Background
According to the authors, the long-term response of grasslands to the ongoing rise in the air's CO2 concentration may depend heavily on the responses of legumes and their associated root symbioses, wherein certain bacteria (collectively referred to as rhizobia) and arbuscular mycorrhizal fungi (AMF) enhance the biological acquisition of nitrogen (N), most conspicuously via N fixation in nodules located on the legumes' roots.  So what might be expected to happen in this regard as time progresses?  In answer to this question, they suggest that since atmospheric CO2 enrichment often leads to decreases in plant N concentrations in the absence of this phenomenon, there is reason to believe that atmospheric CO2 enrichment may favor mycorrhizal fungal strains that are more efficient at nitrogen acquisition, as this trait obviously would benefit plants growing in CO2-enriched air.

What was done
In a 73-day pot study designed to test this hypothesis under controlled environmental conditions, Gamper et al. grew well-watered white clover (Trifolium repens L.) plants that they inoculated with AMF strains they obtained from plants in the Swiss FACE experiment, where they had been growing for the prior eight years in either ambient air or air enriched to an atmospheric CO2 concentration of 600 ppm.

What was learned
This unique experiment revealed, in the scientists' words, that "T. repens plants had 10% higher foliar N concentrations when inoculated with AMF isolates from field plots with elevated CO2 conditions (elevCO2 isolates), compared with plants associated with isolates from plots with ambient CO2 conditions (ambCO2 isolates)."  In addition, they report that "elevCO2 isolates stimulated both biological N2 fixation in the nodule symbiosis and nitrate uptake from the growth substrate," such that "nitrogen uptake from soil was nearly twice as high in plants colonized by elevCO2 isolates as in plants colonized by ambCO2 isolates."

What it means
Gamper et al. say the finding "that only eight years of CO2 fumigation had favored AMF strains with improved N nutritional capabilities" is "particularly noteworthy for asexual, slow-growing organisms where adaptation to environmental change is thought to be slow."  They also note it is significant in that "alleviation of mineral nutrient deficiency by AMF strains could be particularly relevant for nonlegumes, which are known to experience severe N-growth limitation under elevated atmospheric CO2."  Hence, they conclude that "rising atmospheric CO2 may select for fungal strains that will help their host plants to meet increased N demands."