title: Effects of clear-cutting and soil preparation on natural 15N abundance in the soil and needles of two boreal conifer tree species.
abstract: This study presents the impacts of clear-cutting and site preparation on soil and needle 15N-fractionation of Scots pine (Pinus sylvestris, L.) and Norway spruce (Picea abies (L.), Karst). Three microsites on different methods of site preparation were used: (i) mound (broken O/E/B horizons piled upside down over undisturbed humus), (ii) deep (exposed C-horizon) and (iii) shallow (exposed E/B horizon). We found significant differences between species, between closed forest and clear-cuts as well as between different site preparations. For instance, in the context of interspecific variations, the mean needle nitrogen concentrations of both seedlings (1.15,+/-0.10 %) and mature (1.09,+/-0.07 %) pine trees were significantly higher compared to corresponding needle concentrations of seedlings (0.88,+/-0.06 %) and mature trees (0.79,+/-0.02 %) of spruce. Similarly, we observed significantly more 15N-enriched needles of mature spruces (-4.0,+/-0.20 per thousand) as well as of seedlings (-5.0,+/-0.11 per thousand) relative to that of mature pine needles (-5.6,+/-0.10 per thousand) and seedlings (-6.0,+/-0.31 per thousand). These variations were assumed to be caused by the variation in mycorrhizal associations between the species. We assume that the proportion of mycorrhizal N-uptake of pines might have been larger than that of spruce. Regarding the clear-cut effects on N and 15N of both tree species, we observed that, in the mature natural stand, needle N concentrations of both pine (1.09,+/-0.07 %) and spruce (0.79,+/-0.02 %) tree species did not change significantly after clear-cutting (pine: 1.01,+/-0.06 %; spruce: 0.74,+/-0.04 % ). However, clear-cutting resulted in the significant increase in needle 15N natural abundance of both pine (-2.70,+/-0.06 per thousand) and spruce (-2.09,+/-0.05 per thousand) in comparison to that of natural stand (pine:-5.60,+/-0.10 per thousand; spruce:-4.00,+/-0.20 per thousand), which is assumed to be due to the increased level of nitrification and leaching of nitrate after clear-cutting. In the context of site preparation methods, soil and needle N were observed to be more 15N-enriched in deep and shallow treatment sites compared to that of closed forest site and untreated clear-cut site, which indicated that the main source of N uptake seems to be mainly directly from the soil of the corresponding horizons of mineral soil with higher delta15N.
authors.affiliation: Department of Forest Ecology, University of Helsinki, Latokartanonkaari 7, PL 27, FIN 00014 Helsinki, Finland. shambhu.sah@helsinki.fi
authors.forename: Shambu P
authors.initials: SP
authors.lastname: Sah
authors.forename: Hannu
authors.initials: H
authors.lastname: Ilvesniemi
mesh_terms: D016468:Forestry; D009584:Nitrogen / Q000032:analysis / Q000378:metabolism; D009587:Nitrogen Isotopes; D028222:Picea / Q000502:physiology; D028223:Pinus / Q000502:physiology; D012987:Soil; D064028:Tracheophyta / Q000737:chemistry* / Q000502:physiology
chemical_list: D009587:Nitrogen Isotopes; D012987:Soil; D009584:Nitrogen