Organic matter degradation state affects dissimilatory nitrate reduction processes in Knysna estuarine sediment, South Africa

Purpose: The quality of organic matter influencing sediment nitrate (NO₃⁻) reduction processes in estuarine zones is not well understood. This study aimed to assess the denitrification (DNF), anaerobic ammonium oxidation (ANA), and dissimilatory reduction of nitrate to ammonium (DNRA) in estuarine zones of South Africa, and to understand the effects of organic matter fractions and degradation states on anaerobic NO₃⁻ reduction processes. Materials and methods: We measured the anaerobic NO₃⁻ reduction process rates using ¹⁵N isotope-tracing techniques in Knysna Estuary, South Africa. Total hydrolyzable amino acids and fractions and geochemical parameters were also measured. The correlation analysis and structural equation model were used to evaluate the key environmental factors driving NO₃⁻ reduction processes. Results and discussion: Potential DNF, ANA, and DNRA rates in Knysna Estuary varied from 3.59 to 16.62, 0.28 to 1.16, and 1.52 to 8.38 nmol g⁻¹ h⁻¹, respectively, with a large spatial variation. The variations in NO₃⁻ reduction process rates can largely be explained by sediment water content, dissolved organic carbon, and amino acid–based degradation index, while the total organic carbon and inorganic nitrogen contents were not related to the NO₃⁻ reduction processes. The DNF process contributed 47.28–79.34% total NO₃⁻ reduction, as compared to 17.59–47.58% for DNRA and 2.53–5.76% for ANA. The retention of reactive nitrogen (N) attributed to the DNRA process was approximately 42 t N km⁻² year⁻¹. Conclusions: This study reported the first simultaneous investigation of the anaerobic NO₃⁻ reduction processes in estuarine areas of South Africa, implying that the qualities of substrate were more important in regulating NO₃⁻ reduction processes than substrate quantities and highlighting that DNRA played an important role in reactive N retention.