Spatial distribution and driving factors of microbial necromass carbon in coastal wetlands of China

Microbial necromass carbon (MNC) constitutes a vital component of soil carbon pools and plays an essential role in the global soil carbon cycle. However, the mechanisms driving MNC concentration in coastal wetlands, particularly across different habitat types (such as mangroves, salt marshes and mudflats), remain poorly understood. In this study, we conducted a field-based investigation in 101 coastal wetlands along the entire coastline of China to investigate variations in MNC and its main driving factors. Additionally, we integrated global and China field-derived data from cropland, forest, grassland, paddy and coastal wetland habitats for a comparative analysis to explore the specificity of carbon storage mechanisms. We found mangroves exhibited the highest MNC concentration (10.90 mg/g) among the three coastal wetland habitats, primarily regulated by pH and nutrient availability (i.e. available phosphorus). In salt marshes, sediment grain size and chlorophyll-a representing biomass of algae in surface sediment had the highest influence on MNC accumulation (1.89 mg/g), while in mudflats, soil moisture played a dominant role in driving it (2.60 mg/g). Overall, plant biomass was the strongest predictor of MNC accumulation across all coastal habitats, whereas mean annual temperature influenced MNC indirectly through mediating primary productivity. However, we found no significant differences in the contribution of MNC to soil organic carbon (SOC) among the three coastal wetland habitats. Additionally, the MNC concentration (2.58 mg/g) and its contribution to SOC (26.3%) in coastal wetlands were significantly lower than that in other ecosystems (i.e. forest, cropland, grassland), except for paddies (4.67 mg/g; 24.9%). These results suggest that the unique environmental conditions shared by paddies and coastal wetlands, such as anaerobic environments, suppress the decomposition of both microbial and plant-derived carbon, resulting in lower MNC levels and a reduced contribution of MNC to SOC. While microbial necromass plays a relatively minor role in coastal SOC pools, it remains a distinct component of blue carbon, whose dynamics are not fully captured by terrestrial models. Recognizing its role is essential for understanding the stability and complexity of soil carbon dynamics in these ecosystems. Read the free Plain Language Summary for this article on the Journal blog.