Abstract:Through a three-year field experiment, the effects of nitrogen reduction and straw returning on the yield of spring maize, soil nitrate nitrogen and ammonia oxidizing microorganisms in dryland of the southern Loess Plateau were studied. The study set up fivetreatments, including no fertilizer (CK), traditional nitrogen application (N250), traditional nitrogen application combined with straw return to field (N250+S), nitrogen reduction 20% (N200), and nitrogen reduction 20% combined with straw return (N200+S). The results showed that:(1) Nitrogen reduction by 20% and straw returning to the field for 3 consecutive years can increase maize yield and control soil nitrate leaching.Compared with N250 treatment, N200 treatment increases maize yield by 5.9%, and N200+S treatment increases maize yield compared with N200 treatment 7.4%, compared with N250 treatment, N250+S treatment increased maize yield by 9.1%; In the 0-300 cm soil layer, compared with N250, N200 reduced nitrate nitrogen residue by 51.3%. And the N200+S reduced nitrate nitrogen residue by 18.0% compared with N200. The residual nitrate nitrogen in N250+S was reduced by 41.2% compared with N250. (2) Nitrogen reduction and straw returning reduced soil nitrification potential. Compared with N250, N200 reduced soil nitrification potential by 8.8%, and N250+S reduced it by 14.2%.The soil nitrification potential in N200+S was reduced by 20.4% compared with N200. (3) Returning straw to thefield significantly increased soil microbial biomass carbon and nitrogen (SMBC, SMBN). Compared with N250, the SMBC and SMBN in N250+S were increased by 17.5% and 24.0%, respectively. Compared with N200, SMBC and SMBN in N200+S were increased by 18.4% and 31.3% respectively. (4) Nitrogen application and straw returning had little effect on ammonia oxidizing archaea (AOA), but increased the abundance of ammonia oxidizing bacteria (AOB). (5) The abundance of ammonia oxidizing bacteria (AOB) was extremely significantly positively correlated with nitrate nitrogen, ammonium nitrogen, SMBC, SMBN and nitrification potential, while ammonia oxidizing archaea (AOA) was abundant. There was no obvious correlation between degree and impact factor.These results could provide a scientific basis for nitrogen fertilizer management and agricultural sustainable development in the spring maize planting area of the Loess Plateau.