| The rapid acceleration of urbanisation has injected vitality into the economy and society,but has also caused many problems for the ecological environment,such as atmospheric nitrogen deposition,acid deposition and soil pollution and other environmental loads.The Pearl River Delta region is one of the first and most urbanised regions in China,and its red soil characteristics determine the vulnerability of the soil,making the region prone to forest soil degradation,which leads to soil erosion.Soil agglomerate stability is an indicator of the soil’s resistance to erosion and is an important storage vehicle for organic carbon.Soil aggregates are inhabited by micro organisms,organic matter is an important cementing substance for the stability of the aggregate structure,and micro organisms act on the formation and turnover of organic matter to influence the formation and stability of the aggregates.Therefore,the study of the coupling relationship between soil aggregates,organic matter and microorganisms can help enrich the theories on the stability of forest soil aggregates in the PRD region and provide a theoretical basis for soil erosion control and forest carbon sequestration and sink enhancement in the region.In this paper,three typical forest soils under the urban-suburban-rural gradient(urban-rural gradient)in the Pearl River Delta(PRD)region were studied to explore the seasonal characteristics of their soil agglomerate composition and distribution,stability,and the mechanism of microbial influence on the stability of soil agglomerate structure and organic carbon sequestration.The main results showed that:(1)The physical and chemical properties of soils in different forest stands in the PRD are closely related to environmental gradients and seasons.Urbanization increased the range of mean values of soil water content in the three forest stands in different seasons.Soil water content and p H vary seasonally with pinus massoniana forest(PF)-mixed coniferous forest(MF)-broadleaved evergreen forest(BF),but soil organic carbon,total nitrogen and total phosphorus content have a phenological effect.Vegetation and soil microbial growth and development in the PRD are limited by phosphorus.(2)The main composition and distribution of 0-30 cm soil agglomerates in the three forest stands under the urban-rural gradient in the PRD were not seasonally dependent,but closely related to external forces.Under dry sieving,large agglomerates(>0.25 mm)dominated in the 0-30 cm soil layer agglomerates of the three forest stands in the urban-rural gradient;under wet sieving,large agglomerates dominated in the 0-30 cm soil layer agglomerates of the mixed coniferous and broad forests in the rural gradient,except for microagglomerates(<0.25 mm)in the rural gradient.The water stability of soil aggregates in the three forest types under the urban-rural gradient is higher in the rainy season than in the dry season,and the stability of aggregates(mechanical and water stability)in the 0-10 cm soil layer is more influenced by the season than in the 10-30 cm soil layer.Soil agglomeration stability is significantly influenced by urban-rural gradient,stand type,season and soil layer.The urban-rural gradient and the type of forest stand have a more significant influence on the composition and distribution of soil aggregates and their stability than the soil layer.(3)The community composition of the dry season soil bacterial phylum at the taxonomic level in all three forest stands under the urban-rural gradient in the PRD is dominated by Proteobacteria,Acidobacteriota,Actinobacteriota and Chloroflexi,all accounting for more than 80% of the total relative abundance.The community composition of the fungal phylum is dominated by Basidiomycota,Ascomycota,etc.,all accounting for more than 70% of the total relative abundance.The communities of soil bacteria and fungi are complex and diverse at all taxonomic levels.The stability of soil bacterial communities is positively correlated with their diversity,and negatively correlated with the diversity of fungal communities.(4)The rainy season reduced the activities of four microbial extracellular enzymes(β-glucosidase,chitinase,acid phosphatase and catalase)in the 0-10 cm soil layer of three forest stands under the urban-rural gradient in the PRD.The rainfall season reduced the activities of four microbial extracellular enzymes(β-glucosidase,chitinase,acid phosphatase and catalase)and increased the activities of four microbial extracellular enzymes in the 10-30 cm soil layer of three forest stands under the urban-rural gradient in the PRD.It also increased the microbial biomass carbon and nitrogen content and decreased the microbial biomass phosphorus content in the 0-30 cm soil layer of three forest stands under the urban-rural gradient.Elemental thresholds(TER C/N,TER C/P)had no significant effect(p>0.05)on soil bacterial community diversity and stability and fungal community diversity in the dry season,while they had a significant effect(p<0.05)on the water stability of soil aggregates.Soil microbial carbon,nitrogen and phosphorus use efficiency was closely linked to season,gradient environment,stand and soil layer.(5)The main driver of soil aggregate stability(mechanical stability and water stability)was microbial extracellular enzyme activity,as indicated by the partial least squares pathway model.Soil microbial extracellular enzyme activity was significantly negatively correlated with agglomerate mechanical stability(p<0.01)and positively correlated with agglomerate water stability(p<0.01).From the stepwise regression model,it is clear that the mechanical stability of soil aggregates is mainly influenced by soil bulk weight,p H,mass water content and titinase activity,while their water stability is mainly influenced by soil microbial biomass phosphorus content,acid phosphatase activity and soil-microbial carbon and nitrogen stoichiometry imbalance.(6)Soil microorganisms can adjust the element utilization efficiency of carbon and nutrients according to their own growth nutrient needs,improve the soil–microbe element limitation,affect the content of soil microbial biomass carbon and nutrients and microbial metabolites(extracellular enzymes),and in turn affect The formation and turnover of soil cementitious substances(organic matter,etc.),which in turn affects the stability of soil aggregates.The sequestration of soil organic carbon mediated by soil microorganisms affects its sequestration of organic carbon through soil element limitation,microbial community diversity and its stability,and abundance of key enzymes for carbon fixation.In summary,it is relevant to explore the mechanisms of microbial influence on carbon,nitrogen and phosphorus cycles and soil functions at different scales with the help of new technologies and theories.In areas of phosphorus(P)limitation in forest soils in the context of "double carbon" and urbanisation,it is important to promote the succession of Horsetail pine forests to mixed coniferous forests,or to create tree species with stronger P distribution capacity and higher P use efficiency,in order to reduce the intensity of P limitation on plants and microorganisms,enhance forest soil quality and improve soil structure.The aim is to improve the water stability of forest soil aggregates,enhance the ability of forest soils to sequester carbon and increase sinks,serve the national goal of "carbon neutrality",and provide reference for research on global climate change response mechanisms. |
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