NO emission from a temperate forest soil during the freeze-thaw period: A mesocosm study.

Nitrous oxide (NO) is an important greenhouse gas and is involved in the destruction of ozone layer. However, the underlying mechanisms of the high soil NO 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 NO emission. We found the high NO 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 NO was an important mechanism for the higher NO 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 NO fluxes even at the freezing stage, which were trapped under the ice layer and released at the thawing stage, resulting in high soil NO emission during the FT period.