Direct evidence that symbiotic N-2 fixation in fertile grassland is an important trait for a strong response of plants to elevated atmospheric CO2

Although legumes showed a clearly superior yield response to elevated atmos pheric 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 (Medica go sativa L.) plants with a very similar genetic background were grown in c ompetition with each other on fertile soil in the Swiss FACE experiment in order to monitor their CO2 response. Under elevated atmospheric pCO2, effec tively nodulating lucerne, thus capable of symbiotically fixing N-2, strong ly increased the harvestable biomass and the N yield, independent of N fert ilization. In contrast, the harvestable biomass and N yield of ineffectivel y nodulating plants were affected negatively by elevated atmospheric pCO(2) when N fertilization was low. Large amounts of N fertilizer enabled the pl ants to respond more favourably to elevated atmospheric pCO(2), although no t as strongly as effectively nodulating plants. The CO2-induced increase in N yield of the effectively nodulating plants was attributed solely to an i ncrease in symbiotic N-2 fixation of 50-175%, depending on the N fertilizat ion treatment. N yield derived from the uptake of mineral N from the soil w as, however, nor affected by elevated pCO(2). This result demonstrates that , in fertile soil and under temperate climatic conditions, symbiotic N-2 fi xation per se is responsible for the considerably greater amount of above-g round biomass and the higher N yield under elevated atmospheric pCO(2). Thi s supports the assumption that symbiotic N, fixation plays a key role in ma intaining the C/N balance in terrestrial ecosystems in a CO2-rich world.