Fig 1.
Schematic of the nitrogen cycle, featuring the microbial processes of nitrification (NH4+ to NO2– and then to NO3–) and denitrification (NO3– to NO2–, nitric oxide, N2O then to nitrogen gas).
For every mole of NH4+ nitrified to NO3–, two moles of oxygen are consumed (Stoichiometric relationships collectively found in [5,6,10–13]). Note that the proportion of N2O released from nitrification and denitrification is highly variable as indicated by the dashed arrows [14]. Nitrogen assimilation, dissimilatory NO3– reduction to NH4+ (DNRA) and anaerobic NH4+ oxidation (anammox) are excluded from the figure but may be important components of the nitrogen cycle [15].
Fig 2.
Map of Canada with overlays of Saskatchewan and Ontario study sites (map courtesy of Rosa Brannen).
All Saskatchewan sites are in the prairie ecozone, while the Experimental Lakes area sites are in the boreal shield ecozone [42].
Table 1.
Winter nitrification rates and associated data for this study (under ice cover) and for Lake St. George (near surface at 2 m depth, under ice cover; [21]); for surface nitrification rates in Lake Croche (ice-covered; [38]); and for surface estimates of nitrate accumulation in Wisconsin lakes part of the North Temperate Lakes Long-Term Ecological Research (NTL- LTER) study (ice-covered, 30 years of accumulated data; [19]).
Values below limits of quantitation (LOQ) for nitrification rates are reported, including negative values (following [40]) and sample-specific LOQ (as described in Methods and calculated as per [43,44]) are reported.
Table 2.
Winter nitrification rates and associated data for Lake Superior (near surface at 2 m depth, in winter but without ice-cover; [40]).
Summer values of nitrification rates are reported for the two lakes (Western Basin of Lake Superior and Lake Croche) where cross-season study has been performed. Values below limits of quantitation (LOQ) for nitrification rates are reported, including negative values (following [40]) and sample-specific LOQ (as described in Methods and calculated as per [43,44]) are reported.
Fig 3.
Relationship between nitrification rates and NH4+ concentrations for water bodies from this study (ELA and Saskatchewan) and other cold water measurements from Lake Superior (no ice-cover; [40]), Lake St. George (ice-covered; [21]) and Lake Croche (ice-covered; [38]).
Note logged y-axis. The line plotted is the linear model (permutations) for all data from Tables 1 and 2. The linear model and statistics are presented in Table 3. Nitrification rates less than their sample specific LOQ are replaced by their LOQ.
Fig 4.
Concentrations of NH4+ and NO3–, and N2O percent saturation partitioned according to nitrification rates that are above or below 0.11 μg N L-1 d-1.
There are significant differences between the two rate groups for all analyses (NH4+, NO3–, and N2O; Wilcox-Mann-Whitney test, P <0.05). The boxplot and whiskers encompass 95% of the data observed, data points outside of the box and whiskers are outliers. The box itself represents the first and third quartiles, and the center line is the median [65].
Fig 5.
Principal component analysis showing the relationship between measured variables and nitrification rates.
PC1 and PC2 account for 61% of the variance exhibited by the relationship among these variables. Within a PCA, the closer the component vectors are (angle and length) the more closely they are related. Note the association of nitrification rates with NO3–, N2O and NH4+, and of CH4 with chlorophyll and temperature.
Table 3.
Linear model relationships between nitrification rates (μg N L-1 d-1) and NH4+ concentrations (μg N L-1), using the linear permutations modeling approach.
Literature data sources are noted in the caption of Fig 3.