Effects Of Vegetation Recovery On Diazotrophs Community And Nitrogenase Activity In Karst Areas Of Guizhou Province

As a widespread ecosystem in China,karst has deteriorated rapidly in the past 50 years due to rapid deforestation and intensive farming.Under the "Green Grain for Food" project,some disturbed farmland was abandoned in order to restore soil functions and ecosystem services,leading to different stages of vegetation restoration(primary forest,secondary forest,abandoned land and cultivated land).Even with the general lack of nitrogen in karst soils,soil environmental characteristics can make important contributions to “new” nitrogen input to terrestrial ecosystems by adjusting the characteristics of diazotrophs Communities and nitrogen fixation capacity of nitrogenases during post-agricultural succession.In order to investigate the contribution of diazotrophs community and nitrogenase to the nitrogen input of karst soil ecosystem,this paper studied the effects of different vegetation restoration stages on the diazotrophs community and nitrogenase activity at different depths of the soil profile by highthroughput sequencing.The results show:(1)The differences in soil properties between surface and rock-soil interfaces are most significant at different vegetation restoration stages,and the soil properties of primary forests are significantly different from other vegetation restoration stages in the entire soil profile.With the change of vegetation restoration stage,soil water content,total nitrogen,total organic carbon and dissolved organic carbon increased significantly in the later stage of vegetation restoration,but the p H was opposite;ammonium and nitrate nitrogen were increased significantly at the 0-30 cm in soil of the primary forest.Between different soil layers,the p H increases with increasing soil depth,reaching a peak at the rock-soil interface,but the redox potential was opposite;the contents of total nitrogen,ammonium nitrogen and total organic carbon at the rock-soil interfaces were lower than those in upper soil.The nitrogenase activity of the primary forest soil was significantly higher than that of the cultivated land.With the increase of soil depth,the nitrogenase activity of the primary forest and the secondary forest did not differ significantly above the interface,but there was a significant difference at the interface.Nitrogenase activity at the rock-soil interfaces is higher than in the upper soil layer.At the surface layer of soil,there is a significant correlation between various environmental factors and nitrogenase activity,while in the deep soil layer,nitrogenase activity is mainly affected by soil carbon and nitrogen content.(2)At the rock-soil interfaces,the diversity of soil diazotrophs is significantly different at different vegetation restoration stages;with the increase of soil depth,the Shannon index and phylotype richness show significant differences,and diazotrophs diversity at the soil surface is significantly higher than 30-50 cm soil layer.The main family-level diazotrophs groups in the study are Bradyrhizobiaceae,unclassified Proteobacteria,unclassified Alphaproteobacteria,unclassified Rhizobiales,Rhodospirillaceae,unclassified Burkholderiales,Xanthobacteraceae,Burkholderiaceae and Frankiaceae.Bradyrhizobiaceae is mainly affected by the interaction of vegetation restoration and soil depth;the vegetation restoration stage mainly affects unclassified Proteobacteria,unclassified Alphaproteobacteria,and unclassified Rhizobiales in soil;soil depth mainly affects Rhodospirillaceae,unclassified Burkholderiales,Xanthobacteraceae,Burkholderiaceae,and Frankiace.Environmental factors have significant effects on ?-diversity and family level classification of diazotrophs.Among the diversity,phylotype richness is more susceptible to environmental factors;in the classification of diazotrophs,unclassified Proteobacteria,unclassified Burkholderiales,and Burkholderiaceae are more susceptible to environmental factors.(3)The community structure of diazotrophs was significantly different between different vegetation restoration stages and between different soil layers in the same vegetation restoration stage.The community structure parameters indicate that the difference between primary forest and secondary forest,abandoned land and cultivated land were the largest,while the difference between secondary forest and abandoned land was the smallest.In addition,the effect of different soil depths on diazotrophs community structure in the primary forest was greater than that in other vegetation restoration stages.The effect of different soil depths on diazotrophs community structure in cultivated land is minimal.Soil p H,water content,total organic carbon,dissolved organic carbon,and total nitrogen significantly affected the diazotrophs community structure.The composition of diazotrophs community are significantly different at different vegetation restoration stages and soil depths.The diazotrophs composition is distributed in different soil layers in primary forests and cultivated land,while in secondary forests and abandoned croplands are mainly concentrated in upper soil.Overall,soil carbon-to-nitrogen ratio,p H value,and composition of diazotrophs community significantly affected nitrogenase activity.At different vegetation restoration stages,soil p H,redox potential,soil carbon-nitrogen ratio,total nitrogen,water content,diversity of diazotrophs,relative abundance of unclassified Rhizobiales and Xanthobacteraceae have Significant effects on nitrogenase activity.