Effect of turning over Chinese milk vetch (Astragalus sinicus L.) on soil nutrients and microbial properties in paddy fields

【Objectives】Chinese milk vetch（Astragalus sinicus L.）is a major green manure planting in southern paddy fields. In this study, the effects of turning over Chinese milk vetch on crop yield, soil nutrients and microbial properties were investigated to determine the appropriate application rates of chemical fertilizers.【Methods】Rice-Chinese milk vetch (MV) rotation Experiment Station was established in 2009. Five fertilization treatments with three replicates (each replicate plot was 15 m2) were designed as control (CK), 100% chemical fertilizer (100% F), 18000 kg/ha milk vetch + 100% chemical fertilizer (MV+100% F), 18000 kg/ha milk vetch + 60% chemical fertilizer (MV+60% F), 18000 kg/ha milk vetch alone (MV) . Soil samples were collected after rice harvest in October, 2012, and crop yield, soil nutrients, soil microbial biomass carbon (SMBC) and soil microbial biomass nitrogen (SMBN), number of culturable microorganisms and soil enzyme activities were determined. 【Results】1) There was no significant difference in the crop yields between the 100% F and MV+60% F treatments which demonstrated MV could substitute 40% of chemical fertilizer. 2) Compared to the 100% F treatment, the MV+F treatments could increase the contents of soil organic matter and total nitrogen, but not those of total phosphorus and total potassium; the MV+100% F treatment could increase the content of available nitrogen; MV and MV+60% F treatments could decrease the content of available phosphorus. The 100% F treatment had the highest content of available potassium. There were no significant differences in pH values from all treatments. 3) The number of bacteria in the 100% F and MV+100% F treatments were significantly higher than that in the CK treatment, but there were no significant differences between the MV+60% F, MV and CK treatments. These results imply that bacteria were obviously dependent on the soil nutrients. Chemical fertilizer with MV could increase the number of fungi and actinomyces. 4) Correlation analysis showed that the contents of available nitrogen, available phosphorus and available potassium in soils were significantly correlated with the number of bacteria, and NH+\|N content was significantly correlated with the number of fungi and actinomyces. 5) In comparison with 100% F treatment, chemical fertilizer with MV treatments could increase the contents of soil microbial biomass carbon and nitrogen. 6) Soil enzyme activities in chemical fertilizer with MV treatments were significantly higher than those in the 100% F or MV treatments. 7) Soil enzyme activities and microbial biomass were significantly correlated with the soil nitrogen content. Invertase, urease and acidic phosphatase activities were significantly correlated with the rice yield.【Conclusions】Compared to the 100% F, the MV+60%F treatment could achieve equivalent rice yield, indicating that application of 60% fertilizer could be considered as the best fertilization model in our study. Contents of soil organic matter, total nitrogen, available nitrogen, microbial properties and enzyme activities in 100% F treatment were lower than those in chemical fertilizer with MV treatmetns.