Responses of soil CH4 fluxes to nitrogen addition in two tropical montane rainforests in southern China

Background:Atmospheric nitrogen(N)deposition is projected to increase in the next few decades,which may have a marked impact on soil-atmosphere CH4 fluxes.However,the impacts of increased atmospheric N de-positions on soil CH4 flux in tropical rainforests are still poorly understood.From January 2015 to December 2018,a field experiment was conducted in a primary tropical montane rainforest(PTMR)and a secondary tropical montane rainforest(STMR)in southern China to quantify the impact of N additions at four levels(N0:0 kg N-ha-1-year-1;N25:25 kg N·ha-1.year-1;N50:50 kg N-ha-1-year-1;N100:100 kg N·ha-1·year-1)on soil CH4 flux.Results:Four years of measurements showed clear seasonal variations in CH4 flux in all treatment plots for both forest types(PTMR and STMR),with lower rates of soil CH4 uptake during the wet season and higher rates of soil CH4 uptake during the dry season.Soil CH4 uptake rates were significantly and negatively correlated with both soil temperature and soil moisture for both forest types.Annual CH4 uptake for the N0 plots from the PTMR and STMR soils were-2.20 and-1.98 kg N·ha-1-year-1,respectively.At the PTMR site,mean CH4 uptake compared with the N0 treatment was reduced by 19％,29％,and 36％for the N25,N50,and N100 treatments,respectively.At the STMR site,mean CH4 uptake compared with the N0 treatment was reduced by 15％,18％,and 38％for the N25,N50,and N100 treatments,respectively.High level N addition had a stronger inhibitory impact on soil CH4 uptake than did the low level N addition.Conclusion:Our data suggest that soil CH4 uptake in tropical rainforests is sensitive to N deposition.If atmospheric N deposition continues to increase in the future,the soil CH4 sink strength of tropical rainforests may weaken further.