Climate and soil properties cogovern rice arsenic: A longitudinal study

Elevated accumulation of arsenic (As) in rice has recently been proposed as a tangible consequence of climate and environmental changes. This requires validation at global scale or at least regional scale. Unfortunately, no dataset is available that enables a direct connection of the levels of As in rice grain and climatic conditions. In this study, we collected soil and rice samples from 162 sites crossing a longitudinal transect (from latitude 8 to 22° N) that covers approx. 2000 km coastal line and numerous deltas in Vietnam territory. The sampling transect reflects the differences in terms of climate conditions, e.g., warm-wet (in the South including the Mekong River delta), warm-dry (South Central) and cold-wet (in the North Central and North). It was found that rice grain assimilated As at the concentrations ranging from 11.6 to 806.7 µg kg^-1 ( = 153.1 µg kg^-1). While rice tends to assimilate less As in the South ( = 110.3 µg kg^-1), North ( = 154.9 µg kg^-1) and North Central ( = 188.8 µg kg^-1), peak accumulation of As was observed for rice grain in the South Central ( = 265.0 µg kg^-1) where is warm-dry and soil As contents were lowest. This observation implies that rice in the South Central likely act less effectively to prevent the translocation of As from soil to rice. It is likely that climatic conditions through their geochemical effects, particularly in the fallow periods (when water is drained) have affected the translocation of As from soil to rice. This scenario is supported by statistical analysis that indicates close relations of grain As with Si, S, P and Fe in soils. However, more empirical evidence that demonstrates direct effects of climatic factors (such as temperature and humidity) on transformation/translocation of As in the soil-rice system are still required; and mesocosm experiments will be included in our future works.

Keywords: Arsenic; rice; climate; paddy soil; geochemistry