Effect of Mn-Modified Biochar on the Characteristics of Aggregate Structure and the Content of Cd in Weakly Alkaline Cd-Contaminated Soil

Abstract

Field experiments were conducted to investigate the effects of Mn-based modified rice husk biochar on soil organic carbon, aggregate structure, mass load, and the content of available Cd in aggregates. The results showed that the concentration of soil organic carbon increased gradually with additional modified biochar, resulting in a 3.2%-32% increase compared with the CK. Modified biochar improved the composition structure and stability of soil aggregates, and increased the amount of large aggregates (5-8 mm and 2-5 mm), while micro-aggregate production was inhibited (≤0.25 mm). Compared with the CK, mean weight diameter (MWD), geometrical mean weight (GMD), and soil aggregate structural body (R0.25) of the soil aggregates increased by 15.1%-20.3%, 8.1%-22.4%, and 0.43%-7.6%, respectively. Cd was preferentially enriched on small aggregates, and the mass loading of Cd in soil aggregates increased gradually with decreasing particle size. The distribution factor of Cd showed significant enrichment in the 0.5-1 mm and 0.25-0.5 mm grain sizes, whereas it showed a clear loss in particle sizes of 5-8 mm and 2-5 mm. The addition of modified biochar reduced the content of DTPA-Cd in soil aggregates, causing 7.6%-15.1%, 15.6%-24.3%, 3.6%-13.8%, and 11.6%-13.7% reductions in 5-8, 2-5, 0.5-1.0, and 0.25-0.5 mm particle sizes, respectively. In general, modified biochar not only has a favorable optimizing function on the structure of soil aggregates, but also decreases the content of available Cd in different sizes of soil aggregates, increasing its significance in the remediation of Cd-contaminated soil.