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

Abstract

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 ammonium pool, 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.