Effects Of Grazing Intensity On Soil Organic Carbon Fractions And Its Stability In Desert Steppe

In terrestrial ecosystem,grassland has huge carbon storage,most of which is accumulated in soil.Soil organic carbon fractions play an important role in grassland ecosystem carbon cycle and regional climate regulation.Grazing is one of the important factors affecting the carbon function of grassland ecosystems,Stipa breviflora desert steppe of Inner Mongolia is located in an arid and semi-arid region,the carbon pool is in a low level state,which is sensitive to grazing and belongs one of the most unstable grassland type.The analysis of the impact of grazing intensity on the fraction and stability of organic carbon in desert steppe will help to better understand the response mode of soil in desert steppe to the grazing.This study focused on the desert steppe in Siziwang Banner,Inner Mongolia,adopting a completely random block design,and sets up four different grazing treatments,namely,they are control(CK),light grazing(LG),moderate grazing(MG),and heavy grazing(HG),and each treatment was repeated 3 times,based on the measurements of soil organic carbon and its fractions in different grazing plots,studied the changes of above-ground biomass of plant community,soil physical and chemical properties,soil organic carbon fractions and soil organic carbon stability of desert steppe under different grazing intensities,revealed the relationship between soil organic carbon and its fractions and different grazing intensities,and analyzed which grazing intensity is most suitable for soil organic carbon stability.The results are as follows:(1)Moderate and heavy grazing intensity significantly reduced the above-ground biomass of the plant community,compared with CK,the average aboveground biomass of HG community decreased by 51.6% during the experimental period.Grazing significant decreased the soil moisture content,in 2021,the moisture content of HG was significantly lower than that of CK,in 2022,it was significantly lower in all grazing treatments compared with the CK.In terms of Soil p H,which increased with increasing grazing intensity,with the most significant higher in MG and HG.The total nitrogen content in the soil gradually decreases with increasing grazing intensity,with HG is significant lower than other grazing treatments,and the interaction between year and grazing treatment has a significant impact on its content.(2)The effect of grazing on Total organic carbon and organic carbon fraction content had no significant difference between years,but grazing had changed the content of soil organic carbon fractions.MG and HG significantly reduced the content of soil particulate organic carbon(POC),mineral associated organic carbon(MOC),light fraction organic carbon(LFOC),dissolved organic carbon(DOC),readily oxidation organic carbon(ROC),and microbial biomass carbon(MBC).LG has increased the content of soil mineral bound organic carbon.The results of sensitivity index of soil organic carbon fraction showed that ROC is the most sensitive,and followed by LFOC.(3)The ratio of particulate organic carbon to mineral bound organic carbon represents the stability of soil organic carbon.In 2021,this ratio for 2-years show that LG is the smallest compated with other treatments,which indicating that the soil organic carbon was the most stable under LG.This ratio in different soil layers show that its value gradually decreases as the soil layer deepens,which indicating that soil organic carbon becomes more stable as the soil layer deepens.(4)The correlation analysis showed that soil p H value was significantly negatively correlated with soil moisture content,POC,LFOC,DOC,ROC and MBC,there was a significant positive correlation between soil organic carbon and its fractions and above-ground biomass.There is a significant positive correlation between the fractions of soil organic carbon and soil total carbon,organic carbon,and total nitrogen.The principal component analysis results showed that the fractions with the greatest impact on soil organic carbon were LFOC,ROC,and DOC.In conclusion,grazing has negative impact on above-ground biomass,soil p H,moisture content and total nitrogen in desert steppe.Overgrazing will reduce the content of soil total carbon,organic carbon and its fractions,and then affect stability of soil organic carbon,while light grazing can pormote the accumulation and stability of soil organic carbon.The main physical components that affect soil organic carbon content were POC and LFOC,while the chemical components were DOC and ROC,and biological fractions had no significant effect on them.The readily oxidation organic carbon was the most sensitive to the response of grazing intensity and can be used as an important indicator to evaluate the soil stability.