Changes in Sediment N Mineralization and Immobilization in Association with Spartina Alterniflora Invasion in Mangrove Wetland

Nitrogen (N) mineralization and immobilization are two important N biogeochemical processes in estuarine and coastal sediments. However, the changes of those two processes in sediment following Spartina alterniflora (S. alterniflora) invasions into mangrove wetlands under aerobic and anaerobic conditions remain limited. We used a 15N stable isotope dilution technique to investigate sediment gross N mineralization (GNM) and NH4+ immobilization (GAI) under aerobic and anaerobic conditions in mangrove communities, mangrove communities mixed with S. alterniflora, and pure S. alterniflora communities in the Zhangjiang Estuary, China. GNM and GAI rates varied spatially and seasonally. Summer N mineralization and immobilization rates under aerobic and anaerobic conditions were both significantly higher than those in winter, and the high rates were observed mainly in mangrove sediments and decreased with S. alterniflora invasion. GNM rates in aerobic conditions were significantly higher than those in anaerobic conditions, while GAI rates in aerobic conditions were less than those in anaerobic conditions. GNM and GAI rates were related closely to temperature, water content and organic matter. S. alterniflora invasion can weaken GNM and GAI rates by reducing organic matter accumulation, and S. alterniflora invasion could have reduced sediment N mineralization (2.10 × 103 t N yr−1) and immobilization (1.46 × 103 t N yr−1) in mangrove wetlands of China from 1980 to 2020. Our results highlight the importance of S. alterniflora invasion and hydrological conditions in controlling N mineralization and immobilization processes in mangrove sediments and are essential for the evaluation of N-balance in this N-limited ecosystem.