"The CO2 fertilization effect is likely to have a significant impact on plant growth in the humid tropics in a CO2 enriched atmosphere, yielding extra soil organic matter through litterfall and crop residues, and especially through a more vigorous root growth .  The CO2 antitranspiration effect would be of particular significance in the semiarid regions of the tropics and subtropics: plants would grow more vigorously with the same amount of water, and some plant growth would become possible where hitherto the land surface [has been] bare, due to climate- or salinity-induced aridity. A better ground cover would be the result, limiting soil-erosion hazards, lowering the soil-surface temperatures and providing fresh organic matter for incorporation in the soil."

As research in this area has progressed, CO2-induced carbon sequestration has been observed in the soils of several agro-ecosystems, while other studies have demonstrated the potential for this phenomenon to occur on abandoned agricultural land and natural grasslands and savannas, especially in conjunction with the CO2-enhanced nitrogen fixation that is provided by grain and forage legumes. 

Reference
Sombroek, W.G., Nachtergaele, F.O. and Hebel, A.  1993.  Amounts, dynamics and sequestering of carbon in tropical and subtropical soils.  Ambio 22: 417-426.