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 (<i>Pinus sylvestris</i> L.) and Norway spruce (<i>Picea abies</i> 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.