Mangrove ecosystems represent globally significant carbon sinks, with particular importance in their belowground rhizosphere processes, lower DOC alone cannot explain the 41% carbon deficit if productivity is equivalent.

The role of microbial biomass emerges as central to understanding carbon recovery constraints. Microbial biomass carbon was 32% lower in restored forests, and microbial biomass explained more variance in soil carbon stocks than any other single variable (r = 0.68). Structurally, this low MBC could reflect two mechanisms: ^[1] reduced microbial abundance, limiting total carbon stabilization through microbial necromass; or (2) reduced microbial efficiency in converting labile carbon to stable biomass (20). The microbial quotient analysis provides insight here lower MBC: SOC ratios in restored forests (0.38% vs. 0.51%) suggest that the carbon present is less associated with active microbial biomass, indicating reduced microbial presence relative to carbon stocks. This pattern implies that restored forest rhizospheres support smaller microbial populations, potentially explaining slower carbon accumulation.

The shift toward fungal-dominated communities in restored mangroves, evidenced by increased fungal PLFA proportions and lower bacterial-to-fungal ratios, has implications for carbon cycling. Fungi typically mineralize organic matter more slowly than bacteria and produce more persistent extracellular polymers (21). While this might intuitively favor carbon accumulation, our observation that total microbial biomass (fungal + bacterial) is lower in restored forests suggests that the community shift reflects overall biomass reduction rather than a shift toward more conservative carbon metabolism. The lower B:F ratio may indicate substrate quality changes with restored systems dominated by more recalcitrant substrates preferentially utilized by fungi or reduced nutrient cycling efficiency.

Lower soil respiration rates in restored forests (Figure 2A) indicate reduced heterotrophic microbial activity, consistent with the lower microbial biomass. However, respiration rates increase with restoration age, approaching natural forest levels in the oldest restored sites. This suggests that microbial community establishment and activity gradually recover over time, and that time alone may be sufficient for recovery if ecological constraints are not severe.