Direct evidence that symbiotic N 2 fixation in fertile grassland is an important trait for a strong response of plants to elevated atmospheric CO 2

Summary Although legumes showed a clearly superior yield response to elevated atmospheric pCO 2 compared to nonlegumes in a variety of field experiments, the extent to which this is due to symbiotic N 2 fixation per se has yet to be determined. Thus, effectively and ineffectively nodulating lucerne ( Medicago sativa L.) plants with a very similar genetic background were grown in competition with each other on fertile soil in the Swiss FACE experiment in order to monitor their CO 2 response. Under elevated atmospheric pCO 2 , effectively nodulating lucerne, thus capable of symbiotically fixing N 2 , strongly increased the harvestable biomass and the N yield, independent of N fertilization. In contrast, the harvestable biomass and N yield of ineffectively nodulating plants were affected negatively by elevated atmospheric pCO 2 when N fertilization was low. Large amounts of N fertilizer enabled the plants to respond more favourably to elevated atmospheric pCO 2 , although not as strongly as effectively nodulating plants. The CO 2 ‐induced increase in N yield of the effectively nodulating plants was attributed solely to an increase in symbiotic N 2 fixation of 50–175%, depending on the N fertilization treatment. N yield derived from the uptake of mineral N from the soil was, however, not affected by elevated pCO 2 . This result demonstrates that, in fertile soil and under temperate climatic conditions, symbiotic N 2 fixation per se is responsible for the considerably greater amount of above‐ground biomass and the higher N yield under elevated atmospheric pCO 2 . This supports the assumption that symbiotic N 2 fixation plays a key role in maintaining the C/N balance in terrestrial ecosystems in a CO 2 ‐rich world.