Abstract
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•Study showed how changes in redox and SEP pools can control As availability.•As bound to Fe oxide phases remained intact in aerobic soil conditions.•Fe amendment enhanced the retention of As in amorphous, crystalline Fe oxide phases.•As fractions in soil solid phases can preferably model As availability to rice.•Cd response in rice to cultivation methods was different in wet and dry seasons.
Solid-phase As speciation and bioavailability during rice paddy cultivations depend on changes in cropping techniques and types of soil amendments. Field trials tested the impact of agronomic factors on As fractionation and resulting availability to rice, including aerobic (non-flooded) vs anaerobic rice cultivation (flooded); NPK, calcium silicate, ferrous sulfate, farmyard manure (FYM) and vermicompost (VC) addition. A modified sequential extraction procedure was used to determine As concentrations in the following soil fractions: exchangeable (F1); specifically sorbed (F2); amorphous iron oxide (F3); crystalline iron oxide (F4); organic matter and sulfide (F5); and residual bound (F6). Significant proportions of As were determined in F3, F4, and F6. Exchangeable fraction (F1) contained the lowest proportion of As. Aerobic cultivation practices significantly reduced soil As availability in F1 and resulted in higher retention of As in F2, F3, and F4. Application of ferrous sulfate increased As concentrations in F3 and F4 but when combined with FYM and VC, significantly lowered As concentrations in F1; a potential benefit for rice quality and consumer’s health. Linear regression analyses show that As concentrations in F1, F2, F3, and F4 are the most important factors influencing As accumulation in rice. Arsenic concentration in F1 is a good predictor of As accumulation in rice grain. The phytoavailability coefficient (PC) based on As retained in different fractional pools was determined. The addition of calcium silicate and ferrous sulfate under aerobic conditions reduced the PC in the soil, which could be responsible for reduced As accumulation in rice. There were insignificant variations in rice grain Cd concentrations between aerobic and anaerobic cultivations for monsoon trials, but significantly higher Cd concentrations were detected under aerobic cultivation than flooded cultivation for the post-monsoon trials. These data show that water management under different rice cropping practices can influence mobility and bioavailability of elements like As and Cd during rice paddy growth and can inform agricultural policy intended to attenuate element toxicity in edible crops, reducing their introduction into the food chain.