Mon, Aug 15, 2022: 4:30 PM-4:45 PM
513D
Background/Question/MethodsVascular plants in boreal forests compete for a small pool of available nitrogen (N), much of which originates from bryophyte-diazotroph N2-fixation. However, Earth system models currently do not account for bryophyte-diazotroph N2-fixation in boreal forests, based on assumptions that fixed N is locked in bryophyte tissue and becomes available to plants over very long time scales. Nevertheless, previous work has speculated that recently fixed N can be pilfered by mycorrhizal fungi or leached into the soil, therefore providing a direct pathway for vascular plant uptake of recently fixed N, but evidence for this proposed pathway is limited. To test for evidence of recently fixed N uptake, we applied a 15N2 pulse-labeling approach to boreal mesocosms containing spruce (Picea abies) seedlings growing in 2 × 2 factorial combinations of feathermoss (Hylocomium splendens) and bilberry (Vaccinium myrtillus) presence or removal, with all other plants removed. We incubated the mesocosms with 15N2 for one week, and then quantified foliar 15N enrichment of spruce and bilberry. We hypothesized that spruce seedlings would acquire recently fixed N when feathermoss was present, and that seedlings grown without feathermoss would acquire a negligible amount of recently fixed N from free-living or endophytic N2-fixing diazotrophs.
Results/ConclusionsResults revealed that spruce seedlings acquired recently fixed N in all treatments, but, contrary to our hypothesis, seedlings grown without feathermoss acquired 2.4- to 3.0-fold more 15N than seedlings grown with feathermoss, suggesting a substantial contribution from free-living or endophytic N2-fixing diazotrophs. Moreover, bilberry acquired an average of 2.5- to 3.0-fold more 15N than co-occurring spruce, regardless of feathermoss presence. We also found that the absence of feathermoss reduced spruce total foliar N by ~28.5%, which in turn was somewhat negatively correlated with 15N uptake. These findings suggest that vascular plants in boreal forests can quickly acquire N that has been recently fixed by free-living, endophytic, or feathermoss-associated N2-fixing diazotrophs, a phenomenon that has been overlooked in boreal N cycling components of Earth system models.
Results/ConclusionsResults revealed that spruce seedlings acquired recently fixed N in all treatments, but, contrary to our hypothesis, seedlings grown without feathermoss acquired 2.4- to 3.0-fold more 15N than seedlings grown with feathermoss, suggesting a substantial contribution from free-living or endophytic N2-fixing diazotrophs. Moreover, bilberry acquired an average of 2.5- to 3.0-fold more 15N than co-occurring spruce, regardless of feathermoss presence. We also found that the absence of feathermoss reduced spruce total foliar N by ~28.5%, which in turn was somewhat negatively correlated with 15N uptake. These findings suggest that vascular plants in boreal forests can quickly acquire N that has been recently fixed by free-living, endophytic, or feathermoss-associated N2-fixing diazotrophs, a phenomenon that has been overlooked in boreal N cycling components of Earth system models.