Effects of Different Organic Amendments on the Activity of Heavy Metals in Contaminated Soils

In order to understand the effects of the application of organic amendments on the behavior of heavy metals in contaminated farmland soils, field plot experiment was conducted to investigate the effects of the addition of six different organic amendments on the activity of heavy metals in soils, concentration of heavy metals in surface water, and heavy metal uptake by rice plants. The tested six organic amendments were composted rice straw, fresh chicken manure, composted chicken manure, fresh pig manure, composted pig manure and commercial organic fertilizer. The results showed that application of all of the six organic fertilizers could obviously increase the contents of water soluble heavy metals in the soils and concentrations of heavy metals in the surface water at early stage of the experiment, as compared with the control without application of organic manure. The increase in the contents of water soluble heavy metals in the soils and concentrations of heavy metals in the field surface water were highest for fresh pig manure, followed by fresh chicken, and the lowest for commercial organic fertilizers. The contents of heavy metals in soils and the concentrations of heavy metals in field surface water decreased with the increase of the experimental time. The effects of applying organic fertilizer on the accumulation of heavy metals in rice grains was not obvious. Correlation analysis showed that the contents of water soluble heavy metals in soils, and concentrations of heavy metals in field surface water were all significantly correlated with the corresponding DOC concentration, suggesting that the increase of DOC could promote the activation of heavy metals in soils. The difference in the activity of heavy metals in soils added with different organic amendments might be due to the difference in the degree of decomposition and release DOC capacity of the added organic amendments. But the research also showed that the soil DOC decreased gradually with the increasing time of organic fertilizer application. Therefore, the activation of heavy metals in farmland soils caused by organic manure was temporary, and the accumulation of heavy metals in the grain during the late rice growth was not obvious.