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 ‰) as well as of seedlings (−5.0,±0.11 ‰) relative to that of mature pine needles (−5.6,±0.10 ‰) and seedlings (−6.0,±0.31 ‰). 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 ‰) and spruce (−2.09,±0.05 ‰) in comparison to that of natural stand (pine:−5.60,±0.10 ‰; spruce:−4.00,±0.20 ‰), 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 δ15N. †Revised version of a paper presented at the VIII Isotope Workshop of the European Society for Isotope Research (ESIR), 25–30 June 2005, Leipzig-Halle, Germany