Mature temperate and boreal forest productivity is understood to be nitrogen limited. Sources of nitrogen to balance losses over time are not well understood, but include symbiotic N2 fixation in mosses and understory species, bedrock nitrogen where sedimentary rocks occur, and deposition of nitrogen pollution near areas of intensive land use. We recently identified the potential for endophytic bacteria in conifer foliage to provide a new nitrogen source in coniferous and mixed forests regardless of understory composition, bedrock, or proximity to land use. To determine how widespread this source could be and potential host and soil constraints, we examined the potential for foliar nitrogen fixation in bishop pine (Pinus muricata) and lodgepole pine (Pinus contorta ssp. bolanderi and ssp. contorta) growing along a gradient in soil age and associated variation in soil fertility at the “ecological staircase” in Mendocino, California. We hypothesized that potential rates of fixation would be greater in younger soils with higher soil phosphorus availability, and would not differ between the two pine species which co-occur at the sites.
Results/Conclusions
Preliminary results from the acetylene reduction assay on surface sterilized foliar samples confirmed the potential for nitrogen fixation in the pine species (p < 0.0001). Potential rates did not vary across sites differing in soil age and other properties (p > 0.65). Average rates of ethylene production in Pinus contorta at the ecological staircase were more than twice as high as those measured in limber pine foliage (Pinus flexilis) in Colorado (0.58 vs 0.21 nmol g-1 hr-1), but are still low compared to rates found in root nodules in other systems. In the same field sites, fixation rates by free-living bacteria in decomposing litter averaged 0.092 ± 0.040 nmol g-1 hr-1. Nitrogen fixed in the foliage may be more available to the plant and less susceptible to loss than nitrogen fixed in litter and soil. More work is needed to quantify variation in rates of foliar N2 fixation, potential benefits to the plant, and consequences for ecosystem nitrogen budgets.