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

Abstract Microbial activity plays a central role in nitrogen (N) cycling, with effects on forest productivity. Although N biotransformations, 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. 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 —N and —N fluxes for rainfall (RF) and throughfall (TF) and used δ 15 N, δ 18 O and Δ 17 O to elucidate sources of . Finally, we characterized microbial communities and abundance of nitrifiers on foliage, RF and TF water through metabarcoding and quantitative polymerase chain reaction respectively. NO 3 —N fluxes at the site were larger in TF than RF, suggesting a contribution from dry deposition, as also supported by δ 15 N and δ 18 O. However, Δ 17 O indicated that about 20% of in TF derived from canopies nitrification in August, after a severe drought, with a lower proportion in September (≈8%). This seasonal partitioning between biologically and atmospherically derived coincided with a decreasing trend of the abundance of archaeal nitrifiers. Tree canopies and TF had more diverse microbial communities than RF. Yet, RF showed higher variability in microbial composition, likely associated with the origin of air masses. Synthesis . Atmospheric N deposition is significantly altered after passing through tree canopies. While nitrification has been proposed as one of the mechanisms responsible for these changes, very few studies directly investigate its occurrence. Here, we showed that nitrification by epiphytic leaf microbes contributed to increasing NO 3 in TF and that nitrifiers' activity was reduced going from the dry and hot summer to the cool winter. Overall, these results highlight the power of coupling microbial community analysis, functional gene amplification and stable isotope approaches to examine ecosystem‐scale processes.