Cadmium accumulation in rice straws and derived biochars as affected by metal exposure, soil types and rice genotypes

Straw residues, as one of the conservation farming practices, are being strongly encouraged in China, including some cadmium-polluted areas. Nowadays, a portion of this plant residue is promoted to be removed for reducing excess metal(loid) in the soil and to be used for bioenergy production. Nevertheless, the possible influences of contaminated straw or the burial of its derived biochars on Cd accumulation in soil and data based on health risk assessment associated with different status and extent of soil contamination were relatively unknown. Thus it is important to provide a more systematical understanding of contaminated straw burial at specific contamination zones, which may provide useful guidance for straw utilization. In this study, we harvested two genotypes of rice straw from 6 contaminated levels among three soil types to comprehensively study the total Cd contents in straws and its derived biochars and correlate the sets of straw characteristics and Cd contents in three different contamination zones. The total Cd concentration in straws grew at a steady rate relatively with increasing soil Cd contamination levels, compared to those in biochars which performed more fluctuate due to the strong burning. According to correlational analyses, three-way ANOVA showed that the moisture, ash, volatile and fixed carbon were all significantly affected by straw Cd_Total contents (p < 0.001). Such relationships were attributed to guide straw removal portions for gasification. Meanwhile, there was a significant correlation between straw Cd concentration and soil types (p < 0.001), confirming that it might be worth determining soil remediation by straw removal according to site-specific farmland conditions. This work will help to assess efforts toward predicting Cd concentration in the paddy soils related to kinds of contamination status and would also give useful guidance to make sustainable management strategies for crop straws in polluted regions.Novelty statement This work provided data on how much rice straw is needed to remove to ensure the minimal amount to control soil contamination and reduce costs according to site-specific conditions and soil Cd contamination status. It also explains the correlations between straw characteristics related to bioenergy use and soil conditions which would give guidance to balance using crop straw for increased bioenergy production and the need to also protect, preserve, and enhance soil resources.