Aman Ullah

In order to investigate the effects of Cl⁻ and SO₄²⁻ based fertilizers on the accumulation of cadmium (Cd) in rice plants, a long-term experiment, which has been conducted since 1975, and a short-term pot experiment were designed. The results of the long-term experiment showed that the highest total grain Cd was found in the treatment of fertilizers with rich Cl⁻, which was 72.7% higher compared to conventional fertilization (CF). However, there was no significant difference between the CF and fertilization with rich SO₄²⁻ treatments. This phenomenon can be explained by the concentrations of the EDTA extractable Cd being significantly increased by 60% under Cl⁻ treatment, while SO₄²⁻ treatment showed no significant effect. In the short-term trial, compared to CF, Cd concentrations in the roots increased by 1.07 and 0.93 times in the Cl⁻ and SO₄²⁻ treated soils, respectively, under Cd1.2 exposure. Meanwhile, Cd concentrations in the shoots enhanced by 96% in Cl⁻ treated soil but decreased by 34.6% in SO₄²⁻ treated soil. It was therefore concluded that fertilizer-borne Cl⁻ significantly increased the Cd concentration in rice grains in the long-term experiment, but fertilizer-borne SO₄²⁻ had no significant effect on the Cd concentration in rice grains. However, in the pot experiment, SO₄²⁻ based fertilizers decreased Cd transport to the shoots of a rice plant grown in a Cd contaminated soil. These findings will improve the rational fertilization of Cd contaminated soils and the production of safer rice.

Although inorganic and organic manures with high concentrations of heavy metals can lead to accumulation or contamination of heavy metals in soils, there are few reports on the effects of long-term application of swine and green manures on the accumulation of heavy metals in rice grains in paddy soils. A long-term field experiment, which was established in 1990 in paddy soil in Hangzhou, China, was used to investigate the effects of inorganic and organic manures on the availability and accumulation of heavy metals in soil and uptake by rice plant. The results showed that long-term application of nitrogen, phosphorus and potash (NPK) plus green manure or swine manure, and swine manure only increased 202%, 146%, and 100% for total Cd, and 5.5%, 7.6%, and 6.6% for total Cu in rice grains, respectively compared to the control without fertilization. Total Zn in rice grain was significantly increased by 13.9% for the treatment of NPK plus green manure. The accumulation of Cd, Zn, and Cu in rice grains after long-term application of swine and green manures is due to the combined effects of the increased concentrations of total and EDTA extractable Cd, Zn, and Cu in soil and the changes of soil properties. Furthermore, the highest bioconcentration factor for Cd was found in the treatment of NPK plus green manure while for Zn and Cu it was observed in NPK treatment. Thus, it may be concluded that green manure and manure with increased Cd, Zn, and Cu in rice grain results in a potential risk of metal accumulation in paddy soils.