Nitrous oxide emission from denitrification in stream and river networks

Abstract

Nitrous oxide (N(2)O) is a potent greenhouse gas that contributes to climate change and stratospheric ozone destruction. Anthropogenic nitrogen (N) loading to river networks is a potentially important source of N(2)O via microbial denitrification that converts N to N(2)O and dinitrogen (N(2)). The fraction of denitrified N that escapes as N(2)O rather than N(2) (i.e., the N(2)O yield) is an important determinant of how much N(2)O is produced by river networks, but little is known about the N(2)O yield in flowing waters. Here, we present the results of whole-stream (15)N-tracer additions conducted in 72 headwater streams draining multiple land-use types across the United States. We found that stream denitrification produces N(2)O at rates that increase with stream water nitrate (NO(3)(-)) concentrations, but that <1% of denitrified N is converted to N(2)O. Unlike some previous studies, we found no relationship between the N(2)O yield and stream water NO(3)(-). We suggest that increased stream NO(3)(-) loading stimulates denitrification and concomitant N(2)O production, but does not increase the N(2)O yield. In our study, most streams were sources of N(2)O to the atmosphere and the highest emission rates were observed in streams draining urban basins. Using a global river network model, we estimate that microbial N transformations (e.g., denitrification and nitrification) convert at least 0.68 Tg·y(-1) of anthropogenic N inputs to N(2)O in river networks, equivalent to 10% of the global anthropogenic N(2)O emission rate. This estimate of stream and river N(2)O emissions is three times greater than estimated by the Intergovernmental Panel on Climate Change.

Fig. 1.

(A) Box plots of stream N₂O production rates via denitrification of water column NO₃⁻ by catchment land use (reference, agricultural and urban). Benthic N₂O production rates reported in other studies are also shown. Significant differences between land-use types were determined with a one-way ANOVA followed by Tukey's post hoc test (P = 0.004) and are displayed as different lowercase letters above the box plots. See SI Materials and Methods for references. (B) Relationship between stream water NO₃⁻ and rates of N₂O production via denitrification (r² = 0.68, P < 0.001). (C) Nitrous oxide emission rates from streams. Significant differences between land use types were determined with a one-way ANOVA (P = 0.002) followed by Tukey's post hoc test and are displayed as different lowercase letters above the box plots. (D) Relationship between stream water NO₃⁻ concentrations and N₂O emission rates. The vertical dashed line represents a NO₃⁻ threshold (95 μg N·L⁻¹) below which N₂O emission rates are unrelated to NO₃⁻ (two-dimensional Kolmogorov–Smirnov test). Above the threshold N₂O emission rates are positively related to NO₃⁻ concentrations as represented by the least-squares best-fit line (solid black). (E) Percentages of stream N₂O emissions attributed to direct denitrification. Values >100% indicate N₂O was accumulating in the water column. There was no effect of land use (P = 0.13). (F) Variation in the percentage of stream N₂O emissions attributed to direct denitrification is partially explained by stream water NO₃⁻ concentration (r² = 0.32, P < 0.001).

Fig. 2.

Denitrification N₂O yields (percentage of denitrified N released as N₂O) measured in this study in comparison with other ecosystems. Data are displayed in box plots unless there were fewer than nine observations, in which case each observation is represented by a solid circle. See SI Materials and Methods for references.

Fig. 3.

Average N₂O fluxes estimated in this study (all units are μg N·m⁻²·h⁻¹). Black arrows represent fluxes that were directly measured and the white arrow with dashed boundaries represents fluxes that were estimated by mass balance. (A) N₂O produced in the stream, or imported to the stream via groundwater, temporarily resides in a pool of dissolved N₂O before being emitted to the atmosphere. (B) Direct denitrification is the conversion of stream water nitrate to N₂ and N₂O. Less than 1% of stream water nitrate subject to direct denitrification is converted to N₂O, but this is the source of 26% of the N₂O emitted by the stream. (C) The balance of N₂O emission in excess of that produced via direct denitrification (e.g., 37 − 9.6 = 27.4) must have entered the stream via another mechanism. Likely mechanisms include indirect denitrification (e.g., the denitrification of nitrate generated within the sediments), nitrification, and inputs of N₂O-supersaturated groundwater.

Fig. 4.

Relationship between the percentage of dissolved inorganic nitrogen (DIN) inputs to river networks that are converted to N₂O via denitrification and the amount of DIN delivered to the river network from the catchment. Data are from 866 river networks included in a global river network model. Data points are split into rivers draining basins >100,000 km² (solid black circles) and those draining between 10,000 and 100,000 km² (open red circles).