Seasonal Dynamics Of Rhizosphere Soil Microbial Carbon Use Efficiency During Vegetation Restoration In The Loess Hilly Region

Rhizosphere microorganisms are an important part of plant root microeco-system,which play an important role in soil organic matter decomposition,nutrient absorptionand vegetation growth.The growth and metabolism of microbial growth and metabolism in the rhizosphere changes periodically with the climate during seasonal alternation,which is significant for the carbon cycle of terrestrial ecosystems,.However,at present,the research on seasonal dynamic changes of rhizosphere soil microorganisms is scarce at home and abroad.Afforestation has a profound impact on terrestrial ecosystems,in which soil microbial communities respond most rapidly and change most significantly.Therefore,in this paper,Robinia pseudoacaci forest land with different restoration years in loess hilly region is taken as the research object,and farmland soil is taken as the control,and the change characteristicsof rhizosphere soil nutrients,microbial biomass,extracellular enzymes,microbial nutrient limitation and carbon use efficiency in three seasons,namely spring(March 2021),summer(June 2021)and autumn(October 2020),are studied by using the method of ecological stoichiometry theory.At the same time,Pearson correlatio analysis and redundancy analysis(RDA)were used to explore the mutual coupling relationship between Robinia pseudoacacia forest,soil and microorganism from the perspective of rhizosphere microecology,and to clarify the changing rules and control factors of soil microbial carbon utilization efficiency under seasonal changes.The main research progress and conclusions are as follows:(1)Seasonal changes significantly affected rhizosphere soil nutrients,microbial biomass extracellular enzymes,nutrient limitation and microbial carbon use efficiency.Among them,organic carbon(SOC),organic carbon: total nitrogen(SOC: TN),organic carbon:total phosphorus(SOC:TP),β-1,4-glucosidase(BG),β-1,4-xyloside(BX),Th stoichiometric ratio of carbon-phosphorus enzyme(EEAC:P)and nitrogen-phosphorus enzyme stoichiometric ratio(EEAN:P)with seasons was summer>spring>autumn,which increased by 6.79%-166.32% in summer compared with spring,an increase of11.04%-208.60% in spring compared with autumn.The seasonal variation characteristics of total nitrogen(TN),nitrate nitrogen,dissolved organic nitrogen(DON)and microbial biomass nitrogen(MBN)were spring>summer>autumn.Compared with summer,spring increased by 13.15%-51.01%,and summer increased by 30.80%-83.93% compared with autumn.The trend of total phosphorus(TP),available phosphorus,and microbial biomass phosphorus(MBP)with the seasons was spring>autumn>summer,with an increase of21.28%-118.32% in spring compared with autumn,and an increase of 2.83%-12.15% in autumn compared with summer.The seasonal variation characteristics of vector length(VL),vector angle(VA)and microbial carbon use efficiency(CUE)were autumn>spring>summer.Compared with spring,autumn increased by 4.92%-21.66%,and spring increased by 1.32%-248.76% compared with summer.(2)Under seasonal dynamics,SOC:TP explained 37.5% of the variation of extracellularenzymes,and was the main factor affecting the variation of extracellular enzymes.Rhizosphere soil nutrients and extracellular enzymes significantly affect microbial nutrient limitation and microbial carbon use efficiency(CUE).Among them,EEAN:P and DON explain 37.8% and 24.2% of the microbial nutrient limitation variation,respectively,which are the main factors affecting the microbial nutrient limitation variation.main factor.SOC,DOP,ammonia nitrogen,SOC:TP and BG,EEAC:P,EEAN:P had significant negative effects on CUE(P<0.01).(3)The growth and metabolism of microorganisms in rhizosphere soil of Robinia pseudoacacia forest in the study area are restricted by different degrees of phosphorus under seasonal changes,which indicates that soil phosphorus content is the key limiting factor for the growth and development of artificial Robinia pseudoacacia forest in loess hilly region,and phosphorus restriction will increase carbon restriction to some extent.Compared with the rhizosphere soil of Robinia pseudoacacia forest,the microbial relative carbon and phosphorus limitation of farmland soil is stronger,which indicates that compared with farmland soil,returning farmland to forest has alleviated the microbial carbon and phosphorus limitation of soil to some extent.(4)With the vegetation restoration process,the 0-10 cm rhizosphere soil microbial CUE generally showed a decreasing trend,indicating that the rhizosphere microbial carbon sequestration ability gradually weakened with the vegetation restoration process.At the same time,it was found that the rhizosphere microbial CUE of the 16-year-old Robinia pseudoacacia forest was higher than that of the farmland soil.,indicating that the carbon sequestration ability of rhizosphere microorganisms is poor in the latestage of returning farmland to forest.The rhizosphere microbial CUE was the largest in autumn,followed by spring and summer.The results indicated that the rhizosphere microbial carbon sequestration ability of the study area was stronger in autumn,and only the autumn rhizosphere microbial CUE was higher than that of the farmland soil,indicating that the rhizosphere microbial carbon sequestration of Robiniapseudoacacia forest mainly occurred in autumn.