It has been established that the impact of arsenic in irrigated agriculture has become a major environmental concern in Bangladesh. However, to date there is still a limited understanding of arsenic in Bangladeshi paddy soils at a landscape level. Besides, there is no data available for soil arsenic on its geographical distribution, geomorphic variations, and biogeochemical relationships across the landscape. In the present study, paddy (n = 1209) and adjacent nonpaddy (n = 235) soil samples across 10 different physiographic regions, comprising the Holocene floodplains and Pleistocene terraces, were collected, and analysed for arsenic and a suite of 16 other elements. The collected paddy soils were from fields irrigated with groundwater (n = 904) and surface waters (n = 281). Additionally, the soils could be categorised into 6 inundation land types. A set of 30 paddy soil samples from 6 physiographic regions were also studied using the diffusive gradients in thin-films (DGT) to assess the porewater dynamics of arsenic, and other geochemical elements in the soils. The paddy soils had generally 60 percent more arsenic than the matching non-paddy soils, perhaps due to the use of arsenic contaminated groundwater for paddy irrigation. Compared to the groundwater irrigated paddy soils, the surface water irrigated paddy soils had lower arsenic concentrations, but higher concentrations of most of the other elements. Within the topologically different inundation land types, the concentrations of arsenic and other elements, including the toxic metals, were found to be elevated in more deeply and prolonged flooded low-lying soils. The soils in the different physiographic regions had variability in arsenic concentrations as well as in their indigenous biogeochemical characteristics. The inherent concentrations and variability in arsenic and most other elements, including nutrients, were greater in the Holocene floodplain soils compared to the Pleistocene terrace soils. Paddy soils in Bangladesh have a high potential for arsenic resupply from soil solid phase to soil solution phase. In the physiographically different soils across the landscape, there is less difference in nutrient/ toxin bioavailability in the paddy soils than might be predicted based solely on the total concentrations in the soils. Therefore, distinctions of soils based just on total concentrations are perhaps misleading, particularly, when elements mobility under reducing paddy environment is concerned. While the bioavailable/ phytoavailable fractions of the elements in soils are of major concern with respect to the uptake by the growing plants, it is of utmost importance to consider the labile concentrations of elements in soil solution rather than the soil total concentrations. The present study substantiates that arsenic is simply associated with less well weathered/ leached soils and sediments, suggesting that it was either due to the geological newness of Holocene sediments or differences between the sources of sediments that gave rise to the arsenic problems in soils of Bangladesh. The inherent biogeochemical variability along with the complexity of the nature and properties of the soils at local and regional levels across the landscape of the dynamic sedimentary depositional environment in Bangladesh should be considered, in any future research on arsenic in the soil-water-crop systems in Bangladesh environment.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:704531 |
| Date | January 2016 |
| Creators | Chowdhury, Md Tanvir Ahmed |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=231266 |
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