Effects Of Different Land Use On Soil Nitrogen-Fixing Bacteria And Ammonia Oxidizer Diversities In The Horqin Meadow Grassland

In order to determine the effects of different land use on soil nitrogen-fixing bacteria and ammonia oxidizer diversities in the Horqin meadow grassland, four different contiguous land use (paddy field, woodland, natural meadow, upland field) were chosen as the objects for studying the diversities of nitrogen-fixing bacteria nijH gene, AOA amoA gene and AOB amoA gene of soils with the methods of PCR-DGGE and clone.The results showed that the community structure and diversities of nitrogen-fixing bacteria, AOA and AOB were all influenced by the change of land use. The diversity index of nifH gene varied little and most of the dominate nitrogen-fixing bacteria keep the same in all soils, while a few ones had become non-dominate since the natural meadow use was changed. However, two new dominate nitrogen-fixing bacteria appeared in paddy field. All the identified nitrogen-fixing bacteria by cloning and sequencing belonged to Azotobacter chroococcum of y-Proteobacterium, Sinorhizobium of a-Proteobacterium and Desulfovibrio vulgaris of S-proteobacterium, while the major one was Azotobacter chroococcum.The diversity index of AOA amoA gene presented an order with upland field> woodland> natural meadow> paddy field. Land use changed from natural meadow showed obvious effects on the communitity structure of AOA with significant different dominate AOA. Some dominate AOA existed in natural meadow had becoming non-dominate ones under other land use soils. While on the contrary, those non-dominate AOA had becoming more and more dominate in changed soils. All the identified AOA belonged to soil branch of Crenarchaeota.The community structure of AOB varied obviously along with the order which was presented with natural meadow> woodland> paddy field> upland field. Ammonium-oxidating bacteria within Nitrosospira and Nitrosomonas were widely distributed in all the soils of different land use. As well as Nitrosomonas, Nitrosospira accounted for50%both in natural meadow and woodland. Moreover, the major AOB was Nitrosospira in upland field, which was opposite to the paddy field with major AOB of Nitrosomonas. The dominate AOB changed in different land use. In general, nitrogen-fixing microbial diversity was influenced little under different land use type, but ammonia oxidizer diversity was significant changed. Natural meadow and woodland maintained relatively high microbial diversity and stable community structure. However, the community structure of soil nitrogen-fixing bacteria and ammonia oxidizer in upland field and paddy field were quite different from former two types, which affected the soil nitrogen cycle.