Relative contributions of different substrates to soil N2O emission and their responses to N addition in a temperate forest

With increasing nitrogen (N) deposition, soil nitrous oxide (N2O) emission is expected to increase, causing positive feedback to global warming. However, the substrates of soil N2O emission, especially their responses to N addition, are still unclear. Here, we conducted an in situ 15N tracing experiment to study the substrates of N2O (i.e., ammonium-derived, nitrate-derived and organic N-derived N2O emission) under N addition treatment in a temperate forest in northeast China. Nitrate derived N2O through denitrification contributed most to the total N2O emission, pointing to the importance of denitrification under ambient N deposition. NH4NO3 addition of 50 kg N ha−1 yr−1 significantly increased organic N derived N2O on the 6th day after N addition, which suggests that heterotrophic nitrification may be the dominating process with higher N deposition rate. However, because soil pH and the examined functional genes did not change after N addition, future studies should be carried out to understand if the increase of heterotrophic nitrification is transient. Our study emphasizes the role of organic N pool in soil N2O emissions, highlighting the importance of considering the heterotrophic nitrification process while studying soil N cycling or modeling soil N2O emission. Soil N2O flux from the mixed Korean pine and broad leaved forest under ambient and N addition treatments from Jun. 14 to Sep. 23, 2014. Values are reported as mean ± standard deviation (n = 12). The insert shows the diurnal dynamics of soil N2O flux (mean ± standard deviation) during the first 24 h after N addition. [Display omitted] •Nitrate derived N2O through denitrification contributed most to the total N2O emission under ambient N deposition.•NH4NO3 addition of 50 kg N ha−1 yr−1 significantly increased organic N derived N2O on the 6th day after N addition.•Heterotrophic nitrification may be the dominating process under higher N deposition rate.