Effects Of N₂-fixing Tree Species And Planting Patterns On The Stability Of Soil Aggregates And Regulation Mechanisms In Rocky Desertification Areas

Vegetation restoration is an important means of ecological management and is extremely important in improving the effectiveness of soil nutrients,increasing biodiversity and enhancing the ecosystem.Soil aggregate is the basic unit for building good soil structure and is a key factor affecting the nutrient content and effectiveness of soil,and its composition and stability are closely related to the nutrient content,microbial community structure and function.In this study,10-year-old pure plantation of Acricarpus fraxinifolius（PA）;the pure plantation of Dalbergia odorifera（PD）;the mixed plantation of A.fraxinifolius and D.odorifera（MP）and the adjacent natural shrub as the control（no tree,CK）were selected as the research objects.We focus on the effects of N₂-fixing tree species and their planting patterns on the stability of surface soil aggregates and soil microbial regulatory mechanisms,aiming to supply scientific and practical support for the application and promotion of N₂-fixing tree species in the ecological recovery and restoration in rocky desertification areas.The results are as follows.（1）Compared with CK,the understory species composition and vegetation diversity were significantly higher in PA,PD and MP,and the carbon（C）,nitrogen（N）and phosphorus（P）contents of litter and fine roots were also increased to different degrees,but the C:N_litter and C:P_litter of litter were significantly lower than those of CK.The results of total porosity,non-capillary porosity,maximum water holding capacity,field water holding capacity and soil aeration（STP,NCP,MWC,FWC,SA）in the four stand types showed MP>PD>PA>PA>CK,except for soil bulk density（BD）which showed CK>PA>PA>MP.Soil organic carbon（SOC）,total nitrogen（TN）,total phosphorus（TP）,available phosphorus（AP）,nitrate nitrogen（NO₃^--N）and ammonium nitrogen（NH₄⁺-N）contents were significantly higher in PA,PD and MP stands than in CK,indicating that the introduction of N₂-fixing tree species for vegetation restoration in rocky desertification areas could improve the soil structural properties and increase the nutrient content and effectiveness of the surface soil,especially MP had the best effect.（2）Soils in four stand types were dominated by large aggregates（>2 mm）.The mean weight diameter（MWD）,geometric mean diameter（GMD）,water-stable aggregate content（WSA）and aggregate stability index（ASI）of PA,PD and MP stands were significantly higher（p<0.05）in PA,PD,and MP stands than in CK for mechanical and water stability of soil aggregate,while their soil erodibility factor（K）,Fractal dimension（D）and aggregate destruction rate（PAD）were consistently smaller than those of CK.The results of aggregate stability for the three treatments（FW,SW and WS）under different disruption mechanisms（LB method）showed consistent variation patterns with the mechanical and water stabilities of soil aggregates;in addition,the variation patterns of MWD and GMD under different disruption mechanisms were all consistently shown as SW>WS>FW,while the variation patterns of K and D were FW>WS>SW.Aggregate mechanical stability,water stability and aggregate stability under different disruption mechanisms showed mostly significant（p<0.05）or highly significant（p<0.01）correlations with plant properties and soil physicochemical properties,especially with N_litter,N_root,SOC and C:N_soil（p<0.01）.（3）Humic acid（HA）,fulvic acid（FA）,HA/FA ratio and humification（PQ）,and humin（HMi,HMc and HMr）contents in both bulk soil and each particle size aggregate showed the highest MP,followed by PA and PD,and the lowest CK.SOC content was significantly positively correlated with soil aggregate stability（p<0.05）,and the increase of HA and huminin（HMi,HMc and HMr）content in large particle size aggregates contributed to the enhancement of the mechanical and water stabilities of soil aggregates,while the accumulation of FA in small particle size contributed to the enhancement of aggregate stability,and the higher the PQ with the decrease of particle size,the more beneficial to the water stability of soil aggregate and the enhancement of the aggregate stability under different disruption mechanisms.（4）Soil microbial biomass was significantly higher in PA,PD and MP than in CK（p<0.05）,but MBC:MBN and MBC:MBP were both lower than in CK.The number of each classification level of bacterial communities was more than that of fungal communities,and theα-diversity indexes（ACE,Chao1,Shannon,PD＿whole＿tree indexes）of both bacteria and fungi showed MP>PD>PA>CK.Among them,Acidobacteria,Proteobacteria and Actinobacteria were the major phyla of bacteria,and then the dominant phyla of fungi were Ascomycota and Basidiomycota;Non-metric multidimensional scaling method（NMDS）analysis revealed highly significant differences in soil bacterial and fungal community structure between different stand types（stress<0.2）.The co-occurrence network analysis showed that the microbial co-occurrence networks of PA,PD and MP were more complex than CK,and there were significant positive interactions between bacterial and fungal communities.Redundancy analysis（RDA）further showed that AP,C:P_litter,HA and NO₃^--N were the main drivers affecting the top ten phyla of the bacterial community,while C:N_soil,TN and C:N_root were the main drivers affecting the top ten phyla of the fungal community.（5）The trends in the abundance of soil bacterial functional gene（16S）,fungal functional gene（ITS）,N cycle-related functional genes（nif H,AOB-amo A,nir G,nir K,nir S,nos Z）and P cycle-related functional genes（pho D,pqq C）almost consistently showed MP>PD>PA>CK;they were significantly correlated with apoplastic and fine root mass（p<0.05）,and most of them were mainly influenced by the physical（BD,STP and MWC）and chemical（SOC,TP,AP,NO₃^--N）properties of the soil;denitrifying microbial functional genes（nir K and nos Z）were significantly correlated mainly with soil humus components（HA,FA,HMi,HMc,HMr,PQ,FA/FA,I/C）.（6）Verrucomicrobia,Actinobacteria and Chloroflexi of bacteria,and Rozellomycota,Mucoromycota and Kickxellomycota of fungi showed significant（p<0.05）or highly significant（p<0.01）positive correlations mainly with HMc,HMi and HMr.The interaction effect of microbial properties combined with soil physicochemical and plant properties is the most important factor affecting the soil aggregate stability.Also,the regulatory mechanism of aggregate stability was analyzed by partial least squares pathway（PLS-PM）,which showed that bacterial functional gene（16S）interacted with humus components（PQ,I/C）to promote the formation and stability of aggregates（aggregate mechanical stability,water stability,aggregates stability under different disruption mechanisms）,where plant properties(C_litter and N_root)and soil properties(SOC,C:N_soil,AP)are key driving factors.In summary,the introduction of N₂-fixing tree species,either Acricarpus fraxinifolius or Dalbergia odorifera,can increase the content of humus components in the surface soil of rocky desertification area to a certain extent,increase theα-diversity of microbial communities and change their community structure,and the interaction between the two can promote the soil aggregate stability,among which the mixed planting pattern of A.fraxinifolius and D.odorifera has the best effect.