Characteristics Of Soil Organic Carbon Fraction And Its Stability During The Transition From Desert Steppe To Shrubland

In the 1970s and 1980s,Leguminous shrub of Caragana korshinskii were introduced in large numbers for revegetation and restoration to control the degradation of desert steppe caused by overgrazing in eastern Ningxia.During the introduction process over past several decades,many mosaics of grasslandshrubland occurred to form a different desert steppe landscape,accompaning with a series of negative phenomena of grassland ecosystem functioning,such as decrease of herbaceous diversity,soil xerificaiton,and change of soil organic carbon and nitrogen stability etc,among which the characteristics and process of soil organic carbon sequestration and loss during the transition from grassland to shrubland are still unclear and need further investigation.Therefore,Based on ecosystem state transition theory and shrub encroachment process theory,a typical mosaic of grassland-shrubland mosaic(desert grassland,grassland edge,shrubland edge and shrubland)over 35 years was selected to conducted a experiment on soil organic carbon fraction(dissolved organic carbon,easily oxidized organic carbon,particulate organic carbon,light fraction organic carbon,microbial biomass carbon),soil carbon pool management index,organic carbon sources(plant sources,microbial sources)and losses(soil respiration,organic carbon mineralization)and further to explore the dynamic characteristics of soil organic carbon fraction and its stability during the transition from desert grassland to shrub with an aim at providing scientific basis for the stability and sustainable management of soil carbon pool in desert grassland in arid and semi-arid regions.The main results are as follows:(1)Along with transition sites from desert grassland to shrubland,aboveground vegetation C and C/N significantly decreased,while N and P content increased significantly(P<0.05).Litter C firstly decreased and then increased,litter N and P significantly increased,and litter C/N significantly decreased with transition(P<0.05).And soil moisture,soil mean temperature,SOC,TN and TP of the 0-20 cm soil layer showed a decreasing trend,while C/N and soil bulk density showed an increasing trend.In addition,MBC increased and reached a maximum in two edge zone(grassland edge and shrubland edge),then decreased to a minimum in the shrubland,while MBN showed a linear decreasing trend along with the transition.(2)Along with transition sites from desert grassland to shrubland,the SOC stock was significantly reduced by 45.72%and the MRT(mean residence time)of SOC was shortened by 3.85 years.As the SOC chemical fractions,DOC was 16.23%higher and EOC deceased in shrubland compared to desert grassland(P>0.05).Among the SOC physical fractions,POC and LFOC,as SOC physical fractions,were significantly reduced by 22.88%and 36.02%with the transition,respectively.The MBC of SOC biological fraction increased significantly at first,reached a maximum in the transition zone(grassland edge and shrubland edge),then decreased to a minimum in the shrubland.Moreover,POM(particulateassociated organic matter)and MAOM(mineral-associated organic matter)were divided from soil organic matter and POM-TC decreased significantly only in the shrubland,but POM-OC,TN and C/N did not change significantly with sites transition.In addition,MAOM-OC,SOC and TN showed a trend of increasing and then decreasing and were the lowest in the shrubland(1.45 g·kg-1,0.56 g·kg-1,0.08 g·kg-1).And C/N showed an increasing trend along the transition,but there was no significant difference among the sites.(3)Along with transition sites from desert grassland to shrubland,efficiencies of dissolved organic carbon easily oxidized organic carbon,and microbial biomass carbon all increased.The proportion of unprotected soil organic carbon and the organic carbon sensitivity index increased,but the carbon management indicator decreased.The structural equation model showed that the active organic carbon fraction,soil properties and floristic characteristics all directly and positively influenced soil organic carbon with a total explanation of 87%,among which the coefficient of influence of floristics characteristics was as high as 0.96.However,vegetation characteristics also had a negative influence on soil,organic carbon chemical and biological fractions,and a significant positive influence on physical fractions.(4)Along with transition sites from desert grassland to shrubland,total soil respiration,autotrophic respiration and heterotrophic respiration all showed an increasing trend,with 29.06%,45.83%and 24.73%higher in shrubland than in desert grassland,respectively.The proportion of soil heterotrophic respiration to total respiration(76.91%～80.03%)was significantly higher than that of autotrophic respiration(19.97%～23.09%)in each site,but the proportion of autotrophic respiration increased with the transition.Soil respiration in transition sites was mainly positively regulated by soil temperature and was significantly positively correlated with DOC and POC(P<0.05).(5)Along with transition sites from desert grassland to shrubland,the order of soil organic carbon mineralization capacity in transition sites was shrubland edge>shrubland>desert grassland>grassland edge.The carbon mineralization ratio and microbial metabolic entropy was consistent with cumulative mineralization.And the first-order kinetics of the cumulative mineralization was well fitted,with R2 ranging from 0.929 to 0.958.The mineralization rate of soil organic carbon in each site was found to be greater under the reduced rainfall gradient than the increased rainfall gradient at the initial stage of incubation,showing the organic carbon mineralization in the shrubland and shrubland margin was more responsive to precipitation change than that in desert grassland and grassland margin.However,with the incubation time increased the mineralization rate under the increased rainfall gradient was greater than that under the reduced rainfall gradient.Therefore,the order of the final cumulative mineralization of organic carbon under different rainfall gradients in each site was+40%>+20%>CK>-20%>-40%,and the organic carbon mineralization ratio and microbial metabolic entropy also showed an increasing trend with the increase of rainfall.In addition,change of rainfall was no longer the main controlling factor of organic carbon mineralization in the middle and late stages due to nutrient and substrate depletion,and the changes of organic carbon mineralization were the same in each site.In summary,with the transition from desert steppe to shrubland soil organic carbon stocks decreased and its mean residence time were shortened.The efficiencies of dissolved organic carbon,easily oxidized organic carbon,and microbial biomass carbon and the proportion of unprotected organic carbon all increased.Soil respiration rate and organic carbon mineralization also icreased.Results obtained indicates that anthropogenic vegetation transition from desert steppe to shrubland reduces the stability of soil organic carbon and accelerates its turnover process.