Partitioning between atmospheric deposition and canopy microbial nitrification into throughfall nitrate fluxes in a Mediterranean forest

1. Microbial activity plays a central role in nitrogen (N) cycling, with effects on forest productivity. Though N bio-transformations, such as nitrification, are known to occur in the soil, here we investigate whether nitrifiers are present in tree canopies and actively process atmospheric N. 2. This study was conducted in a Mediterranean holm oak (Quercus ilex L.) forest in Spain during the transition from hot dry summer to cool wet winter. We quantified NH4+—N and NO3-—N fluxes for rainfall (RF) and throughfall (TF) and used δ15N, δ18O, and Δ17O to elucidate sources of NO3-. Finally, we characterized microbial communities and abundance of nitrifiers on foliage, RF and TF water through metabarcoding and quantitative Polymerase Chain Reaction, respectively. 3. NO3—N fluxes at the site were larger in TF than RF, suggesting a contribution from dry deposition, as also supported by δ15N and δ18O. However, Δ17O indicated that about 20% of NO3- in TF derived from canopies nitrification in Aug...