertilization (Fig. S1b). The averaged NPP was 582.3±22.8 g C m−2 yr−1 in the control plots. N addition increased NPP by 15.45%, 23.51%, and 41.21%, respectively, in the low-, medium-, and high-N plots (Fig. 1). Fine root biomass showed a decreasing trend with fertilization although no significant differences were observed (Fig. S2).
Significant differences among N treatments are indicated by different letters.
SR and its autotrophic and heterotrophic components
Both total SR and HR followed a clear seasonal pattern with the highest rates in June–August and the lowest rates in spring and autumn for all the treatments (Fig. 2). SR was not significantly different among control and fertilized treatment plots in both 2011 (P = 0.43) and 2012 (P = 0.36) (Fig. 3a). There was no significant variance between the control and low-N treatment for AR and HR in both 2011 and 2012 (P>0.05) (Fig. 3b, Fig. 3c). With the fertilization gradient increasing, significantly higher AR (Fig. 3c) and lower HR (Fig. 3b) occurred in the high-N plots compared with the medium-N treatment in 2012.
Significant differences among N treatments are indicated by different letters.
Net ecosystem productivity (NEP)
With a decrease in HR and increased NPP, NEP significantly increased with fertilization, from 149 to 426.6 g C m−2 yr−1 (Fig. 4). The amount of C (kg) fixed by 1 kg N/ha N addition was in the range of 116.6–209.8 kg C per kg N/ha.
Significant differences among N treatments are indicated by different letters.
Plant chemical parameters
N addition significantly increased foliar N concentrations in the herbaceous layer (Fig. S3). However, N concentrations of the tree foliage showed no significant differences among the control and fertilization treatments (Fig. 5). C and N concentrations of foliar litter were significantly higher in the high-N and medium-N plots than in the control (Fig. 5). Fine root N concentrations showed no significant differences while fine root C concentrations increased by 10.28% in the high-N plots compared with the control (Fig. 5).
Significant differences among N treatments are indicated by different letters.
Soil chemical parameters
Soil pH significantly decreased with fertilization (Fig. S4). MBC and MBN were 132.67–145.68 mg C kg−1 dry soil and 48.98–60.59 mg C kg−1 dry soil, respectively. Although there were no significant differences among the control and fertilized treatments, MBC and MBN generally increased along the fertilization gradient (Table 1). Neither bacterial nor fungal biomass varied significantly with N addition (Table 1). The activities of all seven enzymes involving C, N, and P metabolism showed no significant differences between the control and fertilized plots (Fig. 6).
Significant differences among nitrogen (N) treatments are indicated by different letters. AG = α-glucosidase; BG = β-1,4-glucosidase; CB = β-D-cellobiosidase; XS = xylosidase; NAG = N-acetyl-glucosaminidase; AP = acid phosphatase; LAP = leucine aminopeptidase.
Discussion
Effects of N addition on NPP
N fertilization significantly increased NPP by 15.4–41.2% (Fig. 1) through the vertical growth of trees (Fig. S1). This maximum rate of increase of NPP is more than twice the average level of temperate forests (19.5%) [58]. The increased NPP could not be related to fresh foliar N concentrations, because no significant differences occurred between the control and fertilized plots (Fig. 5). This is inconsistent with commonly observed increases in foliar N with N addition [59], [13]–[15]. For instance, May et al. (2005) [15] found that foliar N concentrations averaged 11% higher in fertilization treatments than in the control in a mixed-deciduous forest. In this study, litter N concentrations significantly increased in the fertilized plots relative to the control (P<0.05) (Fig. 5b), suggesting a likely decrease in leaf N resorption by fertilization [15], [60], [61]. Litter with higher N concentration would be easily decomposed by microbes and release large amounts of available N for plant growth, thus potentially increasing forest productivity [62]–[65].
Foliage N concentrations of trees showed no significant differences among control and fertilized plots (Fig. 5), while foliage N concentrations in plants in the herbaceous layer significantly increased (Fig. S3). This may be because of differences in leaf shape. Compared with coniferous trees, the broad-leaved herbaceous plants may invest more N in foliage to produce enzymes and proteins associated with photosynthetic processes or increase their foliage area to improve photosynthesis. Increased foliage N concentrations following N addition have been commonly observed in previous studies of broadleaf species (i.e., Acer rubrum, Liriodendron tulipifera, Prunus serotina, Acer saccharum, and Betula alleghaniensis) [13]–[15]. Therefore, the nutrient use strategy of plants may be closely related to their foliage shape. Thus, foliage shape should be taken into consideration in the future when it comes to assessing the response of ecosystem C sequestration to N deposition.
SR and its autotrophic and heterotrophic components
No significant differences were observed in total SR among the control and fertilized treatments (Fig. 3a), which is inconsistent with a commonly reported reduction in SR following N addition [9], [66]–[68]. Significantly higher AR in high-N plots than in the medium-N treatment was observed, although neither treatment showed any significant differences to the control (Fig. 3c). No significant differences in fine root biomass (Fig. S2) and N concentrations were observed among the different treatments (Fig. 5b), implying that live root respiration may not change with fertilization. Instead, fine root C concentrations significantly increased by 4.89%, 6.72%, and 10.28%, respectively, in the low-, medium-, and high-N addition foliage plots compared with the control treatment (Fig. 5a). This suggests an increased supply of photosynthetic products from above ground to below ground following N addition. Consequently, plant C may prime the growth and activity of mycorrhizal fungi [29] and rhizospheric microbes [23], [69]–[72], thus promoting AR. However, fertilization may also suppress rhizospheric microbial respiration to a greater extent than that in the bulk soil because of the decreased C allocation to root symbionts and exudation [68]. Therefore, there is a need to distinguish different components of AR in the future to accurately explore the internal mechanism underlying the increased AR.
HR in the high-N treatment was significantly lower than that in the medium-N plot after 3 years' fertilization, although it did not significantly differ from the control and low-N plots (Fig. 3c). Decreased HR is believed to be mainly driven by a decreased microbial biomass [29], [30] and depressed phenol oxidase activity (a lignin-degrading enzyme) [31], [73]. In contrast to most previous studies, we did not observe significant variation in the soil microbial biomass (Table 1), extracellular enzyme activity (Fig. 6), and microbial community composition (Table 1). Although soil pH was significantly lower in the high-N plots than in the control plots, it did not significantly differ from values in the medium- and low-N plots (Fig. S4). Hence, the decreased HR in the high-N plot could not be attributable to changes in the microbial biomass, extracellular enzyme activity, microbial community composition, or soil pH. The variation in HR may depend on the C-use efficiency of the decomposers (defined as the ratio of C employed in the new biomass relative to C consumed for respiration) [74]. When N availability is high, microbes may increase their efficiency leading to an efficient increase in biomass and a relatively low release of C to the atmosphere [75]. Because microbial biomass was measured only in July 2012, a greater frequency of measurement of microbial biomass should be conducted to better explore the reasons for the decrease in HR.
Effect of N fertilization on NEP
With the increasing NPP and decreasing HR, NEP greatly increased from the control to the high-N addition plots (149 versus 426.6 g C m−2 yr−1). NEP in our control plot fell within the published range for boreal forests (40–180 g C m−2 yr−1) [76]. The amount of C fixed per unit of added N fertilizer was in the range of 116.6–209.8 kg C/ha per kg N in this study, which is broadly similar to the range proposed by Magnani et al. (2008) (175–225 kg C per kg N) [11]. Thus, short-term N fertilization can greatly increase the NEP of plantations in northern China and enhance the role of plantations as an important C sink.
However, N fertilization may also induce alterations in the availability of other nutrients such as P, potassium, and calcium, because of the intrinsic stoichemical constraints of plant growth. This could have an important influence on NEP. N deposition is also likely to be accompanied by other environmental changes including rising atmospheric CO2 concentrations, global warming, and soil acidification and these changes will interact with N availability in complex ways. The complexity of these interacting controls (i.e., temperature and nutrient availability) further restricts our ability to forecast future C sequestration capacities. In addition, we should be careful when extrapolating our results and mechanisms to systems with long-term N inputs. Finally, the stand age of plantations may also be a potentially influential factor in evaluating their sequestration capacity. This suggests a need for future studies in stands of various ages and incorporating long-term multi-factorial experiments.
Conclusion
This study has comprehensively analyzed the effects of N addition on biomass accumulation, SR, and its autotrophic and heterotrophic components in plantations of northern China. N addition might alter C sequestration capacity through the following possible pathways (Fig. 7): (1) increased litter N concentration because of decreased N resorption by foliage; (2) enhancement of the amount of photosynthetic products transported downward; (3) increased AR through the priming effect of plant C on rhizospheric microbial and mycorrhizal fungi activity; and (4) suppressed HR through increased microbial C use efficiency. Increasing N deposition is likely to stimulate NEP and slow the accumulation of atmospheric CO2. In the context of global atmospheric N deposition, we highlighted that plantations might offer an important role to mitigate the future climate change.
Ra and Rh are autotrophic and heterotrophic respiration, respectively; Rr is live root respiration, Rm is respiration of mycorrhizal fungi, and Rrm is rhizospheric microbial respiration. Thick arrows represent the enhanced process and thin arrows represent the declined progress in the N addition treatment compared with the control.
Supporting Information
Figure S1.
Diameter at breast height (DBH) and height of trees with nitrogen (N) fertilization in 2010 (black) and 2012 (gray).
https://doi.org/10.1371/journal.pone.0087975.s001
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Figure S2.
Fine root biomass among different nitrogen (N) fertilization gradients in 2012. Significant differences among N treatments are indicated by different letters.
https://doi.org/10.1371/journal.pone.0087975.s002
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Figure S3.
Effects of nitrogen (N) addition on foliar N concentrations of herbaceous layer plants. Significant differences among N treatments are indicated by different letters.
https://doi.org/10.1371/journal.pone.0087975.s003
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Figure S4.
Soil pH in control and nitrogen (N) treatments plots after 3 years fertilization. Significant differences among N treatments are indicated by different letters.
https://doi.org/10.1371/journal.pone.0087975.s004
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Author Contributions
Conceived and designed the experiments: WW. Performed the experiments: ZHD WJZ. Analyzed the data: ZHD. Contributed reagents/materials/analysis tools: HZ. Wrote the paper: ZHD.
References
- 1. Le Quéré C, Raupach MR, Canadell JG, Marland G (2009) Trends in the sources and sinks of carbon dioxide. Nature Geoscience 2: 831–836.
- 2. Vitousek PM, Cassman K, Cleveland C, Crews T, Field CB, et al. (2002) Towards an ecological understanding of biological nitrogen fixation. Biogeochemistry 57: 1–45.
- 3. Thomas DC, Zak DR, Filley TR (2012) Chronic N deposition does not apparently alter the biochemical composition of forest floor and soil organic matter. Soil Biology and Biochemistry 54: 7–13.
- 4. Vitousek PM, Aber JD, Howarth RW, Likens GE, Matson PA, et al. (1997) Human alteration of the global nitrogen cycle: sources and consequences. Ecological Applications 7: 737–750.
- 5. Aber JD, Goodale CL, Ollinger SV, Smith ML, Magill AH, et al. (2003) Is nitrogen deposition altering the nitrogen status of northeastern forests? BioScience 53: 375–389.
- 6. Galloway JN, Townsend AR, Erisman JW, Bekunda M, Cai Z, et al. (2008) Transformation of the nitrogen cycle: recent trends, questions, and potential solutions. Science 320: 889–892.
- 7. Magnani F, Mencuccini M, Borghetti M, Berbigier P, Berninger F, et al. (2007) The human footprint in the carbon cycle of temperate and boreal forests. Nature 447: 849–851.
- 8. Pregitzer KS, Burton AJ, Zak DR, Talhelm AF (2008) Simulated chronic nitrogen deposition increases carbon storage in Northern Temperate forests. Global Change Biology 14: 142–153.
- 9. Janssens I, Dieleman W, Luyssaert S, Subke JA, Reichstein M, et al. (2010) Reduction of forest soil respiration in response to nitrogen deposition. Nature Geoscience 3: 315–322.
- 10. de Vries W, Solberg S, Dobbertin M, Sterba H, Laubhahn D, et al. (2008) Ecologically implausible carbon response? Nature 451: E1–E3.
- 11. Magnani F, Mencuccini M, Borghetti M, Berninger F, Delzon S, et al. (2008) Ecologically implausible carbon response? Reply. Nature 451: E3–E4.
- 12. Liu L, Greaver TL (2009) A review of nitrogen enrichment effects on three biogenic GHGs: the CO2 sink may be largely offset by stimulated N2O and CH4 emission. Ecology letters 12: 1103–1117.
- 13. Boggs JL, McNulty SG, Gavazzi MJ, Myers JM (2005) Tree growth, foliar chemistry, and nitrogen cycling across a nitrogen deposition gradient in southern Appalachian deciduous forests. Canadian Journal of Forest Research 35: 1901–1913.
- 14. Elvir JA, Rustad L, Wiersma GB, Fernandez I, White AS, et al. (2005) Eleven-year response of foliar chemistry to chronic nitrogen and sulfur additions at the Bear Brook Watershed in Maine. Canadian Journal of Forest Research 35: 1402–1410.
- 15. May JD, Burdette SB, Gilliam FS, Adams MB (2005) Interspecific divergence in foliar nutrient dynamics and stem growth in a temperate forest in response to chronic nitrogen inputs. Canadian Journal of Forest Research-Revue Canadienne De Recherche Forestiere 35: 1023–1030.
- 16. Aber JD, Nadelhoffer KJ, Steudler P, Melillo JM (1989) Nitrogen saturation in northern forest ecosystems. Bioscience 39: 378–286.
- 17. Aber J, McDowell W, Nadelhoffer K, Magill A, Berntson G, et al. (1998) Nitrogen saturation in temperate forest ecosystems. Bioscience 48: 921–934.
- 18. Driscoll CT, Lawrence GB, Bulger AJ, Butler TJ, Cronan CS, et al. (2001) Acidic deposition in the northeastern United States: sources and inputs, ecosystem effects, and management strategies. Bioscience 51: 180–198.
- 19. Guerrieri R, Mencuccini M, Sheppard L, Saurer M, Perks M, et al. (2011) The legacy of enhanced N and S deposition as revealed by the combined analysis of δ13C, δ18O and δ15N in tree rings. Global Change Biology 17: 1946–1962.
- 20. Tomaszewski T, Sievering H (2007) Canopy uptake of atmospheric N deposition at a conifer forest: Part II-response of chlorophyll fluorescence and gas exchange parameters. Tellus B 59: 493–501.
- 21. van Heerwaarden LM, Toet S, Aerts R (2003) Nitrogen and phosphorus resorption efficiency and proficiency in six sub-arctic bog species after 4 years of nitrogen fertilization. Journal of Ecology 91: 1060–1070.
- 22. Sullivan PF, Sommerkorn M, Rueth HM, Nadelhoffer KJ, Shaver GR, et al. (2007) Climate and species affect fine root production with long-term fertilization in acidic tussock tundra near Toolik Lake, Alaska. Oecologia 153: 643–652.
- 23. Högberg MN, Briones MJ, Keel SG, Metcalfe DB, Campbell C, et al. (2010) Quantification of effects of season and nitrogen supply on tree below-ground carbon transfer to ectomycorrhizal fungi and other soil organisms in a boreal pine forest. New Phytologist 187: 485–493.
- 24. Janssens IA, Crookshanks M, Taylor G, Ceulemans R (1998) Elevated atmospheric CO2 increases fine root production, respiration, rhizosphere respiration and soil CO2 efflux in Scots pine seedlings. Global Change Biology 4: 871–878.
- 25. Pregitzer KS, Laskowski MJ, Burton AJ, Lessard VC, Zak DR (1998) Variation in sugar maple root respiration with root diameter and soil depth. Tree Physiology 18: 665–670.
- 26. Jia SX, Wang ZQ, Li XP, Sun Y, Zhang XP, et al. (2010) N fertilization affects on soil respiration, microbial biomass and root respiration in Larix gmelinii and Fraxinus mandshurica plantations in China. Plant and Soil 333: 325–336.
- 27. Burton AJ, Jarvey JC, Jarvi MP, Zak DR, Pregitzer KS (2011) Chronic N deposition alters root respiration-tissue N relationship in northern hardwood forests. Global Change Biology 18: 258–266.
- 28. Tu LH, Hu TX, Zhang J, Li RH, Dai HZ, et al. (2011) Short-term simulated nitrogen deposition increases carbon sequestration in a Pleioblastus amarus plantation. Plant and Soil 340: 383–396.
- 29. Craine JM, Morrow C, Fierer N (2007) Microbial nitrogen limitation increases decomposition. Ecology 88: 2105–2113.
- 30. Fierer N, Lauber CL, Ramirez KS, Zaneveld J, Bradford MA, et al. (2011) Comparative metagenomic, phylogenetic and physiological analyses of soil microbial communities across nitrogen gradients. The ISME journal 6: 1007–1017.
- 31. Zak DR, Pregitzer KS, Burton AJ, Edwards IP, Kellner H (2011) Microbial responses to a changing environment: implications for the future functioning of terrestrial ecosystems. Fungal Ecology 4: 386–395.
- 32. Ramirez KS, Craine JM, Fierer N (2010) Nitrogen fertilization inhibits soil microbial respiration regardless of the form of nitrogen applied. Soil Biology and Biochemistry 42: 2336–2338.
- 33. Phoenix GK, Emmett BA, Britton AJ, Caporn SJ, Dise NB, et al. (2012) Impacts of atmospheric nitrogen deposition: responses of multiple plant and soil parameters across contrasting ecosystems in long-term field experiments. Global Change Biology 18: 1197–1215.
- 34. Hagedorn F, Kammer A, Schmidt MW, Goodale CL (2012) Nitrogen addition alters mineralization dynamics of 13C-depleted leaf and twig litter and reduces leaching of older DOC from mineral soil. Global Change Biology 18: 1412–1427.
- 35. Hasselquist NJ, Metcalfe DB, Högberg P (2012) Contrasting effects of low and high nitrogen additions on soil CO2 flux components and ectomycorrhizal fungal sporocarp production in a boreal forest. Global Change Biology 18: 3596–3605.
- 36. Paquette A, Messier C (2010) The role of plantations in managing the world's forests in the Anthropocene. Frontiers in Ecology and the Environment 8: 27–34.
- 37. Liao C, Luo Y, Fang C, Li B (2010) Ecosystem carbon stock influenced by plantation practice: implications for planting forests as a measure of climate change mitigation. Plos one 5: e10867.
- 38. Harmon ME, Ferrell WK, Franklin JF (1990) Effects on carbon storage of conversion of old-growth forests to young forests. Science 247: 699–702.
- 39. Chen GS, Yang YS, Xie JS, Guo JF, Gao R, et al. (2005) Conversion of a natural broad-leafed evergreen forest into pure plantation forests in a subtropical area: effects on carbon storage. Annals of forest science 62: 659–668.
- 40. Yang YS, Guo J, Chen G, Xie J, Gao R, et al. (2005) Carbon and nitrogen pools in Chinese fir and evergreen broadleaved forests and changes associated with felling and burning in mid-subtropical China. Forest Ecology and Management 216: 216–226.
- 41. Hyvönen R, Ågren GI, Linder S, Persson T, Cotrufo MF, et al. (2007) The likely impact of elevated CO2, nitrogen deposition, increased temperature and management on carbon sequestration in temperate and boreal forest ecosystems: a literature review. New Phytologist 173: 463–480.
- 42. Zhao M, Xiang W, Tian D, Deng X, Huang Z, et al. (2013) Effects of Increased Nitrogen Deposition and Rotation Length on Long-Term Productivity of Cunninghamia lanceolata Plantation in Southern China. Plos one 8: e55376.
- 43. Fang J, Chen A, Peng C, Zhao S, Ci L (2001) Changes in forest biomass carbon storage in China between 1949 and 1998. Science 292: 2320–2322.
- 44. Wang C (2006) Biomass allometric equations for 10 co-occurring tree species in Chinese temperate forests. Forest Ecology and Management 222: 9–16.
- 45. Waring R, Landsberg J, Williams M (1998) Net primary production of forests: a constant fraction of gross primary production? Tree Physiology 18: 129–134.
- 46. DeLUCIA E, Drake JE, Thomas RB, Gonzalez-Meler M (2007) Forest carbon use efficiency: is respiration a constant fraction of gross primary production? Global Change Biology 13: 1157–1167.
- 47. Drake J, Davis S, Raetz L, DeLucia E (2011) Mechanisms of age-related changes in forest production: the influence of physiological and successional changes. Global Change Biology 17: 1522–1535.
- 48. Goulden ML, McMillan A, Winston G, Rocha A, Manies K, et al. (2011) Patterns of NPP, GPP, respiration, and NEP during boreal forest succession. Global Change Biology 17: 855–871.
- 49. Vicca S, Luyssaert S, Penuelas J, Campioli M, Chapin F, et al. (2012) Fertile forests produce biomass more efficiently. Ecology letters 15: 520–526.
- 50. Wang W, Peng S, Wang T, Fang J (2010) Winter soil CO2 efflux and its contribution to annual soil respiration in different ecosystems of a forest-steppe ecotone, north China. Soil Biology and Biochemistry 42: 451–458.
- 51. Brookes P, Landman A, Pruden G, Jenkinson D (1985) Chloroform fumigation and the release of soil nitrogen: a rapid direct extraction method to measure microbial biomass nitrogen in soil. Soil Biology and Biochemistry 17: 837–842.
- 52. Vance E, Brookes P, Jenkinson D (1987) An extraction method for measuring soil microbial biomass C. Soil Biology and Biochemistry 19: 703–707.
- 53. Frostegard A, Tunlid A, Baath E (1993) Phospholipid fatty acid composition, biomass, and activity of microbial communities from two soil types experimentally exposed to different heavy metals. Applied and Environmental Microbiology 59: 3605–3617.
- 54. Bossio D, Scow K (1998) Impacts of carbon and flooding on soil microbial communities: phospholipid fatty acid profiles and substrate utilization patterns. Microbial Ecology 35: 265–278.
- 55. Zelles L (1997) Phospholipid fatty acid profiles in selected members of soil microbial communities. Chemosphere 35: 275–294.
- 56. Frostegard A, Baath E (1996) The use of phospholipid fatty acid analysis to estimate bacterial and fungal biomass in soil. Biology and Fertility of Soils 22: 59–65.
- 57. Saiya-Cork K, Sinsabaugh R, Zak D (2002) The effects of long term nitrogen deposition on extracellular enzyme activity in an Acer saccharum forest soil. Soil Biology and Biochemistry 34: 1309–1315.
- 58. LeBauer DS, Treseder KK (2008) Nitrogen limitation of net primary productivity in terrestrial ecosystems is globally distributed. Ecology 89: 371–379.
- 59. Reich PB, Tjoelker MG, Pregitzer KS, Wright IJ, Oleksyn J, et al. (2008) Scaling of respiration to nitrogen in leaves, stems and roots of higher land plants. Ecology letters 11: 793–801.
- 60. Li X, Hu Y, Han S, Liu Y, Zhang Y (2010) Litterfall and litter chemistry change over time in an old-growth temperate forest, northeastern China. Annals of forest science 67: 206.
- 61. Vergutz L, Manzoni S, Porporato A, Novais RF, Jackson RB (2012) Global resorption efficiencies and concentrations of carbon and nutrients in leaves of terrestrial plants. Ecological Monographs 82: 205–220.
- 62. Vitousek P (1982) Nutrient cycling and nutrient use efficiency. American Naturalist 119: 553–572.
- 63. Wieder WR, Cleveland CC, Townsend AR (2009) Controls over leaf litter decomposition in wet tropical forests. Ecology 90: 3333–3341.
- 64. Wood TE, Lawrence D, Clark DA, Chazdon RL (2009) Rain forest nutrient cycling and productivity in response to large-scale litter manipulation. Ecology 90: 109–121.
- 65. He H, Bleby TM, Veneklaas EJ, Lambers H (2011) Dinitrogen-fixing Acacia species from phosphorus-impoverished soils resorb leaf phosphorus efficiently. Plant, cell and environment 34: 2060–2070.
- 66. Bowden RD, Davidson E, Savage K, Arabia C, Steudler P (2004) Chronic nitrogen additions reduce total soil respiration and microbial respiration in temperate forest soils at the Harvard Forest. Forest Ecology and Management 196: 43–56.
- 67. Burton AJ, Pregitzer KS, Crawford JN, Zogg GP, Zak DR (2004) Simulated chronic NO3− deposition reduces soil respiration in northern hardwood forests. Global Change Biology 10: 1080–1091.
- 68. Phillips RP, Fahey TJ (2007) Fertilization effects on fineroot biomass, rhizosphere microbes and respiratory fluxes in hardwood forest soils. New Phytologist 176: 655–664.
- 69. De Nobili M, Contin M, Mondini C, Brookes P (2001) Soil microbial biomass is triggered into activity by trace amounts of substrate. Soil Biology and Biochemistry 33: 1163–1170.
- 70. Paterson E (2003) Importance of rhizodeposition in the coupling of plant and microbial productivity. European Journal of Soil Science 54: 741–750.
- 71. Marschner P, Crowley D, Yang CH (2004) Development of specific rhizosphere bacterial communities in relation to plant species, nutrition and soil type. Plant and Soil 261: 199–208.
- 72. Talbot JM, Allison SD, Treseder KK (2008) Decomposers in disguise: mycorrhizal fungi as regulators of soil C dynamics in ecosystems under global change. Functional Ecology 22: 955–963.
- 73. Edwards IP, Zak DR (2011) Fungal community composition and function after long-term exposure of northern forests to elevated atmospheric CO2 and tropospheric O3. Global Change Biology 17: 2184–2195.
- 74. Del Giorgio PA, Cole JJ (1998) Bacterial growth efficiency in natural aquatic systems. Annual Review of Ecology and Systematics 21: 503–541.
- 75. Manzoni S, Taylor P, Richter A, Porporato A, Agren GI (2012) Environmental and stoichiometric controls on microbial carbon-use efficiency in soils. New Phytologist 196: 79–91.
- 76. Bonan GB (2008) Forests and climate change: Forcings, feedbacks, and the climate benefits of forests. Science 320: 1444–1449.