Effects of tidal flat reclamation on dissimilatory nitrates reduction processes in an Estuarine wetland

Tidal flat reclamation is a worldwide issue that substantially transformed the structure and function of ecosystems. Nevertheless, the effects of land-use changes on the sediment dissimilatory nitrates (NO_x⁻) reduction processes within coastal wetland remains inadequately understood. Here, we investigated spatiotemporal patterns of NO_x⁻ reduction rates in response to land-use changes in surface soils/sediments (0 −5 cm) of estuarine reed marsh and unraveled the underlying mechanisms by integrating associated gene abundances (nirS, nosZ, ANAMMOX 16S and nrfA) and environmental parameters. Denitrification was the dominant pathway for NO_x⁻ reduction processes in soils/sediments of reed marsh (78.46 ± 6.91 %), vegetable field (86.87 ± 5.28 %), paddy field (84.48 ± 9.22 %) and aquaculture pond (85.82 ± 5.15 %). Without considering confounding factors, tidal flat reclamation substantially reduced sediment organic matter (excluding paddy), nitrogen retention index (NIRI), temperature sensitivity of nitrogen loss rates, NO_x⁻ reduction rates (excluding anammox) and associated gene abundances, while increased the contribution of anammox to nitrogen-loss. The multiple stepwise regression analysis indicated that water content, temperature, and organic carbon were important factors controlling NO_x⁻ reduction rates. In addition, tidal flat reclamation also significantly diminished the NIRI, which respectively decreased by 95.7 %, 65.2 % and 56.5 % in vegetable field, paddy, and aquaculture pond. Similarly, the N-loss rate in reed marsh was 10.81 ± 5.59 μmol kg⁻¹ h⁻¹, which respectively decreased by 88.62 %, 63.00 % and 12.00 % in vegetable field, paddy, and aquaculture pond. Thus, reclamation significantly reduces sediment nitrogen loss in coastal marsh wetlands, which may threaten the marsh buffer function and wetland nitrogen pools protection in estuarine and coastal ecosystems.