ISOTOPIC FRACTIONATION ACCOMPANYING FERTILIZER NITROGEN TRANSFORMATIONS IN SOIL AND TREES OF A SCOTS SPINE ECOSYSTEM

Foliage from a mature stand of Scots pine (Pinus sylvestris L.) receiv ing increasing doses of ammonium nitrate and urea nitrogen was assayed during the five subsequent growing seasons for total N concentration and N-15 abundance. The aim of the study was to examine the potential of the delta(15)N technique to provide estimates on fertilizer N recov ery and its fate in the ecosystem. The N-15 abundance in the foliage i ncreased in proportion to the dose of fertilizer application. This was generally owing to the fact that the delta(15)N of the fertilizer N w as significantly higher than that in the soil inorganic-N pool, as wel l as in the needle biomass of the Scots pine trees on the nonfertilize d plots. Due to N-15 isotope discrimination occurring during N transfo rmations in soil the relationship was however not very close. Calculat ions based on the principle of isotope dilution yielded only rough and , in some cases, even misleading estimates of the fraction of the fert ilizer-derived nitrogen (N-dff) in the needles. This was especially th e case for the urea-N, which undergoes significant isotopic fractionat ion during the process of ammonia volatilization and possibly microbia l NH4+ assimilation in soil. Over five growing seasons, foliar total N concentration peaked at the end of the second season while the N-15 a bundance continued to increase. Although large methodological errors m ay be involved when interpreting natural N-15 abundance, the measureme nt of delta(15)N seems to provide semi-quantitative information about fertilizer N accumulation and transformation processes in coniferous e cosystems. A better understanding of the tree and soil processes causi ng isotopic fractionation is a prerequisite for correct interpretation of N-15 data.