Assessment of the variability in nutrient content of deposited sediment and crop diversity in selected tidal basins of Bangladesh

Abstract

In Bangladesh, coastal polders have been constructed since 1960s to protect the land from floods as well as to intensify crop production. However, the polders altered the sensitive river-floodplain sediment balance and resulted in river siltation, drainage congestion and waterlogging. Therefore, Tidal River Management (TRM) was introduced to solve the waterlogging problem while ensuring sedimentation in the low-lying tidal basins and increasing the potential for crop production. This study has been conducted to assess the variation in nutrient content and composition of the deposited sediment and water quality in selected tidal basins, along with the socio-economic and institutional factors for crop production and diversity in the basins in relation to the level of success in TRM operations in Beel Bhaina (successful TRM), East Beel Khuksia (partially successful TRM) and Beel Pakhimara (unsuccessful TRM). The level of success of a TRM operation is understood by how the local community perceived the TRM operation results considering the extent of reduced waterlogging and drainage congestion problems and the depth of sedimentation inside the beel. Different physical and physicochemical properties of soil e.g., soil texture, pH, electrical conductivity (EC), organic matter (OM) and available nutrient elements such as Ca, Mg, Fe, P, S and Cl were determined for assessing the deposited sediment quality. For understanding the water quality, pH, EC, total dissolved solid (TDS), total suspended solid (TSS), Na+, Ca2+, Mg2+, K+, Cl-, CO32-, HCO3-, SO42-, PO43-, NO3- and NO2- were measured. Sodium adsorption ratio (SAR), sodium percentage (Na%), potential salinity (PS), magnesium ratio (MR), calcium-magnesium ratio (Ca: Mg), permeability index (PI), and Kelly’s index (KI) were calculated to evaluate the suitability of the water for agricultural purposes. The soil and water quality parameters were analyzed following the established methodologies. Participatory Rural Appraisal tools like Focus Group Discussions, Group Discussions, Key Informant Interviews, individual interviews, and field observations were used to gather useful information regarding the socio-economic and institutional factors for crop production and diversity in the study areas. The field data were collected through several visits to the study areas and relevant organizations including upazila agriculture offices, upazila agricultural extension offices and Bangladesh Water Development Board. Soil texture showed similar pattern in the three basins. Among the three beels, the EC of East Beel Khuksia was the lowest which may be one of the reasons for good harvest. The pH of the soils fell under a slightly alkaline class. All the soil parameters were higher in winter than that of monsoon, except for OM. The plant residues from crop production during winter may have increased the OM content in monsoon. Ca was the dominant cation in the basins because it is less susceptible to leaching. As salt affected soils tend to increase the available Fe under waterlogging conditions, Fe was higher in winter than in monsoon. All the water sources (beel, river and GW) from the three basins were suitable for irrigation purpose in terms of pH. The water salinity of Beel Pakhimara was better than the two other beels. The EC of GW in East Beel Khuksia was permissible for irrigation which could be the reason for higher crop production. The highest TDS and TSS were found in winter in the three basins, except for Beel Pakhimara having the highest TSS in monsoon. The water of Beel Bhaina and East Beel Khuksia was categorized as ‘permissible’ and ‘unsuitable’, respectively whilst that of Beel Pakhimara was categorized as ‘good’ in winter for irrigation purpose considering available TDS. The most dominant cation was Na in almost all the water samples due to the seawater influence, evaporation or crystallization and other hydrogeochemical processes. In Beel Bhaina and East Beel Khuksia, severe sodium toxicity was present in water whilst there was no sodium toxicity in Beel Pakhimara. The most dominant anion was Cl- in Beel Bhaina and East Beel Khuksia, whereas it was HCO3- in Beel Pakhimara. The HCO3- dominance could be due to the weathering from calcite parent materials and dissolution of limestone and dolomite. The water from the three beels was categorized as ʻsevereʼ for Cl- toxicity. The SO42- and HCO3- contents were found to be ‘satisfactory’ in both the seasons. Very low amounts of PO43-, NO3- and NO2- were present in the water samples compared to other available anions in the three basins. For Beel Pakhimara, all the SAR values fell under ‘excellent’ category for crop production in both the seasons. Considering PS, the water of Beel Bhaina was ‘unsuitable’ and that of East Beel Khuksia and Beel Pakhimara was ‘suitable’ in both the seasons. The dominant cropping pattern in Beel Bhaina, East Beel Khuksia and Beel Pakhimara was fish-fish-boro rice. The yields of boro rice were 7.2, 7.7 and 6.4 ton/ha in Beel Bhaina, East Beel Khuksia and Beel Pakhimara, respectively. Beel Bhaina and East Beel Khuksia being two adjacent tidal basins, similar crop varieties were also observed on gher embankments, for example beans, spinach, cucumber, and tomato. Moreover, beans were cultivated in abundance during winter and monsoon in Beel Bhaina. In East Beel Khuksia, tomatoes were cultivated in winter and cucumbers in monsoon at a large scale. In Beel Pakhimara, no farming was seen during monsoon. The farmers cultivated boro rice in the elevated lands in winter. The farmers of the three tidal basins reported that they were getting higher crop yield and economic return after the TRM operation. The profit margin in Beel Bhaina, East Beel Khuksia and Beel Pakhimara is found to be 53.6%, 50% and 43.7%, respectively. The variation in crop variety was also due to market competition, economic return and extension services. However, similar scenarios were found considering, access to capital, access to market, and competition in agricultural markets in the three beels regardless of the TRM success.