Abstract

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).

Keywords

Ectomycorrhiza; Forest soils; Nitrogen deposition; N-15 natural abundance

Published in

Biogeochemistry
2014, Volume: 117, number: 2-3, pages: 351-358

SLU Authors

• Högberg, Peter

  • Department of Forest Ecology and Management, Swedish University of Agricultural Sciences
    

• Högberg, Mona N

  • Department of Forest Ecology and Management, Swedish University of Agricultural Sciences
    

Sustainable Development Goals

Protect, restore and promote sustainable use of terrestrial ecosystems, sustainably manage forests, combat desertification, and halt and reverse land degradation and halt biodiversity loss


UKÄ Subject classification

Environmental Sciences


Publication identifier

DOI: https://doi.org/10.1007/s10533-013-9873-x

Permanent link to this page (URI)

https://res.slu.se/id/publ/51988