Search

With increasing nitrogen (N) deposition, soil nitrous oxide (N 2 O) emission is expected to increase, causing positive feedback to global warming. However, the substrates of soil N 2 O emission, especially their responses to N addition, are still unclear. Here, we conducted an in situ 15 N tracing experiment to study the substrates of N 2 O (i.e., ammonium-derived, nitrate-derived and organic N-derived N 2 O emission) under N addition treatment in a temperate forest in northeast China. Nitrate derived N 2 O through denitrification contributed most to the total N 2 O emission, pointing to the importance of denitrification under ambient N deposition. NH 4 NO 3 addition of 50 kg N ha -1  yr -1 significantly increased organic N derived N 2 O 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 N 2 O emissions, highlighting the importance of considering the heterotrophic nitrification process while studying soil N cycling or modeling soil N 2 O emission., Competing Interests: Declaration of competing interest All authors declare that no conflict of interest exists., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Nitrous oxide (N 2 O) is an important greenhouse gas and is involved in the destruction of ozone layer. However, the underlying mechanisms of the high soil N 2 O emission during the freeze-thaw (FT) period are still unclear. Here, we conducted a mesocosm study with high frequency in situ measurements to explore the responses of soil microbes to the FT cycles and their influences on soil N 2 O emission. We found the high N 2 O emission rate during the FT period was mainly due to the release of substrates, the maintenance of high enzyme activities at the freezing stage, and the fast recovery of microbial biomass nitrogen (MBN) and high microbial activities at the thawing stage. Physical isolation of previously produced N 2 O was an important mechanism for the higher N 2 O flux at the thawing stage. With increasing numbers of the FT cycles, MBN at the thawing stage remained stable and potential dehydrogenase activities at the thawing stage also remained stable after the first eight FT cycles and only declined during the last two cycles, suggesting the sustainability of the biological mechanisms. Our study suggests that although MBN declined, microbial enzymes could maintain high activities at a few degrees Celsius below zero in this temperate forest soil and produce high N 2 O fluxes even at the freezing stage, which were trapped under the ice layer and released at the thawing stage, resulting in high soil N 2 O emission during the FT period., (Copyright © 2018 Elsevier B.V. All rights reserved.)