Increasing and Decreasing Nitrogen and Phosphorus Trends in Runoff from Drained Peatland Forests—Is There a Legacy Effect of Drainage or Not?

A recent study on nitrogen (N) and phosphorus (P) exports from drained peatland forests reported increasing concentrations over long time since their initial drainage. Concurrently, some other studies have suggested decreasing trends from drained peatland forests, particularly for P. To evaluate these contradictory findings, we re-analyzed past data and reviewed the literature related to temporal N and P concentration trends in runoff from drained peatland forests. Review of literature indicated that decreasing trends are found particularly in sites where initial P concentrations are high (> 50 μg P l⁻¹), plausibly because of relatively recent fertilization and drainage operations. Decreasing N trends have been found in sites where ditch cleaning temporarily decreased concentrations. Increasing N trends have occurred in sites, where initial concentrations have been low, close to the levels found in pristine peatlands. Complementing past published data with additional data from sites with no recent forestry operations indicated that N concentrations correlated positively with drainage age (years since initial drainage), percentage of drained peatlands in the catchment (drainage proportion), and southern location of the study site. P concentrations correlated most strongly with drainage age. Our study indicated that four factors, in particular, need to be considered when interpreting nutrient concentration trends in runoff from drained peatlands: 1) management history, 2) drainage age, 3) drainage proportion, and 4) site location. Our results supported earlier conclusions in that the estimates which ignore the legacy effect of drainage remarkably underestimate the true impact of forestry on water courses in intensively drained regions.