What was done
Well-watered mixtures of two plants - the legume white clover (Trifolium repens L.) and C4 buffalo grass (Stenotaphrum secundatum (Walt.) Kuntze) - which were initially equal in plantlet size, number and spatial distribution, were grown for 15 months in sand placed within large plastic containers located in greenhouses maintained at different atmospheric CO2 concentrations (360 and 700 ppm) under three sand nutrient conditions (zero-N/low-P, zero-N/high-P, plus-N/high-P), during which time ten separate harvests were made of all plant biomass over a height of 5 cm above the sand surface, after which the total carbon contents of the whole plants and their respective soils were determined.

What was learned
The slightly less than a doubling of the air's CO2 concentration employed in this study led to (1) increases of 27%, 55% and 23% in final-harvest whole plant biomass in the zero-N/low-P, zero-N/high-P and plus-N/high-P soil treatments, respectively, (2) increases of 22%, 41% and 374% in the amounts of new carbon found in the soils of these respective treatments, and (3) corresponding increases of 22%, 53% and 53% in total new mesocosm carbon contents (comprised of soil plus plant carbon).

What it means
Soil P deficiency in this study severely limited the ability of elevated CO2 to stimulate total mesocosm carbon capture, while lack of N was most harmful to soil carbon capture. Consequently, in grasslands that are managed for animal production, Edwards et al. say that "it may be possible to increase their potential to sequester C as atmospheric CO2 increases by altering land management," especially in ways that relieve these nutrient deficiencies, as has also been suggested by Jones and Donnelly (2004).

Reference
Jones, M.B. and Donnelly, A. 2004. Carbon sequestration in temperate grassland ecosystems and the influence of management, climate and elevated CO2. New Phytologist 164: 423-439.