Effects Of Introducing Valuable N₂-fixing Tree Species On Soil Aggregates Phosphorus Forms And Availability In Rocky Desertification Mountainous Regions

Rocky desertification is serious in karst areas in southwest China,with low vegetation coverage and poor soil.As an important ecological management measure,vegetation restoration plays an extremely important role in improving the availability of soil nutrients,increasing biodiversity and improving the ecological environment.In this study,four types of site(the pure plantation of Dalbergia odorifera,PD;the pure plantation of Acricarpus fraxinifolius,PA;the mixed plantation of D.odorifera and A.fraxinifolius,MP;the adjacent natural weed field was taken as the control,CK)in Mashan,Guangxi were selected as the research objects after 8 years of vegetation restoration.The effects of introducing N₂-fixing tree species and there planting patterns on soil aggregates phosphorus forms and availability in rocky desertification mountainous regions were discussed.The purpose of this study could provide theoretical basis and practical support for the application of N₂-fixing tree species in the ecological restoration and reconstruction of rocky desertification.The main results are as follows.(1)After 8 years of vegetation restoration in the rocky desertification mountainous area,the basic physical and chemical properties of the soil changed to different degrees.Soil aggregate mean weight diameter(MWD)and soil phosphorus activation coefficient(PAC)of MP were the largest,which were significantly higher than those of PA,PD and MP(P<0.05).The soil organic matter(SOM)content in bulk soil and different aggregate soils in the study area increased significantly,and the SOM,NO₃^--N and AP contents in MP were significantly higher than those in CK(P<0.05).However,there were no significant differences in TN,TP,N/P,NH₄⁺-N contents and p H values among the four stand types in the bulk soil(P>0.05).(2)After 8 years of vegetation restoration in the rocky desertification mountainous area,the topsoil(0-10cm)microbial biomass carbon(MBC),nitrogen(MBN),phosphorus(MBP),microbial community structure and N and P-Hydrolytic enzyme activities changed to different degrees.Except for the small aggregates(<0.25mm)in MP,the contents of MBC,MBN and MBP in all aggregate soils and bulk soils were significantly higher than those in CK.Compared with CK,the PLFA of total microbes,bacteria,fungi,arbuscular mycorrhizal fungi,actinomycetes and activity of N and P-Hydrolytic enzyme(NAG,LAP,ALP and PDE)in all stands(especially MP)and most aggregate soils increased significantly.(3)After 8 years of vegetation restoration in rocky desertification mountainous area,the organic phosphorus fractions in the bulk soil and most aggregate soil of all stand types increased to different degrees compared with those in CK.The contents of labile organic phosphorus and highly resistant organic phosphorus of MP and PA in bulk soil and all aggregate soils were significantly higher than those of CK(P<0.05).The contents of labile organic phosphorus,moderately resistant organic phosphorus and highly resistant organic phosphorus had no significant difference between PD and CK in the bulk soil and most aggregate soils(P>0.05).The stand types,soil aggregate types and the interaction of stand types×soil aggregate types had significant effects on both labile organic phosphorus and moderately labile organic phosphorus(P<0.05).Redundancy analysis(RDA)shows that alkaline phosphatase(ALP),MBN and Arbuscular mycorrhizal fungi may be the key factors driving the change of soil organic phosphorus fractions in rocky desertification mountainous areas.(4)After 8 years of vegetation restoration in rocky desertification mountainous area,the soil inorganic phosphorus fractions of each stand changed.Compared with CK,the increase of all kinds inorganic P in MP and PA in bulk soil and all aggregates soils almost reached a significant level.The content of inorganic P fractions in the bulk soil and aggregate soils of the four stands generally followed the order of Ca₁₀-P>Fe-P>Al-P>O-P>Ca₂-P>Ca₈-P.Soil aggregate types had significant effect on Ca₁₀-P and Fe-P(P<0.05).Redundancy analysis(RDA)shows that Phosphodiesterase(PDE),MBP and SOM may be the key factors affecting the change of soil inorganic P fractions in rocky desertification mountainous areas.(5)There is a strong synergistic effect between N and P cycle in soil.Correlation analysis showed that in the four stands of CK,PD,PA and MP,there was a very significant positive linear correlation between P-Hydrolytic enzyme ln(ALP),ln(PDE)and N-Hydrolytic enzyme ln(NAG+LAP),respectively(P<0.0001).ln(NAG+LAP):ln(ALP)has a very significant positive linear correlation with MBN:MBP(P<0.001).There was a very significant positive linear correlation between NO₃^--N and AP(P<0.0001).There was a significant positive linear correlation between MBN and MBP(P<0.05).Generally,we found that introduction N₂-fixing tree species could improve the soil nutrient status and soil phosphorus availability in rocky desertification mountainous areas to a certain extent,and the mixed planting pattern has the best effect.