Bark-associated diazotroph communities are a cryptic source of nitrogen in forests
Nitrogen is an essential nutrient limiting forest productivity. Plants cannot access atmospheric N2 directly and rely on diazotrophic bacteria to fix nitrogen into bioavailable forms such as ammonium. Within forests, biological nitrogen fixation (BNF) occurs primarily in soils and root nodules. However it is unclear whether the extensive microbial communities recently discovered in tree bark can also fix nitrogen. Here we combine field measurements, metagenomic profiling, and biogeochemical assays to show that bark-dwelling diazotroph communities are abundant and active across diverse tree species. Bark from eight Australian tree species showed exceptionally high C:N ratios and depleted nitrogen stable isotope signatures ({delta}15N) consistent with locally fixed nitrogen, suggesting strong selection for diazotrophs. Consistently, bark microbial communities harbour phylogenetically and physiologically diverse diazotrophs, averaging [~]1012 cells m-2 and in higher relative abundance than underlying soils. Canonical and alternative nitrogenases were detected across eight bacterial phyla, primarily bark-adapted Alphaproteobacteria, Acidobacteriota, and Verrucomicrobiota, with strong signatures of purifying selection. Stable isotope labelling experiments demonstrated that bark-dwelling diazotrophs fix nitrogen at rates varying with tree species and habitat. In line with the presence of methane-oxidising diazotrophs, BNF was strongly stimulated by methane addition and suppressed by methanotroph inhibitors. Initial upscaling suggests bark-associated nitrogen fixation contributes up to 3.8 Tg N yr-1 globally ([~]6% of natural terrestrial BNF), with further studies required to constrain this budget and its contribution to tree nitrogen demands. Altogether, this discovery of substantial above-ground nitrogen inputs revises our understanding of forest nutrient cycles and redefines the functional scope of the caulosphere.