Arsenic dynamics in paddy soil under traditional manuring practices in Bangladesh

Fertilization with organic matter (farm yard manure and/or rice straw) is thought to enhance arsenic (As) mobilization into soil porewaters, with subsequent As assimilation by rice roots leading to enhanced translocation to the grain. Here, interlinked experiments (field manuring and soil batch culture) were conducted to find the effect of organic matter at a field application rate practiced in Bangladesh (5 t/ha) on As mobilization in soil for paddies impacted by As contaminated groundwater irrigation, a widespread phenomenon in Bangladesh where the experiments were conducted. Total As concentration in a paddy soil (Sonargaon) ranged from 21.9 to 8.1 mg/kg down the soil profile and strongly correlated with TOC content. Arsenic, Fe, Mn, and DOC release into soil solution, and As speciation, are intimately linked to OM amendment, soil depth and temporal variation. Organic matter amendments lead to increased mobilization of As into both soil porewaters and standing surface waters. The As speciation in the porewater was dominated by inorganic As (As_i) (arsenite and arsenate), with traces amounts of methylated species (DMA^V and MMA^V) only being found with OM amendment. It was noted in field trials that OM fertilization greatly enhanced As mobility to surface waters, which may have major implications for the fate of As in paddy agronomic ecosystems.