Potential Global Sequestration of Atmospheric Carbon Dioxide by Semi-Arid Forestation

Résumé : 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.