Cycling and transport of phosphorus and nitrogen from harvested peatland forests and possible mitigation and methods

Abstract

Drainage of peatlands has been part of forestry practice since the early 20th century. In Ireland and the UK, approximately 800,000 ha of peat catchments were drained and afforested between the 1950s and 1990s. A large number of these forests are now approaching maturity and the rate of forest harvesting will undergo a rapid increase in the near future. Nutrient release following harvesting of upland blanket peat forests in western Ireland has become a significant environmental concern, as these forests drain into ecologically valuable, oligotrophic, salmonid rivers. Therefore, the objectives of this study were to assess the mechanisms of release and transport of phosphorus (P) and nitrogen (N) from harvested peatland forests and the effectiveness of various mitigation methods to decrease nutrient export from them. A litterbag study was used to investigate the decomposition and P and N release dynamics of harvest residue needles on different peat sites and a laboratory incubation study assessed the mobilization of P and N from peat due to a rise in water table. Laboratory flume and mini-catchment experiments investigated the effect of harvest residue brash windrows on nutrient export from harvested blanket peatlands and nutrient retention efficiency of blanket peatland buffer areas. The results suggest that P is easily released from decaying harvest residue needles, whereas N is mostly retained or accumulated during the initial stages of decomposition. Therefore, the release of N from harvest residue needles is not a likely source of the increased N export following clear-cutting, but harvest residues may contribute to P losses. High water tables produced anoxic conditions in upper peat layers and resulted in P mobilization from moderately decomposed, nutrient-poor, Irish blanket peat and highly decomposed, nutrient-rich, Irish blanket peat. In peats from Finland, P mobilization occurred only from nutrient-poor, ombrotrophic peats, and not from nutrient-rich, minerotrophic peats. A high water table also caused higher mobilization of ammonium-N (NH4+-N) from nutrient-rich peats compared to nutrient-poor peats. These results suggest that water-logged conditions may cause different patterns of nutrient release from different types of peats depending on their biogeochemical characteristics.The release of P from brash windrows in blanket peat catchments is a significant source of P to receiving waters. Although brash windrows do not release N during the early stages of their decomposition, the decomposition of brash may enhance N leaching and export, probably by enhancing microbial activity and soil N mineralization below brash. The removal of harvest residues and whole-tree harvesting could be an efficient means of decreasing N and P release and transport to water courses from blanket peat catchments. Conventionally-designed buffer areas, with and without seeded grasses, may have limited effect on P retention from through-flow waters in blanket peat catchments, as frequent high flow episodes, typical in blanket peat areas, are disadvantageous for effective P retention. Alternatively, a mini-buffers method, comprising brash mats and windrows positioned perpendicular to the slope, as opposed to along the main slope, and with the strips between the brash windrows seeded with native grasses, is proposed. In contrast to conventional buffer areas, a grassed or non-grassed mini-buffers method might be a feasible means of decreasing P export to water courses from harvested blanket peat sites