Effect of long-term application of organic fertilizer on Cu, Zn, Fe, Mn and Cd in soil and brown rice

A long-term fertilization experiment, designed to have five application treatments of same rates of N, P, K, with or without incorporation of different organic fertilizers to the field, was carried out to determine the effects of these treatments on the concentrations of Cu, Zn, Fe, Mn, and Cd in soil and brown rice. Results showed that soil total Cu, Zn and Cd concentrations in the treatments with incorporation of rice straws (NPKS), 2 folds rice straws (NPKS2), Chinese milk vetch (NPKG) and pig manure (NPKM) were higher than those in the treatment with only application of N, P and K fertilizer (NPK). Compared with the treatment NPK, soil total Cu, Zn and Cd in the treatment NPKM increased by 53.6%, 23.6%, and 406.2%, respectively; however, no significant difference was observed from each other in total soil Fe and Mn concentrations. Likewise, long-term application of organic fertilizers increased available soil Cu, Zn and Cd in the treatments NPKS, NPKS2, NPKG and NPKM. Especially for the NPKM treatment, available soil Cu, Zn and Cd concentrations significantly increased by 335.9%, 320.8%, and 421.4%, respectively, compared with the treatment NPK. The Cd concentrations in brown rice in the treatments NPKS, NPKS2, NPKG and NPKM were higher than the upper limit (> 0.20 mg/kg) of the National Standard for Food Hygiene for Rice Cd concentration. In order to guarantee food safety, much attention should be paid to long-term application of organic fertilizer such as animal manure to the field.