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 fixers 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 CO 2 (1600 ppm) or present CO 2 (400 ppm) 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 y -1 ) as the N level when plants were grown at an ambient CO 2 level. Specific nodule activity was also reduced with an increasing level of soil N. Our results showed there was a greater advantage in being a nitrogen fixer under ancient levels of CO 2 compared with the present CO 2 level.