Mangroves are important ecosystems on the coasts of southeastern and southern China, which have a significant role in altering microbial communities and nutrient cycles. However, the response of sediment nitrogen (N) mineralization (GNM) and ammonium immobilization (GAI) on the different coastal types (muddy and sandy) to mangrove growth remains poorly understood. Here, surface sediment (0–5 cm) GNM and GAI were examined simultaneously in bare flat and mangrove habitats on both muddy and sandy coasts along a coastal transect of South China, using 15N isotope dilution techniques. The results showed that GNM and GAI rates in sediment ranged from 0.31 to 33.94 μg N g−1 d−1 and 0.24 to 41.02 μg N g−1 d−1, respectively, and mangroves significantly increased the two process rates on both muddy and sandy coasts. The magnitude of increase in GNM and GAI was more pronounced for sandy coasts than for muddy coasts. Mangroves also promoted the ratio of GAI to GNM (RAI), but the ratio of GNM to total N (PAM%) generally did not differ between bare flat and mangrove habitats. Sediment total organic carbon, total N, ammonium, ferrous iron, and microbial abundances were crucial factors in regulating GNM and GAI on both two contrasting coasts, but the process rates on sandy coasts were also related to sediment grain size composition and specific surface area. Our results suggested that mangrove growth is more effective in promoting GNM and GAI on sandy coasts than on muddy coasts, which may be important to improving the nutrient cycle and energy flow on sandy coasts. Overall, these findings provide valuable insights into the N transformations and influencing mechanisms on different coastal types influenced by the growth of mangroves and imply that it is significant to advance mangrove restoration on sandy coasts, yet widely overlooked.