Impact Of Water Status On Soil Microbial Biomass And Community Structure In Typical Grasslands Of Inner Mongolia

In semi-arid grassland ecosystem,the soil water availability is the major factor limiting primary productivity.Soil microorganisms are the important media that adjust soil nutrient cycles.In this study,we selected two platforms(long-term enclosure and grazing,and spring precipitation simulation)in Inner Mongolia.The objectives were:1)to investigate seasonal changes of soil microbial C,N,and P,as well as the plant growth under enclosure and grazing treatments 2)to evaluate the gross P fluxes under drying and rewetting cycles using 33P istopic dilution method 3)to study the impact of the increased spring precipitation on primary productivity and the responses of soil microorganisms to the changing soil water conditions by using the combination of PLFA,DNA-based sequencing,and metagenomics analyses.The results showed that:(1)In this semiarid ecosystem approximately two times lower C:P and N:P ratios in microbial biomass(25:1 and 3:1,respectively)compared with global analysis(46:1 and 6:1,respectively).Enclosure from grazing increased MBC and MBN,while the change patterns of microbial pools were affected by season but not pasture management.Consequently,the turnover rates of microbial biomass as calculated from the seasonal fluctuations were similar in enclosure and grazing treatments(around 1.5 year-1 for MBC and MBN,3 year-1 for MBP).Lower mean stock in soil K2SO4-extractable N but similar in MBN compared with total plant N uptake were observed in all treatments,suggesting N deficiency in this region and the vital role soil microbes play as a stable nutrient pool for plant uptake.In contrast,both NaHCO3-extractable P and MBP stocks were much higher than total plant P uptake,suggesting no P deficiency under current N status.(2)Compared with constant moisture(CM)treatment,a tendency of lower physicochemical processes of P fluxes was observed in drying and rewetting(DRW)treatment,but with no significant difference between the two treatments within 30-day incubation.The release of the microbial P accounted for the increase of the water-extractable P.However,the response of microbial P to DRW was only observed in the first cycle.Within the period of the incubation,gross organic P(Po)mineralization contributed 44%of the total isotopically exchangeable P,indicating the importance of biological processes in P transformation in grassland soils.DRW significantly decreased the gross Po mineralization.The gross Po mineralization stopped during dry phase and continued again after rewetting with mineralization rates as similar as in CM(0.3-2.4 mg P kg-1 d-1).Additionally,qPCR analysis showed that the decrease of the Po mineralization in DRW treatment might be related to the declined fungal gene copies.(3)Spring watering improved ANPP and the soil MBC pool throughout the growing season.A significant positive correlation between ANPP and the average MBC was found across the treatments,indicating that MBC could be used as a good indicator for soil fertility under a changing climate.Throughout the growing season,soil water and N availability were the main factors controlling soil microbial dynamics in this semi-arid grassland ecosystem.Nevertheless,the transient decrease in the MB C:P ratio when plant P uptake decreased indicates P competition between plants and microorganisms.(4)PLFA results showed that there were Gram-negative(G-)bacteria and fungi being sensitive to increased spring precipitation.In which,based on the DNA-based high throughput sequencing analyses,we found two phyla of G-bacteria(Proteobacteria and Bacteroidetes)and one phylum of fungi(Ascomycota).Further,with metagenomics analysis,we found that the impact of increased spring precipitation on soil microbial functions can last for a relatively long time.Spring precipitation can influence not only the soil water status,but also plant growth and soil nutrient conditions,and in turn affect the soil microbes.In addition,spring precipitation had negative effects on the main metabolic functions of soil microbes,indicating inadequate nutrient supply under appropriate water status in this ecosystem.In summary,through combining the field investigation and lab incubation,we found that high soil water availability could increase microbial turnover and plant uptake of soil N and P,thereby improve ANPP.In addition,good soil water status promoted soil Po mineralization,which may relate to the relative high abundance of soil fungal community.Miseq sequencing data indicated that the main phylum of fungi that sensitive to soil water was Ascomycota.The results could provide valuable supports for the sustainable management of grassland in China.