Sprecher
Beschreibung
Nitrogen (N) is a crucial nutrient making up ~80% of Earth’s atmosphere. Despite this, most is in the biologically inert form N2—making nitrogen a limiting nutrient. No eukaryotes and only a few prokaryotes (<15%) possess the necessary molecular machinery to split atmospheric N2 and “fix” it to bioavailable forms. While not able to fix nitrogen themselves, certain clades of plants (including liverworts, hornworts, actinorhizal plants, legumes and cycads[MK1.1]) forge symbiotic relationships with N-fixing prokaryotes to procure nitrogen in nitrogen-poor ecosystems. The origin and drivers of the evolution/expression of this trait are not well understood. However, nitrogen-fixation in both living and fossil plants may be empirically observed using nitrogen-isotope fingerprinting (15N/14N, expressed as δ15N). Foliar nitrogen isotopes reflect their source: when it derives from nitrogen fixation this will be the atmospheric value δ15N=0, otherwise it will resemble soil (δ15N>0). Previous work using growth chambers has demonstrated that with increasing pCO2, plants exhibit greater nitrogen use efficiency (evidenced by higher foliar C/N ratios). This could imply that plants in the high-CO2 world of the early Cenozoic had less demand for nitrogen (and thus less need for nitrogen-fixing symbionts), while in the modern low-CO2 world the same taxa may require more nitrogen (and thus engage in nitrogen-fixing-symbioses). To test this hypothesis, we are comparing foliar δ15N in Cenozoic fossil specimens (n>50) and in modern herbarium specimens (n>100) to assess whether declining CO2 impacted the evolution and expression of symbiotic-nitrogen-fixation in cycads, legumes, and alders