Effect Of Vegetation Restoration On Turnover And Stability Of Soil Organic Carbon In Degraded Red Soil Region

Forest restoration and reconstruction has become an important measure to improve soil ecological function in the degraded subtropical red soil region.In recent decades,a great deal of research achievements have been made in the study of vegetation restoration organic carbon in the degraded red soil,mainly focusing on soil carbon pool and carbon storage,organic carbon content of different components,organic carbon stability and so on.Little is known about the input of new carbon and the turnover of old carbon after forest restoration.Taking the seriously degraded grassland(mainly C₄)as a control,this paper selected Pinus massoniana(coniferous forest)and Schima superba(broad-leaved forest)planted for 25 years as forest restoration objects in Taihe County of Jiangxi Province,and analyzed soil vegetation carbon,soil total organic carbon,soil labile carbon and non-labile organic carbon with stable carbon isotope method.Soil organic carbon sources after vegetation restoration were compared.The effect of soil microbial community on soil organic carbon turnover was discussed by analyzing soil microbial community structure in situ.It provides valuable reference for forest restoration in red soil area.The main findings are as follows:(1)The content of OC in 0-10cm soil layer of broad-leaved forest was significantly higher than that of grassland and coniferous forest in the control area.The order of soil organic carbon pool size is broadleaf forest>grassland>coniferous forest.The order of SOC pool size is broadleaf forest>coniferous forest>grassland.The content of soluble organic carbon and microbial biomass carbon in broadleaf forest was the highest,followed by coniferous forest and grassland.The order of non-labile carbon content in 0-20cm soil layer was broadleaf forest>coniferous forest>grassland.?¹³C_DOC and?¹³C_MBC in degraded grassland were significantly higher than those in coniferous forest and broad-leaved forest,and lower than that in whole soil.In coniferous and broad-leaved forests,the proportion of organic carbon in the light part of soil was the highest,exceeding 70%;the proportion of soluble carbon from new carbon was the middle,and the proportion of microbial biomass carbon was the lowest,not exceeding 35%.This indicated that the utilization of C₃ and C₄vegetation carbon was different among different components of active organic carbon.(2)After the grassland was transformed into coniferous forest and broad-leaved forest,the organic carbon from litter?¹³C to soil surface?¹³C increased by 7.41 and10.89‰respectively,which exceeded the normal range of increase(0.5?1.5‰),indicating that SOC of forestwere the mixture of the former grassland and the current restored forest.There was a significant linear regression relationship between the natural logarithm of soil organic carbon concentration and the value of soil?¹³C in artificial broad-leaved forest.The decomposition rate of old carbon in broadleaf forest was lower than that in coniferous forest.The retention time of organic carbon in coniferous forest and broadleaf forest was 42 and 66 years respectively.Therefore,the selection of Schima superba for forest restoration in degraded grassland can not only better protect the old carbon,but also help the new carbon the soil.(3)The proportion of macro-aggregates and micro-aggregates in the surface soil of the three vegetation types were the largest and the proportion of micro-aggregates was the smallest,which indicated that the vegetation without human disturbance could improve the soil structure and improve the function of soil and water conservation in the red soil degradation area.For different vegetation types,the proportion of soil macroaggregates in0-20 cm layer of broadleaf forest is higher than that in coniferous forest and grassland,which is more conducive to enhancing soil resistance to hydraulic erosion.The content of organic carbon in surface soil silt and clay of grassland and plantation was the highest;Soil organic carbon of aggregates in the grassland surface soil was significantly lower than that of the whole soil,while that in broad-leaved forest was smaller;the total organic carbon and new carbon in soil were the largest in aggregates,the next in silt and the smallest in micro-aggregates.The turnover time of organic carbon in coniferous forest and broad-leaved forest in aggregates was 41-53 years.(4)?¹³C of soil exhaled CO₂ decreased with the extension of incubation time after 5days,indicating that microorganisms used C₄ vegetation more frequently in the process of soil mineralization of grassland.The activities of cellulase,sucrase and amylase were significantly increased and the activities of polyphenol oxidase were decreased after grassland was transformed into broadleaf and coniferous forests.A significant positive correlation was found between the three hydrolases,which had an effect on the transformation of soil organic carbon.Total organic carbon and total nitrogen significantly affected the activities of amylase and sucrase,and microbial biomass carbon significantly affected the activities of amylase,cellulase and beta-glucosidase.The total PLFAs of soil microbial community in the three vegetation types were broadleaf forest>coniferous forest>grassland.Bacteria were the main microbial group,and Gram-positive bacteria were the dominant microbial group.Amylase was significantly correlated with microbial community groups and total PLFAs,sucrase was most closely correlated with microbial community structure.Total organic carbon and total nitrogen mainly affected bacteria,actinomycetes,arbuscular mycorrhizal fungi and total microbial community.In summary,grassland vegetation with broad-leaved forest not only increased the content of total organic carbon,active carbon and non-labile carbon in the whole soil,but also increased the content of organic carbon in aggregates.The total amount of microbial communities closely related to carbon turnover increased significantly,indicating that the evergreen broad-leaved tree lotus could restore vegetation directly in this region.Organic carbon from broad-leaved forests can rapidly enter the soil and play an active role in improving carbon sequestration capacity in degraded red soil areas.However,the vegetation restoration effect of Pinus massoniana,a pioneer tree species,is weaker than that of broad-leaved forests,and forest management measures such as fertilization should be taken to promote coniferous forests.