Response of soil fertility and Cu and Cd availability to biochar application on paddy soils with different acidification levels

The activation of toxic heavy metals caused by soil acidification has shown detrimental effect on rice safety and quality in paddy soil of southern China. Previous reports have shown that biochar had great potential in controlling heavy metal pollution in soil, but the effect of biochar on soil fertility and heavy metal availability in different soil acidity levels remains unknown. In our study, rice straw (RSBC) and rice husk (RHBC) were selected to produce biochar. An incubation experiment with three biochar application rates, namely 0 (CK), 20 g kg−1 (C1), and 50 g kg−1 (C2) combined with four soil acidification levels, pH value 4.09 (L1), 4.48 (L2), 4.68 (L3), and 5.04 (L4), was conducted in this study. The soil pH, total nitrogen (N), soil organic matter (SOM), available phosphorus (P), available potassium (K), available Cu and Cd concentrations were determined after incubation for 60 days. Results showed that the RSBC is greater in improving the pH and nutrients in acidified soil than RHBC, and the increase depends on biochar dosage. Compared with RHBC, the RSBC showed greater performance in decreasing the availability of Cu and Cd in four soil acidity levels (L1, L2, L3, and L4); the available Cu reduced by 41.32, 25.09, 21.42, and 5.80%, and available Cd by 15.69, 13.73, 17.65, and 17.65%, respectively. In addition, higher soil acidification degree induced smaller increase range of soil pH and available P concentration by biochar. We suggest that biochar can be applied in acidified paddy field to remediate heavy metal pollution in Southern China rice production. At the same time, we need to further ascertain the impact of these two biochars on the bioavailability of heavy metals in different soil acidity levels.