Soil Aggregates Stability And Organic Carbon Content Changes And Its Mechanism In Horqin Degraded Grassland

Soil organic carbon(SOC)represents the largest pool of terrestrial carbon containing twice greater than atmospheric carbon stocks,so slight changes in soil carbon stocks will have profound effects on the provision of ecosystem services and climate regulation.Grassland store20?30%of global soil organic carbon stocks.Currently,a majority of grasslands have suffered degradation due to intensive grazing,which often associated with soil carbon loss as greenhouse gases.Therefore,understanding the mechanisms of SOC loss in degraded grassland is essential for the development of effective measures to mitigate climate change.The soil carbon stock is determined by the balance between carbon inputs from plants and carbon outputs by microbial-mediated decomposition and stabilization of SOC.As the basic unit of soil structure,soil aggregates play a key role in soil carbon cycle.In this study,we selected non-degraded,lightly degraded,moderately degraded and severely degraded grasslands in Horqin grassland to form a sequence of degraded grasslands,and analyzed the plant biomass and community composition,soil aggregate stability,microbial community structure,hydrolyzed enzyme activities and other soil properties to investigate the main factors influencing SOC loss in different degradation levels of Horqin grasslands.The main results of this study are as follows.(1)The SOC content,and nitrogen and phosphorus content decreased with the intensification of degradation.Grassland degradation caused the disintegration of macroaggregates,and the proportion of macroaggregates and their carbon and nitrogen contents decreased along the degradation gradient.Soil aggregate stability(i.e.,mean weight diameter and geometric mean diameter)decreased with the intensification of degradation.The results of variance partitioning analysis and structural equation modeling indicated that the decreases in plant root biomass and exchangeable Ca²⁺and Mg²⁺were the main factors driving the soil aggregate stability during grassland degradation.(2)Grassland degradation led to significant differences in microbial community structure within different aggregate size fractions.The main factors causing differences in microbial community structure were dissolved nitrogen content,total phosphorus content and p H for macro-aggregates,dissolved organic carbon content,organic carbon content,total nitrogen content,total phosphorus content and p H for micro-aggregates,and dissolved organic carbon content,organic carbon content and p H for silt-clay aggregates.(3)With the intensification of grassland degradation,the decrease in plant uptake and increase in released from mineral weathering resulted in increase of the available phosphorus content.Moreover,soil aggregate stability weakened and exchangeable Ca²⁺and Mg²⁺decreased,leading to microorganisms C and P-limitation mitigated and shifted to N-limitation with grassland degradation.The structural equation modeling indicated that the increases in available phosphorus content caused less energy losses in microbes occurred during C and P acquisition by deploying extracellular enzymes,which represent a cost hindering microbial carbon use efficiency.Correlation analysis also confirmed that microbial carbon use efficiency was negatively correlated with the ratio of G⁺/G^-.(4)Grassland degradation reduced organic matter input into the soil,as a consequence of the aboveground,belowground and litter biomass decreased,and those were positively correlated with SOC content.Microbial carbon use efficiency increased and microbial biomass decreased along the degradation gradient,which caused a less C-acquisition enzymes produced,indicating that the mineralization of microorganisms on SOC decrease with the he intensification of degradation.The result of boosted regression tree model suggested that exchangeable Ca²⁺and mean weight diameter are the two most influential variables on SOC among the 12 studied variables.In addition,soil properties were the most important factors,accounting for 51.8%in controlling SOC content,revealing that stabilization mechanisms linked to soil aggregation and chemical stabilization by polyvalent cations were important controls on SOC stocks during grassland degradation.In summary,grassland degradation caused significant differences in microbial community structure within different aggregate size fractions and resulted SOC loss via weaken physical protection of SOC and microorganisms mitigated soil carbon loss by changes in their physiology,activity and shifts in community structure.This study furthered understanding of soil aggregate scale microbial communities and reveals the mechanism SOC loss in degraded grasslands,emphasizing the important role of physico-chemical protection in soil carbon cycling,which are beneficial for sustainable management of degraded grasslands.In future studies we need to integrate plant communities,microbial communities and functions,and soil geochemical properties to better forecast soil C dynamics under changing environments.