Comprehensive cadmium input-output mass balances in two contaminated paddy fields: Implications for soil pollution...

Soil cadmium (Cd) contamination poses a serious threat to food safety and human health. Quantifying Cd inputs and outputs accurately is necessary for revealing the causes of contamination and for predicting future trend. A two-year field experiment was conducted in two heavily Cd-contaminated paddy fields in southern China to quantify Cd inputs and outputs. Irrigation water was the main source of Cd contamination, contributing 122-163 g ha^-1 and approximately 95% of the total Cd inputs. Crop removal and leaching represented 12-47% and 53-88% of the total Cd outputs, respectively. Retention of rice straw as stubbles reduced crop Cd removal considerably, whereas mid-season draining followed by reflooding increased Cd leaching flux substantially. The Cd mass balance was positive in one field and negative in another, with the difference being caused by different levels of Cd contamination, soil pH and cropping systems (single versus double rice cropping). Cd concentrations in brown rice exceeded the national Cd limit (0.2 mg kg^-1) by 5-35 times, indicating a high risk to food safety. To protect paddy soils from contamination, Cd inputs, especially from irrigation water, must be reduced markedly.