The influences of mangrove grown on sedimentary nitrate reduction activities are more pronounced in sandy coasts compared to muddy coasts

Mangrove forests have the potential to colonize and grow in both muddy and sandy coastal areas, but the effects of mangrove presence or absence on nitrate (NO_x⁻) reduction processes in these two types of coastal sediments have not been well studied.

The dynamics of NO_x⁻ reduction processes were studied in surface sediments (0–5 cm) of mangrove and the adjacent bare flat in muddy and sandy coasts of China using stable isotope (¹⁵N) pairing and quantitative PCR.

Sediment NO_x⁻ reduction rates (denitrification, anammox and DNRA) and their relative contributions (DEN%, ANA% and DNRA%) exhibited significant spatial and temporal variations, while the function-related genes (nirS, ANA 16S rRNA and nrfA) only exhibited significant differences in space. Sediment grain size, organic matter, nutrients, Fe²⁺/Fe³⁺, sulfide and temperature were critical factors controlling spatial and temporal variability in NO_x⁻ reduction rates. Mangrove grown in muddy coasts only had a significant impact on DNRA rates (p < 0.05), while the impact on denitrification and anammox rates were both not significant (p > 0.05). The N retention index (NIRI) variation indicated that the presence of mangrove had a more significant positive role of N retention in sandy coasts than in muddy coasts.

Mangrove grown in sandy coast is more effective at promoting both sediment N-loss and N retention activities compared to those in muddy coast. And our results highlight the impact of mangroves on NO_x⁻ reduction processes in muddy and sandy sediments, which is crucial for effective management and conservation efforts in nearshore ecosystems.