EC₅₀ of soil S^2- inhibiting rice growth and soil microbial activities

【Objectives】S2- is one of the important limiting factors for rice growth in cold waterlogged paddy fields which are typical low yield paddy fields. Characteristics of ecological toxicity of S2- to system of soil-rice under S2- stress were investigated in order to provide a theoretical basis for future research to reveal the dose critical value of S2- toxicity and its toxic mechanism in cold waterlogged paddy soil. 【Methods】 Effect of exogenous S2- treatment on rice physiological indices(rice physiological growth, chlorophyll, proline and the activities of the antioxidant enzyme system in the rice leaves), soil microbial activities and their bioecological characters at the seedling and tillering stages in the interactive system of soil-rice-S2- and microbe were studied by a greenhouse pot experiment which is set to simulate S2- toxicity in waterlogged paddy soil. A single season conventional late japonica rice Xiushui 134 was selected as crop material, and S2- was applied at seven different levels in one paddy soil, namely 0, 10, 50, 100,300, 600 and 1200 mg/kg. 【Results】 The addition of exogenous S2- gradually inhibits the growth of rice and decreases soil microbial activities in the normal tested paddy soil in which the background S2-concentration is 12.08 mg/kg. The plant height and dry matter accumulation of rice decrease sharply when the exogenous S2- concentration exceeds 50 mg/kg. However, SPAD value of chlorophyll content in rice leaves, proline accumulation and activities of antioxidant enzyme systems are increased significantly. These results suggest that exogenous S2- of 50 mg/kg is the critical concentration under the tested condition to poison rice growth. Meanwhile, soil microbial activities are declined rapidly with the increase of the concentration of exogenous S2-. The turning point of decreasing soil microbial activities can be presumed as 40 mg/kg of soil S2- concentration by analyzing its toxicant concentration that inhibits a microbe-mediated ecological process by 50%(EC50). Soil microbial activities and rice growth in the system of soil-rice-S2- and microbe are significantly associated and synchronously affected by S2- in the tested soil. Overall, soil S2- of 40 mg/kg is the critical concentration to inhibit the system of soil-rice-soil microbe in the tested soil. 【Conclusion】 When the concentration of S2- in soil exceeds a certain concentration, S2- would lead to significantly negative effects on the system of soil-rice-soil microbe. Soil S2- of 40 mg/kg is the critical concentration to inhibit the system of soil-rice-soil microbe in the tested soil. Therefore, some reasonable agronomic measures should be taken to control the negative effects once the concentration of S2- is over 40 mg/kg in the test paddy soil.