Recent experiments suggest that foliar diazotrophic endophytes in the needles of conifers may contribute to tree nitrogen (N) budgets in N limited ecosystems. Sierra lodgepole pine (Pinus contorta var. murrayana) thrive in a broad spectrum of soil types ranging from water-logged organic soils to well-drained glacial outwash, but often dominate wet and poorly drained N limited soils unable to support other species. Foliar endophyte biological nitrogen fixation (BNF) may help explain how Sierra lodgepole pine persists on N poor soils. However, the rate of BNF in pine foliage is poorly understood, and how rates of BNF vary with host tree development is unknown. We conducted a series of acetylene reduction assays in Yosemite National Park to indirectly assess the rate of BNF by foliar diazotrophic endophytes in seedlings and mature branches (Aug 2016), and intermediate-aged trees (Aug 2017), by measuring the conversion of acetylene to ethylene. 17 groups of 5 seedlings as well as six branches from each of ten mature and intermediate Sierra lodgepole pine were excised and incubated in 16 oz septa-fitted mason jars (volume = 473ml) for two hours in a 10% v/v acetylene atmosphere. Gas samples were analyzed for ethylene concentration on a Shimadzu-GC with an FID. The conversion rate of acetylene to ethylene is equal to the net increase in moles of ethylene in the headspace during the 2-hour incubation.
Results/Conclusions
Positive rates of ethylene production were detected in Sierra lodgepole pine seedlings (0-8.68 µmol C2H4 gdw-1 hr-1), intermediate aged trees (0.5-1.5 µmol C2H4 gdw-1 hr-1), and mature trees (0-0.94 µmol C2H4 gdw-1 hr-1) compared to controls. Endogenous ethylene production in samples were below detection limit. Seedlings contained the highest rate and broadest range of ethylene production, while mature trees showed the lowest rates of ethylene production across the narrowest range. Our data reflects rates higher than previously reported values observed in limber pine (Pinus flexilis) in Niwot, CO (0.22 ± 0.02 nmol C2H4 g-1 needle hr-1), and were also greater than the 30-110 mmol C2H4 gdw-1 observed in deciduous white cottonwood (Populus trichocarpa) foliage. Overall, our results indicate that nitrogenase activity occurs in the needles of Sierra lodgepole pine, and that rates of ethylene production may decline with host tree age.