Response Of Greenhouse Gas Emissions To Different Hydrothermal Conditions Under Different Grazing Intensities In The Seasonal Frozen Soils For A Typical Steppe

As greenhouse gases(GHGs)are increasing at a supring rate,global warming is becoming one of the most serious environmental problems in our planet.Grasslands cover about 40.5%of the terrestrial surface which could become a big contributor to atmospheric GHGs,and soils play an important role in this contribution.Therefore,it is necessary to evaluate GHG distributions in different soil layer,GHG emission characteristics and controlling factors in grassland ecosystem.In this study,three sites with different grazing history(ungrazed since 1979 or UG79,ungrazed since 1999 or UG99,and continuously grazing or CG)were selected in the Xilin gol Steppe.A situ gas collection system and static chamber method were used to determin GHG concentrations in the soil profile and GHG fluxes above ground repectively.At the same time,soil water content and soil temperature were recorded by automatic probe at 30-min intervals to reveal the effect of environmental factors on GHGs.The main results are as follows:1)Characteristics of GHG cencentration in the soil profiles for different grazing exclusion sites.The characteristics of CO₂ concentration in the soil profile were obvious.Although CO₂concentration remained almost constant in the freezing period and witnessed a slight increase in the freeze-thawing period,it climbed to the highest in the growing period.Besides,CO₂concentrations in the top soil(0-10cm)was significantly lower than deep soil at all sites.The effect of grazing on CO₂ concentration was mainly to decrease its concentrations,but not to change distribution characteristics.In addition,it is noticeable that CO₂ concentration showed a higher value in the year with more precipitation,compared with the year with less precipitation.The seasonal variation of N₂O concentration in soil profile was different.It increased dramaticaly at the beginning of freeze-thaw period and post rain period,especially in the early stage of freeze-thaw period.This period is the key period for greenhouse gas production in typical grassland of Inner Mongolia.Besides,The N₂O concentration in different years is mainly affected by weather.It is higher in the year with more precipitation compared with the year with less precipitation.In addition,it was clearly that grazing increased the N₂O concentration in the soil.The results also showed that higher N₂O concentration was observed in the deep soil layer compared with surface layer,but there was little difference.2)GHG emissions for different grazing exclusion sites.Our results showed that Xilin gol steppe is the source of atmospheric CO₂ and N₂O,the sink of CH₄.The emissions of N₂O,CO₂ and CH₄ showed obviously seasonal pattern in different grazing sites.CO₂ emissions and CH₄ absorption mainly occured in the growing period,but N₂O emissions were controlled by freeze-thaw cycles in spring and rainfall events in growing periods.In addition,the global warming potential of grazing exclusion sites were slightly higher than that of grazing sites,but there was no significant difference among them.3)Effect of grazing exclusion on N₂O emissions during freeze-thaw cycles(FTCs)in a typical steppe of Inner MongoliaCompared with grazing,short-term grazing exclusion led to a three-fold increase in N₂O emissions during Spring-FTCs,but long-term GE had no significant effect on N₂O emissions and that even showed decreased trends under certain climatic conditions(such as less snow in winter).Although N₂O fluexes were affected by many factors during freeze-thaw period,soil water content,N₂O concentration and microbial biomass were the main factors controlling N₂O emission during this period.4)Effect of soil water and soil temperature on GHGsOur results showed that CO₂ fluxes and CO₂ concentration was positively correlated with soil temperature and water content(P<0.01).it means that CO₂ fluexes were significantly affected by soil water content and temperature in our sites.However,the correlation between soil temperature and N₂O concentration was weak.It is said that soil temperature had less significant effect on N₂O fluxes.The results also showed that soil moisture was significantly correlated with soil profile N₂O concentration in certain soil layers in grazing exclusion sites,thus grazing exclusion may promote the effect of soil moisture content on N₂O.At the same time,soil temperature was significantly correlated with N₂O fluxes in CG sites,it means grazing could promote the effect of temperature on N₂O emissions.In addition,Soil temperature and water content had less significant effect on CH₄ emissions.In summary,from the perspective of greenhouse gas emission reduction,moderate grazing should be encouraged.