Belowground Carbon Storage in a Grassland Community
Adair, E.C., Reich, P.B., Hobbie, S.E. and Knops, J.M.H. 2009. Interactive effects of time, CO2, N, and diversity on total belowground carbon allocation and ecosystem carbon storage in a grassland community. Ecosystems 12: 1037-1052.
Results indicated 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."
Given these findings, 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.