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Inorganic carbon, in the form of allogenic (transported) and pedogenic (soil)carbonates in semi-arid soils, may comprise an important carbon sink. Carbondioxide, CO2, originating from the atmosphere and exhaled by tree roots intothe soil, may be hydrated by soil water within the unsaturated zone (USZ) ofsemi-arid soils to produce the carbonic acid (H2CO3) solutes HCO3- bicarbonateand H+ Hydrogen ion. This H+ may then dissolve relict soil CaCO3 carbonate(calcite), to release Ca+2 calcium cations and more HCO3- bicarbonate. Whenconditions allow, one mole of Ca+2 and two moles of HCO3- combine toprecipitate one mole of calcite, and to release one mole of CO2: Ca+2 + 2HCO3---> CaCO3 + CO2 + H2O. However, it has been claimed that such carbonates do notsequester significant amounts of present day atmospheric CO2. The reasons givenwere that they originate in part from the pre-existing limestone; and that forevery mole of calcite precipitated, one mole of CO2 may be liberated to theatmosphere. It was argued that only if the Ca+2 cation is derived from anon-carbonate source can sequestration be assumed. We have tested theseassumptions under field conditions at two semi-arid sites in Israel. We foundthat bicarbonate, originating from root exhalation, is depleted and isincorporated within the USZ as carbonates precipitate. Thus, a netsequestration of atmospheric CO2 does occur under semi-arid forests. Moreover,most of the CO2 liberated in the precipitation reaction may remain in the soil.And Ca+2 in the sediment may also be supplied from sources other thanpre-existing calcite. Forestation can therefore augment pedogenic carbonateformation. By extrapolating our data globally, we suggest that worldwidesemi-arid forests (existing and to be planted) may sequester 5-20% of thecurrent annual anthropogenic increase of atmospheric carbon dioxide aspedogenic carbonate.

Sequestration of atmospheric carbon dioxide as inorganic carbon in the unsaturated zone under semi-arid forests