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 is integral to the global soil carbon cycle. However, the mechanisms driving MNC accumulation in large-scale coastal wetlands, particularly across different habitat types (such as mangroves, saltmarshes, and mudflats), remain poorly understood. In this study, we conducted extensive sampling across 101 sites in China's coastal wetlands, covering a latitudinal gradient of over 2,500 km, to investigate variations in MNC and its drivers. Additionally, we integrated global data from farmland, forest, grassland, and coastal wetland habitats for a comparative analysis to explore the specificity of carbon storage mechanisms in coastal wetlands. Our results reveal significant differences in MNC content among the three coastal wetland habitats, with mangroves exhibiting the highest MNC levels and saltmarshes the lowest. We found that primary productivity, especially plant biomass, is the dominant driver of MNC accumulation in coastal wetlands, while climatic factors have an indirect effect. Furthermore, after accounting for the influence of plant biomass, we observed that edaphic properties also played an important role in driving MNC accumulation within mudflat and saltmarsh habitat. Compared to other habitats, i.e., grasslands, forests, and farmlands, coastal wetlands exhibited lower MNC levels (8.44 mg/g) and a reduced contribution to soil organic carbon (32.16%). In conclusion, these findings demonstrate that plant primary productivity significantly regulates MNC accumulation in coastal wetlands. Furthermore, coastal wetlands exhibit distinct characteristics in carbon storage compared to other ecosystems, underscoring the unique role of blue carbon ecosystems in the soil carbon cycle.

Methods
Soil samples were sampled from 101 sampling sites in Binhai, China. The hydrolysates of the collected soil samples were centrifuged, neutralized, and then centrifuged at 4000 rpm for 10 min. The supernatant was freeze-dried, purified with anhydrous methanol, derivatized with derivatization reagents, and extracted with dichloromethane. The organic phase containing the aminosugar derivative was dried with Nitrogen at 45°C and redissolved in a 1:1 solvent of hexane and ethyl acetate. We quantified the concentrations of GluN, mannosamine (ManN), galactosamine (GalN), and MurA using an Agilent 6890A gas chromatograph/mass spectrometer and a DB-5 capillary column (30 m × 0.25 mm × 0.25 μm) with myo-inositol as an internal standard.