Isotopic fractionation accompanying fertilizer nitrogen transformations in soil and trees of a Scots spine ecosystem

Abstract

Foliage from a mature stand of Scots pine (Pinus sylvestris L.) receiving increasing doses of ammonium nitrate and urea nitrogen was assayed during the five subsequent growing seasons for total N concentration and 15N abundance. The aim of the study was to examine the potential of the δ15N technique to provide estimates on fertilizer N recovery and its fate in the ecosystem. The 15N abundance in the foliage increased in proportion to the dose of fertilizer application. This was generally owing to the fact that the δ15N of the fertilizer N was significantly higher than that in the soil inorganic-N pool, as well as in the needle biomass of the Scots pine trees on the nonfertilized plots. Due to 15N isotope discrimination occurring during N transformations in soil the relationship was however not very close. Calculations based on the principle of isotope dilution yielded only rough and, in some cases, even misleading estimates of the fraction of the fertilizer-derived nitrogen (Ndff) in the needles. This was especially the case for the urea-N, which undergoes significant isotopic fractionation during the process of ammonia volatilization and possibly microbial NH4+ assimilation in soil. Over five growing seasons, foliar total N concentration peaked at the end of the second season while the 15N abundance continued to increase. Although large methodological errors may be involved when interpreting natural 15N abundance, the measurement of δ15N seems to provide semi-quantitative information about fertilizer N accumulation and transformation processes in coniferous ecosystems. A better understanding of the tree and soil processes causing isotopic fractionation is a prerequisite for correct interpretation of 15N data.