Distribution of soil organic matter in freshwater emergent/open water wetlands in the Portland, Oregon metropolitan area

We measured soil organic matter (SOM) concentrations in a large sample (n = 95) of freshwater emergent and open water wetlands in the Portland, Oregon, USA, area as part of a study of the ecological development of mitigation wetlands. Mean SOM concentrations were higher in naturally occurring wetlands (NOWs) than in mitigation wetlands (MWs) at 0-5 cm (SOM = 9.75 and 5.83%, respectively, p = 0.0001) and at 15-20 cm (SOM = 6.85, 4.68%, p = 0.0551). If temporal accumulation of SOM is occurring, it is slow; we found no significant relationship between SOM and wetland age (p = 0.6003) and no significant change in SOM concentration in soils in MWs sampled in 1987 and 1993. Concentrations of SOM were not significantly related to land use but were related to soil series, texture class, and association, and to hydrogeomorphic class. For a subset of wetlands monitored for hydrology, we also found a significant negative relationship between SOM and the extent of inundation by standing water. Mitigation may be leading to direct loss of SOM, probably resulting from soil management practices during project construction. We also show that hydrologic regime significantly affects SOM. Because most projects in our study were built in pre-existing wetlands and have extensive areas of open water, our results suggest that low concentrations of SOM are likely to persist. For SOM and probably for SOM-supported wetland functions, fundamental goals of mitigation and wetland management (in-kind wetland replacement, no-net-loss of structure and function) are not being achieved, at least in the short term. The success of mitigation, in terms of SOM, could be improved by better project design and better management of soils during project construction.