Research On Nitrogen Forms And Microbial Community Structure Of Grassland Soil In Inner Mongolia

Grasslands are an important ecosystem covering about a quarter of the earth’s surface. Different management practices of grassland ecosystems can have a major impact on the sustainability of these ecosystems. The objective of this study was to determine the impact of different pasture management practices on soil nitrogen forms and soil microbial community structures in the semi-arid grassland ecosystem in Inner Mongolia. The main results were as follows:(1) From investigating the different depth of soil nutrients and the soil nitrogen forms under different pasture management, the results show that:Under different pasture management, the absolute value of soil ammonia nitrogen and amino nitrogen were kept at a higher level under enclosure than under grazing. While there was no significant difference between the ratio of soil ammonia nitrogen and amino acid nitrogen to total nitrogen. Overgrazing pushed up the ratio of acid-insoluble nitrogen to total nitrogen, while reduced the ratio of ammonia nitrogen to total nitrogen. Under same pasture management, the ratio of soil amino acid nitrogen, amino sugar nitrogen and ammonia nitrogen to total nitrogen decreased with depth, while the ratio of soil acid-hydrolyzable unknow nitrogen and acid insoluble-nitrogen to total nitrogen increased with depth. The surface soil N-supplying capacity under different pasture management followed the order:Enclosure from grazing since 1996 (E96)>enclosure from grazing since 1983 (E83)>continuous overgrazing (FG). Results from this study clearly showed that appropriate years’pasture management was benefit for the improvement of the subsoil properties and soil nutrient availability. However, overgrazing and too long time enclosure were both not conducive to soil nutrient recovery.(2) From investigating the grassland soil microbial community structures, we found that the soil microbial community structures of control treatment and light grazing treatment were relatively similar. The microbial community structures were substantially changed due to moderate grazing and severe grazing treatment. Overgrazing obviously decreased the surface soil microbial biomass and diversity of the microbial communities. Enclosure from animal grazing can obviously improve 0-20 cm and 20-40 cm depth of soil microbial community structure, but had a little impact on subsoil (40-60 cm). The changes of grassland soil microbial community structure and diversity were influenced by soil cation exchange capacity, pH, total nitrogen, organic matter, available potassium and other basic properties.(3) Studied on the abundance and community structure of soil nitrogen transformation bacteria on grassland by using qPCR and PLFA isotope tracer technique. Results showed that the soil under different grazing intensity, ammonia oxidizing archaea AOA copy numbers were much higher compared with ammonia oxidizing bacteria AOB copy numbers. Subsoil (0-20 cm) under the different grazing intensity affected by the copy number of AOA showed that in the semi-arid grassland soils, AOA may play an important role in the nitrogen cycle. The PLFAs i 15:0,16:1co7c,16:lω5c, 16:0 and 18:lco7c represent bacteria, and PLFA 10-Me-16:0 represents actinomycetes used the most of 13C-labeled urea. The much higher incorporation of 13C in PLFA 16:0 in the enclosure treatment soils after 3 and 7 days’incubation, compared with the overgrazing treatment soil, would suggest higher metabolic activity in the enclosure treatment soils than the soils from the overgrazing treatment.(4) Through correlation analysis of grassland soil nitrogen transformation and microbial community analysis. Showed that, after urea addition, the soil microbial community, nitrate nitrogen and ammonia nitrogen concentration were significantly correlated; Gram-positive bacteria, fungi and actinomycetes were closely associated with ammonia nitrogen; Actinomycetes, gram-negative bacteria and fungi were closely associated with nitrate nitrogen. Based on different grazing management of organic nitrogen forms and high-throughput sequencing results of RDA redundancy analysis showed that the soil microbial categories were closely related to forms of organic N, microbial biomass was closely related to nitrogen mineralization.Results from this study showed that pasture management influenced soil nutrient availability, microbial biomass, microbial community structure and also the turnover of soil nitrogen.