N-15 ABUNDANCE OF SOILS AND PLANTS ALONG AN EXPERIMENTALLY-INDUCED FOREST NITROGEN SUPPLY GRADIENT

N-15 abundances of soils and a grass species (Deschampsia flexuosa (L.) Trin.) were analysed in a forest fertilization experiment 10 years after the last fertilization. Nitrogen had been given as urea, at seven doses, ranging from 0 to 2400 kg N ha-1. Previously, we have shown that plants in systems experiencing large losses of N become enriched with N-15. This was explained by the fact that processes leading to loss of N, e.g. ammonia volatilization, nitrification followed by leaching or denitrification and denitrification itself, tend to fractionate against N-15. In this experiment, N-15 abundance increased with dose of N applied in both grass and soil total-N, but more so in the grass. This was interpreted to be due to the grass sampling small but active pools of N subject to losses. In contrast, soil total-N largely consists of inactive N that does not immediately exchange with pools of N from which fractionating losses occur. Hence, soil total-N shows a large pretreatment N-15 memory effect, and is, therefore, an integrator of the long-term N balance. When short-term changes (years, decades) in N balances are monitored using variations in N-15 abundance, plants are more suitable indicators of such change than is soil total-N.