Soil Threshold Values for Zn Based on Soil-Rice System and Health Risk Assessment in a Typical Carbonate Area of Guangxi

Zinc (Zn) is enriched in carbonate area related to geological genesis. To ensure safety of rice, soil threshold values of Zn in soil–rice systems were assessed based on analysis of soil–rice Zn concentration in relation to human health risk. Models for the prediction of Zn concentration of early-season and late-season rice grain were accurately established on the basis of significant partial correlations between log 10 (BAFs) and log 10 (soil properties). The rice threshold value ranged from 10.67 to 37.90 mg/kg, which might not suitable for male and urban residents. The soil safety threshold of early-season rice and late-season rice in carbonate area ranged from 148–200 mg/kg, 119–200 mg/kg with pH below 6.5, 148–250 mg/kg, 119–250 mg/kg with pH ranging from 6.5 to 7.5; 148–300 mg/kg, 119–300 mg/kg with pH above 7.5, respectively.