Characteristics of physiological groups of soil nitrogen-transforming microbes in different vegetation types in the Loess Gully region, China

The "Grain for Green" project is one of the important measures used to improve the environment of China's Loess Plateau. In recent years, soil physicochemical properties in the region have been improved, which in turn promoted ecological succession. Different vegetation types change the soil environment to different extents, influencing the biogeochemical cycling of materials such as nitrogen, an essential nutrient for plants. Recently, a growing amount of attention has been paid to the relationship between vegetation and nitrogen cycling in natural ecological systems. Meanwhile, several physiological groups of nitrogen-transforming microbes in soil are closely associated with soil nitrogen cycling. To determine the effect of vegetation restoration (grass stage, shrub-grass stage and tree-brush-grass stage) on the populations and distribution of physiological groups of nitrogen-transforming microbes (ammonifying bacteria, nitrifying bacteria and denitrobacteria), soil samples were collected from depths of 0--10 cm under seven vegetation types from different stages of restoration in the Dongzigou Basin, Ansai, Shaanxi Province and bacteria populations were measured through the most-probable-number (MPN) method. The results show the populations of ammonifying bacteria, nitrifying bacteria, and denitrobacteria increased as vegetative succession proceeded from grass communities to more complex tree-shrub-grass communities, and those populations ranged from 2.49 to 185.79 x 10 super(6) cfu/gdm, 19.43 to 77.21 x 10 super(2) cfu times g super(-1) times dm super(-1) and 1.23 to 38.24 x 10 cfu times g super(-1) times dm super(-1), respectively. For ammonifying bacteria and denitrobacteria, the highest N levels were obtained in the Quercus liaotungensis Koidz. community and the lowest in the Artemisia sacrorum Ledeb. community. The maximum population levels of denitrobacteria and ammonifying bacteria were obtained in the Q. liaotungensis Koidz. community and these ranged between 31 to 74 times higher than the lowest levels which were seen in the Artemisia sacrorum Ledeb. community, respectively. The highest levels of nitrifying bacteria were obtained in the Q. liaotungensis community and these were 4-fold higher than the lowest one in the Syzygium aromaticum (L.) Merr. and Perry community. The effect of vegetation restoration on the population of nitrobacteria was the strongest on ammonifying bacteria, followed by denitrobacteria and nitrifying bacteria. Soil nitrog