Regulation Mechanism Of Soil Phosphorus Pool Composition And Availability In Subtropical Forest Restoration

Phosphorus(P)is an essential mineral nutrient for plant growth and development.Soil P availability is the most limiting factor for primary productivity in terrestrial ecosystems and a key factor in maintaining ecosystem stability and sustainability.The total P(TP)content of soil in subtropical forest is low,and the available P(AP)content is even lower.There is still no consensus on whether forest restoration alleviates or exacerbates soil P limitation,and the key influencing factors and regulatory mechanisms for soil P availability remain unclear.Therefore,the dynamic change of soil P composition and availability with the restoration of forest in subtropical has attracted more attention from scholars.In this context,this paper presents a study of the law of variation of soil P availability and its regulation mechanism during forest vegetation restoration,which is of scientific importance for the formulation of long-term vegetation restoration strategies and sustainable utilization methods of soil P resources.The space-time substitution method was used to select Pinus massoniana forest(PM),Pinus massoniana+Lithocarpus glaber theropencedrymion(PL),Choerospondias axillaris deciduous broad-leaved forest(CA)and Lithocarpus glaber evergreen broad-leaved forest(LG)in the forest restoration sequence in the hilly region of central Hunan.The plots were set and samples of humus and mineral layers were collected.The changes of soil P fractions and availability,vegetation factors,soil physicochemical factors,and microbiological characteristics were analyzed across restoration stages.Starting from the correlation between the change of soil P component and its availability and multiple-unit of factors,this paper explores the regulation mechanism of soil P component and availability change on the time scale of several decades of forest vegetation restoration,providing scientific basis for making full use of soil potential P resources,forest vegetation restoration and reconstruction,and the sustainable management of existing forest ecosystem.The major results as follows:(1)TP content in humus layer and mineral layer increased from PM(336.4,214.7 mg/kg)to LG(363.9,242.8 mg/kg),and reached the peak at CA(364.4,259.9 mg/kg).It is stablilised by a balance between the decomposition and release of litters and uptake of tree growth.With the forest restoration,labile P(LP)content was reduced 5.9-14.3%because of P removal by plant uptake,moderately labile P(MP)contents was increased 2.4-64.6%due to organic matter accumulation,occluded P(OP)increased for 2.8-9.4%because soil weathering intensity and strong fixation of Fe and Al oxides,Available P(AP)content increased by 1.1%to 49.7%,and the humus layer and mineral layer were the highest in evergreen broad-leaved forest and deciduous broad-leaved forest(198.7,93.3 mg/kg).respectively,indicating that forest ecosystem transit from P acquiring to P recycling system with the restoration of forest vegetation.The LP/TP of humus layer and mineral layer decreased significantly from PM(21.8%,8.8%)to LG(17.5%,6.7%),and MP/TP increased significantly from PM(31.3%,20.3%)to LG(36.8%,26.1%).OP/TP decreased from PM(46.9%,70.9%)to LG(45.7%,67.2%),indicating that forest restoration increased soil P availability.With forest restoration,soil labile Po and moderately labile Po significantly affected the change of labile Pi,the LP/MP and LP/OP decrease significantly,while AP/OP and MP/OP increase significantly,indicating that forest restoration increases the absorption and utilization of LP,promotes the transformation of soil stable P component into labile component,Po mineralization is an important way to alleviate soil P limitation.(2)Over the course of forest restoration,the MBC,MBN and MBP of humus layer increased from PM to LG by 23.7%,52.4%and 20.2%,respectively,and the MBC,MBN and MBP of mineral layer increased by 8.9%,22.6%and 48.1%,respectively.Indicating that forest restoration enhances soil microbial activity,improved the turnover rate of soil organic matter and the potential of available nutrient supply,and affects the stoichiometric properties of microbial biomass.The levels of dominant phylum of bacterial and fungal communities in humus and mineral layers are overall similar.Proteobacteria(26.0-37.1%),Actinobacteria(19.9-28.4%),Acidobacteriota(14.8-23.6%)and Chloroflexi(2.4-21.9%)were the dominant bacterial phylum among communities,while Basidiomycola(21.1-60.2%)and Ascomycota(21.2-43.0%)were the dominant phylum of fungal communities,and the major different species of bacteria and fungi were concentrated in CA.The relative abundance of Gram-positive bacteria(G+,22.3-37.0%),Gram-negative bacteria(G-,63.1-72.0%),ectomycorrhizal fungi(ECMF,1.4-60.8%)and arbuscular mycorrhizal fungi(AMF,0.1-1.9%)also changed significantly with forest restoration.The vegetation community tree species composition and soil nutrients change significantly,which leads to changes in the microbial community composition and relative abundance of groups with respect to their nutrient strategies,and thus affects the ability of microbes to activate P.(3)Soil P-solubilizing bacteria at different stages of forest restoration mainly include eight genuses,among which Rhizobium and Burkholderia of Proteobacteria,Streptomyces of Actinomyces,and Bacillus of Firmicutes are the dominant genuses.P-solubilized fungi mainly include Penicillium of Ascomycota and AMF of Glomeromycota are the dominant genuses.Compared with P-solubilizing bacteria abundance of PM,PL,CA and LG increased by 0.5%.106.1%and 18.1%in humus layer,and increased by 1 1.7%,17.2%and 38.5%in mineral layer,respectively.Compared with P-solubilizing fungi abundance of PM,PL,CA and LG decreased by 0.9%,24.4%and 63.8%in the humus layer,and PL,CA increased by 36.8%,65.0%,LG decreased by 27.9%in mineral layer,respectively.During forest restoration,there are significant differences in the factors affecting the soil P-solubilizing microbial abundance in different soil layers.It was shown that forest restoration and soil depth affect the abundance of P-solubilizing microorganisms by altering the living environment of the soil microbial community,and thus the soil P availability.(4)With forest restoration,the activities of C acquisition enzyme(BG+CBH)and N acquisition enzyme(NAG)decreased,the activities of P acquisition enzyme(ACP+ALP)increased in humus layer.The enzyme activity of C,N,and P in mineral layers shows an overall increasing trend,reflecting the changing nutrient requirements of microbial activities.In the process of forest restoration,the logarithmic conversion ratios of C,N and P acquisition enzyme activities in humus layer and mineral layer were 1:0.86:1.12 and 1:0.82:1.53,respectively,indicating that the forest soil microorganisms in the study region were jointly limited by C and P.With forest restoration,the vector length of humus layer decreased and the mineral layer increased,and the vector angles of both soil layers increased,indicating that the C limit of humus layer decreased,the C limit of mineral layer increased,and the P limit of both soil layers increased.LPi suppresses the increase in ALP activity,MPi promotes the increase in ACP activity,and MPo promotes the increase in both ACP and ALP activity,thus enhancing the rate of soil Po mineralization.Forest restoration indirectly affects soil enzymatic activity and its stoichiometric characteristics by improving soil substrates and facilitating the conversion of available soil nutrients,thereby alleviating the nutrient deficiencies in forest ecosystems.(5)In the process of forest restoration,LTN,CNP stoichiometry of litter layer,C/N ratio of soil,ACP activity,G+/G-,Bacillus and Rhizobium significantly affected AP components and P availability in two soil layers.NH4+-N,DOC,TA1,MBP,MBC/MBN,EC/N,Penicillium,Burkholderia abundance in the humus layer,and fine root biomass,silt percentage,sand percentage,moisture content,TFe,MBC,C acquisition enzyme,Paecilomyces abundance in mineral layer were the primary driving factors influencing the AP components and P availability.Among the direct effects of environmental factors on LP,MP,AP,LP/TP,MP/TP and AP/TP,soil physical and chemical factors significantly positively affected the MP and AP contents,MP/TP in the two soil layers and AP/TP in the mineral layer.As the soil depth increases,the effect of vegetation factors on LP content and LP/TP changes from positive to negative.The positive effect of soil microbial factors on AP content and AP/TP was enhanced.The abundance of P-solubilizing microorganism changes from a negative to a positive effect on the LP content and LP/TP,a positive effect on the MP and AP content increases,and the effect on the MP/TP and AP/TP changes from positive to negative.Among the combined effects of environmental factors on LP,MP,AP,LP/TP,MP/TP and AP/TP,the interpretation rates of humus layer were 51.3%,86.4%,70.9%,68.1%,76.3%and 6.0%,respectively.The interpretation rates for the mineral soil layer were 85.1%,87.3%,88.9%,64.9%,86.9%and 86.9%,respectively.The soil depth affects the processes and strength of biochemical reactions related to P,and the effect of different groups of factors on the availability of P is significantly different.With forest restoration,the abundance of P-solubilizing microorganisms in the mineral soil layer significantly increased the impact of soil AP components and P availability,suggesting that microbial action is an important way to alleviate soil P limitation.Litter return promoted soil P accumulation and P availability during vegetation restoration.The soil nutrient content increases,the activity of the microorganism increases,and then the soil P conversion rate increases.The P availability in mineral soil layer was low,microbial community structure was affected by nutrient changes,which regulate the abundance changes of P-solubilizing bacteria and fungi,enhance phosphatase activity and promote mineralization of Po,increasing soil AP components and P availability,and thus alleviating soil P restriction.The results show that the restoration of subtropical forests enhances the accumulation and retention of soil TP and improves the availability of soil P.