Impact of Re-wetting of Forestry-Drained Peatlands on Water Quality—a Laboratory Approach Assessing the Release of P, N, Fe, and Dissolved Organic Carbon

A laboratory column study with peats from four sites from south-central Finland and two sites from blanket peats in the west of Ireland was established to assess the factors contributing to P, N, Fe, and dissolved organic carbon (DOC) transfer to receiving water courses from restored forestry-drained peatlands. The study indicated that the DOC and Fe release from re-wetted peats are likely governed by the amount of Fe in peat and the degree of Fe reduction upon re-wetting. In contrast to our other hypothesis concerning DOC, high degradability of organic matter was not related to high DOC release. Nitrate release was found to largely cease along with oxygen depletion, but ammonium release was considerable from a site with high nitrification potential before wetting. The release of P from anoxic peat was complicated in the sense that it appeared to be controlled by many factors simultaneously. In the nutrient-poor sites, the P release increased following re-wetting, probably because of their high easily soluble peat P content and low Al and Fe content, resulting in high anoxic P mobilization, but limited re-sorption of the mobilized P. Among the three nutrient-rich sites, there was either no P release upon re-wetting or higher P release than from the nutrient-poor sites. Low risk for P release following re-wetting in nutrient-rich sites was associated with low content in peat of easily soluble P and a high molar Fe/P ratio.