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

Abstract

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 (δ13Corg), 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 δ13Corg 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.