雪被和土壤水分对典型半干旱草原土壤冻融过程中CO2和N2O排放的影响

Abstract

Soil freeze-thaw cycle is very common in the regions at mid-high latitude and high altitude. Soil physical properties,chemical composition and microbial activities could be affected significantly during this cycle. Increases in greenhouse gas fluxes have been frequently reported during soil freezing and thawing,both in laboratory and field experiments. Moreover,some studies indicated that the accumulative fluxes during soil freezing and thawing could contribute a lot to the annual budget,especially of N2 O. However,many soil incubation studies have introduced experimental artifacts that diminish the realism and relevance of the freeze-thaw cycle. Most of past laboratory studies adjusted soil moisture beforesoil freezing,so that the effect of snow cover and water from melting snow was not considered during soil freeze-thaw cycles.In order to simulate field conditions more closely in the laboratory,the effects of snow cover and soil moisture on CO2 and N2O fluxes during freeze-thaw cycles were evaluated for a typical semi-arid grassland soil. Three different soil moisture levels were established either prior to soil freezing or by adding fresh snow to the soil surface after freezing. Our results showed that the dynamics of soil CO2 fluxes during three freeze-thaw cycles under snow cover and watering treatments were not significantly different. Soil CO2 emissions were generally enhanced during the first freeze-thaw cycle,and gradually decreased with successive cycles. However,snow cover had significant effect on soil N2 O fluxes during freeze-thaw cycles when the soil water-filled pore space( WFPS) were around 50%. The highest emissions of N2 O were observed during the first freeze-thaw cycle in watering treatment,while in snow cover treatment a repetition of the freeze-thaw cycles resulted in a further increase of N2 O emissions. The reasons for the significant differences in N2 O performance between two treatments might be the different soil water dynamics and microbial properties along the soil profile. CO2 emissions were a function of soil moisture,with emissions being largest around 50% WFPS and smallest at 32% WFPS. Emissions of N2 O during soil freeze-thaw increased with increasing soil moisture,which suggests that denitrification,instead of nitrification,might be the main process in the soil producing N2 O during these periods. No significant N2 O emissions were observed during freeze-thaw cycles under relatively low soil water conditions,which indicate that the enhanced N2 O emissions might only become significant when the soil moisture reaches a certain threshold. In addition,the dynamics of soil air concentration of CO2 and N2O along the soil profile were positively correlated with soil-surface fluxes and could provide additional information on the N and C turnover processes in the soil.