Impacts of biochar and silicate fertilizer on arsenic accumulation in rice (Oryza sativa L.)

Human exposure to arsenic (As) through rice consumption is a global food safety issue, especially in Southeast Asia. To investigate the impacts of biochar amendment (rice husk and smooth cordgrass-derived biochar) and/or silicate fertilizer on As mobility/phytoavailability in soil and on As accumulation in rice, pot and microcosm experiments were conducted. The results showed that both single application of low doses of biochar (0.5%, w/w) and coapplication of biochar with silicate fertilizer decreased As levels in grain (brown rice) by 14–16%, but not in straw and roots. The biodilution of As in grain resulting from increased grain biomass (by 6–21%) could be mainly a response to the decline in grain As levels with biochar and/or silicate fertilizer amendment. However, both applications exerted limited effects to decrease the overall As uptake by rice grain and straw, potentially due to the small changes in As mobility/phytoavailability in amended soil relative to the control, although plant-available silicon (Si) from amendment could potentially inhibit As uptake. Furthermore, microcosm-based anaerobic incubation experiments demonstrated that As levels in soil solution increased (up to 11–14-fold) with increasing doses of biochar amendment (up to 5%, w/w), possibly due to biochar enhancing the reductive dissolution of iron (oxyhydr) oxides via an increase in the total number of iron-reducing bacteria (up to 1.6–3.2-fold). Our findings suggested that a low application rate of biochar may not be a very effective approach for mitigating As accumulation in rice, while a high application rate could enhance the health risk of As in As-contaminated flooded soil.