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

Numerous studies have highlighted the importance of nitrates (NO) reduction processes in estuarine and coastal ecosystems over the past decades. However, the biotic and abiotic factors sediment NO reduction processes in mangrove of varying ages are still not fully understood. Here, we investigated the dynamics of sediment NO 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 N stable-isotope pairing techniques and quantitative PCR. The denitrification (2.64-11.30 nmol g h), anammox (0.06-0.83 nmol g h), and dissimilatory nitrate reduction to ammonium (DNRA, 0.58-16.34 nmol g h) rates varied spatially and seasonally, but their contributions to the total NO 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) and reduced N loss (481.32 t N yr) 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.