Effects of long-term soil warming on soil organic and inorganic nitrogen cycling in a temperate forest soil as assessed by measurements of natural 15N abundances of soil N pools

The capacity of forest soils and trees to sequester C is closely linked to soil nitrogen (N) bioavailability, a major control of microbial and plant growth and functioning. Recent meta-analyses indicated that both, soil organic C and N cycling, intensify with climate/soil warming, though little studies investigated this in long-term (decadal) warming experiments. Changes in N cycling processes have been addressed by measuring total and labile N pools, net and gross N process rates, and changes in extracellular enzyme activities. An alternative approach, integrating over longer time intervals, is to study the natural ¹⁵N abundances of different soil and plant N pools. In this study we quantified the natural ¹⁵N abundances (d¹⁵N values) of coarse and fine litter, fine roots, soil organic N, extractable organic N, microbial biomass N, ammonium and nitrate at the long-term soil warming experimental site in Achenkirch (Tyrol, Austria). This site is one of the few climate manipulation experiments in forests operating for more than 14 years and has provided unique insights into the effects of global warming on forest ecosystem processes. We analyzed ecosystem compartments across three seasons (May, August, October 2019), to investigate the consistency of warming effects on soil N cycle processes. Moreover, we developed an isotope fractionation model to decipher the isotope fractionations of the studied soil N processes and the fractions transformed by them, i.e. for depolymerization, microbial uptake, N mineralization, nitrification and soil N losses. Overall, the consistent increase in fine root δ¹⁵N in warmed soils indicated a general opening of the soil N cycle (greater N losses), which was mirrored in increased ammonium d¹⁵N values, the latter implying increased fractions of ammonium being oxidized to nitrate. Higher fractions of ammonium being nitrified makes labile N more amenable to N losses, either by leaching of nitrate or by denitrification losses. Since nitrification and denitrification exhibit strong isotope fractionation effects against ¹⁵N, the lost N is concomitantly ¹⁵N depleted, while residual substrates remaining in the ecosystem become ¹⁵N enriched, thereby explaining the ¹⁵N enrichment with increasing N cycling and N loss rates in warmed soils.