Effect of Aquaculture Reclamation on Sediment Nitrates Reduction Processes in Mangrove Wetland

Sediment denitrification, anaerobic ammonium oxidation (anammox), and nitrate dissimilation to ammonium (DNRA) play an important role in controlling the dynamics of nitrates (NO_x⁻) and their fate in estuarine and coastal ecosystems. However, the effects of land-use change on NO_x⁻ reduction processes in mangrove sediments are still unclear. Here, we used a mud experiment method combined with a ¹⁵N stable isotope tracer method to study the mechanism and ecological environment of the change of land use pattern on the sediment NO_x⁻ reduction processes in mangrove wetlands. Our study showed that most physicochemical parameters, NO_x⁻ reduction rates, and their gene abundances varied considerably. The denitrification, anammox, and DNRA rates in mangrove sediment cores were in a range of 1.04–4.24 nmol g⁻¹ h⁻¹, 0.14–0.36 nmol g⁻¹ h⁻¹, and 0–2.72 nmol g⁻¹ h⁻¹, respectively. The denitrification, anammox, and DNRA rates in aquaculture sediment cores were in a range of 1.06–10.96 nmol g⁻¹ h⁻¹, 0.13–0.37 nmol g⁻¹ h⁻¹, and 0–1.96 nmol g⁻¹ h⁻¹, respectively. The highest values of denitrification, anammox, DNRA, the contribution of denitrification and DNRA to total NO_x⁻ reduction (DEN% and DNRA%), gene abundances (<i>nirS</i>, Amx 16S rRNA, and <i>nrfA</i>), total organic carbon (TOC), total nitrogen (TN), and TOC/TN in sediments were generally found in the top layer (0–5 cm) and then decreased with depth, while the contribution of anammox to total NO_x⁻ reduction (ANA%), Fe²⁺, and Fe²⁺/Fe³⁺ were generally increased with sediment depth in both mangrove and aquaculture ecosystems. When mangrove wetlands are transformed into pools, some properties (including TOC, TN, and Fe³⁺), DNRA rates, DRNA%, and <i>nrfA</i> gene abundances were decreased, while some properties (including NH₄⁺, TOC/TN, Fe²⁺, and Fe²⁺/Fe³⁺), denitrification rates, DEN%, <i>nirS</i>, and ANAMMOX 16S gene abundances were increased. Sediment organic matter (TOC and TN) content and Fe²⁺ both affected NO₃⁻ reduction rates, with organic matter the most prominent factor. Thus, aquaculture reclamation enhances N loss while reducing N retention in sediments of mangrove wetlands, which plays an important role in regulating the source and fate of reactive N in mangrove ecosystems.