Spatio-temporal variation in the potential ammonia oxidation rates and microbial communities of mangrove wetlands with different sediment textures in South China

Mangroves play an important role in regulating the biogeochemical cycle of nitrogen (N) in coastal wetlands. However, their effects on the N cycle, especially the potential ammonia oxidation rates (PARs) contributed by ammonia-oxidizing archaea (AOA) and bacteria (AOB) in different sedimentary textures remain unclear. Here, we analyzed the PARs, AOA and AOB amoA gene abundances, and stable carbon isotope ratios (δ¹³C_org) of mangrove wetland sediments collected in summer and winter from the muddy and sandy coasts in southern China. The results showed higher sediment PARs and amoA gene copies during summer, and greater values were found on mangrove-vegetated sediments than unvegetated bare flats. PARs and the amoA gene copies were negatively correlated to salinity and pH, which agrees with previous research. However, sediment texture (i.e. organic matter content and grain size) also plays a role, of which muddy sediments were characterized by higher PARs and amoA gene abundances than sandy sediments. Vegetated sediments have a lower pH, smaller grain size, and a higher organic matter and nutrient content. This suggests that mangroves may enhance ammonia oxidation and microbial abundance by altering sediment textures. A greater effect of mangroves on nutrient cycling of sandy coasts implies that mangroves are instrumental in promoting the nutrient levels of sandy coasts. Furthermore, AOA represented <10 % of the amoA gene copies but contributed ca. 40 % of PARs, indicating AOA's central role in ammonia oxidation. Our study showed that mangrove forests are important for microbial abundance and N cycling in different coastal sediment textures.