Nitrogen retention dynamics in a large floodplain river: a case study on the Padma River, Bangladesh

Large tropical floodplain rivers act as important pathways of nitrogen transport from land to the sea. In the present study, a mass balance approach was used to evaluate nitrogen retention over a two-year period from a 50 km reach of the Padma River in Bangladesh. The relationship between concentration and discharge was estimated from 58 nitrogen concentration and discharge measurements. Daily nitrogen flux was then calculated from the hydrological inflow and outflows of the reach, and total nitrogen (TN) retention was estimated based on the flux difference of TN inflow and outflows. To validate mass-balance measurements, retention processes of nitrogen loss due to water retention (NLWR), sedimentation, potential denitrification rate (PDR), and nitrogen fixation rate (NFR) were estimated from the water column of the river. Monthly mass-balance measurements revealed substantial seasonal variation in nitrogen retention, indicating river discharge as the main controlling factor. Estimated maximum retention values (tonnes per month) of NLWR, sedimentation, PDR, and NFR were all associated with the monsoons, with 86 % occurring during that period. However, the percentage of PDR and NFR to TN retention was higher in non-monsoon months (post-monsoon, dry/winter and pre-monsoon), suggesting retention mechanisms varied seasonally. TN retention via NLWR accounted for the largest portion of total TN retention, that consistently exceeded 50 %, followed by sedimentation. PDR in submerged geomorphic units was the second-most important retention mechanism in the dry/winter and pre-monsoon seasons. The present research provides a benchmark for nitrogen-budget modelling in tropical rivers, supporting planning for sustainable river management.