Decomposition of harvest residue needles on peatlands drained for forestry – Implications for nutrient and heavy metal dynamics

In the boreal and temperate zones about 15 million hectares of peatlands and wetlands have been drained for forestry purposes and a large number of these forests are now approaching their commercial thinning or regeneration age. One of the major concerns raised in connection with an increased harvesting of drained peatland forests is the deterioration of the downstream water quality due to an enhanced nutrient transport. Harvest residues left on the site are a potential high source of nutrients to recipient water bodies and both increased N and P exports have been reported after conventional stem-only clear-cutting. We studied the decomposition of Picea abies and Pinus sylvestris harvest residue needles at two clear-cut areas and two uncut forested areas on drained peatlands at two locations in southern Finland. Our results indicated that P is easily released from harvest residue needles. After the first three growing seasons, spruce and pine needles had lost approximately 31% and 47% of their initial P contents, respectively. There was no clear gain or loss of nitrogen. Most of the heavy metals accumulated in the needles as the decomposition process proceeded, especially at the more southern study location near the heavily industrialized Helsinki capital area with high atmospheric deposition. The conclusion is that harvest residue needles are not a likely source of the increased N export that has been observed to occur from peat soils soon after clear-cutting, but that P release from harvest residues may be a cause for the reported high P losses.

► We studied the decomposition of Picea abies and Pinus sylvestris harvest residue needles.
► P was easily released from needles.
► No clear gain or loss of nitrogen during the first few years of decomposition.
► Most of the heavy metals accumulated, especially at the site with higher atmospheric deposition.
► Soon after clear-cutting, needles may initially contribute to P export but not likely N export.