Variability of copper availability in paddy fields in relation to selected soil properties in southeast China

Copper (Cu) is an essential nutrient element for plant growth and is a toxic heavy metal in excess concentrations. As such, its concentration and availability in soils are of great agricultural and environmental concern. Availability and spatial pattern of copper in relation to selected soil properties in surface soils were evaluated for an agricultural region in southeastern China. A total of 224 topsoil samples (0–15 cm) were collected from paddy fields in a study area of 731 km2. We measured total Cu and DTPA-extractable Cu (available Cu) concentrations, soil pH, soil organic matter content (SOM), total nitrogen, available phosphorus, available potassium, and cation exchange capacity (CEC). We estimated Cu availability by calculating the ratio of available Cu to total Cu concentration. The results of our chemical analyses indicated that both total Cu and available Cu concentrations had a wide range throughout the study area. In addition, we measured slight Cu accumulation in paddy fields of the study area in comparison to background levels at Zhejiang Province scale. Correlation analysis revealed that available Cu concentration was positively correlated with total Cu concentration, CEC and SOM as indicated by moderate to high correlation coefficients (r = 0.64–0.82), and Cu availability was directly correlated with SOM, pH and Cu concentration with moderate to high positive correlation (r = 0.47–0.82) at 0.01 level of significance. Spatial distribution maps illustrated that total Cu concentration and available Cu concentration had similar distribution trends with the highest concentrations in the northeast region and low concentrations in the southwest region of the study area. Copper availability ratio had a spatial distribution trend with high ratios in the northeast region and low ratios in the central region of the study area. Soil properties influencing the spatial distribution of Cu availability were SOM and pH, in addition to the concentration of available Cu.