Objectives Soil enzyme activity is closely related to soil nutrient turnover and sometimes is used to evaluate the fertility of soil. We investigated the effect of typical winter green manures on soil enzyme activities in south China.

Methods The localized field experiment in red paddy soil from 2015 located in Jinxian county, Jiangxi Province. At maturing stages of late rice in 2018 and 2019, soil samples were collected in the treatment plots of winter fallow (CK), planting in winter and incorporation of milkvetch (Astragalus sinicus L.), and rape (Brassica napus L.). The organic matter and available N, P and K content were measured, and the activities of urease (Urea), α-glucosidase (AG), glucosaminidase (NAG), β-1,4 glucosidase (BG), β-cellobiosidase (CBH), β-xylosidase (BXYL), acid phosphatase (ACP), phenoloxidase (POX) and peroxidase (PER) were analyzed. The importance of enzyme activity driving soil fertility change was evaluated using redundancy analysis (RDA). And the relationship between soil enzyme activity, soil fertility and rice yield were analyzed using partial least squares path mode (PLS-PM).

Results Compared with CK, both rape and milk vetch treatment increased the annual rice yield by 15.05%−15.10% and 11.60%−11.82% in 2018 and 2019 respectively, and increased soil microbial biomass carbon and nitrogen, available N, P and K content, but there was no significant difference between the two treatments. The rape incorporation was more efficient than milkvetch in enhancing soil enzyme activities in the two years, although the effect was not completely consistent in the two years. In 2018, the Urea, AG and CBH activities in rape treatment were 22.57%, 17.59% and 64.89% higher than in milkvetch, and in 2019, the Urea, AG, BG, CBH, ACP and POX showed higher activities of 54.24%, 8.19%, 28.84%, 28.05%, 64.68% and 56.46% in rape treatment. The results of RDA indicated that AG and CBH activity were the key factors driving soil fertility change. Moreover, PLS-PM analysis showed that soil enzyme activity and microbial biomass carbon and nitrogen regulated rice yield by affecting soil organic matter, N, P and K contents; the influence of enzyme activity on soil N, P and K contents was obviously higher than on soil organic matter.

Conclusions The planting and turnover of winter green manures induce the change of soil enzyme activities, the activities of AG and CBH play key roles in driving soil fertility change in red paddy soil, and soil enzyme activity regulate rice yield through enhancing soil organic matter and available nutrient contents. Planting in winter and incorporation of rape is more efficient than milkvetch in increasing soil enzyme activities.