Effects Of Consecutive Turnover Of Milk Vetch (Astragalus Sinicus L.) On Paddy Soil Microbial Properties

Soil quality is the basis of sustainable development of agricultural ecosystem, and soil microbial properties are important characterizations of soil quality. In this study, we investigated soil microbial properties such as soil enzyme activites, soil microbial biomass, soil microbial diversity and the relationship between soil quality in order to estimate the effects of consecutive turnover of milk vetch on paddy soil microbial properties as well as illustrate the function of soil microbial properties as bio-indicator of soil quality. Short-term experiment based on rice-milk vetch rotation system was carried out since 2008 in Baisha, Minhou, Fuzhou province, which includes six treatments of control (CK),100% chemical fertilizer (NPK), milk vetch (18000kg/hm2)+100% chemical fertilizer (NPKM1), milk vetch (18000kg/hm2)+60% chemical fertilizer (NPKM2), milk vetch (18000kg/hm2) alone (MV), milk vetch (18000kg/hm2)+40% chemical fertilizer+straw (NPKMS). To assess whether seasonal fluctuations of measured properties might mask the effects of fertilization, soil samples were collected three times within a growing season (1th, April,2012; 8th, August,2012; 13th, October,2012). The main results obtained were as follows:(1) Crop yield. Compared to CK, treatments with fertilizer significantly increased rice grain yield by 10.7% to 25.0%, and the highest in NPKM1, NPKMS treatments. Chemical fertilizer combined with milk vetch or straw effectively promoted the increasement of rice grain yield and reduced the use of chemical fertilizer, rice grain yield of NPKM1 was 6.6% higher than NPK, while NPKMS decreased 60% chemical fertilizer dosage.(2) Soil fertility. Compared with CK and chemical fertilizer alone, chemical fertilizer plus milk vetch increased the content of soil organic matter (OM), total nitrogen (TN), ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3--N), water-soluble organic carbon (WSOC), available K (AK), and available P (AP) deficiency in all treatments was seen after 4 years of rice planting except NPKMS treatment. Soil pH under chemical fertilizer application were slightly lower than CK and MV treatments. However, there was no acidification trend observed in the surface soil by chemical fertilizer utilization.(3) Soil enzyme activities and microbial biomass. Chemical fertilizer with milk vetch incorporated greatly enhanced soil microbial biomass carbon (SMBC), soil microbial biomass nitrogen (SMBN), soil enzyme activities (urease activities, invertase activities, acid phosphatase activities). Chemical fertilizer alone (NPK) obviously decreased the content of SMBC and SMBN, while milk vetch alone (MV) lowered qMB.(4) Microbial community structure. Denaturing gradient gel electrophoresis (DGGE) patterns revealed that the fertilization resulted in an obvious change in the bacterial and ammonia-oxidizing bacterial (AOB) community structure but little effects on the community structure of fungi and ammonia-oxidizing archaea (AOA). For bacterial community structure, turnover of milk vetch increased the abundance of Proteobacteria, however, decreased the abundance of Actinobacteria. Shannon index (H) and richness (5) of NPKM1 and NPKM2 were highest in all treatments. Redundancy analysis (RDA) revealed that the AOB community structure in the fertilization treatments was significantly different from control treatment (CK), and the values of diversity index in fertilization treatments observably higher than CK, indicating that fertilization had a great impact on the AOB community structure. Nevertheless, there was no significant change caused by milk vetch.(5) Population of microorganisms. Turnover of milk vetch changed the population of soil cultivable microorganisms, the population of bacteria, fungi in NPKM1 and NPKMS treatments were significantly bigger than other treatments. Compared to NPK treatment, chemical fertilizer plus milk vetch or straw increased the amoA gene copy numbers of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA). AOA were more abundant than AOB, the ratio of AOA to AOB ranging from 2.5 to 52.6. Moreover, the population size of AOA stimulated by ammonium (NH4+) and significant correlation was found between archaeal amoA gene copy numbers and ammonium content, implying that AOA probably drove ammonia oxidation in acidic paddy soil. The ratio of AOA to AOB was remarkably related to pH and rice grain yield, respectively, and enhanced by chemical fertilizer.(6) Soil enzyme activities, microbial biomass and the population of cultivable microorganisms were significantly related to the content of soil basic nutrients (TN, AN, WSOC, NH4+, NO3-, AK) and rice grain yield, respectively, indicating that all of the microbial properties together reflected the level of paddy soil fertility and productivity and could be used as potential bio-indicators for soil quality evaluation.