What was done
Perennial ryegrass (Lolium perenne L. cv. Preference) swards were grown in large containers at two different levels of soil nitrogen supply over a period of two years in ventilated plastic tunnels maintained at ambient and elevated (700 ppm) CO2 concentrations, as well as at ambient and elevated (+3°C) temperature.  The soils were maintained at field capacity via irrigation, but the water supply was reduced in summer to simulate characteristic summer water deficits.  Throughout the experiment, a number of measurements were made on the plants and soils, including canopy gas exchanges, plant water soluble carbohydrates and biomass production.

What was learned
The authors found that "a relatively large part of the additional photosynthetic carbon is stored below-ground during the two first growing seasons after exposure to elevated CO2, thereby increasing significantly the below-ground carbon pool."  At low and high soil nitrogen supply, for example, the elevated CO2 increased soil carbon storage by 32 and 96%, respectively, "with no significant increased temperature effect."  In fact, the elevated temperature actually helped to increase soil carbon storage, because the enhanced soil desiccation at +3°C helped restrict below-ground respiration.

What it means
The authors state that "this stimulation of the below-ground carbon sequestration in temperate grassland soils could exert a negative feed-back on the current rise of the atmospheric CO2 concentration."