Soil organic carbon controls dissimilatory nitrate reduction to ammonium along a freshwater-oligohaline gradient of Min River Estuary, Southeast China

Quality and source of soil organic carbon controlling DNRA in estuarine wetlands have not been well understood. We used a ¹⁵N isotopic tracer approach to investigate DNRA rates and determined the effects of organic carbon quality and source on magnitudes and distribution of DNRA along a freshwater-oligohaline gradient of Min River estuary, Southeast China. DNRA rates ranged from 0.45 to 2.92 nmol g⁻¹ h⁻¹ and were significantly higher in summer than in winter. DNRA was well predicted by dissolved organic carbon (DOC), organic carbon isotope (δ¹³C_org), total OC as the main predictor variables, which explained 63%, 6.0% and 7.0% of DNRA variances, respectively. Water content and temperature were crucial for DOC availability with important implications on DNRA. Soil δ¹³C_org increased from freshwater to oligohaline wetlands and showed a positive correlation with DNRA. We therefore conclude that high DOC availability enhances DNRA and can best predict spatial distribution in subtropical estuary.