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?

摘要

High delta 15N 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 delta 15N of the soil, but attributed the high delta 15N 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 15N of trees and the soil profile, because we have previously found higher delta 15N 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 delta 15N 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 delta 15N 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).