Prior decomposition of organic matter in amendments under aerobic soil conditions attenuates As dissolution after soil submersion

Application of organic amendments (OAMs) in paddy fields has several advantages. One such advantage is the supply of essential nutrients to rice plants. However, the application of OAMs enriches microbial respiratory substrates as decomposable organic matter, possibly promoting soil reduction and As dissolution after soil submersion. Therefore, this study evaluated the effect of prior aerobic decomposition of organic matter in OAM-mixed soils on the degree of soil reduction and As dissolution under anaerobic soil conditions. The soil used was from a non-polluted paddy field with a light clay texture and a moderate quantity of iron oxides. Rice straw compost and fermented chicken dung (FCD) were mixed with the paddy soil. The OAM-mixed soils were aerobically incubated for 5, 16, or 37 days. Carbon dioxide emissions during the aerobic period indicated that the organic matter in OAMs, especially FCD, was consumed by microbial aerobic respiration. Following aerobic incubation, the soils were anaerobically incubated for 10, 40, or 82 days. The addition of OAMs, particularly FCD, facilitated soil reduction and enhanced As dissolution from anaerobic soils. However, the effect of OAM addition on soil reduction and As dissolution after submersion was considerably weakened by a prolonged aerobic period because of the decrease in easily decomposable organic matter. Our results indicate that applying OAMs to soils earlier and maintaining aerobic conditions for a longer period can be an effective measure to attenuate OAM-enhanced As dissolution in submerged paddy soils. More comprehensive and field-scale investigations with various combinations of OAMs and soils are needed.