Effects Of Different Land Use Types On Soil Aggregate Stability And Organic Carbon Fractions In The Karst Areas Of Southern Yunnan

Land use types are important factor affecting soil structure and soil organic carbon pool by regulating carbon fluxes between the terrestrial biosphere and atmosphere.Five typical land use types including farmland,grassland,shrubland,artificial forest and natural forest were selected to explore the responses of soil structure and soil organic carbon pool to land use types and environmental factors in the karst faulted basin,China.Soil aggregate stability,aggregate organic carbon(AOC)contents and the contents,stocks and distribution ratios of soil organic carbon(SOC)and labile organic carbon(LOC),namely dissolve organic carbon(DOC),easily oxidated carbon(EOC)and microbial biomass carbon(MBC),were analyzed along soil profile(0-60 cm)under different land use types.The main results are as follows:(1)The contents and stability of aggregate were affected by land use types and soil depths(P<0.01).Aggregate stability of grassland and shrubland was higher than artificial forest and natural forest.The stability of soil aggregates was significantly correlated with aggregate size distribution and SOC contents(P<0.01),the stability increased with the number of >1 mm aggregates and SOC contents.Path analysis showed that SOC,C/P ratio,exchangeable calcium and sand were the key factors affecting the stability of soil aggregates.(2)The AOC contents of farmland increased with the increase of aggregate size along soil profile.The AOC contents of grassland,shrubland,artificial forest and natural forest decreased with the increase of aggregate size,and the AOC contribution of >2 mm decreased gradually,and that of 0.25-2 mm and 0.053-0.25 mm increased gradually,the contribution of <0.053 mm aggregates did not change significantly between soil layers.The AOC contents of >2 mm was positively correlated with aggregate stability and SOC(P<0.01).Redundancy analysis indicated that the AOC contents were mainly affected by sand,C/P ratio,exchangeable potassium,total phosphorus,available phosphorus and p H.(3)The SOC,EOC and MBC contents decreased with soil depths under the five land use types,but the decline rates of EOC and MBC contents in soil layers were obviously higher than SOC.The contents and stocks of SOC,EOC and MBC were the highest in grassland and shrubland on average,followed by artificial forest and natural forest.Land use types and soil depths had no significant effects on DOC contents(P>0.05).The ratios of labile organic carbon to soil organic carbon were affected by land use types and soil depths(P<0.01).The EOC/SOC and MBC/SOC ratios of artificial forest were lower than those of grassland,shrubland and natural forest.Pearson correlation analysis showed that SOC was positively correlated with LOC(P<0.01).Path analysis further identified that the variation in the SOC and EOC were significantly related to the changes of C/P ratio,total phosphorus,sand and exchangeable calcium and MBC was mainly influenced by sand and C/P ratio.These findings suggested that the aggregate stability,AOC and LOC contents of grassland and shrubland were higher than those of farmland,artificial forest and natural forest.The ecological restoration mode with grass irrigation as the main utilization mode could be the main measure of ecological restoration in this study.SOC,C/P ratio and sand contents were the key factors affecting soil aggregate stability,aggregate organic carbon and labile organic carbon in different land use types.Moreover,more attention should be paid to soil phosphorus and sand contents in the future research on soil organic carbon pool.