Biochar Alleviates Heavy Metal Contamination in Paddy Fields- A Review

Rice (Oryza sativa L.) is an indispensable food for Asian countries and is grown on about 11% of world's agricultural land. Since rice cultivation requires long term flooding, the plant and paddy soils easily accumulate heavy metals (HMs), which are sourced from various compartments and affect its productivity, especially in the mining areas. Furthermore, with a rapidly increasing population, the major concern of current situation is to find a balance between environmental sustainability, and rice production in the areas limited with toxic metal stress. HM contamination in crop field has become a global concern as it causes direct impact on human health due to its non-degradability, biomagification, and consequent bioaccumulation in almost all trophic levels of the food web. Presence of non-essential (e.g. Cd, Pb, As, Hg, and Cr) and excess of essential (e.g. Cu, Zn, Fe, Mn, V, Mo, and Se) HMs have negative impact on rice grain quality and quantity. Therefore, it is imperative to manage the bioavailability of HMs to limit their entry into the food web. Biochar is a pyrolysed plant biomass with its wide use to promote soil carbon sequestration, metal chelation, and nutrient (e.g. P, N, and K) availability to crop plants. Soil microbial diversity, crop growth, and yield are positively influenced by biochar amendments. A number of reports are also available increased sorption of HMs and their decreased availability to crop plants in the agroecosystems. However, there is no comprehensive review to date with critical analysis of application of biochar and its impact in managing HM stress, especially in rice field soils. The present review aims to summarize published reports on the effect of biochar application for alleviating the HM stress in rice field. This review has focused on the hitherto achievements in the biochar amendments for bioremediation of metal toxicity and suggested future research on field application. The mechanism of action of biochar in regulating the bioavailability and mobility of HMs has also been evaluated.