| Abstract
| - Arsenic-rich groundwater from shallow tube wells iswidely used for the irrigation of boro rice in Bangladeshand West Bengal. In the long term this may lead to theaccumulation of As in paddy soils and potentially haveadverse effects on rice yield and quality. In the companionarticle in this issue, we have shown that As input intopaddy fields with irrigation water is laterally heterogeneous.To assess the potential for As accumulation in soil, weinvestigated the lateral and vertical distribution of As in ricefield soils near Sreenagar (Munshiganj, Bangladesh) andits changes over a 1 year cycle of irrigation and monsoonflooding. At the study site, 18 paddy fields are irrigatedwith water from a shallow tube well containing 397 ± 7 μgL-1 As. The analysis of soil samples collected beforeirrigation in December 2004 showed that soil As concentrationsin paddy fields did not depend on the length of theirrigation channel between well and field inlet. Withinindividual fields, however, soil As contents decreased withincreasing distance to the water inlet, leading to highlyvariable topsoil As contents (11−35 mg kg-1, 0−10 cm). SoilAs contents after irrigation (May 2005) showed that mostAs input occurred close to the water inlet and that most Aswas retained in the top few centimeters of soil. Aftermonsoon flooding (December 2005), topsoil As contentswere again close to levels measured before irrigation. Thus,As input during irrigation was at least partly counteractedby As mobilization during monsoon flooding. However,the persisting lateral As distribution suggests net arsenicaccumulation over the past 15 years. More pronouncedAs accumulation may occur in regions with several ricecrops per year, less intense monsoon flooding, or differentirrigation schemes. The high lateral and vertical heterogeneityof soil As contents must be taken into account in futurestudies related to As accumulation in paddy soils and potentialAs transfer into rice.
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