The effects of tidal sediment deposition on soil fertility and rice productivity in southwestern Bangladesh
Bruin, J.M. de
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In southwestern Bangladesh, construction of polders aimed to protect the coastal region from flooding and salinity intrusion, while reclaiming land to improve agricultural productivity. However, these polders ignored the natural sediment dynamics, consequently resulting in river siltation and land subsidence that is associated with widespread waterlogging. Managing sediments by re-allowing tidal flooding is found to effectively improve the health of rivers, solve waterlogging problems, and support agricultural productivity. Additionally, it is often mentioned that tidal sediments improve soil fertility. There is, however, no literature that supports this hypothesis. To better understand the sustainability of sediment management strategies, this study examined the effects of tidal sediment deposition soil fertility and rice productivity in three tidal basins. To study soil fertility, a rice growth experiment was set up to determine rice productivity in fresh sediments and old sediments from Beel Pakhimara, Beel Khuksia, and Polder 32. In this experiment, rice is grown in the tidal sediments, compost mixtures, and after applying a washing treatment. Next, rice productivity is compared to the differences in sediment characteristics, plant tissue composition, and nutrient availability in the pore water. The results find that rice productivity is significantly higher in the old sediments from Beel Pakhimara and Polder 32, and there are no significant differences between fresh and old sediments in Beel Khuksia. Rice productivity in the compost mixtures is significantly higher in all study locations compared to the original sediments. No significant differences are found in the applied washing treatments. In conclusion, tidal sediments do not improve soil fertility, and compost application is necessary to support rice productivity. Nitrogen deficiencies are observed in all study locations, and indicate the need for additional nitrogen fertilization. Phosphorus is the main controlling nutrient, and is mobilized by two interacting biogeochemical processes related to iron and sulphate reduction. The impact of salinity on rice productivity is limited, and can be explained by the capability of the salt tolerant Boro variety used in this study. To fully understand the impacts of tidal sediments on soil fertility, seasonal variability and other rice varieties should be included in further studies.