  Carbon sequestration was studied in the planted Israeli Yatir forest, a 28
${km}^2$ Aleppo pine forest growing at the semi-arid timberline (with no
irrigation or fertilization). The organic carbon sequestration rate (above and
below ground) was measured as 550 gram $CO_2 {m}^{-2} {yr}^{-1}$, by Eddy
Covariance flux and Carbon Stock counting methods. Assuming that the soil
composition at Yatir is representative, we estimate a global organic
sequestration rate of roughly 3.0 billion tons $CO_2$ per year, after future
global forestation, by extrapolating to 20% of the global semi-arid area.
Consider now the inorganic carbon sequestration rate. $CO_2$ exhaled into the
soil by tree roots is hydrated by soil water to produce mainly bicarbonate
${{HCO}_3}^-$, which combines with soil ${Ca}^{2+}$ to precipitate calcite
${CaCO}_3$. We quantify the annual sequestration rate in Yatir soil cores by
measuring the decreased density versus depth, of bicarbonates in the liquid
phase of the soil unsaturated zone (USZ). We found that the bicarbonate
concentration decreases with depth, as the calcite precipitates and is
incorporated within the USZ. The depth profiles were converted to time
profiles, taking into account the $\approx 11 cm/yr$ annual rate of downward
water percolation at Yatir. In 1 Liter of sediment, using data from a core
depth of 2.2 meters, the calcite precipitation rate was measured as 22 mg $CO_2
{yr}^{-1} L^{-1}$. Taking 6 m as the global semi-arid average depth of root
respiration, extrapolating as above, roughly 0.8 billion tons of $CO_2$ could
potentially be sequestered globally each year. The total organic plus inorganic
sequestration rate is then $\approx 4$ billion tons $CO_2$ per year. This
demonstrates the global potential, the need for further measurements, and the
need to begin implementing a global land management policy of widespread tree
planting in semi-arid regions.
