Effects Of Different Land Use Types On Soil Organic Carbon And Its Fractions Of A Typical Small Watershed In Danjiangkou

Soil carbon pool is the largest carbon pool in terrestrial ecosystems and has a longlasting carbon sequestration function.It plays an important role in regulating climate change.As a relatively active part of soil organic carbon,soil labile organic carbon are closely related to the activities of plants and microorganisms.Different land use types directly lead to differences in surface vegetation and soil properties,which have an important impact on soil organic carbon and its fractions.This study takes the small watershed of Qingtang,Xijiadian Town,Danjiangkou City as the research area,and selects five common land use types(weedland,dry land,orchard,arbor forest,and shrub forest)as the research objects.Through field investigation,sampling and indoor analysis to study the plant roots characteristics,soil properties and soil organic carbon fractions under different land use types.Using random forest analysis,linear regression equations and structural equations to reveal the influence laws of plant roots characteristics and soil properties on soil organic carbon and its fraction.In order to provide a basis for evaluating the benefits of regional vegetation restoration and optimizing the land use structure.The main research conclusions are as follows:(1)Plant roots characteristics and soil properties in different land use typesThe study found that under the five land use types,the plant roots were dominated by fine roots diameter less than 0.4 mm,which were concentrated in the surface soil of0-20 cm.The root length density of the diameter class less than 1 mm under each land use type accounted for 76.08% to 98.92% of the total root length density.The root density showed the changing law of shrub forest > arbor forest > orchard > weedland >dry land.With the increase of soil depth,the specific root length of orchard and weedland decreased first and then increased,while that of dry land,arbor forest and shrub forest showed a decreasing trend.The organic carbon and glomalin-related soil protein content of shrub forest were significantly higher than those of other land use patterns(P<0.05).There were significant differences in total nitrogen and total phosphorus contents under different land use patterns.The total nitrogen content of shrub forest was significantly higher than that of other land use patterns(P<0.05),and the total phosphorus content of shrub forest and arbor forest was lower.Root density was significantly positively correlated with organic carbon,total nitrogen,and glomalin-related soil protein content(P<0.01),while there was a significant correlation between root density and soil texture(P<0.05).(2)Characteristics of soil organic carbon fractions in different land use typesThere were significant differences in soil organic carbon fractions in different land use types(P<0.05).Overall,the soil labile organic carbon fractions(particulate organic carbon,easily oxidized organic carbon,dissolved organic carbon,and microbial biomass carbon)were the highest in shrub forest,and the lowest in dry land and weedland.The fractions of soil active organic carbon in different soil depths and land use types showed a decreasing trend with the increase of soil depth.The particulate organic carbon fraction of the shrub forest accounts for about 50% of the total soil organic carbon.Under different land use types,the soil carbon efficiency(labile organic carbon fractions/soil total organic carbon)of the shrub forest is the highest,and the soil carbon stability index is the lowest,indicating that the soil organic carbon of the shrub forest has high activity and low stability,and soil organic carbon is easy to mine decompose.(3)Effects of biotic and abiotic factors on soil organic carbon fractionsThe soil organic carbon fractions in the study area were closely related to soil properties and roots characteristics.There was a significant correlation between soil organic carbon,soil total nitrogen,glomalin-related soil protein,root density characteristics(root weight density,root length density,root surface area density,root volume density,root length density of D≤0.5 mm,root length density of 0.5<D≤2mm,root length density of D>2 mm)and labile organic carbon fractions(particulate organic carbon,easily oxidized organic carbon,dissolved organic carbon)(P<0.01).Mineral-associated organic matte was significantly positively correlated with clay,soil organic carbon,and easily extractable glomalin-related soil protein(P<0.01).Random forest results showed that glomalin-related soil protein had strong predictive power for soil organic carbon.For soil organic carbon fractions,abiotic factors such as soil organic carbon and easily extractable glomalin-related soil protein were the most predictive,and biological factors such as root surface area density and root length density in diameter classes of 0.5<D≤2 mm were more predictive.The results of structural equation modeling showed that for particulate organic carbon and mineral-associated organic matter,the root system could not only directly affect their content,but also indirectly affect them by affecting soil nutrients and glomalin-related soil protein.For easily oxidized organic carbon and dissolved organic carbon,the root system can not only directly affect their content,but also indirectly affect them through affecting soil nutrients.For microbial biomass carbon,the root system mainly affects its content by affecting soil nutrients.