Ancient CO2 Levels Favor Nitrogen Fixing Plants over a Broader Range of Soil N Compared to Present

Small inreases in CO 2 stimulate nitrogen fixation and plant growth. Increasing soil N can inhibit nitrogen fixation. However, no studies to date have tested how nitrogen fixing plants perform under ancient CO 2 levels (100 MYA), when nitrogen fixing plants evolved, with different levels of N additions. The aim of this study was to assess if ancient CO 2 , compared to present, favors nitrogen fixers over a range of soil nitrogen concentrations. Nitrogen fixers ( Alnus incana ssp. rugosa , Alnus viridis ssp. crispa , and Alnus rubra ) and their close non-nitrogen fixing relatives ( Betula pumila , Betula papyrifera , Betula glandulosa ) were grown at ancient (1600 ppm) or present (400 ppm) CO 2 over a range of soil N levels, equivalent to 0, 10, 50, and 200 kg N ha −1 year −1 . The growth of non-N fixing plants increased more than N fixing plants in response to the increasing N levels. When grown at an ancient CO 2 level, the N level at which non-nitrogen fixing plant biomass exceeded nitrogen fixing plant biomass was twice as high (61 kg N ha −1 year −1 ) as the N level when plants were grown at the ambient CO 2 level. Specific nodule activity was also reduced with an increasing level of soil N. Our results show there was a greater advantage in being a nitrogen fixer under ancient levels of CO 2 compared with the present CO 2 level.