Zinc and iron enrichment of vermicompost can reduce the arsenic load in rice grain: An investigation through pot and field experiments

Abstract

The heavy metalloid arsenic (As), occurring in both trivalent and pentavalent is extremely toxic and has detrimental effect on humans through water-soil-crop transfer. Previously organic and inorganic amendments have been used separately for mitigation of As in rice but there exists a research gap regarding use of them simultaneously. In this study, both pot and field scale investigations were undertaken for four consecutive years in As-contaminated locations to assess the efficacy of zinc (Zn) and iron (Fe)-enriched vermicompost in reducing the As uptake in rice grain. Altogether seven types of vermicompost and three application rates were evaluated. The treatment combination V4D1 (enriched vermicompost V4 applied to soil at 3 t/ha) recorded the lowest soil available As (2.525 mg kg−1) and the highest soil available As (2.982 mg kg−1) was observed with V5D3 (enriched vermicompost V5 applied to soil at 1.5 t/ha). Application of enriched and non-enriched vermicompost reduced As in grain by 58.14 and 31.40% respectively over no vermicompost (control). The partial dependence plot from stepwise regression modelling of different fractions of As revealed that an increase in organically bound As resulted in a decrease in the availability of As and hence uptake by rice. Further, Zn and Fe-enriched vermicompost resulted in increase of iron plaque formation on the root. A significant positive relationship (r = 0.462) was observed between dithionite-citrate-bicarbonate (DCB) extractable -Fe andAs. A significant negative correlation (r = ˗0.410) between DCB-Fe and grain As, advocates better root plaque formation resulting a higher capacity to sequester As onto the root surface and reducing its's entry into the rice system. The carcinogenic risk somewhat was benign (TCR of 2.69 × 10−3 and SAMOE of 0.101) against no vermicompost (TCR of 6.64 × 10−3 and SAMOE of 0.04). Therefore enriching vermicompost with ZnSO4 and FeSO4 at 10% dry weight basis (V4) of the composting substrate can lower arsenic build-up in rice grains without affecting yield.