What was done
The authors employed mass balance calculations to quantify the effects of biodiversity, atmospheric CO2 concentration and soil nitrogen (N) content on the total amount of C allocated belowground by plants (total belowground C allocation or TBCA), as well as ecosystem C storage, in an eight-year experiment that was part of the BioCON study of a periodically-burned Minnesota grassland.

What was learned
Adair et al. determined that annual TBCA increased in response to all three treatment variables - "elevated CO2, enriched N, and increasing diversity" - and that it was also "positively related to standing root biomass." Upon removing the influence of root biomass, however, they found that the effects of N and diversity became neutral or even negative (depending on the year), but that "the effect of elevated CO2 remained positive." In the case of years with fire, on the other hand, they found that "greater litter production in high diversity, elevated CO2, and enhanced N treatments increased annual ecosystem C loss."

What it means
Under normal non-fire conditions, elevated CO2, N and biodiversity generally tend to increase ecosystem carbon gain; but if grasslands are frequently burned, they could actually remain neutral in this regard.