Potential risk of cadmium in a soil-plant system as a result of long-term (10 years) pig manure application

Animal manure may be a primary source of cadmium (Cd) to Chinese farmlands because abnormally high values of Cd were observed in various manures. In this study, we evaluated the potential risk of Cd in soil-plant (maize and soybean) system as a result of the long-term (10 years) application of pig manure (PM). During 10 years, the loading rate of Cd through PM application ranged from 26.33-131.50 g/ha/year, while the crops removal rate of Cd was relatively small in comparison to the quantity of Cd supplied by PM application (1.03-4.36 g/ha/year). The PM application significantly increased total Cd concentration in soil. Although the Cd levels did not exceed the Chinese soil quality criteria (1.0 mg/kg dry matter (DM)), it would only take less than two years to reach this limit at high PM application rate. The same trend was also observed for the Cd concentration in maize and soybean. More seriously, Cd concentration in grain of soybean was higher than the threshold values for animal and human ingestion (0.2 mg/kg DM). Based on a mass balance calculation, we found that atmospheric deposition was also an important source of Cd in the experimental area (10.27 g/ha/year). Moreover, the application of PM enhanced the leaching loss of Cd, but they were still fairly small (0.34-0.73 g/ha/year).