Long-term soil warming causes acceleration of soil nitrogen losses in a temperate forest studied by 15N isotope fractionation

Climate warming was shown to strongly affect the biogeochemical cycles in global forests, reducing soil carbon storage and accelerating soil nitrogen (N) and phosphorus cycling. In a long-term soil warming experiment in a temperate old-growth forest in Achenkirch, Austria, we recently showed faster root turnover and growth, decreases in microbial biomass, carbon use efficiency and soil carbon storage, increases in ecosystem phosphorus limitation, and varied responses of the soil N cycle in warmed plots (+4 ° C above ambient for 14 years). In this study we therefore employed natural stable isotope techniques to better understand ecosystem-level responses of the N cycle in Achenkirch, studying the abundance of ¹⁵N and ¹⁴N (expressed as δ¹⁵N values) in a wide range of soil nitrogen pools (bulk soil N, root N, microbial biomass N, extractable organic N, ammonium, nitrate) and employed isotope fractionation models to explain the patterns found dependent on soil warming. Specific N cycle processes such as mineralization, nitrification and denitrification cause substantial isotope fractionation (against the heavy stable isotope ¹⁵N), leading to ¹⁵N enrichment of the residual substrates and ¹⁵N depletion of the cumulative products, depending on the fraction on substrates consumed and the isotope fractionation factor of that process. Other processes such as diffusion, (de)sorption and depolymerization exert negligible isotope fractionation. We found a significant warming effect on the isotopic signatures of root N and the soil ammoniumpool, i.e. a ¹⁵N enrichment in these pools. ¹⁵N enrichment of tree fine roots, considered to be isotopic integrators of the plant available N pool, suggest increased soil N cycling and greater soil N losses in warmed plots causing a ¹⁵N enrichment of the soil inorganic N pool (ammonium and nitrate). The increased ¹⁵N enrichment in ammonium of warmed soils highlights an increased activity of nitrifiers, with greater fractions of ammonium oxidized to nitrate causing the observed ¹⁵N enrichment of ammonium. However, soil nitrate did not show the expected ¹⁵N depletion imparted by nitrifiers but matched or even exceeded δ¹⁵N values of soil ammonium. Isotope fractionation calculations indicated that >50% of the soil nitrate produced was lost, particularly through denitrification promoting gaseous N losses in the form of NO, N₂O and/or N₂ and less through nitrate leaching. Natural ¹⁵N abundance studies thereby hold great potential for evaluating the status quo of the complex N cycle in terrestrial ecosystems and to monitor in situ responses to climate change with minimal invasion and improved time integration.