Effect Of Grazing Intensities On Soil Organic Carbon Characteristics In Desert Steppe And Their Mechanisms

Inner Mongolia desert steppe ecosystem is fragile,and grazing is the main utilization of this type of steppe.However,the large-scale degradation of grassland caused by long-term overgrazing has induced a series of soil environmental issues,especially the loss of soil organic carbon and the change of its components.Accordingly,the present study was conducted in the experimental platform of long-term grazing intensity control in desert steppe of Inner Mongolia Agricultural University（established in 2004,with four grazing treatments:no grazing,light,moderate and heavy grazing）.In this study,soil environmental indicators,soil organic carbon and its components were measured by field sampling combined with in-lab analysis.We used amplicon sequencing and gene chip technology to analyze the changes in soil microbial community structure and functional genes related to carbon cycling.We observed soil organic carbon mineralization and temperature sensitivity by laboratory incubation.We used solid-state ¹³C NMR to analyze soil organic carbon microstructure and examined the changes of soil organic carbon chemical functional structure under different grazing intensities.In this study,we analyzed the microbiological mechanism of the effect of different grazing intensities on soil organic carbon and its components,and investigated the driving factors of the effect of different grazing intensities on soil organic carbon mineralization and temperature sensitivity.The objectives of this study were to reveal the effects of long-term different grazing intensities on soil organic carbon characteristics in desert steppe and its regulatory mechanisms.The main results are as follows:1.Soil organic carbon content under light grazing was not significantly different from no grazing（P>0.05）,but both were significantly higher than moderate and heavy grazing（P<0.001）,and there was no significant difference between moderate and heavy grazing（P>0.05）.Soil inorganic carbon and total carbon were significantly higher under light grazing than no grazing,moderate and heavy grazing（P<0.001）,and there was no significant difference between moderate and heavy grazing and no grazing（P>0.05）.Soil active organic carbon content was in the order from high to low:light grazing>ungrazed>moderate grazing>heavy grazing（P<0.001）.Soil slow organic carbon content gradually decreased with increasing grazing intensities,while there was no significant difference between light grazing and no grazing（P>0.05）.Soil passive organic carbon content was lowest under light grazing,which was opposite to the trend of active organic carbon.Soil activity index and carbon pool management index both decreased significantly with increasing grazing intensities（P<0.001）,with the same trend as that of active organic carbon.Soil fungal necromass carbon,bacterial necromass carbon,microbial necromass carbon and microbial biomass carbon content all decreased significantly with increasing grazing intensities（P<0.001）,and there was no significant difference between light grazing and no grazing（P>0.05）.2.Soil saturated water holding capacity,soil capillary water holding capacity and field water holding capacity under light grazing were not significantly different from no grazing（P>0.05）,but all were significantly higher than moderate and heavy grazing（P<0.001）,and there were no differences between moderate and heavy grazing（P>0.05）.Soil capillary porosity decreased significantly under moderate and heavy grazing（P<0.001）,and the trend was the same as that of soil water holding indicator.Soil non-capillary porosity and total porosity were both highest under light grazing and significantly higher under ungrazed than under both moderate and heavy grazing（P<0.001）,with no difference between moderate and heavy grazing（P>0.05）.Soil bulk density increased significantly under both moderate and heavy grazing（P<0.001）,in contrast to the trend of soil capillary porosity.Soil macroaggregates content decreased significantly only under heavy grazing（P<0.001）.Soil small aggregates decreased significantly under moderate and heavy grazing,with no significant differences between grazing intensities（P>0.05）.Soil microaggregates increased significantly with increasing grazing intensities（P<0.001）,and there was no difference between light grazing and no grazing（P>0.05）.Soil geometric mean diameter decreased significantly with increasing grazing intensities（P<0.001）,in contrast to the trend of small aggregates.Grazing led to a significant decrease in soil p H and total nitrogen,and heavy grazing led to a significant increase in soil conductivity（P<0.001）.3.Grazing altered the alpha diversity of soil microbial communities.The relative abundance of soil bacteria and fungi,Proteobacteria,Bacteroidetes and Ascomycota were significantly higher in ungrazed and lightly grazed soils than in moderately and heavily grazed soils（P<0.001）,but there were no differences between light and ungrazed,moderate and heavy grazing（P>0.05）.The relative abundance of Acidobacteria,Actinobacteria and Basidiomycota was lowest under ungrazed,light and heavy grazing,respectively（P<0.05）.The absolute abundance of genes related to soil carbon degradation,fixation and methane metabolism were all highest under no grazing and lowest under light grazing,and all showed a gradual increase with increasing grazing intensities.All grazing changed theβ-diversity of bacteria,fungi and functional genes,and the structure of soil microbial communities and functional genes changed significantly with different grazing intensitis.4.Soil organic carbon was significantly negatively correlated with soil p H,conductivity,bulk density and total phosphorus,and significantly positively correlated with soil moisture and porosity indicators（P<0.05）.Soil organic carbon was significantly and positively correlated with microbial abundance and the abundance of two dominant bacterial phyla,Proteobacteria and Bacteroidetes,and negatively correlated with the abundance of three dominant bacterial phyla,Verrucomicrobia and Planctomycetes,and the dominant fungus,Mortierellomycota（P<0.05）.Soil organic carbon was significantly negatively correlated with the absolute abundance of carbon degradation genes cdh,man B,sga,naglu and iso-plu,carbon fixation genes acs B,mct and smt A,and methane metabolism genes mmo X and pqq-mdh（P<0.05）.Soil microbial necromass carbon was positively regulated by microbial biomass carbon,and its content and the decrease in microbial biomass carbon content directly led to the decrease in organic carbon content.With the increase of grazing intensities,soil capillary water holding capacity and large aggregates decreased,and the increase of soil p H,bulk density,small aggregates and microaggregates led to the decrease of soil bacterial and fungal richness,which indirectly regulated the decrease of organic carbon content.5.Soil carbon mineralization varied under different grazing intensities,with the highest cumulative mineralization of ungrazed soil organic carbon,which decreased with increasing grazing intensities,with significant differences between grazing intensities（P<0.001）.The chemical functional structure of soil organic carbon had the highest O-alkyl carbon content,followed by alkyl carbon,aromatic carbon and carbonyl carbon.Grazing significantly reduced soil O-alkyl carbon（P<0.05）,but there was no difference between grazing intensities（P>0.05）.Both alkyl carbon and carbonyl carbon were significantly higher under light grazing than ungrazed,moderate and heavy grazing（P<0.05）.Aromatic carbon was significantly（P<0.05）lower under ungrazed and light grazing than moderate and heavy,but there was no difference between ungrazed and light;moderate and heavy grazing（P>0.05）.Grazing reduced Q10（the temperature sensitivity of soil carbon mineralization）（P<0.001）,which showed the same trend as the change in aromatic carbon.Soil organic carbon content was significantly and positively correlated with carbon mineralization,while it was significantly and negatively correlated with soil temperature sensitivity（P<0.05）.6.With the increase of grazing intensities,soil carbon mineralization decreased,but the factors affecting soil carbon mineralization differed with different grazing intensities.Soil carbon mineralization under light grazing was mainly influenced by changes in microbial community structure.Changes in soil environment and microorganisms under moderate grazing jointly regulate carbon mineralization.Heavy grazing soil abiotic factors,aggregate structure and microbial community coupled to regulate the reduction of carbon mineralization.Collectively,the changes in soil physicochemical properties,microbial community structure,and functional genes related to carbon cycling caused by different grazing intensities in desert steppe over a long period of time are the main reasons for regulating changes in soil organic carbon and its components.Soil carbon mineralization is an important characteristic of organic carbon changes,and grazing leads to the reduction of organic carbon mineralization,but the driving mechanisms are different under different grazing intensities.