Aggregation but Not Organo‐Metal Complexes Contributed to C Storage in Tidal Freshwater Wetland Soils

Core Ideas Water‐stable aggregates may assist in the preservation of organic carbon in wetland soils. Soil C was more evenly distributed among the aggregate‐size classes in the restored but not the natural site. Macroaggregate‐C more closely resembled plan detritus whereas mineral‐C was enriched with N‐containing compounds. One of the many goals of wetland restoration is to promote the long‐term storage of carbon (C) in the terrestrial biosphere. Unfortunately, soil C reservoirs in restored wetlands are slow to accumulate even after hydrology and plant communities are reestablished. Oftentimes wetland restoration changes the soil matrix and thus can dramatically alter how soil C is stored and processed. Our research investigated whether soil organic matter (SOM) preservation theories derived from studies in non‐wetland soil systems can be extended to wetland soils. We examined C associated with water‐stable soil aggregates, minerals, and metal oxides within habitats of one natural and one restored tidal freshwater wetland. This study revealed that a majority of the soil C in the natural site was associated with large macroaggregates (>2000 μm), and soils from the restored site stored more C in small macroaggregates (≥250 to <2000 μm). Despite these different associations, the chemical composition of SOM followed similar patterns across each aggregate‐size class. Results from the sequential extraction procedure suggest organo‐metal oxide complexes do not contribute to C stabilization in these habitats. This research is one of the few studies that have examined C stabilization related to soil structure in wetland soils. Our results suggest soil aggregate formation may be an important mechanism driving C stabilization, and that disruption to macroaggregates may limit C accumulation in restored wetlands. Additional empirical research and long‐term field monitoring are needed to confirm linkages between aggregate‐C stabilization and accumulation in wetland soils.