NITROGEN ISOTOPES IN MODERN AND FOSSILIZED CYCAD LEAVES: EVOLUTIONARY AND GEOLOGICAL IMPLICATIONS

The cycads (Division: Cycadophyta) are sometimes referred to as “living fossils.” These ancient plants have their evolutionary origins in the late Paleozoic, and have been persistent fixtures of understory flora since their first appearance in the fossil record. Here we attempt to exploit their long fossil record in order to investigate the antiquity of a hallmark of cycad physiology: nitrogen fixation. All extant cycad species have near-surface coralloid roots that contain symbiotic populations of cyanobacteria. These bacteria provide cycads with nitrogen, which the cyanobacteria fix from atmospheric N₂ (δ¹⁵N = 0‰). In the process of fixing atmospheric nitrogen into biomass, cyanobacteria impart little isotopic fractionation (<2‰). Therefore, the nitrogen isotopic composition of cycad plant material should closely resemble that of the atmospheric nitrogen reservoir. We have measured the nitrogen isotope ratios of extant and fossilized cycad leaves in an effort to 1) gauge the viability of fossil cycads as archives of atmospheric nitrogen isotope ratios, and 2) assess the longevity of the cycad-cyanobacterial symbiosis. Our preliminary results suggest that wild populations of modern cycads do faithfully record atmospheric nitrogen isotope ratios, but cycads grown under fertilizer-induced nitrogen excess conversely show variability likely reflecting soil uptake. The fossil record, while still incomplete, shows variability that suggests either fluctuations in atmospheric nitrogen isotopes or temporal differences in the primary method of nitrogen assimilation in cycads (fixation vs. soil uptake).