Export of phosphorus and nitrogen from lodgepole pine (Pinus contorta) brash windrows on harvested blanket peat forests

Abstract

Enhanced nutrient export from blanket peats and other peat soils after forest harvesting has been reported by several studies. As part of forest practices on afforested blanket peat, harvest residues are utilized as brash mats to improve the soil carrying capacity against heavy machinery to extract the harvested timber to the roadside, before being collected together to form brash windrows, between which the second rotation trees are planted. Three types of experiments were conducted to assess the impact of brash windrows, derived from lodgepole pine forests, on phosphorus (P) and nitrogen (N) release in blanket peat catchments. First, a field survey was conducted in 6 sites to study the easily extractable P and N contents in soil below brash windrows and respective brash free areas. The dynamics of N and P release when water flows from a windrow to a brash-free area was assessed using laboratory flume studies. Finally, the role of windrows in N and P export to adjacent watercourses under field conditions was assessed using artificially isolated mini-catchments with and without harvest residues. The study showed that windrows in blanket peat catchments are a significant source of P release to soil and soil water underneath and act as hot spots for P export to receiving waters. The windrows may act as a sink rather than a source for N during the early stages of their decomposition. However, more N was exported from brash-covered than brash-free areas, the probable reason being that microbial activity and soil N mineralization was increased under brash. Forest management options other than windrowing brash should be developed and studied to decrease N and P release and transport to watercourses from blanket peat catchments.