Biological Nitrogen Fixation And Its Influencing Factors Of Pueraria Lobata Communities In Early Stage Of Succession In Northwest Guangxi

Nitrogen(N)is a major element limiting net primary production in natural terrestrial ecosystems.Dynamics of N are crucial in governing ecological structures and processes.N availability plays a crucial role in determining ecosystem carbon(C)sequestration,since ecosystem C and N cycling are tightly coupled.For ecological restoration,N availability to some extent determines the rate and direction of ecological restoration.Biological nitrogen fixation(BNF)is a major pathway of new nitrogen(N)inputs for terrestrial ecosystems,including symbiotic nitrogen fixation(SNF)and free-living nitrogen fixation(FNF).Therefore,it is crucial to reveal the responses of BNF to environmental changes for understanding or predicting dynamic changes of N input to ecosystems under global changes.However,current measured datas on both rates of SNF and FNF are still lacking.The understanding on relative contribution of SNF and FNF in different ecosystems is still insufficient.Residing environments and resource acquiring are multiple for SNF and FNF diazotrophs,leading to influencing factors of both processes may quite different.In this study,we investigated Pueraria lobata community in early successional in Northwest Guangxi.The rates of Kudzu nodule SNF and soil FNF were measured by acetylene reduction method and ¹⁵N₂ culture method,respectively.Meanwhile,soil physical-chemical proprieties,kudzu leaf traits,diazotroph nif H gene abundance,diversity and community composition were measured to determine the controls of SNF and FNF.The results would be helpful to understand the BNF rate and regulation mechanisms of kudzu community,and provide scientific and technological support for the restoration of vegetation in the Northwest Guangxi.Main results are as follows:(1)Annual rates of SNF and FNF were not significantly.Total N₂ fixation rates ranged from 7.12 to 61.93 kg N ha^-1 yr^-1 with an average of 22.93 kg N ha^-1 yr^-1,and the rates of annual mean SNF and FNF were 13.38 kg N ha^-1 yr^-1 and 9.55 kg N ha^-1 yr^-1,respectively.Among the eight study sites,the contribution of SNF and FNF to total BNF of Pueraria lobata community was similar.(2)The diazotroph community composition in nodules and soil was similar.Bradyrhizobium was the dominant diazotroph.Bradyrhizobiaceae and Rhodocyclaceae were the dominant populations in the nodule and soil,respectively,and were also the most closely related populations with other families.(3)The controlling factors of nodules and soil diazotrophs were different.The diversity of nodules diazotroph was limited by soil NO₃^--N and available Mo,while the abundance of soil diazotroph was limited by soil available iron and p H.(4)The major controlling factors for SNF and soil FNF rates were different in kudzu community.SNF was more affected by biotic factors than abiotic factors,but was mainly restricted by the abundance of diazotroph nif H gene.Soil FNF was more affected by abiotic factors than biotic factors,and was mainly restricted by soil water content.In conclusion,both SNF and FNF were equally important for the kudzu communities of Northwest Guangxi.The main factors affecting the rates of SNF and soil FNF or symbiotic and free-living diazotroph communities were different.