Microbial necromass accumulation and amino sugar signatures in Amazonian mangrove soils

Mangrove soils are among the most efficient natural sinks of soil organic carbon (SOC), yet the processes controlling carbon stabilization in tropical mangroves remain poorly understood. In particular, the role of microbial necromass in blue carbon systems influenced by strong fluvial and marine interactions is still underexplored. Here, we quantified amino sugars (muramic acid, glucosamine and galactosamine), measured by high-performance liquid chromatography (HPLC), as biomarkers of microbial necromass in surface soils (0–15 cm) from four mangrove forests along the Amazon coast (Amapá, Brazil), a region characterized by intense sediment dynamics and heterogeneous physicochemical conditions driven by Amazon River discharge and tidal forcing. Total amino sugar concentrations varied markedly among sites, ranging from 283 ± 52 to 678 ± 180 µg g⁻¹ soil, indicating strong spatial heterogeneity in microbial residue accumulation. Microbial necromass carbon (MNC) showed a similar pattern, with values between 2.26 ± 0.50 and 5.35 ± 1.46 g kg⁻¹ soil. Across all mangroves, fungal necromass carbon consistently dominated total microbial necromass, highlighting the prevalence of fungal-derived residues in these systems. The close correspondence between amino sugar contents and MNC emphasizes the central contribution of microbial residues to SOC pools in mangrove soils. Overall, these results demonstrate that microbial necromass constitutes a substantial and spatially variable component of soil organic carbon in Amazonian mangrove soils, highlighting the relevance of microbial residues for understanding carbon storage patterns in tropical coastal environments.