Effects Of Land Use Change On Community Of Soil Bacteria And Ammonia-oxidizing Microorganisms In Semi-arid Areas Of Northern China

Over farming leads to rapid degradation of already fragile semi-arid ecosystem, and restoration of such degradated ecosystem has been proven difficult. Soil ecosystem bridges atmosphere, hydrosphere, lithosphere, and biosphere. The sustainable development of soil ecosystem involves increasing soil stability and resilience while maintaining productivity, achieved with the help of soil microorganisms, which play an important role in carborn and nitrogen cycle. Wuchuan Region in Inner Mongolia and Ili Region in Xinjiang were the typical semi-arid areas in Northern China. This study used the two aeres as examples, measured land use effects on soil bacterial and ammonia-oxidizing microbial community, and analyzed the interactions between microbial community, land use change, and environmental factors.The methods for restoration that are typically adopted in semi-arid regions include abandoning farmland, growing drought tolerant grass(Lolium perenne L.), and growing shrubs (Caragana korshinskii Kom.). In the present study, the effects of these restoration methods on the abundance and diversity of soil bacteria were evaluated by quantitative real-time PCR (qPCR), Miseq next-generation sequencing of 16s rRNA genes. Soil bacteria community and physico-chemical properties had been significant changed between three main restoration lands and the farmland. Restoration increased the abundance of soil bacterial. Specifically, the abundances of K-selected bacterial communities such as Acidobacteria were increased after restoration, while the abundances of R-selected bacterial communities such as β- and γ-Proteobacteria were decreased. Compared to farmland, the relative abundances of Cyanobacteria speices in restoration land were incereased, while the relative abundances of Nitrospira and Ntrosospira speices were decereased.The effects of these restoration methods on the abundance and diversity of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) were evaluated via qPCR, terminal restriction fragment length polymorphism, and clone library analysis of amoA genes. Both the abundance and community composition of AOB were significantly different between the restored lands and the adjacent control. Significantly lower nitrification activity was observed for the restored land. Clone library analysis revealed that all AOB amoA gene sequences were affiliated with Nitrosospira. Abundance of the populations that were associated with Nitrosospira sp. Nv6 which had possibly adapted to high concentrations of inorganic nitrogen, were decreased on the restored land. Only a slight difference in the AOB communities was observed between the restored land with and without the shrub (Caragana korshinskii Kom.). A minor effect of land restoration on AOA was observed in this study.Compared with conventional farming (CF), organic farming (OF) and Grazing Pasture (GP) in Ily Region both increased soil TN and SOC stock but via contrasting mechanisms. The abundance of AOB under cropland was higher than those of GP. The AOB sequences, belonging to Nitrosospira cluster 1 which is adaptable to high mineral N content in cold region, were increased in CF compare to other land uses. Conversion of GP to cropland did not affect the abundance, but the community structure of AOA. The abundance of AOB and AOA in large macroaggregate and silt and clay were significantly lower than those in small macroaggregate under cropland. In cropland, the small macroaggregate provides the microenvironment for the growth of AOB and AOA, thereby serving as a potential hotspot for ammonia oxidation.In summary, land restoration negatively affected the abundance of AOB and soil nitrification activities, while reclamation of grassland to cropland significantly increased the AOB abundance, and shifted the community structure and spatial distribution variation of AOB and AOA at the soil aggregates. The results indicated that soil N cycle could be substantially altered by land restoration or grassland reclamation, and this should be well integrated in the improvement of regional land utilization.