Soil Microbial Communities And Their Effects On Carbon And Nitrogen Processes In Typical Sand-fixing Shrublands

Soil microorganisms,as the core of soil ecosystem,are closely related to a range of ecosystem processes.They directly affect formation and maintenance of soil fertility and fuction of ecosystem.In the plant-soil system,plants tend to select specific soil microbial community composition to affect microbial functions.In semiarid and arid areas of China,revegetation with xerophilous shrubs is an effective measure for controlling desertification.Although many studies have evaluated effects of shrublands on soil properties and microclimate,information about soil microbial community and their functions still remains unclear.In this study,we investigated the activity and composition of soil microbial community,and illustrate the roles of soil microbes on soil organic carbon?SOC?accumulation and soil nitrogen?N?transformation,in three revegetation shrublands?Artemisia ordosica,Salix psammophila,and Caragana microphylla?,using high-throuhput sequencing techolody,stable isotope tracing techolody,and laboratory analysis.The main results and conclusions are as follows:?1?The soil enzyme activity,microbial biomass,and microbial divisity were higher in the three shrublands than that in bare sandland.In the three shrublands,soil collected in the A.ordosica shrubland exhibited the highest microbial enzyme activity and biomass.The bacterial diversity was higher in the A.ordosica than that in the S.psammophila and C.microphylla shrublands,and the fungal diversity was higher in the A.ordosica and C.microphylla shrublands.The highest relative abundance of bacterial phylum in the A.ordosica,S.psammophila,and C.microphylla shrublands were Proteobacteria?36.55%?,Actinobacteria?28.01%?,and Bacteroidetes?8.73%?,respectively.The highest relative abundance of fungal phylum in the A.ordosica,S.psammophila,and C.microphylla shrublands were Ascomycota?71.57%?,Basidiomycota?87.11%?,and Ascomycota?77.54%?,respectively.?2?The SOC mineralization experiment showed that the cumulative CO₂ emissions in the A.ordosica,S.psammophila,and C.microphylla shrublands were 176.21 mg C?kg^–1,308.04 mg C?kg^–1,and 252.65mg C?kg^–1,respectively.The S.psammophila shrubland had the highest rates of mineralization of the slow SOC pool(1.18 mg C?kg^–1?day^–1).The relative abundance of microbial function genes involved in degrading recalcitrant C-complexes was significantly higher in the S.psammophila shrubland.Most C-degradation genes were assigned to three bacterial phyla,Actinobacteria,Proteobacteria,and Acidobacteria.The mineralization rate of the slow SOC pool and the microbial taxonomic and functional composition displayed a significant correlation.?3?Soil microbial carbon use efficiency was highest in the A.ordosica shrubland?glucose,58.65%;phenol,13.03%?,and was lowest in the S.psammophila shrubland?glucose,30.82%;phenol,6.29%?.The microbial carbon use efficiency was significantly ccorrelated with soil C:N and N availability.The substrate?glucose and phenol?addition experiment showed that microbial carbon use efficiency had stronger effect on glucose assimilation.In the A.ordosica shrubland,the higher microbial carbon use efficiency facilitated assimilation of more substrste to microbial biomass carbon and mineral associated organic C,thereby improving SOC pool stabilitzation.?4?Ammonification rates were similar across the shrublands,and all shrublands showed net ammonium immobilisation.The C.microphylla shrubland had the higher rates of gross nitrification than the A.ordosica and S.psammophila shrublands.A lower net nitrate immobilisation capacity by microbes in the C.microphylla shrubland might contribute to nitrate accumulation.The relative abundance of nitrogen cycling genes was significantly different in the three shrubaldans.The C.microphylla shrubland had the lowest relative abundance of nir D?involved in the dissimilatory nitrate reduction pathway?and the highest relative abundance of nir K?involved in the denitrification pathway?.The N transformation rates in the shrublands were related to soil microbial biomass nitrogen and the N-cycling genes responsible for nitrification as well as assimilatory and dissimilatory nitrate reduction.The microbial biomass nitrogen had greater explanatory power for the gross N transformation rates.Our results show that different shrublands had their corresponding specific microbial community compositions and functions.The soil microbial communities in A.ordosica,S.psammophila,and C.microphylla shrublands facilitated to stable organic carbon formation,recalcitrant organic carbon decomposition,and available nitrogen loss,respectively.From the points of soil organic carbon accumulation and nitrogen transformation,the A.ordosica shrubland was more conducive to the improvement of soil fertility.This study provided soil microbial mechanism underlying the effect of revegetation shrublads on soil fertility formation and ecological processes,and provided the basis for evaluating the ecological function of sand-fixing shrublands.