Published: June 22, 2011 r2011 American Chemical Society 6080 dx.doi.org/10.1021/es2003765 | Environ. Sci. Technol. 2011, 45, 6080–6087 ARTICLE pubs.acs.org/est Organic Matter—Solid Phase Interactions Are Critical for Predicting Arsenic Release and Plant Uptake in Bangladesh Paddy Soils Paul N. Williams, †, * Hao Zhang, †, * William Davison, † Andrew A. Meharg, ‡ Mahmud Hossain, ‡ Gareth J. Norton, ‡ Hugh Brammer, § and M. Rafiqul Islam || † Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, England ‡ Institute of Environmental Biology, Aberdeen University, Aberdeen, AB24 3UU, Scotland § FAO advisor (retired), Bangladesh ) Department of Soil Science, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh b S Supporting Information ABSTRACT: Agroecological zones within Bangladesh with low levels of arsenic in groundwater and soils produce rice that is high in arsenic with respect to other producing regions of the globe. Little is known about arsenic cycling in these soils and the labile fractions relevant for plant uptake when flooded. Soil porewater dynamics of field soils (n = 39) were recreated under standardized laboratory conditions to investigate the mobility and interplay of arsenic, Fe, Si, C, and other elements, in relation to rice grain element composition, using the dynamic sampling technique diffusive gradients in thin films (DGT). Based on a simple model using only labile DGT measured arsenic and dissolved organic carbon (DOC), concentrations of arsenic in Aman (Monsoon season) rice grain were predicted reliably. DOC was the strongest determinant of arsenic solid-solution phase partitioning, while arsenic release to the soil porewater was shown to be decoupled from that of Fe. This study demonstrates the dual importance of organic matter (OM), in terms of enhancing arsenic release from soils, while reducing bioavailability by sequestering arsenic in solution. ’ INTRODUCTION Producing safe and nutritious foods in ever-greater quantities is a major global priority. 1 Arsenic contamination of food crops threatens this goal, with small, yet prolonged doses of this element via the daily rice meal being a serious public health concern for some parts of Asia. 2 This scenario is epitomised in the Bengal delta, where burdens from rice are significant, and, even for those drinking arsenic elevated water, account for ca. 50% of the total intake. 2 The food security issue is compounded further, as arsenic can perturb rice growth; severely diminishing harvested yields, 3 while simultaneously reducing the concentra- tion of essential mineral nutrients in the grain. 4 The agricultural lands in parts of south-central Bangladesh have been severely degraded as a consequence of irrigation with arsenic enriched groundwaters used in irrigation of paddy fields. 5 Yet, elevated arsenic in rice grain is a problem not confined to this region or just to soils most greatly impacted by arsenic in irrigation waters. Lu et al. 6 demonstrated that even baseline con- centrations (averages between 1.6 and 6.6 μg As g 1 , depending on soil type) in Bengal delta soils can be problematic, probably due to naturally high arsenic bioavailability 6,7 combined with rice’s highly efficient shoot arsenic assimilation and the effective transfer of this arsenic to the grain when shoot tissue content is low. 8 Gauging the ability of Bangladesh soils to produce low arsenic rice grain is of paramount importance. Soil porewater and chemical extraction techniques have not proved useful in this task. 7 A holistic approach which has had documented success in monitoring element availability in flooded soils is DGT. 9,10 The DGT measurement integrates a wider range of key soil properties that impact on release/adsorption than other single measure- ment approaches. 9 Importantly, it emulates the dominant pro- cesses occurring in soils during plant uptake by lowering the solute concentration locally and inducing diffusive supply and release from complexes and the solid phase. 9 Although DGT has not previously been used to investigate arsenic uptake in rice, DGT devices with ferrihydrite as the binding phase have been shown to be effective for measuring both arsenate and arsenite species. 11,12 Laboratory soil measurements by DGT have been shown to be a robust predictor of cationic metal concentra- tions in field cultivated rice. 10 The DGT measurements outper- formed soil solution, acetic acid, and calcium chloride extractions in their capacity to explain rice root and grain metal variability. Element release from Bangladesh paddy soils is only under- stood in the general sense, with arsenic mobility in most soils being governed by reductive dissolution of Fe(III) hydroxides, and hence redox conditions and the reactivity of the Fe(III) phases. However, as metal oxides have a strong affinity with OM, due to ligand exchange reactions with COOH and phenol/ catechol OH functional groups, they can compete with arsenic for absorption sites. 1315 Additionally, adsorption affinities can be affected by OM mediated change in redox conditions, thus favoring arsenic transport. 15 OM and dissolved OM-Fe com- plexes also bind arsenic, which may promote desorption of Received: February 1, 2011 Accepted: May 25, 2011 Revised: May 23, 2011