Distribution and Speciation of Chromium and Cadmium in an Organic and Inorganic Fertilized Chernozem

Fertilizers are widely applied in agricultural practice to improve crop yield and quality. However, they can also alter the behavior of soil pollutants. A field experiment was conducted on a chernozem in Heilongjiang Province, China. Various proportions of inorganic nitrogen (N), phosphorus (P), and organic (manure) fertilizers were applied. Soil samples were collected four times over 2 years, and the total concentrations and speciation of chromium (Cr) and cadmium (Cd) were analyzed. Inorganic fertilizer addition had little effect on the total concentration of Cr and Cd, while organic fertilizer addition reduced the total concentrations of both Cr and Cd. Inorganic fertilizers increased the concentration of Cr in the exchangeable form, but decreased that of the residual form (P < 0.05). Addition of inorganic or organic fertilizer alone decreased concentrations of exchangeable Cd (P < 0.05), but the combination of inorganic and organic fertilizers had the opposite effect. High concentration of total Cd exceeding the upper limits of the Environmental Quality Standard for Soils proposed by the Ministry of Environmental Protection of China could result in an environmental risk, and fertilizer application could lower that risk. Results of risk assessment code calculated as the percentage of the sum of metal concentrations in the exchangeable and carbonate-associated form in total metal concentration showed that Cr and Cd risks were lowered when organic fertilizer was applied alone or in combination with inorganic fertilizers. Our study highlighted that organic fertilizer should be applied alone or in combination with inorganic fertilizers to lower the environmental risks of Cr and Cd pollution in the chernozem.