Understanding Arsenic Uptake By Rice As a Function Of Field Water Regime.

Arsenic-contaminated groundwater is widely used to irrigate dry-season rice (boro) in Bangladesh. This has greatly improved food security but is leading to increased As levels in soils and rice and may cause yield reductions in the long term. Owing to the gradual accumulation of As in paddy soil, the adverse effects of groundwater irrigation may also extend to monsoon rice (aman) cropped under rain-fed or naturally flooded conditions. Since arsenic is redox-sensitive and predominantly mobilized under reducing conditions, irrigation management during the dry season and the water regime prevailing during the monsoon may significantly influence As uptake by boro and aman rice, respectively.

Here we present data from field studies in Jessore and Munshiganj Districts (Bangladesh), in which vertical and temporal trends in soil porewater chemistry and plant As contents were assessed in relation to water management. The study in Jessore, focused on As uptake by aman under intermittently and continuously flooded conditions. Compared to continuous flooding, porewater As concentrations were significantly lower under intermittent flooding, and average As straw and grain contents were lower by factors of 3 and 2.5, respectively.

The study in Munshiganj assessed two water-saving cultivation systems, alternate wetting and drying (AWD) and raised beds (RB), in terms of their potential to reduce As uptake by boro compared to conventional continuous flooding (CF) and to current local farmers' practice. To sufficiently resolve porewater dynamics, micro suction cups, allowing sampling on time scales of hours to days as well as porewater samplers based on solute exchange were deployed. We discuss porewater dynamics of As and of elements influencing plant As uptake (Fe, P, Si) at the time scale of irrigation events and at different periods of crop growth, and relate these to As contents in straw and grain obtained under the experimental treatments (AWD, RB and CF) and under local farmers' practice.