Turnover Characteristics And Influencing Factors Of Soil Aggregate Carbon Pool In Multi-generation Eucalyptus Plantation

Eucalyptus,as one of the three fast-growing and high-yield trees in China,has developed rapidly in the south of China in recent decades.However,due to the blind expansion of eucalyptus planting,coupled with short cycle,mountain preparation,and high intensity management,a series of problems such as the decrease of soil organic carbon content and soil fertility have occurred in eucalyptus plantation.What are the turnover characteristics of soil aggregate organic carbon pools in eucalyptus plantation under multi-generation continuous cropping pattern?Are there differences in the influencing factors of soil aggregate organic carbon pool turnover in different generations of eucalyptus plantations?What is the relationship between different influencing factors and turnover characteristics of soil aggregate organic carbon pools?In order to explore the characteristics of organic carbon turnover in soil aggregates of several generations of eucalyptus plantations and the relationship at aggregate level between the chemical components and mineralization characteristics of organic carbon with various influencing factors,four generations of eucalyptus plantations with the same age were selected as experimental objects in major Eucalyptus producing areas of China,with 16-year uncultivated eucalyptus forest land and evergreen broad-leaved forest land as the control.Different active organic carbon components and organic carbon mineralization of each particle size aggregate were measured,and the differences of different active organic carbon components,organic carbon mineralization,physical and chemical properties,iron oxides and biological indicators of different generations of eucalyptus plantation aggregate levels were compared.Correlation analysis,redundancy analysis and random forest were used to study the turnover characteristics and influencing factors of organic carbon in soil aggregates of eucalyptus plantations from three aspects:soil intrinsic properties,physical and chemical protection and microbial activity.The main conclusions are as follows:(1)Multi-generation continuous planting of eucalyptus significantly changed the storage and turnover of soil aggregate organic carbon in forest land.The organic carbon content and mineralization capacity increased significantly in the early stage,after two or three generations of continuous planting,the organic carbon content and mineralization capacity decreased significantly,but increased 5.62%-33.81%compared with the first generation.The total organic carbon content(SOCc)and concentration(SOCp),active carbon fraction 1(LCF1),active carbon fraction 2(LCF2)and recalcitrant carbon fraction(RCF)at any aggregate size showed the change characteristics of first significant increase and then significant decrease with the increase of successive generations,and reached the highest value in the second or third generation.The cumulative mineralization and the cumulative mineralization rate of organic carbon at any aggregate size increased at first and then decreased with the increase of successive planting generations,which reached a significant difference level at the aggregate size of 0.25-2 mm and<0.25 mm,which indicated that the mineralization capacity of organic carbon in small aggregates and micro-aggregates significantly improved in the early stage,and decreased after the third generation.(2)The influence of multi-generation continuous planting management mode of eucalyptus plantation on the chemical properties of aggregates was consistent with that of the bulk soil.Most nutrient contents increased significantly in the early stage of continuous planting,and decreased significantly from the third generation.There are significant differences in total nitrogen content(TN),C/N ratio(C/N),total phosphorus content(TP)and total potassium content(TK)among successive generations at any aggregate level.With the increase of successive planting generation,TN,C/N and TP showed a significant increase at first,with an increase range of 6.67%-57.26%,and then decreased significantly.(3)Continuous planting of eucalyptus for several generations caused significant changes in the iron oxide in soil aggregate level,and the organic carbon bonded iron oxide increased significantly in the early stage and then decreased significantly,which indicated that continuous planting of eucalyptus for several generations would affect the combination of organic carbon and iron oxide in soil aggregate level,thus affecting the fixation and turnover of organic carbon.The concentration of amorphous iron oxide(Feo),the ratio of amorphous iron oxide to free iron oxide(Fed)(Feo/Fed)and the content of iron oxide bound carbon(CFe)among different successive generations in any aggregate showed significant differences,which increased at first and then decreased with the increase of generations.However,Fed showed no significant difference among the first three generations,but decreased significantly in the fourth generation eucalyptus plantation.with a decrease range of 6.25%-12.63%.(4)Most of the enzyme activities in soil aggregate level increased at first and then decreased with the increase of successive generations of eucalyptus plantation,and reached a significant level in 0.25-2 mm aggregate,which indicated that continuous planting of eucalyptus significantly increased the microbial activity of small aggregates in the early stage and decreased the microbial activity of small aggregates in the later stage.The activities of β-glucosidase,cellobiase,β-xylosidase,βacetylaminoglycosidase and acid phosphatase in any size aggregates first increased and then decreased with the increase of successive generations,and they were the highest in the second or third generations.(5)The results show that SOCp,TN and C/N were positively correlated with different active organic carbon components in any size aggregate,and they were selected as the main prediction indexes,which indicated that the organic carbon concentration and total nitrogen content of aggregates had important effects on the storage and turnover of organic carbon.CFe showed extremely significant positive correlation with recalcitrant organic carbon content in>2 mm and<0.25 mm aggregates,and it was selected as an index that had important influence on different active organic carbon components and organic carbon mineralization in<0.25 mm aggregates,which indicated that iron oxide binding was an important mechanism of organic carbon fixation in macro-aggregates and micro-aggregates,especially microaggregates.Enzyme activities in any particle size were selected as indexes that had an important influence on organic carbon and mineralization of organic carbon with different activities,which indicated that enzyme activity had an important predictive effect on organic carbon turnover of aggregates.