Nitrogen Absorption And Assimilation Of Stipa Breviflora And Soil Microorganisms Under Different Stocking Rates In A Desert Steppe

There is an inseparable relationship among plant-soil-microorganism in grassland ecosystems.The impact of grazing on grassland ecosystems is complex.On the one hand,livestock affect the morphological characteristics of plants through feeding.On the other hand,they affect the physical and chemical properties of soil through trampling and return of feces and urine,and thereby affect the community structure and function of microorganisms in soil.Change in microbial community structure and function will further regulate the nutrient cycle process of soil,and directly affect nutrient absorption by plants.Nitrogen is one of the important limiting factors for the healthy growth of plants in desert steppe.Therefore,in order to systematically explain the response strategy of plant nitrogen absorption characteristics and growth characteristics to stocking rate,it is particularly important to study the rhizosphere and bulk soil microbial communities and functions of grassland plants.In this study,Stipa breviflora desert steppe in Inner Mongolia was selected as the research area,and a representative constructive species(Stipa breviflora)was studied to explore the effects of different stocking rates on population characteristics,nitrogen absorption and assimilation processes,microbial community structure and function and nitrogen mineralization rate in the rhizosphere and bulk soil of Stipa breviflora,and comprehensively analyze the relationship between them.The research aimed provide a theoretical basis for the sustainable management and utilization of grassland.The main conclusions were as follows:(1)Heavy grazing increased the density of Stipa breviflora.The effect of stocking rate on the aboveground productivity of Stipa breviflora was regulated by rainfall and temperature in the growing season.The density structure of Stipa breviflora population in the control area(no grazing),light,moderate and heavy grazing areas were classified as decreasing,stable,stable and growing,respectively.(2)Different stocking rates changed the activities of nitrogen assimilation related enzymes in Stipa breviflora roots and leaves.Compared with the control,light and moderate grazing significantly increased the activities of nitrate reductase(NR),glutamine synthetase(GS),glutamate oxaloacetate aminotransferase(GOT)and glutamate pyruvate aminotransferase(GPT)in Stipa breviflora leaves,as well as the activities of GS,GOT and GPT in roots,while heavy grazing significantly reduced the activities of GS in Stipa breviflora leaves and NR in roots.NR,GOT and GPT activities in Stipa breviflora leaves and roots were significantly positively correlated with nitrogen content,soluble protein,free amino acid and proline content.(3)Heavy grazing reduced the nitrification rate and net nitrogen mineralization rate of Stipa breviflora rhizosphere and bulk soil.The net nitrogen mineralization rate and nitrification rate were significantly correlated with soil physical and chemical properties(water content,p H,soil aggregate,compactness),microbial community structure(kingdom level: relative abundance of bacteria and eukaryotes;phylum level: relative abundance of microorganisms of Acidobacteria,Proteobacteria,Chloroflexi,Cyanobacteria,Bacteroidetes and Candidatus rokubacteria)and microbial functional genes(more than 70% of functional genes were related to nitrogen metabolism).(4)Stipa breviflora rhizosphere soil microorganisms were more sensitive to stocking rate than bulk soil microorganisms.With increase in stocking rate,the relative abundance of bacteria in rhizosphere soil decreased gradually,while the relative abundance of eukaryotes and viruses gradually increased.The stocking rate mainly affected the relative abundance of functional genes related to dissimilatory nitrate reduction and complete nitrification processes.There was a significant correlation between soil properties(p H,ammonium nitrogen,total carbon,microbial biomass carbon and nitrogen,soil aggregate and compactness)and kingdom and phylum level microorganisms and functional genes related to nitrogen metabolism.More than 80% of functional genes related to the nitrogen cycle in rhizosphere and bulk soil were relatively more abundant in the control and light grazing treatments.In conclusion,the change of soil nitrogen mineralization rate driven by microorganisms in desert steppe under different stocking rates is the main factor affecting the decrease of available nitrogen content in soil,and thereby affected the nitrogen absorption and assimilation of Stipa breviflora.And at the same time,in response to livestock grazing,Stipa breviflora has formed its own morphological and physiological adaptation strategies.This study suggested that light grazing in desert steppe is conducive to soil nitrogen turnover and plant nitrogen absorption and assimilation.