Forest streams are important sources for nitrous oxide emissions

Streams and river networks are increasingly recognized as significant sources for the greenhouse gas nitrous oxide (N(2)O). N(2)O 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 N(2)O emissions from forest streams although they constitute a major part of the total stream network globally. Here, we compiled N(2)O 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 N(2)O 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 N(2)O concentrations and environmental variables were difficult to discern, the percent saturation of N(2)O 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 N(2)O 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 N(2)O emissions. An estimate of the N(2)O emission from low‐order streams at the national scale revealed that ~1.8 × 10(9) g N(2)O‐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 N(2)O sources in the landscape with 800 × 10(9) g CO(2)‐eq emitted annually in Sweden, equivalent to 25% of the total N(2)O emissions from the Swedish agricultural sector.