Seasonal Dynamics Of Soil Microbial Nutrient Utilization Strategies After Restoration Of Robinia Pseudoacacia Forest On The Loess Plateau

Plantation restoration is one of the main measures for the restoration of the ecosystem in the Loess Plateau,the process of which has inevitable influence on the most active microorganisms in the soil.In addition,strategies for nutrient utilization of microorganisms（element using efficiency,extracellular enzymes）,as the most critical link in soil carbon cycle,are also affected by temperature,precipitation,sunshine,and so forth,the climatic factors of which are taken into consideration on the foundation of seasonal dynamics.Since the conversion of farmland to forest in 1999,the quality of ecological environment in the Loess Plateau has been significantly improved,and the vegetation coverage has increased from 31.6%to 62.8%.It is a typical representative of China’s fragile ecosystems,which experienced massive change through vegetation restoration,providing a natural experimental platform for strategic research.However,there is still a lack of research on the response of microorganisms’strategies for nutrient utilization to seasonal dynamics,and the conclusions of existing experimental studies on seasonal dynamics are not uniform due to differences in vegetation types and the number of years for restoration.Based on this,this study selected 4 different numbers of periods(46th year,30th year,20th year,and16th year for restoration of soils in Robinia pseudoacacia forest in Ansai District,Yan’an as the research objects,aiming to clarify the variation characteristics of the nutrient content in soil,extracellular enzyme activities and utilization efficiency of elements based on seasonal dynamics（spring,summer,autumn）.The study revealed the effects of seasonal dynamics on physicochemical properties of soil and strategies for nutrient utilization of microbial microorganisms in the hope of providing a reference for further understanding of the carbon cycle process in the context of"double carbon".The main conclusions of the study are as follows:（1）After the plantations restoration of different periods,the soil nutrient content and composition had significant differences in seasonal dynamics,and increased significantly compared with farmland soil.In the four restoration periods,SOC,DOP,C:N and C:P were significantly greater in summer than in spring and autumn.Among them,SOC has an increase range of133%-392%in summer compared with spring,and has the largest increase in the 16th year of vegetation restoration,and an increase range of 352%-616%compared with autumn,which has the largest increase in the 30th year;DOP increased by 483%-658%in summer compared with spring,the largest increase in the 16th year,and 15%-731%compared with autumn,and the largest increase in the 46th year.In the four recovery periods,DON and DON:DOP were significantly lower in summer than in spring and autumn,and the range of DON in summer was 17%-65%and 22%-70%lower than that in spring and autumn.Both saw the largest decline in the 46th year.Compared with the farmland soil,the SOC content in each restoration period increased by 309%on average.（2）There were significant differences in the seasonal dynamics of soil hydrolase activity and their vector angles in different restoration periods,but there were no significant differences in oxidase activity.The seasonal dynamics of soil C acquisition enzyme activities（BG,BX,CBH）and N acquisition enzyme activities（LAP,NAG）in each restoration year were significantly greater in summer than in spring and autumn.Among them,the C-acquisition enzyme activity increased by 19%-97%in summer compared with spring,with the largest increase in the 16th year,and 71%-271%compared with autumn,with the largest increase in the30th year;N-acquired enzyme activity in summer compared with in spring,the increase range is 31%-96%,the largest increase in the 30th year,and the increase range compared with autumn is 22%-129%,the largest increase in the 16th year.The seasonal dynamic performance of P acquisition enzyme activity（AKP）was significantly lower in summer than in spring and autumn,and the decline ranges were 65%-82%and 30%-81%,with the largest decline in the 20th year.The seasonal dynamics of the extracellular enzyme vector angle（VA）in each recovery year were lower than 45°in summer,but higher than 45°in spring and autumn,which indicated that the soil was in strong N limitation in summer,and was relatively in strong P limit in spring and autumn.（3）There were significant differences in the seasonal dynamics of element use efficiency in different restoration periods,but CUE,NUE,and PUE showed different responses to seasonal changes.The seasonal dynamic performance of CUE is that summer is significantly lower than that of spring and autumn,the average CUE in summer is 0.09 in the four recovery periods,and the CUE in spring and autumn are 0.31 and 0.53;while the seasonal dynamic performance of NUE and PUE is significantly higher in summer than in spring and autumn,the average NUE and PUE in summer are 0.94 and 0.96,the NUE and PUE in spring are 0.76 and 0.77,and the NUE and PUE in autumn are 0.40 and 0.25.This indicates that microorganisms have more C retention and N and P mineralization in spring and autumn,and more CO₂emissions and N and P retention in summer.（4）Seasonal dynamics and Restoration period can regulate microbial nutrient utilization strategies by changing soil nutrient conditions.Seasonal dynamics can change soil nutrient composition through climatic factors,acting on C and N acquisition enzyme activities and element use efficiency.However,the recovery periods can only indirectly affect the activity of N-acquisition enzymes by changing soil organic carbon components.CUE,NUE and PUE are mainly affected by C:N imbalance,C and N acquisition enzymes are mainly affected by SOC and DOC,and P acquisition enzymes are mainly affected by TN,DON,TP and DOP.