Is the high 15N natural abundance of trees in N-loaded forests caused by an internal ecosystem N isotope redistribution or a change in the ecosystem N isotope mass balance?

Abstract

High δ15N of tree foliage in forests subject to high N supply has been attributed to 15N enrichment of plant available soil N pools after losses of N through processes involving N isotope fractionation (ammonia volatilization, nitrification followed by leaching and denitrification, and denitrification in itself). However, in a long-term experiment with high annual additions of NH4NO3, we found no change in the weighted average δ15N of the soil, but attributed the high δ15N of trees to loss of ectomycorrhizal fungi and their function in tree N uptake, which involves redistribution of N isotopes in the ecosystem (Hogberg et al. New Phytol 189:515–525, 2011), rather than a loss of isotopically light N. Here, we compare the effects of additions of urea and NH4NO3 on the δ15N of trees and the soil profile, because we have previously found higher δ15N in tree foliage in trees in the urea plots. Doing this, we found no differences between the NH4NO3 and urea treatments in the concentration of N in the foliage, or the amounts of N in the organic mor-layer of the soil. However, the foliage of trees receiving the highest N loads in the urea treatment were more enriched in 15N than the corresponding NH4NO3 plots, and, importantly, the weighted average δ15N of the soil showed that N losses had been associated with fractionation against 15N in the urea plots. Thus, our results in combination with those of Hogberg et al. (New Phytol 189:515–525, 2011) show that high δ15N of the vegetation after high N load may be caused by both an internal redistribution of the N isotopes (as a result of change of the function of ectomycorrhiza) and by losses of isotopically light N through processes fractionating against 15N (in case of urea ammonia volatilization, nitrification followed by leaching and denitrification).