Global nitrogen input on wetland ecosystem:The driving mechanism of soil labile carbon and nitrogen on greenhouse gas emissions

Greenhouse gas emissions from wetlands are significantly promoted by global nitrogen input for changing the rate of soil carbon and nitrogen cycling,and are substantially affected by soil labile carbon and nitrogen conversely.However,the driving mechanism by which soil labile carbon and nitrogen affect greenhouse gas emissions from wetland ecosystems under global nitrogen input is not well understood.Working out the driving factor of nitrogen input on greenhouse gas emissions from wetlands is critical to reducing global warming from nitrogen input.Thus,we synthesized 72 published studies(2144 paired observations)of greenhouse gas fluxes and soil labile compounds of carbon and nitrogen(ammonium,nitrate,dissolved organic carbon,soil microbial biomass nitrogen and carbon),to understand the effects of labile carbon and nitrogen on greenhouse gas emissions under global nitrogen input.Across the data set,nitrogen input significantly promoted carbon dioxide,methane and nitrous oxide emissions from wetlands.In particular,at lower nitrogen rates(<100 kg ha^(-1)$·yr^(-1))and with added ammonium compounds,freshwater wetland significantly promoted carbon dioxide and methane emissions.Peatland was the largest nitrous oxide source under these conditions.This meta-analysis also revealed that nitrogen input stimulated dissolved organic carbon,ammonium,nitrate,microbial biomass carbon and microbial biomass nitrogen accumulation in the wetland ecosystem.The variation-partitioning analysis and structural equation model were used to analyze the relationship between the greenhouse gas and labile carbon and nitrogen further.These results revealed that dissolved organic carbon(DOC)is the primary factor driving greenhouse gas emission from wetlands under global nitrogen input,whereas microbial biomass carbon(MBC)more directly affects greenhouse gas emission than other labile carbon and nitrogen.