The Effects Of The Soil Aggregate Stability And Phosphorus Fractions After The Close-to-nature Silviculture Of Cunninghamia Lanceolata Pure Plantation

The degradation of soil quality caused by long-term cultivation of single species of Cunninghamia lanceolata is not conducive to the benign circulation of soil nutrients and the sustainable management of high productivity of forest land.Exploring a forest management model compatible with forestry production and ecological protection is important for improving soil quality,land nutrient availability and sustainable management of ecosystem structure and function in south subtropical plantations.In this paper,pure plantation（Cunninghamia lanceolata pure plantation,PP）and coniferous and broad-leaved tree species mixed plantation（Cunninghamia lanceolata,Castanopsis hystrix and Michelia hedyosperma mixed plantation,MP）soil as the research object,focusing on the exploration of close-to-nature silviculture of Cunninghamia lanceolata pure plantation after introducing broadleaved tree species（Castanopsis hystrix and Michelia hedyosperma）:（1）Changes in understory vegetation composition and physicochemical properties of plants and soil,（2）Changes of soil microbial biomass carbon（MBC）,microbial biomass nitrogen（MBN）,and microbial biomass phosphorus（MBP）,microbial diversity,community structure and microbial enzyme activity,（3）Variation characteristics and influencing factors of soil humus（HS）components,（4）soil aggregates stability indexes with main driving factors,and（5）Variation characteristics of soil phosphorus（P）fractions and the relationship between aggregate stability and P fractions.Through the study of the above contents,the purpose is to reveal the impacts of plant-soil-microorganism coupling on the soil aggregates stability after the close-to-nature modification of subtropical Cunninghamia lanceolata plantation,as well as the main regulatory pathways and key driving factors,from the perspective of physiological and biochemical,microorganism structure and diversity.Hedley P grading method was used to reveal the characteristics of P fractions and the relationship with soil aggregate stability after the close-to-nature silviculture.To provide scientific advice and guidance for improving soil quality,promoting the benign circulation of soil nutrients and improving the sustainable management of forest productivity.Following are the main results:（1）Compared with PP,the litterfall biomass（LF）,C/N_LF and fine root biomass（FR）in MP were significantly increased（P<0.05）.Soil organic matter（SOM）,total nitrogen（TN）,ammonium nitrogen（NH₄⁺-N）,available phosphorus（AP）,carbon to nitrogen ratio of soil(C/N_soil)and carbon to phosphorus ratio of soil(C/P_soil)were significantly increased in the bulk soil and each size of soil aggregate（P<0.05）.Nitrate nitrogen（NO₃^--N）and total phosphorus（TP）were significantly increased in<0.25 mm and 0.25-1 mm aggregates,respectively（P<0.05）.（2）The>2 mm aggregate was the major component in both PP and MP.Compared with PP,the stability index of soil aggregates in MP was changed,the soil structure was effectively improved.Meanwhile,the MWD,GMD,and R_>0.25 of soil aggregates was significantly increased（P<0.05）.The PAD,RSI and RMI of aggregates were significantly decreased（P<0.05）.（3）Compared with PP,the contents of soil MBC,MBN and MBP in MP were significantly increased in the bulk soil and each size of aggregates（P<0.05）.The activities of hydrolase（AG,BG,CB,NAG,LAP,and ACP）related to C,N,and P and oxidase（PO and PER）were significantly increased in the bulk soil and each size of aggregates,respectively（P<0.05）.The fungal Pielou index andα-diversity indexes（Chao1 and Shannon）of MP were significantly higher than those of PP（P<0.05）,while the bacterial Pielou,Chao1 and Shannon indexes were not significantly different between PP and MP.Non-metric multidimensional scale analysis（NMDS）showed that there were significant differences in the bacterial and fungal community structure between PP and MP（P<0.05）.Linear discriminant analysis（LEf SE）showed that there were significant differences between PP and MP（P<0.05）,and the composition of bacterial and fungal communities had significant changes in different degrees（P<0.05）.Compared with PP,Verrucomicrobia,Chloroflexi and Gemmatimonadetes in the MP soil bacterial community and Ascomycota,Kickxellomycota and Glomeromycota in the fungal community.The relative abundance of Glomeromycota was increased significantly（P<0.05）;Planctomycetes and Firmicutes in MP soil bacteria community were associated with Basidiomycota,Mortierellomycota,Mucoromycota and Rozellomycota in fungal community.The relative abundance of Rozellomycota was significantly decreased（P<0.05）.The topological property analysis of microbial co-occurrence network showed that the number of nodes and the number of edges in MP network were greater than PP network topology parameters.（4）The close-to-nature silviculture of PP accelerated the process of soil humification.Compared with PP,HAs in MP soil and each size of aggregates and PQ values representing that the conversion rate of HAs were significantly increased（P<0.05）.The content of FAs in 0.25-1 mm aggregate was significantly decreased（P<0.05）.The content of HMi was significantly increased in the bulk soil and>2 mm,1-2 mm and 0.25-1 mm soil aggregates（P<0.05）.The contents of HMc,HMr and HMs in the bulk soil and each size of aggregates were significantly increased（P<0.05）.However,the I/C ratio was significantly decreased in the>2 mm,0.25-1 mm and<0.25 mm soil aggregates（P<0.05）.（5）The soil quality and aggregate stability of Cunninghamia lanceolata pure plantation were improved after close-to-nature silviculture.As the main organic cementing material in the formation of soil aggregates,the contents of HAs and the（PQ）are positively correlated with aggregate stability indexes MWD,GMD and R_>0.25,but negatively correlated with PAD,RSI,RMI and AS.In the bacterial and fungal communities,the relative abundances of Chloroflexi and Ascomycetes were positively correlated with the distribution proportion of HAs,but negatively correlated with the distribution proportion of FAs.Redundant analysis（RDA）results showed that Chloroflexi in bacterial community and Ascomycetes in fungal community were the most important factors driving the distribution of>2 mm soil aggregates and explained 70%and79%of the variation,respectively.Correlation analysis and Partial least squares path models（PLS-PM）showed that soil properties（SOM）,fungalα-diversity index（Chao1）,bacterial relative abundance and humus characteristics（HAs/SOM and PQ）had positive effects on mechanical stability（MWD_d）,water stability（MWD_w）and aggregate stability under different crushing mechanisms(MWD_FW,MWD_WS and MWD_SW).The effect sequence of all factors on the stability of soil aggregates was as follows:Fungalα-diversity>SOM>Humus characteristics>Relative abundance of the Chloroflexi.Therefore,the close-to-nature silviculture of Cunninghamia lanceolata pure plantation directly causes the change of SOM,which affects the soil microbial community and HS component,and finally promotes the formation and stability of soil aggregates.（6）Resin P content in MP soil was significantly increased by 9.43%（P<0.05）,Na HCO₃-Pi,Na OH-Pi and Sonic-Pi contents were extremely significantly increased by 9.43%,8.06%,11.89%and 15.37%（P<0.05）,respectively.Microbial-Pi content was significantly increased by 13.73%（P<0.05）,and HCl-Pi content was significantly increased by 26.13%（P<0.05）.PLS analysis showed that Soil properties(SOM and C/N_soil),soil aggregate stability(MWD_d,MWD_w,MWD_FW,MWD_WS and MWD_SW),microbial characteristics（MBC,MBN,MBP,relative abundance of bacterial Chloroflexi andα-diversity index of fungi Chao1）,and P conversion capacity（PAC and ACP）for P fractions.All factors together accounted for 86.9%of the change in P fractions.In conclusion,this study systematically evaluated the plant and soil properties,microbial characteristics,HS component characteristics,soil aggregate stability and the changes of different P fractions in soil caused by the close-to-nature silviculture of Cunninghamia lanceolata pure plantation.The results showed that the introduction of broad-leaved tree species in coniferous plantations could effectively increase the soil HAs content and promote the transformation of FAs to HAs,changing the composition of bacterial and fungal communities,increasing theα-diversity of fungi significantly（P<0.05）,and improving the stability of soil aggregates and changing the P fractions.Positive changes in the relative abundance of Chloroflexi and Ascomycetes may promote the cementation of microaggregates by stimulating the accumulation of HAs and accelerating the rapid humification of SOM,thus promoting the formation and stability of macro-aggregates.At the same time,the changes of biology and biological factors caused by the close-to-nature silviculture of Cunninghamia lanceolata pure plantation changed the P fractions of soil and increased the AP supply potential of soil.This study provided important theoretical support for tree species selection,transformation model and management measures to improve soil quality,nutrient availability and system productivity of Cunninghamia lanceolata plantation,in order to satisfy the multi-objective and multifunctional plantation sustainable management strategy.