Forest streams are important sources for nitrous oxide emissions

Abstract

Streams and river networks are increasingly recognized as significant sources for the greenhouse gas nitrous oxide (N2O). N2O is a transformation product of nitrogenous compounds in soil, sediment and water. Agricultural areas are considered a particular hotspot for emissions because of the large input of nitrogen (N) fertilizers applied on arable land. However, there is little information on N2O emissions from forest streams although they constitute a major part of the total stream network globally. Here, we compiled N2O concentration data from low‐order streams (~1,000 observations from 172 stream sites) covering a large geographical gradient in Sweden from the temperate to the boreal zone and representing catchments with various degrees of agriculture and forest coverage. Our results showed that agricultural and forest streams had comparable N2O concentrations of 1.6 ± 2.1 and 1.3 ± 1.8 µg N/L, respectively (mean ± SD) despite higher total N (TN) concentrations in agricultural streams (1,520 ± 1,640 vs. 780 ± 600 µg N/L). Although clear patterns linking N2O concentrations and environmental variables were difficult to discern, the percent saturation of N2O in the streams was positively correlated with stream concentration of TN and negatively correlated with pH. We speculate that the apparent contradiction between lower TN concentration but similar N2O concentrations in forest streams than in agricultural streams is due to the low pH (<6) in forest soils and streams which affects denitrification and yields higher N2O emissions. An estimate of the N2O emission from low‐order streams at the national scale revealed that ~1.8 × 109 g N2O‐N are emitted annually in Sweden, with forest streams contributing about 80% of the total stream emission. Hence, our results provide evidence that forest streams can act as substantial N2O sources in the landscape with 800 × 109 g CO2‐eq emitted annually in Sweden, equivalent to 25% of the total N2O emissions from the Swedish agricultural sector.

Headwater Stream Microbial Diversity and Function across Agricultural and Urban Land Use Gradients

“…The N 2 O concentrations measured in this study, at 0.22 to 4.41 g N 2 O liter Ϫ1 (58 to 1,217% saturated), were comparable to the values reported for agricultural streams in Illinois (74, 75) but lower than values published for agricultural drainage waters in Scotland, UK (76) and higher than values reported for other forested and agricultural streams (42,(76)(77)(78). N 2 O production is known to vary by land use, with higher production from denitrification in streams in agricultural and urban basins (41,76,79), and changes in community composition have been shown to influence denitrification rates in agricultural (80) and urban streams (21). However, Audet et al found no difference in N 2 O concentrations measured in forested and agricultural streams in Sweden (76).…”

“…However, Audet et al found no difference in N 2 O concentrations measured in forested and agricultural streams in Sweden (76). Stream N 2 O concentrations are often correlated with dissolved nitrogen concentrations (41,76,79,81); however, variability in this relationship is often observed between sites (74, 75, 77). In this study, N 2 O concentrations were not correlated with microbial community compo-sition, but rather, N 2 O production was likely elevated in streams indirectly due to high rates of denitrification in response to NO 3…”

“…N 2 O production is known to vary by land use, with higher production from denitrification in streams in agricultural and urban basins (41,76,79), and changes in community composition have been shown to influence denitrification rates in agricultural (80) and urban streams (21). However, Audet et al found no difference in N 2 O concentrations measured in forested and agricultural streams in Sweden (76). Stream N 2 O concentrations are often correlated with dissolved nitrogen concentrations (41,76,79,81); however, variability in this relationship is often observed between sites (74, 75, 77).…”

An intense precipitation event causes a temperate forested drainage network to shift from N2O source to sink

“…Spatially, urban (McMahon and Dennehy 1999; Beaulieu et al 2010; Beaulieu et al 2011) and agriculture (Beaulieu et al 2011; Audet et al 2017; Mwanake et al 2019) landcover types have been associated with high terrestrial nitrogen loading and subsequent high lotic N 2 O emissions. However, a recent study showed that forested rivers have similar N 2 O concentration as agriculture rivers (Audet et al 2020). Furthermore, within‐site N 2 O concentration and emission variability are difficult to predict.…”

Carbon dioxide dynamics in an agricultural headwater stream driven by hydrology and primary production

“…To explore how representative the SBM catchment is for streams draining agricultural areas in the region, a snapshot sampling survey was performed across 10 streams (denoted region UPP 2 in the study by Audet et al (2019)) of various sizes (catchment area 8.5-740 km 2 ) and agricultural influence (30-86%) distributed within a radius of 10 km from the city center of Uppsala (Table S1).…”