Global Role of Vanadium for Cryptogamic Nitrogen Fixation in Extratropical Forests

Biological nitrogen fixation (BNF) by nitrogenase is often assumed to rely on molybdenum as an enzymatic cofactor, despite molybdenum scarcity in terrestrial ecosystems relative to vanadium and iron, two alternative cofactors. Findings that cyanolichens across northeastern American boreal forests can rely substantially on vanadium nitrogenase (V-Nase) for BNF suggest that V-Nase is used by other cryptogams, which collectively contribute a large share of terrestrial BNF. Here, we show global-scale vanadium-based nitrogen fixation in common cryptogams from deciduous and needleleaf extratropical forests, including remote and urban areas. Measurements demonstrate V-Nase activity in bryophytes and cyanolichens from 44 sites across three continents. V-Nase is regulated by molybdenum content and nitrogen fixation rate, a marker of nitrogen demand. Extrapolations based on nutrient deposition suggest hotspots for V-Nase activity at higher latitudes and nonsignificant contributions in urbanized areas (>40% and <10% sample BNF, respectively) and a likely decrease in sample-specific activities by 30% relative to preindustrial times. This newly discovered global role of vanadium forces a re-evaluation of the past, present, and future nitrogen cycle. Including nitrogenase enzymatic heterogeneity can help close the nitrogen budget, predict future forest productivity, and model the response of the terrestrial carbon sink to global change, particularly at high latitudes.