Background
The authors write that "symbiotically fixed N has been reported to increase under elevated CO2 (Edwards et al., 2006; Cheng et al., 2011)," but they say "there has been no systematic synthesis of the overall effects of CO2 on the amount of N fixed by crop and pasture legumes and the associated N2 fixation parameters in these systems." And, therefore, they go on to provide such an analysis.

What was done
While perusing pertinent papers published between 1976 and February 2012 - which employed either growth chambers, open-top chambers or free-air CO2 enrichment facilities - Lam et al. obtained a total of 366 sets of data from 127 different experiments that reported the effects of elevated CO2 on N dynamics in grain crop and legume pasture systems.

What was learned
For a mean atmospheric CO2 concentration increase of approximately 300 ppm, the team of five Australian researchers found that "under elevated CO2 there was a 38% increase in the amount of N fixed from the atmosphere by legumes, which was accompanied by greater whole plant nodule number (33%), nodule mass (39%) [and] nitrogenase activity (37%)." But they add that the true CO2-induced enhancement may have been even greater; for they say that "fixed N in nodulated roots was not examined in some studies included in [their] database." In addition, they say that the grain yield of legumes increased 38.7% under elevated CO2, while the grain yield of C3 non-legumes was 23.0% and that of C4 crops was 23.7%.

What it means
Lam et al.'s findings suggest that the extra N fixed by legumes in CO2-enriched air can partly compensate for the additional N removed by the enhanced harvest of grain in grain/legume rotations in a CO2-enriched world of the future; and they thus conclude that "planting legume cover crops will be important in regions where grain growers are reluctant to apply a high rate of fertilizer due to unreliable rainfall and unpredictable crop response." And these changes in natural processes and mankind's informed responses to them will prove to be of tremendous help in meeting the challenge of doubling world food production between now and the mid-21st-century mark, when agricultural production everywhere will have to rise by that amount in order to adequately feed the planet's anticipated population at that not-so-distant point in time.

References
Cheng, L., Booker, F.L., Burkey, K.O., Tu, C., Shew, H.D., Rufty, T.W., Ficus, E.L., Deforest, J.L. and Hu, S. 2011. Soil microbial responses to elevated CO2 and O3 in a nitrogen-aggrading agroecosystem. PLoS ONE 6: e21377.

Edwards, E.J., McCaffery, S. and Evans, J.R. 2006. Phosphorus availability and elevated CO2 affect biological nitrogen fixation and nutrient fluxes in a clover-dominated sward. New Phytologist 169: 157-167.