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

• Published in: Plant and soil 1994, Vol.158 (2), p.169-182
• Main Authors: Nommik, H, Pluth, D J, Larsson, K, Mahendrappa, M K
• Format: Article
• Language: English
• Subjects: abonos nitrogenados; ammonia; ammoniac; ammonium nitrate; amoniaco; aplicacion de abonos; engrais azote; fertilisation; fertilizer application; feuille; hojas; leaves; nitrate d' ammonium; nitrato de amonio; nitrogen fertilizers; Pinus sylvestris; soil; sol; suelo; technique des traceurs; tecnicas de trazadores; tracer techniques; urea; uree; volatilidad; volatilite; volatility
• ISSN: 0032-079X; 1573-5036

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 super(15)N abundance. The aim of the study was to examine the potential of the delta super(15)N technique to provide estimates on fertilizer N recovery and its fate in the ecosystem. The super(15)N abundance in the foliage increased in proportion to the dose of fertilizer application. This was generally owing to the fact that the delta super(15)N 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 super(15)N 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 (N sub(dff)) 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 NH sub(4) super(+) assimilation in soil. Over five growing seasons, foliar total N concentration peaked at the end of the second season while the super(15)N abundance continued to increase. Although large methodological errors may be involved when interpreting natural super(15)N abundance, the measurement of delta super(15)N 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 super(15)N data.