FERTILIZATION OF TERRESTRIAL BIOMASS DURING THE LAST 120 MILLION YEARS

Global models, as well as stable isotope analyses of paleoindicators, have determined elevated levels of atmospheric carbon dioxide, relative to today, for the last 120 million years. Quantitative estimates for pCO₂ exceed 2500 ppm with the highest values occurring during the Cretaceous and generally decreasing into the Cenozoic, and decreasing further into the Quaternary. Modern-day increases in greenhouse gases have spurred multiple ecological studies to assess the effect of elevated CO₂ on plant biomass. Twenty such studies published since 1990 grew plants under elevated CO₂ levels between 550 and 750 ppm, which is equal to 1.4 to 1.9 times today's pCO₂ level. Most of these studies, performed on different types of plants with different methods, revealed biomass fertilization of ~46% under elevated pCO₂. Since pCO₂ levels have been several times higher in the geologic past than tested in previous experiments, additional experiments are needed to assess the potential effect of atmospheric fertilization on the terrestrial biosphere through time. In order to pursue this, we grew radish (Raphanus sativus L.) in growth chambers that maintained controlled environmental conditions and pCO₂ levels ranging from that of today’s atmosphere (389 ppm) to ~5x that of today (1791 ppm); upon harvest we measured total biomass and δ¹³C in both above and below ground biomass. As we have seen in many other studies, δ¹³C of plant tissue was tightly correlated with the δ¹³C of the CO₂ supplied for growth (R² = 0.99); however striking patterns in biomass fertilization were observed. When we compared the average biomass of plants grown near today's pCO₂ levels with those grown at 2 to 5 times today's pCO₂ levels, we saw a 33% increase in above ground biomass and a 118% increase in below ground biomass. In order to explain enhanced fertilization at very high CO₂ levels, we are examining δ¹³C measurements as a way to assess changes in below ground tissue function. During this presentation we will report on continued experiments reaching pCO₂ levels = 4200 ppm, which is equivalent to ~11x today's level, spanning the full range of estimates for the last 120 million years.