Sequestration of Atmospheric Carbon Dioxide as Inorganic Carbon in the Unsaturated Zone of Semi-Arid Forests

The global atmospheric carbon dioxide (CO2) concentration has risen from the pre-Industrial Revolution values of approximately 270 ppmv (parts per million by volume) to current values of ~400 ppmv. More recently, the global atmospheric CO2 reservoir (~3130 billion tons) has been increasing annually by approximately 20 billion tons CO2 per year. This rise has occurred predominantly through the burning of fossil fuels, and secondarily through the processes of deforestation and desertification. Considering the urgency of limiting global warming and ocean acidification and rising sea levels, engineering projects are being evaluated worldwide, and will hopefully soon be employed to retard the increasing atmospheric CO2 concentration. We propose tackling part of the needed CO2 reduction using an economical and sustainable, low-tech method to reverse the secondary processes of deforestation, specifically in semi-arid regions. These areas, which comprise ~17% of the global land area, are characterized by erratic and low amounts (25-70 cm/yr) of precipitation. Although agriculture is marginal, these areas will support grasses, shrubs and small trees. These plants develop deep root systems into the underlying upper soil section of the thick water unsaturated zone (USZ) that extends down to the top of the subsurface aquifer. Studies in the semi-arid Yatir forest in Israel have tracked atmospheric CO2, as it transits through the soil, until it is sequestered as carbonate salts for tens to hundreds of thousands of years in the USZ. In addition, large amounts of carbon are removed by photosynthesis, and then retained in living plant biomass or accumulated as plant litter. This building of plant biomass (today's coal fields) was quite effective in reducing atmospheric CO2 during the Carboniferous Period. However, above ground sequestration is less germane today; for the processes of decay and decomposition of leaves and trees return much of the carbon to the atmosphere as CO2 over time periods of years to decades, although some carbon is indeed removed for longer periods. In the USZ, sequestration of appreciable quantities of carbon from CO2 can be long term. Because plant roots respire, they continuously pump CO2 from the atmosphere into the USZ (along with decay of organic matter in soils which also produces CO2), such that the CO2 concentration in soil gas can be orders of magnitude greater than in the atmosphere. The resulting USZ aqueous solution contains dissolved inorganic carbon (DIC) compounds in dynamic equilibrium: CO32- carbonate, HCO3- bicarbonate, CO2, and carbonic acid. Bicarbonate in the DIC interacts with soil minerals to form and then precipitate a variety of secondary carbonate salts. While some of these salts (for example, lime CaCO3) are re-dissolved by percolating rainwater, radiocarbon dating of these precipitates yields ages of thousands of years, proving that their removal is long term. The Yatir forest (GPS: 31.348051, 35.050801) in the northern Negev of Israel is an example of reforestation in a semi-arid zone. The forest is situated at an elevation of 650 m above sea level, along the southwestern flanks of the Judean Hills. It comprises mainly of Aleppo pine (Pinus Halepensis Mill.), with some subordinate cypress and other pine species that were planted in the 1960s. The tree density is approximately 300 trees per hectare, of mean height 11 m. The forest's trees act as pumps, taking in CO2 through stomatal apertures (pores) in their leaves, building bio-mass via photosynthesis, and then pumping out CO2 through root respiration underground into the thick water unsaturated zone (USZ). Via the process of transpiration, trees absorb water through their roots, and then lose water vapor through their stomata. Interestingly, the increase in atmospheric CO2 concentration allows these trees to more efficiently use their limited semi-arid water resources, since the amount of CO2 that they require can be inhaled via smaller stomatal openings, thereby minimizing water loss. Consider a volume in the USZ of this forest (1 km^2 area and 6 m depth: 6x10^9 Liter), with growing trees on top and roots in the USZ (breathing CO2). Our measurements at Yatir show that the carbonate salt precipitation within this volume of sediment is approximately equivalent to 38 tons CO2 per year. Sequestration of these inorganic carbonate salts has so far received little attention. The world's semi-arid zones cover approximately 2.3 x 10^7 km2. Taking the Yatir forest data as representative of semi-arid zones, our very rough potential global CO2 annual sequestration rate estimate in the world's (to be planted) semi-arid forests is then ~1 billion tons CO2/yr (38x2.3x10^7 tons), precipitated as inorganic carbonate salts within the USZ. This value represents ~5% of the 20 billion tons CO2/yr rate by which the CO2 in the atmosphere is currently increasing. Tentatively, based on Yatir data, sequestration of atmospheric CO2 as inorganic carbon under semi-arid forests represents a sustainable and economic method to help suppress the rate by which CO2 is increasing in the atmosphere. Our estimates should be checked and refined by extending the Yatir forest studies to other semi-arid regions.