Characterizing nitrogen dynamics and their response to sediment dredging in a lowland rural river

It is challenging to quantify nitrogen (N) balance in a lowland rural river covered by macrophytes due to its multiple N sources and complex cycling processes. In order to address the challenge, we developed a N-cycling model (River-N) for lowland rural rivers covered by macrophytes, and coupled it with Nitrogen Dynamic Polder (NDP) and Environmental Fluid Dynamics Code (EFDC) model. A two-year (2020-2021) dataset was used to calibrate and validate the coupled model. The coupled model was used to investigate a lowland rural river's N imports, exports and its response to sediment dredging, and resulted in an acceptable performance for total N concentrations (TN) (R2 = 0.57). Our investigation conducted in the study river revealed a considerable N retention (difference between imports and exports) ability (955.5 kg yr-1). The river's N imports were mainly from connected-river import (55.6 %), surrounding-polder export (31.4 %), sediment N release (7.4 %). N exports were mainly from connected-river export (59.0 %), denitrification (31.7 %) and macrophyte uptake (4.4 %). Compared with scenario without dredging, the rates of particulate nitrogen (PN) resuspension and settling increased, and the rates of ammonia nitrogen (NH4+) release, denitrification and macrophyte uptake decreased under the scenario with dredging. During dredging period, the rates of NH+4 release, PN resuspension, PN settling were 2.6, 7.0 and 4.3 times higher than that during pre-dredging period, respectively. The denitrification rate during dredging period was decreased to 43.5 % of that during pre-dredging period. Our investigation results implied that sediment dredging can be a pathway to remove high-N sediment, but did not necessarily decrease TN in river due to external N imports. Considering the critical role of macrophytes in N-cycling, macrophytes should be a critical component requiring our attention in water management practices.

Automated summary

This study developed a N-cycling model for lowland rural rivers covered by macrophytes and investigated the N imports, exports, and response to sediment dredging. The findings showed a considerable N retention ability in the study river, with significant N imports from connected rivers and surrounding polders. Sediment dredging increased particulate nitrogen resuspension and settling rates, while decreasing ammonia nitrogen release, denitrification, and macrophyte uptake rates.