Abstract: In order to explore the effect of combined application of chemical fertilizer on soil enzyme activity and its main driving factors in the wheat field under long-term straw returning, a four-year long-term positioning experiment in Guangshui was carried out. Six treatments were set up,including straw （S, the amount of 6 000 kg/hm², the same as other treatments）, straw+100%N （S100N, 187.5 kg/hm² N, the amount of other fertilization treatments was converted according to this amount）, straw+80%N （S80N）, straw+60%N （S60N）, straw+80%N + microbial inoculant （S80Nm, the amount of the straw decomposition microbial agent was 30 kg/hm²）, straw+60%N+microbial inoculant （S60Nm, the dosage of the microbial agent was the same as S80Nm）. Soil enzyme activity, soil physical and chemical properties and wheat yield were measured. The results showed that compared with S treatment, S100N and S80N significantly increased wheat yield by 182.82% and 179.55%（P<0.05）, respectively. The addition of the straw decomposition microbial agent had no significant effect on soil physical and chemical properties and wheat yield. Compared with S100N treatment, S80N increased the activities of soil phosphatase （Phos）, sulfatase （Sul）, β-glucosidase （βG）, β-xylosidase （βX）, α-glucosidase （αG）, acetylglucosaminidase （NAG） and leucine aminopeptidase （LAP）. Redundancy analysis （RDA） showed that soil organic matter and alkali hydrolyzable nitrogen were the main influencing factors of soil enzyme changes under the condition of straw returning. Therefore, from the point of view of reducing amount and increasing efficiency of fertilizer, S80N was a more suitable management measure to improve soil quality.

Key words: wheat field, straw returning to field, chemical fertilizer, soil physical and chemical properties, soil enzyme