Abstract: Soil functional microorganisms can promote nutrient transformation in cropland ecosystems and maintain the stability of soil environment. Based on a long-term experiment in Changwu Agro-ecological Experimental Station with six typical fertilization treatments, including no fertilizer (CK), medium nitrogen fertilizer (N₁₂), high nitrogen fertilizer (N₂₄), medium phosphate fertilizer (P₁₂), high phosphate fertilizer (P₂₄), and medium nitrogen and phosphorus fertilizer (N₁₂P₁₂), we explored the effects of fertilization managements on the abundance of soil nitrogen-transforming bacteria and phosphate-solubilizing bacteria by the traditional coating plate count method. The results showed that there were significant differences in the abundance of soil microorganisms involved in nitrogen cycling under different fertilization treatments. The abundance was decreased in the order of aerobic free-living nitrogen-fixing bacteria, ammonia oxidizing bacteria, nitrite bacteria, denitrifying bacteria. Ammonia oxidizing bacteria and nitrite bacteria had a similar trend in single fertilization treatments and they were reduced with the increasing nitrogen and phosphate application levels, while aerobic free-living nitrogen-fixing bacteria showed opposite responses. Inorganic phosphate solubilizing bacteria were more sensitive to nitrogen fertilizer than to phosphate fertilizer. Phosphate fertilizer decreased the abundance of organic phosphate solubilizing bacteria and promoted organic phosphorus mineralization. There were correlations between soil nitrogen-transforming bacteria, phosphate-solubilizing bacteria and soil physical and chemical parameters. The abundance of ammonia oxidizing bacteria was significantly negatively correlated with nitrate content, while positively with pH and field water-holding capacity. The abundance of aerobic free-living nitrogen-fixing bacteria was significantly positively correlated with soil total phosphorus, available phosphorus contents and field water-holding capacity. The abundance of nitrite bacteria was positively correlated with total nitrogen content. The abundance of denitrifying bacteria was significantly positively correlated with soil water content. The abundance of inorganic phosphate solubilizing bacteria was positively correlated with total nitrogen content, but negatively with soil water content. The abundance of organic phosphate solubilizing bacteria was positively correlated with total organic carbon content. Redundancy analysis results showed that soil total phosphorus content had the strongest effect on the abundance of soil nitrogen-transforming bacteria and phosphate-solubilizing bacteria, accounting for 33.2% of the variation in soil bacterial abundance (F=13.9,P=0.002). Our results indicated that the abundance of soil nitrogen-transforming bacteria and phosphate-solubilizing bacteria was significantly affected by the long-term fertilizer management in the rainfed cropland of the Loess Plateau, and that there was a closely synergistic effect between soil nitrogen-transforming bacteria and phosphate-solubilizing bacteria.

Key words: long-term fertilization, Loess Plateau, nitrogen-transforming bacteria group, phosphate-solubilizing bacteria, cropland ecosystem.