Distinct Accumulations of Microbial Necromass Carbon in Mangrove, Salt Marsh, and Bare Flat Wetlands

As a key component of stable soil organic matter, microbial necromass carbon (MNC) serves as a pivotal contributor to long-term carbon sequestration in coastal blue carbon ecosystems. Despite its recognized importance, the ecosystem-specific characteristics and accumulation mechanisms of MNC remain poorly understood across various coastal wetlands. In the present study, surface soils contained higher MNC contents than subsurface soils across mangroves, salt marshes, and bare flats. Additionally, distinct accumulations of MNC were found across the three types of wetlands. Mangrove wetlands exhibited a significantly higher MNC contents, especially bacterial necromass carbon (BNC) contents, compared with salt marsh and bare flat wetlands. Nitrogen availability, especially total nitrogen (TN), microbial biomass nitrogen (MBN), and the ratio of microbial biomass carbon to MBN, emerged as key drivers of necromass formation, emphasizing the importance of nutrient regulation. Furthermore, BNC accumulation in mangrove soils may be improved by iron-mediated stabilization. Interestingly, BNC exhibited the opposite trend from fungal necromass carbon (FNC), which contributed less to soil organic carbon (SOC) in mangroves than in salt marshes and bare flats. The overall proportions of MNC in the SOC pool are similar across all wetland types despite these disparate contributions, most likely as a result of a dilution effect resulting from the accumulation of refractory, plant-derived organic carbon in mangroves. Elucidating these dynamics is crucial for advancing our understanding of wetland carbon restoration of these ecosystems to enhance their carbon sequestration potential.