Dec 15, 2011 - Helen M. Baulch,1,2 Sherry L. Schiff,3 Roxane Maranger,4 and Peter ...... Cunjak, R. A., T. D. Prowse, and D. L. Parrish (1998), Atlantic salmon.
GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 25, GB4013, doi:10.1029/2011GB004047, 2011
Nitrogen enrichment and the emission of nitrous oxide from streams Helen M. Baulch,1,2 Sherry L. Schiff,3 Roxane Maranger,4 and Peter J. Dillon5 Received 1 February 2011; revised 9 August 2011; accepted 12 September 2011; published 15 December 2011.
[1] Nitrous oxide (N2O) is a potent greenhouse gas produced during nitrogen cycling. Global nitrogen enrichment has resulted in increased atmospheric N2O concentrations due in large part to increased soil emissions. There is also a potentially important flux from streams, rivers and estuaries; although measurements of these emissions are sparse, and role of aquatic ecosystems in global N2O budgets remains highly uncertain. Using the longest-term measurements of N2O fluxes from streams to date, we found annual fluxes from 14 sites in five streams of south-central Ontario, Canada varied widely–from net uptake of 3.2 ± 0.2 (standard deviation) mmol N2O m−2 d−1 to net release of 776 ± 61 mmol N2O m−2 d−1. N2O consumption was associated with very low nitrate concentrations (0.80 were averaged to obtain
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Denitrification Nitrification
Increased dissolved organic carbon
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Depends on temperature, range of O2 [Silvennoinen et al., 2008a] Increase [Cavigelli and Robertson, 2000]c, [Betlach and Tiedje, 1981]c, [Weier et al., 1993]c Decrease [Goreau et al., 1980]c; [Khalil et al., 2004]c Decrease [Cavigelli and Robertson, 2000]c, [Martikainen, 1985]c Max at pH 6.5 [Stevens et al., 1998]c Decrease [Jiang and Bakken, 1999]c
Decrease [Kemp and Dodds, 2002; Silvennoinen et al., 2008a] Increase [Kemp and Dodds, 2002] Increase [Müller et al., 1980]c Max at pH 7.5 [Strauss et al., 2002] Max at pH 8.5 ([Warwick, 1986]– note reviewc)
Nitrification Denitrification Nitrification
b
Unknown
Unknown
Unknown
Unknown
Unknown
Decrease [Maag and Vinther, 1996]c
Denitrification
Unknown
Decrease [Silvennoinen et al., 2008a]
Unknown Unknown
Decrease [Weier et al., 1993]c Unknown/ no effectd
Increase [Richardson et al., 2004] Decrease [Bernhardt and Likens, 2002; Strauss and Lamberti, 2000; Strauss et al., 2002] No effect [Strauss et al., 2002; Strauss et al., 2004] Increase to maximum [Silvennoinen et al., 2008a] Increase [Garcia-Ruiz et al., 1998b; Martin et al., 2001] No effect [Martin et al., 2001] Increase [Starry et al., 2005] No effect [Kemp and Dodds, 2002]
Possible increase
Increase
Resulting Effects on N2O Production
Decrease, or no effect [Jiang and Bakken, 1999]c
No effect [Beaulieu et al., 2010a]
Increase [Silvennoinen et al., 2008b]
Effects on N2O Yieldb
Increase [Kemp and Dodds, 2002; Strauss and Lamberti, 2000; Strauss et al., 2002] No effect [Strauss et al., 2004]
Increase [Beaulieu et al., 2010a; Garcia-Ruiz et al., 1998b; Inwood et al., 2005; Kemp and Dodds, 2002; Martin et al., 2001; Richardson et al., 2004; Silvennoinen et al., 2008b]
Change in Rate
Where indicated, we cite results from studies performed in ecosystems other than rivers or streams. Nitrification: N2O:NO3−, denitrification N2O:N2. c Indicates an ecosystem other than a river or stream. d If the DOC affects nitrification rates by increasing competition for NH4+ [Strauss et al., 2002], no effects on N2O:NO3− may be expected.
a
Increased pH
Increased oxygen
Nitrification
Denitrification
Nitrification
Increased ammonium
Increased temperature
Denitrification
Rate Affected
Increased nitrate
Environmental Change
Table 1. Possible Effects of Changes in Environmental Conditions on Rates of N Cycling, N2O Yields, and N2O Productiona
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Figure 1. Study catchments in Ontario, Canada (area of detailed map is shown as inset). Clockwise from bottom catchments are Stouffville Creek, Black River, Layton Creek (smaller adjacent to Mariposa), Mariposa Brook (larger adjacent to Layton), and Jackson Creek. Sampling sites are shown as white dots with the most upstream site as 1, and increasing numbers downstream. The approximate overall direction of flow is indicated with an arrow. the k value. These multiple results varied by an average of 0.006% (coefficient of variation (CV); maximum CV was 0.02%). Model results constrained by O2 isotopes, and where the model was run without these data varied by 95%) of gases were emitted prior to passage to adjacent downstream sites. However, in Stouffville Creek, gas concentrations at the second site were periodically influenced by the adjacent upstream site. [17] We estimated time-weighted yearly fluxes for each site and each year. Time-weighted fluxes were calculated by averaging the instantaneous flux at the start and end of
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a sampling interval, multiplying this measurement by the duration of the sampling interval, then summing results for a 365 day period. 13–22 flux measurements were used in each annual flux estimate. We assumed no emissions occurred during periods of complete ice cover [Macdonald et al., 1991]. [18] We then calculated the mean flux for a stream across years and sites. Annual fluxes were calculated as described for 365 day periods ranging from June 2006–2007, June 2007–2008 and October 2006–2007 and October 2007–2008 for all sites where we had data. Recognizing the overlap in some annual estimates, we took the mean of any overlapping annual periods, and then the mean within a site (across years), then finally, the mean of sites within a stream. [19] We excluded the third site on Mariposa Brook from our mean stream flux estimates because of concern that high fluxes of N2O are not indicative of the study reach. Due to access limitations at this site we measured gas transfer velocity downstream of our sampling site. While this reflects a real flux, it appears to reflect N2O accumulated in a deeper, slow-flowing upstream section, then degassed as it passes through this more turbulent downstream riffle. 2.4. Statistical Analyses [20] We used best-subset multiple linear regression (SYSTAT 13.0) to develop models of annual N2O flux based on stream chemistry (NO2+3−, NH4+, total phosphorus [TP], DOC), accepting models where DAIC was ≤2 (from the minimum AIC). [21] Next, we compiled N2O flux and NO3− concentration data from all published studies of streams where annual values were reported. Although discharge, depth, velocity and stream order were not reported, all of the streams assessed were small streams. Discharge in these systems (where reported) was typically 0.999), and NO2− never composed a large portion of the NO2+3− pool. Ammonium concentrations were typically lower than nitrate concentrations and varied less among sites. DOC concentrations were typically higher at sites downstream of wetlands. SO42− concentrations followed the reverse pattern. [27] Best subset regression results for annual N2O flux data suggest several alternative models where the DAIC is ≤2 (from the minimum AIC; Table 3). We report linear regression results for the simplest model which uses only
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Figure 4. Mean flux (tabulated over annual windows) as a function of nitrate concentration during different annual periods. Error bars represent standard deviation based on predictions of three calibrated gas transfer models. In some cases error bars are smaller than symbols, hence are not visible. Data from the third site on Mariposa Brook are not plotted. These data would lie at 136 mM NO2+3−, 776 mmol N2O m−2d−1 (spring 06–07) and 122 mM NO2+3−, 679 mmol N2O m−2d−1 (fall 06–07). NO2+3− as a predictor (Table 4), to allow comparison to global data, where available data do not include all parameters across all supported models. The predictive capability of this model (r2adjusted = 0.59) is equal to the lowest AIC model which is more complex (incorporating NO2+3−, TP and DOC; r2adjusted = 0.59), and similar to another alternative model which incorporates NH4+ and NO2+3− (r2adjusted = 0.58; Table 3). The relationship between NO2+3− and (logtransformed) N2O flux is also supported if we include the site from Mariposa Brook where we believe fluxes exceed true values at the reach scale (r2adjusted = 0.80 for 5 streams), and exclude Stouffville Creek sites where gas concentrations are influenced by the reservoir (r2adjusted = 0.87 for 5 streams). If we extend our analysis to include all streams for which there are published annual data, we find a significant positive relationship between log-transformed NO3− concentrations and log-transformed N2O flux (Figure 5, Table 4, r2adjusted = 0.39). Exclusion of an outlier noted by Beaulieu et al. [2008] yields a much stronger relationship (Table 4, r2adjusted = 0.61).
Table 3. Results of Best Subset Regression for Variable Selection to Predict Under-Ice N2O % Saturation and Annual N2O Fluxes Variables
AIC
TP, DOC, NO2+3− NO2+3− NH4+, NO2+3− DOC, NO2+3− TP, NH4+, NO2+3−
Annual Flux Data 10.2 11.7 11.8 12.2 12.2
DOC, NO2+3− DOC, SO42−, NO2+3− TP, DOC, SO42−, NO2+3− SO42−, NO2+3− Organic N, SO42−, NO2+3− NO2+3−
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Under-Ice Data 159.3 159.8 160.2 160.5 160.8 161.2
r2adjusted 0.59 0.59 0.58 0.54 0.38 0.91 0.91 0.91 0.90 0.90 0.89
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Table 4. Results of Regression Analyses Assessing Predictors of Annual N2O Flux Among Streams, and Under-Ice N2O Concentrationsa Analysis Annual N2O fluxes, all streams (this study) Under-ice N2O % saturation
Resulting Equation This Study Log (N2O flux) = 2.40 + 0.02[NO2+3−] N2O % saturation = 136 + 1.43[NO2+3−]
20 Low Order Streams With Annual Flux Estimates All streams Log (N2O flux) = 2.34 + 0.948 ⋅ log [NO2+3−] All streams excluding outlier noted by Beaulieu et al. [2008] Log (N2O flux) = 1.38 + 1.42 ⋅ log [NO2+3−]
Model F-Ratio r2adjusted
N
p
6.7 112
0.59 0.89
5 0.08 15
