Effects Of Close-to-nature Change Management In Pinus Massoniana Plantation On Soil Aggregates Carbon And Nitrogen Fractions And Transformation

The ecological problems caused by the long-term large-scale planting of single tree species of Pinus massoniana have always attracted much attention.To explore the forest management method with high ecological benefit and taking into account wood production and other ecological services to replace the traditional pure forest management model of Pinus massoniana is an important means to solve the problems caused by the management of Pinus massoniana plantation in southern China,such as soil degradation,low productivity,reduced biodiversity and so on.In this study,the pure plantations of Pinus massoniana pine（PP）and the close-to-nature forest（CP）formed by introducing broad-leaved tree species（i.e.Castanopsis hystrix Miq.and Michelia hedyosperma Law.）into pure forest of Pinus massoniana were studied.The research method of combining field monitoring with laboratory experiment is adopted.The physical and chemical properties,soil microbial community structure and function,soil carbon（C）and nitrogen（N）components in top soil aggregates of two kinds of forest types were measured and analyzed.The purposes in our study are exploring the changes of different C and N fractions in soil aggregates and their relationship with environmental factors.It provides an important scientific basis for effectively improving the stability of soil aggregates,afforestation model of C and N accumulation and transformation,tree species allocation and sustainable management technology of Pinus massoniana plantation.The main results are as follows,（1）12 years after the introduction of broad-leaved tree species into Pinus massoniana plantation.The soil organic carbon（SOC）,ammonia nitrogen（NH₄⁺-N）,available phosphorus（AP）and C/N ratio of soil(C/N_soil)in bulk soil and aggregates,increased significantly（p<0.05）in CP compared to PP.There was no significant difference in nitrate nitrogen（NO₃-^-N）,total phosphorus（TP）and p H value in bulk soil and aggregates.The mean weight diameter（MWD）of soil aggregates was significantly higher in CP than in PP（6.51%,p<0.05）.In addition,microbial biomass carbon（MBC）,microbial biomass nitrogen（MBN）,and microbial respiration（MR）of CP were significantly improved（p<0.05）in the bulk soil and aggregates,while microbial metabolism entropy（q CO₂）was significantly decreased（p<0.05）in bulk soil,>2mm and<0.25mm agglomerates.Microbial total PLFAs,bacteria（Bacteria）,gram-positive bacteria（G⁺）,gram-negative bacteria（G^-）,fungi（Fungi）and arbuscular mycorrhizal fungi（AMF）in bulk soil and aggregates in CP was significantly higher than those in PP（p<0.05）,while fungi/bacteria ration only in the bulk soil in CP was significantly higher than that in PP（p<0.05）.MWD showed a significant positive correlation（p<0.05）with the microbial communities of the bulk soil and most of soil aggregates.Principal component analysis（PCA）showed that the close-to-nature transformation management significantly changed the soil microbial community structure in the bulk soil and aggregates.Redundancy analysis（RDA）indicated that SOC and MWD were the main factors driving changes in soil microbial communities.（2）Compared with PP,the activities of hydrolase which related to soil C and N transformation in CP changed to some extent,i.e.β-glucosidase（BG）,cellobiohydrolase（CB）and N-acetylglucosaminidase（NAG）increased significantly in bulk soil and all aggregates,while L-leucine aminopeptidase（LAP）increased significantly only in bulk soil and 0.25-2mm soil aggregate.Phenol oxidase（PO）in CP was significantly higher than that in PP in bulk soil and all aggregates,while Peroxidase（PER）in the two stands had no significant difference in bulk soil aggregates.Except for LAP and PER,there was a very significant positive correlation between BG,CB,NAG,PO and each microbial community.There was a extremely significant positive correlation between soil C hydrolase BG and CB,oxidase PO and soil N hydrolase（LAP,NAG）.The hydrolytic enzymes（BG and NAG）and oxidase（PO）related to C and N cycle in bulk soil and aggregates were positively correlated with MWD.（3）Compared with PP,labile organic carbon（LOC）,lntermediate organic carbon（IOC）and lecalcitrant organic carbon（ROC）of bulk soil and aggregates of CP increased significantly or extremely significantly,and the recalcitrant carbon index（RCI）increased significantly in bulk soil and other aggregates except 0.25-2mm aggregate.Labile organic nitrogen（LON）and recalcitrant organic nitrogen（RON）of CP increased significantly in bulk soil and>2mm soil aggregate,intermediate organic nitrogen（ION）increased significantly in bulk soil and aggregates,while recalcitrant nitrogen index（RNI）in CP showed a downward trend in bulk soil and all aggregates,and reached a significant level in<0.25mm soil aggregate.There was a significant or extremely significant positive correlation between unstable organic carbon（LOC+IOC）and MWD in bulk soil and all aggregates,and there was a significant positive correlation between RCI and MWD only in>2mm soil aggregate.There was a significant positive correlation between active organic nitrogen（LON+ION）and MWD in 0.25-2mm and<0.25mm soil aggregates,while there was a negative correlation between RNI and MWD in bulk soil and all aggregates,and reached a significant level in0.25-2mm and<0.25mm soil aggregates.（4）C and N enzyme stoichiometry of PP and CP were positively correlated with soil ratio of C/N,LOC/LON,IOC/ION and（LOC+IOC）/（LON+ION）,whereas negatively correlated with MBC/MBN,and had no correlation with ROC/RON.Variance decomposition showed that the main factors affecting SOC and its components were the interaction among soil physical and chemical properties,microbial characteristics and plant characteristics,which accounted for 66.8%of the changes,while the most important factors affecting N and its components were the interaction between soil physical and chemical properties and microbial characteristics,which explained 45.8%of the changes.RDA showed that Fungi,TN and q CO₂ were the main factors driving the variation of SOC and its components,while Actinomycete,q CO₂,p H,and soil bulk density（BD）were the most important factors driving the variation of N and its components.In conclusion,the close-to-natural transformation of introducing broad-leaved tree species into Pinus massoniana plantation can effectively improve the particle size structure and stability in soil aggregates,increase soil nutrient quality,and enhance the retention,accumulation and transformation of soil C and N.This study provides a new method and a new way to effectively promote the formation of soil aggregate stability and improve the accumulation and transformation of soil C and N components in Pinus massoniana plantation.