| Driven by the implementation of policies such as “converting cropland to forest(grassland)” and “planting structure adjustment”,a large number of grassland ecosystems in the southwest karst region have been transformed into artificial agroforestry ecosystems through projects such as converting cropland from barren mountains,converting cropland to forest,and controlling rocky desertification,which has led to drastic changes in the carbon storage of ecosystems in this area.However,little is known about the extent and pattern of these changes on soil active organic carbon(AOC).In this paper,two typical types of grassland ecosystem transformation types of grassland into forest and grassland into cropland in karst mountain areas were taken as the objects.The soil organic carbon(OC)and AOC components,soil carbon,nitrogen and phosphorus cycle related enzymes,and microbial community composition and structure were measured and analyzed,and the correlation mechanism between AOC and microorganisms under the transformation of grassland utilization patterns in karst mountainous areas was discussed.The main results are as follows:(1)The conversion of grassland to cropland and grassland to forest had significant effects on the components of OC and AOC in surface soil and subsoil(p < 0.05),in which grassland conversion to cropland significantly increased the content of oxidized carbon(Oxidizable OC),dissoved organic carbon(DOC),mineralizable carbon(Mineralizable C),and microbial biomass carbon(MBC)in the surface soil(p <0.001),and there was a significant increase in the content of OC and particulate organic carbon(POC)in subsoil(p < 0.001);The conversion of grassland to forest significantly increased the content of OC and MBC in surface soil(p < 0.001).(2)The conversion of grassland to cropland and grassland to forest had significant effects on the physical and chemical properties of surface soil and subsoil(p < 0.05).The conversion of grassland to cropland significantly increased the p H,soil water content(WC),and soil bulk density(BD)of surface soil(p < 0.001),and significantly increased the p H,total soil nitrogen(TN),total soil phosphorus(TP),and soil effective nitrogen(AN)of subsoil(p < 0.001);The conversion of grassland to forest significantly increased WC and TP in surface soil(p < 0.01).(3)Both grassland conversion to cropland and grassland conversion to forest had significant affects on the activities of enzymes related to carbon,nitrogen and phosphorus cycling in surface soil and subsoil(p < 0.05),and the conversion of grassland to cropland significantly increased the activities of β-glucosidase(βGC),soil amylase(AMS),soil cellulolytic enzyme(CEL),and soil cellulose disaccharide hydrolase(CBH)in surface soil(p < 0.001)and βGC,and n-acetyl-glucoaminidase(NAG)in subsoil(p < 0.001);The conversion of grassland to forest significantly increased the activities of βGC,soil acid phosphatase(ACP),soil sucrase(IVT),and CBH in surface soil(p < 0.001),and NAG in subsoil(p < 0.001).(4)The AOC components were significantly correlated with soil bacterial and fungal diversity index(p <0.05),in which DOC and POC were positively correlated with F-simpson(p <0.05),while Oxidizable OC,LOC,MOC and POC were negatively associated with B-simpson(p <0.05),so we can manage AOC pools by managing bacterial and fungal simpson diversity in the future.(5)Structural equation model(SEM)showed that soil depth and B-simpson and F-simpson diversity indices had significant effects on the variation of AOC reserves(p < 0.05).Soil depth had a significant effects on POC,LOC,Oxidizable OC,and MBC by changing B-simpson(p < 0.05);Soil depth also significantly affected DOC by changing F-simpson(p < 0.05).In addition,soil depth had a significant effect on B-simpson(p < 0.05).In summary,compared with ecosystem types,soil depth was an important factor affecting the content of AOC. |
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