Effects of exogenous ferrous on rice growth and soil microbial activities

In this study, the effect of exogenous ferrous (Fe2+) on the rice physiological indices, soil microbial activities and its bioecological characteristic at seedling and tillering stage in the interaction system between soil-rice-ferrous and microbe was investigated by a pot experiment in greenhouse, where the ferrous toxicity in waterlogged paddy soil was simulated. Ferrous with six different levels including 0, 100, 200, 400, 800 and 1600 mg/kg was applied respectively in paddy soil. The results showed that the addition of exogenous Fe2+ gradually inhibited the growth of rice and decreased the activities of soil microorganism, while the background concentration of Fe2 + in the normal tested paddy soil was 207.77 mg/kg. The plant height and dry matter accumulation of rice decreased significantly when the concentration of exogenous Fe2+ exceeded 100 mg/kg. However, SPAD value representing the content of chlorophyll in the leaf of rice, proline accumulation and activities of antioxidant enzyme systems increased obviously, suggesting that exogenous Fe2+ 100 mg/kg was the critical concentration leading to damage on growth in this experiment. Meanwhile, soil microbial activities declined rapidly and then steadily decreased with the increase of the concentration of exogenous Fe2+. The critical level in decreasing soil microbial activities can be presumed as 100 mg/kg of exogenous Fe2+ 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-ferrous and microbe were significantly associated and synchronously affected by the ferrous in tested soil. Overall, exogenous Fe2+ of 100 mg/kg was the critical concentration in inhibiting rice growth and soil microbial activities in tested soil. Therefore, based on this result, the concentration of ferrous toxicity in tested paddy soil can be presumed as about 300 mg/kg, which includes the background level. Some reasonable agronomic measures should be considered in controlling the negative effects when the concentration of ferrous iron exceeds such concentration in the test paddy soil.