Understanding the seasonal variations in active heavy metal components and sensitive impact factors is of importance for the ecological risk reduction during the agricultural production processes. Paddy fields, vegetable lands, and hilly forests were selected as three main land cover types to assess the seasonal characteristics of Cd bioavailability and reveal how or to what extent it was affected by the physiochemical parameters of soils, under different land-use types in a typical Cd-contaminated watershed in the middle and lower reaches of Xiangjiang River. One-year in situ monitoring results showed that natural rainfall pH in winter and spring was lower than in summer and autumn in the study region. The total Cd content of paddy soils was significantly higher than that of the vegetable soil, while the hilly forest soil showed the lowest total Cd value. Similar seasonal variations in total Cd content were found in three soil types with slightly lower summer and autumn concentrations than spring and winter values, but no obvious correlation was detected between the total and the available Cd components. The paddy soil available Cd concentration during the 5-9-month crop growth season was significantly lower than the other months of the year, while vegetable cultivation and hilly forest soils showed the opposite trend. Eh was the key factor that had a positive influence on the Cd activity in paddy soil. Soil TOC concentration was negatively correlated with soil activity in vegetable soil. TOC, water soluble organic carbon, showed a significant positive correlation with Cd effectiveness. The results provide scientific references for Cd contamination control and safe agricultural production.
Keywords: Cd contamination; acid rainfall region; available Cd; key influencing factor; seasonal variation.