Background
The authors write that "Berner and Kothavala (2001) pointed out that the weathering of soil carbonate minerals may serve as a sink for increasing atmospheric CO2," which can act as a negative feedback to global warming; and in a study published that same year, they say that Andrews and Schlesinger (2001) conducted an experiment in the Duke forest and found that "an increase of 55% in atmospheric CO2 over two years resulted in a 33% increase in the flux of dissolved inorganic C to groundwater." In addition, they report that in an open-top chamber study of the effect of elevated atmospheric CO2 on mineral weathering reactions in mid-latitude forest soils, Williams et al. (2003) found that "the CO2 treatment had a significant effect on the concentration of dissolved inorganic C, which was 12% higher in elevated than ambient CO2 chambers."

What was done
Working in Guangzhou City, Guandong Province, China, with model forest ecosystems that each contained eight one- to two-year-old seedlings of each of six of the most widely distributed tree species in southern China, which they transplanted into 3-m diameter open-top chambers having an above-ground height of 3 meters and a below-ground depth of 0.7 meter (which contained soil removed from a nearby evergreen broad-leaved forest), Liu et al. studied the effect of elevated atmospheric CO2 (700ppm) on the dynamics of inorganic C in the chambers' soil leaching water over a period of three years after the treatments started.

What was learned
The eight researchers report that compared to the ambient CO2 treatment, net biocarbonate-C loss increased by 42%, 74% and 81% in the high CO2 concentration treatment in 2006, 2007 and 2008, respectively, due to both increased inorganic C concentrations in the leaching water and a greater amount of leaching water, the latter of which phenomena was due to the fact that the elevated CO2 concentration also increased soil moisture contents and resulted in greater volumes of leaching water during the high rainfall events of the area's monsoon season.

What it means
In discussing the significance of their findings, Liu et al. say their results suggest that "tropical forest soil systems may be able to compensate for a small part of the atmospheric CO2 increase through the accelerated processing of CO2 into biocarbonate-C during soil mineral weathering, which might be transported in part into ground water or oceans on geological timescales."

References
Andrews, J.A. and Schlesinger, W.H. 2001. Soil CO2 dynamics, acidification, and chemical weathering in a temperate forest with experimental CO2 enrichment. Global Biogeochemical Cycles 15: 149-162.

Berner, R.A. and Kothavala, Z. 2001. GEOCARB III: a revised model of atmospheric CO2 over Phanerozoic time. American Journal of Science 301: 182-204.

Williams, E.L., Walter, L.M., Ku, T.C.W., Kling, G.W. and Zak, D.R. 2003. Effects of CO2 and nutrient availability on mineral weathering in controlled tree growth experiments. Global Biogeochemical Cycles 17: 10.1029/2002GB001925.