Changes in geomorphic units alter nitrogen fixation in a large lowland tropical river

The present investigation was carried out over 50 km reach, the Padma River of Bangladesh, downstream of the confluence of the Ganges and Brahmaputra rivers. The study area is highly dynamic, with diverse geomorphic units, high rates of bank erosion, and sediment abstraction. Sentinel-2 imagery (2019-2020) was processed using NDVI values to classify in-channel geomorphic units (GUs) of the study area. The identified GUs categorised as primary and secondary channels (C&S) islands (VI), bars (L, T and SB), vegetated bank (EK), dry channel (ED) and water depression (WD). Monsoonal water flow was considered an essential factor for reshaping GUs and supplying nutrients to GUs. A field study was carried out in low flow (dry/winter) season to validate the classification of GUs and to measure the nitrogen fixation rate in each type of GUs. Vegetation cover, soil/sediment moisture, bulk density, soil texture and elevation were determined for each GUs. Regression analysis showed that only bulk density and elevation significantly impact nitrogen fixation estimation and prediction can be improved using linear mixed models. Later on, based on seasonal variation of GUs, the distribution of nitrogen fixation was estimated in GUs in different seasons. Results showed that changes in the number and surface area of GUs across seasons could alter nitrogen fixation. This type of systematic investigation of the spatial and temporal distribution of geomorphological and nitrogen fixation rate measuring and monitoring will be useful for planning river restoration and ecosystem management programs.