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?

High delta N-15 of tree foliage in forests subject to high N supply has been attributed to N-15 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 delta N-15 of the soil, but attributed the high delta N-15 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 delta N-15 of trees and the soil profile, because we have previously found higher delta N-15 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 N-15 than the corresponding NH4NO3 plots, and, importantly, the weighted average delta N-15 of the soil showed that N losses had been associated with fractionation against N-15 in the urea plots. Thus, our results in combination with those of Hogberg et al. (New Phytol 189:515-525, 2011) show that high delta N-15 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 N-15 (in case of urea ammonia volatilization, nitrification followed by leaching and denitrification).