Sediment accretion and accumulation of P, N and organic C in depressional wetlands of three ecoregions of the United States.

Wetland depressions without surface channel connections to aquatic systems are substantial sinks for nitrogen (N), phosphorus (P) and organic carbon (org. C). We assessed accretion, N, P and org.-C accumulation rates in 43 depressional wetlands across three ecoregions of the USA (Erie Drift Plain, EDP; Middle Atlantic Coastal Plain, MACP; Southern Coastal Plain, SCP) using caesium-137 (137Cs). The mean sediment accretion rate in minimally affected (reference) sites was 0.6 ± 0.4 mm year-1 and did not differ among ecoregions. Accumulation rates for N and org. C averaged 3.1 ± 3.1 g N m-2 year-1and 43.4 ± 39.0 g org. C m-2 year-1 respectively, and did not differ across minimally affected sites. Phosphorus accumulation rates were significantly greater in EDP (0.10 ± 0.10 g P m-2 year-1) than MACP (0.01 ± 0.01 g P m-2 year-1) or SCP (0.04 ± 0.04 g P m-2 year-1) sites. Land-use modality and wetland-type effects were analysed in SCP, with few differences being found. Depressional wetlands sequester substantive amounts of nutrients and C; their cumulative contributions may significantly affect landscape nutrient and C dynamics because of the abundance of wetland depressions on the landscape, warranting further investigation and potential watershed-scale conservation approaches.