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This study presents the latitudinal variation (from 60°30¢ N to 68°2¢ N latitude) of natural abundances of 15 N in the foliage, humus and soils of boreal forests of Finland. Our results clearly showed that N concentration of the foliage did not change significantly with latitudes but their 15 N values were significantly higher in higher latitude sites relative to that of the mid and lower latitude sites, indicating the different forms of N uptake at the higher latitudes compared to the lower latitudes. We assume that the higher foliage d 15 N values of the higher latitudes trees might be due to either more openness of N cycle (greater proportional N losses) in these latitudes compared to the sites of southern latitudes (lower N losses) or the differences in their mycorrhizal associations. Regression analysis showed that the temperature was the main factor influencing the 15 N natural abundance of humus and soil of all forest ecosystems, both before and after clear-cut, whereas rainfall was the main controlling factor to the foliage 15 N. Possible reasons behind the increasing d 15 N natural abundances of plants and soils with increasing latitudes are discussed in this paper. The clear-cut did not show any specific trend on the 15 N fractionation in humus and soil, i.e. both 15 N-enrichment and -depletion occurred after clear-cut.

C than the lower altitude soil. The decreasing trend of C-isotope (as well as the trend of N-isotope) might be attributed to the lower mineralisation rate and net nitrification rate at the higher altitude. The observed differences in isotopic N-and C-contents of needles and soils across altitudinal gradients in this study are a result of the combination of environmentally induced variations in physiological and morphological differences. For example, altitudinal variation in needle δ 15 N and δ 13 C at the research site indicates that the needle isotopic composition is related to nitrogen availability at a site.

To test the reliability of the radiocarbon method for determining root age, we analyzed fine roots (originating from the years 1985-1993) from ingrowth cores with known maximum root age (1-6 years old). For this purpose, three Scots pine (Pinus sylvestris L.) stands were selected from boreal forests in Finland. We analyzed root 14 C age by the radiocarbon method and compared it with the abovementioned known maximum fine root age. In general, ages determined by the two methods (root 14 C age and ingrowth core root maximum age) were in agreement with each other for roots of small diameter (\0.5 mm). By contrast, in most of the samples of fine roots of larger diameter (1.5-2 mm), the 14 C age of root samples of 1987-1989 exceeded the ingrowth core root maximum age by 1-10 years. This shows that these roots had received a large amount of older stored carbon from unknown sources in addition to atmospheric CO 2 directly from photosynthesis. We conclude that the 14 C signature of fine roots, especially those of larger diameter, may not always be indicative of root age, and that further studies are needed concerning the extent of possible root uptake of older carbon and its residence time in roots.