Comparison of Soil C Pools and Microbial Activity in a Drained and Natural Freshwater Forested Wetland of Coastal North Carolina.

Wetland ecosystems account for a small proportion of terrestrial ecosystems (2-5 %) but store upwards of 25 % of all terrestrial carbon (C), making them a significant contributor to the global C cycle. Drainage of coastal wetlands for silvicultural purposes alters the capacity of these ecosystems to sequester C, by increasing soil aeration and thereby stimulating oxidative breakdown of soil organic C (SOC).  We compared basic soil chemical and physical properties (e.g., bulk density, pH, and redox potential), soil C pools (e.g., SOC, microbial biomass C, and Δ¹³C), and microbial activity in soils profiles from a drained and undrained wetland.  Two field sites in coastal North Carolina were used for this study: 1) freshwater forested wetland located in the Alligator River National Wildlife Refuge; and 2) drained forested wetland in the fourth rotation of loblolly pine.  Soil samples were collected from both sites between January and March 2016 from organic and mineral diagnostic horizons (e.g., Oi, Oe, Oa1, Oa2, Oa3, A, AB, and B).  Overall, the drained wetland had higher pH (4.88), Eh (428 mV), and bulk density (0.93 g cm^-3) compared to undrained wetland (4.50, 370 mV, and 0.38 g cm^-3, respectively).  On average, the peat surface had subsided 28 cm, the C density had increased by 0.5 g cm^–3, and the C stores had increased by 337 Mg C ha^-1 after drainage.  Soil Δ¹³C showed a clear pattern of enrichment with soil depth in the drained wetland but not in the undrained wetland, indicating the greater degree of microbial processing that has occurred at the drained site.  Indices of microbial activity indicated that the drained site had a greater microbial biomass C and enzyme activity.  It is possible that peat depth prior to drainage was greater than current peat depths in the forested wetland, leading to observed increases in soil C pools as soils subsided.  Inputs of new tree-derived C post-drainage, via litter input and root turnover, may also play a role in impacting long-term C balances of drained wetland.