Old-growth temperate and boreal coniferous forests often grow in nutrient poor soils. In these systems, the origin of accumulated nitrogen (N) is unclear because N-fixing plants occur at low abundance; there is more N in soil and vegetation than can be explained by known sources. The disparity between accumulated N and low abundance of N-fixing plants suggests an unknown diversity and activity of N-fixing bacteria. Recent findings indicate that a novel N uptake pathway involving endophytic bacteria in foliage may account for some of the missing N. To probe the ubiquity of this pathway and potential host and soil constraints, we examined the potential for foliar N-fixation in bishop pine (Pinus muricata) and lodgepole pine (Pinus contorta ssp. bolanderi and ssp. contorta) along a natural soil fertility gradient at the “ecological staircase” in Mendocino, California. We conducted acetylene reduction assays on twigs from mature trees at this site from 2015-2017 and performed a 15N2 labeling chamber experiment with seedlings of both species transplanted from the field.
Results/Conclusions
Our measurements show that lodgepole and bishop pine foliage has active nitrogenase enzymes capable of fixing N, strengthening the case for atmospheric nitrogen uptake through this pathway. Rates of ethylene production across all sampling events ranged from 0.002 – 5.9 nmol hr-1 g-1 twig in lodgepole pine and 0.02-5.1 nmol hr-1 g-1 twig in bolander pine. There appear to be effects of soil fertility, sampling date, and tree species on ethylene production (p < 0.05). Preliminary data from isotope ratio mass spectrometry measurements of 15N-labeled tissue suggests that there is 15N accumulation in seedlings, with greater accumulation occurring in new needles that emerged during the experimental period and significant accumulation in root tissue as well as foliar tissue. To better understand the ecological significance of this association, future work will focus on scaling up and calibrating rates of N fixation to determine contributions of this N accumulation pathway to ecosystem N budgets.