Effects Of Cunninghamia And Castanopsis Mixed Plantation On Phosphorus Composition And Transformation In Soil Aggregates

Phosphorus（P）,as an essential element in the process of plant growth,has always been considered as one of the limiting nutrient elements in subtropical forests,which has seriously hindered the sustainable development of plantations in this area,especially in the long-term large-area single-species planting model of fir plantations.Therefore,how to efficiently utilize and fully tap the potentially effective P sources in the soil and find a more suitable forestry model for the retention and availability of P in subtropical soils will better improve the productivity and ecosystem services of Chinese fir plantations.This study will focus on the key scientific question of how to improve soil P supply potential and availability in fir plantations,and select South subtropical Cunninghamia lanceolata/Castanopsis hystrix mixed forests of different ages as the research object,and take the pure unmodified fir forest was used as the control（CK）,Using a combination of field monitoring and laboratory experiments,the physicochemical properties,soil microbial community structure and function,organic phosphorus（P_o）components and inorganic phosphorus（P_i）components and other indicators were measured and analyzed,focusing on exploring the variation characteristics of Po and Pi components in soil aggregates and their relationship with environmental factors.The research on the above issues provides important scientific basis for tree species selection,planting patterns and sustainable management measures to improve soil aggregate stability,soil quality and P availability in Chinese fir plantations.The results of this study are as follows:（1）After 12 years,the introduction of Castanopsis hystrix hybrid transformation in pure fir forest,the soil organic carbon（SOC）,total nitrogen（TN）,ammonium nitrogen（NH₄⁺-N）,available phosphorus（AP）and soil carbon-phosphorus ratio(C/P_soil)in MP was significantly higher than that of PP（P<0.05）in both the original soil and the soil aggregates of four particle sizes（>2 mm,1-2mm,0.25-1 mm and<0.25 mm）.Compared with PP;nitrate nitrogen（NO₃^--N）was significantly increased in>2 mm and 0.25-1mm soil aggregates（P<0.05）,soil carbon-nitrogen ratio(C/N_soil)in bulk soil and>2 mm,<0.25 mm soil aggregates were significantly increased（P<0.05）,but total phosphorus（TP）,soil nitrogen-phosphorus ratio(N/P_soil)and p H were not significantly changed in the bulk soil and soil aggregates of all particle sizes.The means weight diameter（MWD）of MP was significantly higher than that of PP（P<0.05）.（2）Compared with PP,the microbial biomass carbon（MBC）of MP was significantly increased by 30.95%in the bulk soil,and there was no significant difference in the remaining four particle sizes.Microbial biomass nitrogen（MBN）and microbial biomass phosphorus（MBP）in the bulk soil and each particle size were significantly increased by 73.84%,77.89%,41.15%,67.13%,25.75%and73.01%,48.77%,37.62%,38.00%,94.09%（P<0.05）.There was a significant positive correlation between MBN and MWD in the bulk soil,>2mm and<0.25mm soil aggregates（P<0.05）,and there was a significantly positive correlation between MBP and MWD in the bulk soil and aggregates of various particle sizes（except 0.25-1mm）（P<0.05）while there was no significant correlation between MBC and MWD in both the original soil and the four particle sizes.Microbial total PLFAs,bacteria,gram-positive bacteria（G⁺）,gram-negative bacteria（G^-）,Fungi,actinomycetes（Act）,arbuscular mycorrhizal fungi（AMF）of MP significantly higher than PP in bulk soil and most particle size aggregates（P<0.05）,while fungi/bacteria（F/B）was significantly higher than PP in bulk soil only（P<0.05）.Microbial total PLFAs,Bacteria,Fungi,Act and MWD were significantly positively correlated in bulk soil and soil aggregates with most particle sizes（P<0.05）,and F/B and MWD were significantly positively correlated in bulk soil and<0.25mm（P<0.05）,AMF,G^-and MWD were not significantly correlated in the bulk soil and soil aggregates of all particle sizes.Bacteria,Fungi,Act were positively correlated with soil physical and chemical properties(SOC,TN,TP,AP,NH₄⁺-N,NO₃^--N,C/N_soil and C/P_soil)（P<0.05）.Principal component analysis（PCA）showed that the soil microbial community structure in the bulk soil and soil aggregates of various particle sizes was significantly changed after 12 years,Castanopsis hystrix mixing in the Chinese fir.finally,determined by redundancy analysis（RDA）.carbon-phosphorus ratio(C/P_litter)is the most important environmental factor driving changes in soil microbial community structure.（3）Compared with PP,the hydrolase activities related to soil C,N,and P conversion in MP changed to varying degrees,specifically:β-glucosidase（BG）,N-acetyl-glucosidase（NAG）was significantly increased in original soil and aggregates of all particle sizes（P<0.05）;acid phosphatase（ACP）was also significantly increased in original soil and aggregates of all particle sizes（except0.25-1 mm）（P<0.05）;while leucine aminopeptidase（LAP）was significantly increased only in 0.25-1mm and<0.25mm soil aggregates（P<0.05）.Correlation analysis showed that the activities of C hydrolase（BG）and N hydrolase（NAG,LAP）were significantly positively correlated with P hydrolase（ACP）（P<0.01）;BG,NAG,LAP and ACP,there was a significant positive correlation with soil physicochemical properties（SOC,TN,TP,AP,and NH₄⁺-N）（P<0.05）,while the carbon-nitrogen ratio of litter(C/N_litter)was significantly negatively correlated with the activities of these four hydrolases enzyme（P<0.05）;BG,NAG,LAP and ACP were significantly positively correlated with Bacteria,G^-and Fungi（P<0.05）.（4）Compared with PP,the labile organic phosphorus（LO-P）of MP was significantly increased in both the bulk soil and soil aggregates of four particle sizes（P<0.05）,the moderate labile organic phosphorus（MLO-P）and the medium The moderately resistant organic phosphorus（MRO-P）was significantly increased in the original soil and all particle sizes（except 1-2 mm）（P<0.05）,and the highly resistant organic phosphorus（HRO-P）was significantly increased in the original soil and three particle sizes（>2 mm,1-2 mm,<0.25 mm）were significantly increased（P<0.05）.LO-P was significantly positively correlated with MWD in all particle size soil aggregates（P<0.05）,and HRO-P was significantly correlated with MWD in bulk soil and three particle size aggregates（>2mm,1-2mm,<0.25mm）.MWD was significantly positively correlated（P<0.05）,while MLO-P and MRO-P were only significantly positively correlated with MWD in soil aggregates>2mm and 0.25-1mm（P<0.05）.The results of variance decomposition analysis showed that the main factors affecting the changes of Po components came from the common effects of soil physicochemical properties,microbial characteristics and plant characteristics.The environmental factors were sorted by redundancy analysis（RDA）,and finally determined.the nitrogen-phosphorus ratio of litterfall(N/P_litter),Fungi,BG,LAP,NH₄⁺-N were the main environmental factors driving the variation of Po components.（5）Compared with PP,the contents of aluminum-phosphorus（Al-P）,occluded phosphorus（O-P）,calcium-phosphorus（Ca-P）in the Pi fraction in MP soil were not significantly increased in the bulk soil and soil aggregates of all particle sizes,while soil iron-bound-phosphorus（Fe-P）content increased significantly（P<0.05）in the bulk soil and in soil aggregates of all particle sizes（except 0.25-1 mm）.Fe-P was significantly positively correlated with MWD in three particle sizes（>2mm,1-2mm,<0.25mm）（P<0.05）,and O-P was significantly positively correlated with MWD only in>2mm soil aggregates（P<0.05）,while Al-P,Ca-P and MWD had not significant correlation.The results of variance decomposition analysis showed that the main factors affecting the changes of Pi components mainly came from the independent effect of soil physical and chemical properties,followed by the interaction effect of the three.The environmental factors were sorted by RDA,and it was finally determined that TN,BG,N/P_litter,and LAP were the most important environmental factors driving the variation of Pi components.（6）Structural equation modeling（SEM）results showed that the introduction of Castanopsis hystrix hybrid transformation in Chinese fir pure forest mainly drives AMF and Fungi by changing C/N_litter,TN and SOC,and finally affect the transformation of P.Considering the direct effect and indirect effect,SOC is the most critical factor to promote the growth of AMF and ultimately improve the conversion of P.In conclusion,the introduction of Castanopsis hystrix hybrid transformation in Chinese fir forest can effectively improve the particle size structure and stability of soil aggregates,improve soil nutrient quality,and enhance soil P retention,accumulation and transformation.This study provides new ideas and methods for effectively promoting the formation of soil aggregates in pure Chinese fir forests and improving the accumulation and transformation of soil P components.