Effects of Carbon Dioxide Enrichment and Nitrogen Addition on Inorganic Carbon Leaching in Subtropical Model Forest Ecosystems

Soil mineral weathering may serve as a sink for atmospheric carbon dioxide (CO2). Increased weathering of soil minerals induced by elevated CO2 concentration has been reported previously in temperate areas. However, this has not been well documented for the tropics and subtropics. We used model forest ecosystems in open-top chambers to study the effects of CO2 enrichment alone and together with nitrogen (N) addition on inorganic carbon (C) losses in the leachates. Three years of exposure to an atmospheric CO2 concentration of 700 ppm resulted in increased annual inorganic C export through leaching below the 70 cm soil profile. Compared to the control without any CO2 and N treatments, net biocarbonate C (HCO3−-C) loss increased by 42%, 74%, and 81% in the high CO2 concentration treatment in 2006, 2007, and 2008, respectively. Increased inorganic C export following the exposure to the elevated CO2 was related to both increased inorganic C concentrations in the leaching water and the greater amount of leaching water. Net annual inorganic C (HCO3−-C and carbonate C: CO32−-C) loss via the leaching water in the high CO2 concentration chambers reached 48.0, 49.5, and 114.0 kg ha−1 y−1 in 2006, 2007, and 2008, respectively, compared with 33.8, 28.4, and 62.8 kg ha−1 y−1 in the control chambers in the corresponding years. The N addition showed a negative effect on the mineral weathering. The decreased inorganic C concentration in the leaching water and the decreased leaching water amount induced by the high N treatment were the results of the adverse effect. Our 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 HCO3−-C during soil mineral weathering, which might be transported in part into ground water or oceans on geological timescales.