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?

• Published in: Biogeochemistry Vol. 117; no. 2-3; pp. 351 - 358
• Main Authors: Högberg, Peter, Johannisson, Christian, Högberg, Mona N
• Format: Journal Article
• Language: English
• Published: Cham Springer-Verlag 01.03.2014; Springer; Springer International Publishing
• Subjects: ammonia; ammonium nitrate; Animal and plant ecology; Animal, plant and microbial ecology; Biogeosciences; Biological and medical sciences; Boreal forests; Coniferous forests; denitrification; Earth and Environmental Science; Earth Sciences; Earth, ocean, space; Ecosystems; ectomycorrhizae; Environmental Chemistry; Environmental Sciences; Exact sciences and technology; Forest ecosystems; Forest soils; forests; Fundamental and applied biological sciences. Psychology; Fungi; isotope fractionation; Isotope geochemistry; Isotope geochemistry. Geochronology; Isotopes; leaching; Leaves; Life Sciences; long term experiments; Miljövetenskap; nitrification; Nitrogen; nitrogen content; Soil horizons; soil profiles; Soils; Surficial geology; Synecology; Terrestrial ecosystems; Trees; urea; volatilization; Ectomycorrhiza; Nitrogen deposition; Forest soils; N-15 natural abundance; experimental studies; minerals; mass balance; vegetation; trees; denitrification; urea; enrichment; forest soils; soil profiles; soils; Thallophyta; ammonia compounds; stable isotopes; N-15/N-14; nitrogen; Plantae; N-15; concentration; organic minerals; leaching; forests; fungi; ecosystems; isotope fractionation; volatilization
• ISSN: 0168-2563; 1573-515X; 1573-515X
• DOI: 10.1007/s10533-013-9873-x

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 (Högberg 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 Högberg 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).