Abstract: A field experiment with four treatments was conducted in sierozem of the Gansu Yellow River irrigation area in 2017-2021, including conventional rotary tillage (RT), rotary tillage with maize straw returning (RTS), subsoil tillage (ST), subsoil tillage with maize straw returning (STS). After harvesting maize in 2017-2021, topsoil (0-20 cm) and subsoil (20-40 cm) samples were collected. Soil physicochemical properties, soil microbial biomass carbon and nitrogen, soil extracellular enzyme activities involved in soil carbon, nitrogen, and phosphorus cycling, and maize yield were measured. The results showed that compared with RT and RTS, ST and STS significantly increased maize yields, with the highest yields in STS. ST and STS significantly reduced soil bulk density and soil compaction of 20-40 cm soil layer, pH and EC of 0-40 cm soil layer, with the lowest in STS. Compared with RT, STS significantly increased SWC, SOC, TN, AHN, and AP in 0-40 cm soil layer. RTS, ST and STS significantly increased MBC and MBN in 0-20 cm soil layer, with the highest in STS. STS and RTS significantly increased MBC/SOC. In 20-40 cm soil layer, STS significantly increased MBC and MBN, and RTS significantly increased MBN. In 0-20 cm soil layer, ST, RTS and STS significantly increased the activity of BG and CBH, and STS significantly increased the activity of BXYL, NAG and AKP, while RTS significantly increased the activity of NAG. In 20-40 cm soil layer, STS increased the activity of BG, BXYL, CBH, NAG and AKP, RTS increased the activity of BG, and ST increased the activity of BXYL. Correlation analysis showed that, in 0-40 cm soil layer, MBC, MBN, SOC, TN, AHN, AP, SWC, BD, and EC showed significant or extremely significant correlation with the activities of various extracellular enzymes. Redundancy analysis showed that MBC and SOC were the main factors affecting soil enzyme activities, followed by MBN and pH. In conclusion, subsoil tillage with straw returning is the best tillage mode to improve soil physical properties, increase soil fertility, soil microbial biomass and enzyme activity, and thus enhance corn yield.

Key words: subsoil tillage, straw returning, soil microbial biomass, soil enzyme activity, soil physicochemical property