Changes in natural¹⁵N abundance highlight warming-induced stimulation of soil nitrate losses by coupled nitrification–denitrification in an old-growth montane forest

Climate warming alters biogeochemical cycles, especially in high-altitude forests where warming accelerates soil organic matter decomposition and CO₂ efflux. Faster nitrogen (N) mineralization can enhance N availability to plants but may also increase N losses if soil microbial N use efficiency declines. However, long-term data on soil N loss mechanisms remain scarce. Key N cycling processes affect the natural 15N:14N isotope ratio (δ15N) differentially, with (de)nitrification yielding 15N-depleted products and leaving residual pools 15N-enriched. We investigated belowground N cycling after 14 years of soil warming (+4 °C) in a temperate old-growth forest in Achenkirch, Austria, by measuring δ15N values in belowground N pools (root N, bulk soil N, microbial biomass N, ammonium, nitrate) through isotope ratio mass spectrometry. Warming had no effect on δ15N of bulk soil N, microbial biomass N, and nitrate, but significantly increased δ15N in root N (−5.0 to −4.1‰) and in soil ammonium (−2.9 to 1.1‰). Root δ15N, reflecting inorganic soil N, indicates that warming-induced N losses caused 15N enrichment of inorganic soil N. Elevated ammonium δ15N points to increased rates of nitrification, while nitrate δ15N patterns imply denitrification (60–65% of nitrate sink) exceeding leaching as the main loss pathway, which aligns with available field observations. Coupled plant–soil δ15N analysis thus revealed decadal warming-driven changes in N cycling and identified coupled nitrification–denitrification as a key pathway of soil N loss, which is otherwise difficult to measure directly.