Long-term effects of biochar on trace metals accumulation in rice grain: A 7-year field experiment

Abstract

Biochar (BC) is an emerging amendment for minimizing the phytoavailability of trace metals in soil-crop system, but its relevant durability and mechanism are unclear. A 7-year field study was conducted in double rice paddy land of mid-subtropical China. Wheat straw BC was once applied at rates of 24 and 48 Mg ha−1 (LBC and HBC, respectively). Results showed that the increase in soil total organic C (TOC, 41.1–54.6%), pH (0.41–0.86 units) and cation exchange capacity (13.7–23.3%) for the HBC was kept for 14, 11 and 4 seasons (p < 0.05), respectively. Meanwhile, the reduction in soil CaCl2-extractable Cd, Pb, Zn and Mn (29.5–61.6%, 38.2–81.0%, 36.5–70.7%, 19.1–56.7%, respectively, p < 0.05) was maintained for 9–11 seasons. And, the reduction in grain Cd, Pb, Zn and Mn (23.0–59.5%, 15.2–49.8%, 17.5–22.9%, 13.4–34.2%, respectively, p < 0.05) was maintained for 4–6 seasons. Similar but weaker effects were observed for the LBC. And, their durable periods (except for TOC) for the LBC were about 1/3–1/2 of those for the HBC. Phytoavailability of Cd, Pb, Zn and Mn in soil-rice system was negatively correlated with soil pH and TOC (p < 0.05). Therefore, the immobilization durability of the BC on trace metals in soil-rice system are mainly attributed to the primary fixation of stable microstructure and functional groups, and mediated by the associated changes in soil pH. Alkaline materials should be taken as supplemental agents to strengthen the immobilization durability of the BC on trace metals in soil-rice system.