The Effect Of Different Grazing Intensities On Greenhouse Gas Fluxes(GHGs) In Semi-arid Grassland Of The Agro-pastoral Ecotone Of Northern China

Grazing is one of the most important ways for utilization of natural grassland in the northern China.There were different studies focused in on grazing methods effect on the structure and function of ecosystems,such as rotational grazing and forbidden grazing in early spring.Grazing affects soil structure and function through biological and physical processes such as animal trampling,feeding and excretion,and further affects greenhouse gas fluxes.There are less researches on greenhouse gas fluxes by grazing intensities in semi-arid grassland ecosystems,especially in the grassland ecosystems in the agro-pastoral ecotone.In this study,four different grazing intensities(No Grazing,Light Grazing,Moderate Grazing,and Heavy Grazing)were set in the semi-arid grassland ecosystem of agro-pastoral ecotone in Northern China(Youyu,Shanxi).The influences of different grazing intensity on the GHGs fluxes,were studied by measuring greenhouse gas fluxes in growing season with the opaque static chamber.The results showed that:1.The seasonal mean values of CO2,CH4 and N2 O fluxes in the first year of grazing had no significant effect(P>0.05),but had obvious seasonal dynamics in 2017.Grazing decreased soil water content that compared with UG(P<0.05),MG significantly decreased microbial biomass carbon(MBC,P<0.05)and the MG and HG reduced microbial biomass nitrogen(MBN,P<0.05).Analyzing the correlation between greenhouse gas flux and biological and abiotic factors,it was found that significant positive correlation between CO2 fluxes and soil temperature,below-ground biomass,litter and microbial biomass carbon,but not with grazing intensity.While the correlation between CO2 fluxes and soil nitrate nitrogen,inorganic nitrogen and water content depended on grazing intensity,specifically as follows: The CO2 fluxes was increased significantly with the increasing of nitrate nitrogen in MG and HG,inorganic nitrogen had significantly positive correlation with CO2 emission under UG,MG,and HG,there were significantly positive relationship between soil moisture and CO2 fluxes in LG and HG(P<0.05).For CH4 fluxes,there were significantly positive relationship between CH4 fluxes nitrate nitrogen,ammonium nitrogen,belowground biomass,and litter,but not with grazing intensity(P<0.05).In addition,a significant positive correlation between CH4 fluxes and soil temperature only under light grazing(P<0.05).The N2 O fluxes was significantly increased with the increasing trend of soil temperature and microbial biomass.Inorganic nitrogen had significantly positive correlation with N2 O fluxes only under LG treatment(P<0.05).2.In the second year of grazing treatment(2018),no significant effect on CO2 fluxes by different grazing intensities,but LG and MG significantly increased N2 O fluxes,and LG and HG significantly increased CH4 uptake in 2018.Grazing significantly reduced above-ground biomass(P<0.05),HG decreased below-ground biomass,while MG and HG significantly reduced litter(P<0.05).The correlation between greenhouse gas flux and biological and abiotic factors was analyzed.It was found that there were significant positive relationship between CO2 fluxes and soil temperature,below-ground biomass,litter,MBN and microbial C/N ratio,but not with grazing intensity(P<0.05).The CH4 fluxes was inhibited significantly with the decreasing of below-ground biomass.There was a significant positive correlation between soil water content and CH4 fluxes whatever grazing or no grazing(P<0.05).There was a significant positive correlation between CH4 uptake and ammonium nitrogen under HG,and between inorganic nitrogen and CH4 uptake under UG,CH4 uptake was a negative correlated with soil temperature under UG and LG(P<0.05).N2 O fluxes was positively correlated with microbial biomass,below-ground biomass and litter(P<0.05).There was a significant positive correlation between N2 O fluxes and soil temperature under UG,And significant positive correlation between N2 O fluxes and ammonium nitrogen under HG(P<0.05).In summary,there was a significant inter-annual difference in the influence of grazing on the greenhouse gas fluxes of semi-arid grassland.On the one hand,due to the inter-annual rainfall.On the other hand,the cumulative effect of grazing is also an important reason for the inter-annual difference in the greenhouse gas fluxes.There were not a significantly impact of vegetation and soil structure due to the only two years of grazing.So,there are needed long-term and in-depth studies to explain how grazing affects greenhouse gas fluxes in a mechanistic way on the effects of grazing on ecosystem structure and function and provide scientific management basis for production practice.