Sediment nitrates reduction processes affected by non-native Sonneratia apetala plantation in South China

Numerous studies have highlighted the importance of nitrates (NO_x^-) reduction processes in estuarine and coastal ecosystems over the past decades. However, the biotic and abiotic factors sediment NO_x^- reduction processes in mangrove of varying ages are still not fully understood. Here, we investigated the dynamics of sediment NO_x^- reduction processes and associated gene abundances in mangroves of different ages (including 0-year unvegetated mudflats, 10 and 20-years Sonneratia apetala, as well as >40 years of mature native Kandelia obovate) on the Qi'ao Island using ¹⁵N stable-isotope pairing techniques and quantitative PCR. The denitrification (2.64-11.30 nmol g^-1 h^-1), anammox (0.06-0.83 nmol g^-1 h^-1), and dissimilatory nitrate reduction to ammonium (DNRA, 0.58-16.34 nmol g^-1 h^-1) rates varied spatially and seasonally, but their contributions to the total NO_x^- reduction (DEN%, ANA%, and DNRA%), associated gene abundance (nirS, anammox 16S rRNA, and nrfA), and organic matter only varied spatially. Organic matter and microbial abundances are the dominating factors controlling N loss and retention. Without considering confounding factors, mangroves conservation and restoration significantly increased DNRA rates, NIRI (DNRA/(denitrification + anammox)), organic matter content, and microbial abundances (p < 0.05 for all), but reduced N loss rates. Mangroves conservation and restoration are estimated to have increased sediment N retention (~931.81 t N yr^-1) and reduced N loss (~481.32 t N yr^-1) in coastal wetlands of China over the past 40 years (1980-2020). Overall, our results indicate that mangrove restoration and conservation can significantly increase sediment N retention due to the rapid biomass accumulation, and it can provide more nutrients for mangrove and microorganism growth, thus creating a virtuous cycle in these N-limited ecosystems.