Spatial and temporal variation in moss-associated dinitrogen fixation in coniferous- and deciduous-dominated Alaskan boreal forests

Abstract

Dominant canopy tree species have strong effects on the composition and function of understory species, particularly bryophytes. In boreal forests, bryophytes and their associated microbes are a primary source of ecosystem nitrogen (N) inputs, and an important process regulating ecosystem productivity. We investigated how feather moss-associated N2-fixation rates and contribution to N budgets vary in time and space among coniferous and broadleaf deciduous forests. We measured N2-fixation rates using stable isotope (15N2) labeling in two moss species (Pleurozium schreberi and Hylocomium splendens) in broadleaf deciduous (Alaska paper birch—Betula neoalaskana) and coniferous (black spruce—Picea mariana) stands near Fairbanks, interior Alaska, from 2013 to 2015. N2-fixation rates showed substantial inter-annual variation among the 3 years. High N2-fixation was more strongly associated with high precipitation than air temperature or light availability. Overall, contribution of N2-fixation to N budgets was greater in spruce than in birch stands. Our results enhance the knowledge of the processes that drive N2-fixation in boreal forests, which is important for predicting ecosystem consequences of changing forest composition.