Interspecific variation and impact of clear-cutting on natural 15N abundance and N concentration in the needle-to-soil continuum of a boreal conifer forest

The primary objective of this paper is to study the interspecific variation of 15N natural abundance in two dominant conifer tree species, Scot pine (Pinus sylvestris L.) and Norway spruce (Picea abies L. Karst) in a N-limited mixed boreal coniferous forest in southern Finland. Both a mature tree stand (a natural forest stand) and a stand of seedlings in a clear-cut area were considered. We analyzed %N and δ15N natural abundance in needles, branches, litter and soil of the both above-mentioned stands. Pine needles had higher N concentrations than spruce needles, both in mature forest (pine = 1.09%, SE ± 0.02 and spruce = 0.79%, SE ± 0.02) and in the clear-cut area (pine = 1.01%, SE ± 0.07 and spruce = 0.74%, SE ± 0.04). Furthermore, pine needles were significantly more depleted in δ15N natural abundance than the spruce needles, both in the mature forest (pine = -5.6‰, SE ± 0.1‰ and spruce = -4.0‰, SE ± 0.2‰) and in the clear-cut sites (pine = -2.8‰, SE ± 0.2‰ and spruce = -2.1‰, SE ± 0.3‰). The more pronounced depletion of 15N in the pine foliage might be connected to its different fungal associates in the roots compared to spruce roots. We assume that the mycorrhizal N-uptake is very prominent at the N-limited site. After clear-cutting, needles of the seedlings and the uppermost part of soil organic layer were found to be more 15N-enriched than at the natural forest site. This was attributed to an increased level of nitrification in the clear-cut site soil.