Stimulated or Inhibited Response of Methane Flux to Nitrogen Addition Depends on Nitrogen Levels

Abstract

AbstractUpland forest soil is significant atmospheric methane (CH4) sink that is suggested to be affected by increased nitrogen (N) availability, likely from N deposition or addition. However, the effect of N addition on upland soil CH4 uptake and the underlying mechanisms remain unclear. Here, in a montane forest, frequent in situ sampling over three years revealed that this forest soil was a sink of atmospheric CH4 with an uptake rate of 304.9 ± 26.9 mg CH4/m2/yr under ambient treatment. High N addition suppressed CH4 uptake by 23.2% compared to ambient conditions, whereas low N addition stimulated CH4 uptake by 35.6%. N addition did not change the monthly pattern of CH4 flux, which was quite similar to the pattern of leachate dissolved organic carbon (DOC) concentrations from the organic layer. The linear relationship between the CH4 flux and DOC concentration indicated increased CH4 uptake with an increase in DOC. The induced decrease in CH4 uptake under high N addition was partly related to the high soil N availability and partly contributed to a decrease in DOC due to high N addition. In contrast, the increased CH4 uptake under temporarily low N addition was most likely correlated with a release in N limitation to CH4‐oxidizing bacteria. These results indicated that the effect of N addition on CH4 uptake was dependent on the level of N, and DOC could regulate CH4 uptake in upland soil, which should be investigated in depth when understanding the responses of dynamic CH4 fluxes to N additions.