Depth-Resolved Microbial Functional Diversity and Carbon Utilization Across a Forest–Wetland Gradient

Soil represents the largest terrestrial carbon sink, with a substantial fraction stored in subsoils. Microbial functional diversity regulates ecosystem carbon cycling, yet how microbial traits vary with soil depth and landscape transitions remain poorly understood.  This knowledge gap is particularly relevant in coastal environments, where hydrologic and biogeochemical gradients impose strong selective pressures on microbial metabolism. We investigated microbial functional diversity and carbon utilization patterns across a coastal forest–salt marsh gradient, with a specific focus on depth-resolved trait expression and biogeochemical consequences. Monthly in situ porewater sampling was conducted across forest, wetland, and creek environments, from surface soils to subsurface layers. Porewater chemistry (pH, redox potential, electrical conductivity, dissolved organic carbon, DOC) was monitored to characterize environmental and biogeochemical gradients. Microbial carbon utilization patterns and functional diversity were assessed using Biolog EcoPlates, and key extracellular enzyme activities (β-glucosidase and phosphatase) were measured to evaluate microbial activity. DOC concentration increased from forest to wetland soils, accompanied by shifts in microbial functional traits. Forest soils, wetland surface layers and creek samples supported higher microbial diversity, whereas wetland deep layers retained a strong metabolic capacity for processing complex organic carbon substrates, indicating functional specialization under persistent anoxic and saline conditions. Deep layers showed measurable enzyme activities, indicating active microbial carbon turnover. These findings demonstrate that microbial functional diversity varies across both depth and landscape gradients, with implications for carbon transformation and storage in coastal ecosystems.