Molybdenum and phosphorus limitation of asymbiotic nitrogen fixation in forests of Eastern Canada: Influence of vegetative cover and seasonal variability

The limitation of asymbiotic dinitrogen (N2) fixation by phosphorus (P) is well-documented. Studies on Mo limitation of asymbiotic N2 fixation are, however, scarce. To what extent Mo limits asymbiotic N2 fixation on the global scale is still unclear and the mechanisms controlling the emergence of Mo limitation remain elusive. The aim of this work was to investigate the effect of nutrient additions (P, Mo and P+Mo) on asymbiotic N2 fixation activity in leaf litters from Eastern Canadian forests (cold temperate). We specifically tested how different vegetative covers (deciduous versus coniferous) respond to nutrient additions. We also evaluated on one site (coniferous litters) if nutrient (Mo, P) limitation change during the growing season. We report that the vegetative cover has a strong influence on the emergence of Mo and P limitations; while many sites under coniferous cover responded to different combinations of nutrient addition, none of the sites under deciduous cover responded to any nutrient addition. We also observed that nutrient limitation changed during the growing season; asymbiotic N2 fixation in coniferous litters was limited by P in the early stage of the growing season, by Mo but not P in mid-season and in late season neither P nor Mo were limiting. This Seasonality in nutrient limitation might be an important factor affecting the biological N input in temperate and boreal ecosystems that remain to be fully described. Our results are discussed in the context of litter decomposition and nutrients cycling.