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

📝 Abstract

Inorganic carbon, in the form of allogenic (transported) and pedogenic (soil) carbonates in semi-arid soils, may comprise an important carbon sink. Carbon dioxide, CO2, originating from the atmosphere and exhaled by tree roots into the soil, may be hydrated by soil water within the unsaturated zone (USZ) of semi-arid soils to produce the carbonic acid (H2CO3) solutes HCO3- bicarbonate and H+ Hydrogen ion. This H+ may then dissolve relict soil CaCO3 carbonate (calcite), to release Ca+2 calcium cations and more HCO3- bicarbonate. When conditions allow, one mole of Ca+2 and two moles of HCO3- combine to precipitate 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 not sequester significant amounts of present day atmospheric CO2. The reasons given were that they originate in part from the pre-existing limestone; and that for every mole of calcite precipitated, one mole of CO2 may be liberated to the atmosphere. It was argued that only if the Ca+2 cation is derived from a non-carbonate source can sequestration be assumed. We have tested these assumptions under field conditions at two semi-arid sites in Israel. We found that bicarbonate, originating from root exhalation, is depleted and is incorporated within the USZ as carbonates precipitate. Thus, a net sequestration 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 than pre-existing calcite. Forestation can therefore augment pedogenic carbonate formation. By extrapolating our data globally, we suggest that worldwide semi-arid forests (existing and to be planted) may sequester 5-20% of the current annual anthropogenic increase of atmospheric carbon dioxide as pedogenic carbonate.

💡 Analysis

Inorganic carbon, in the form of allogenic (transported) and pedogenic (soil) carbonates in semi-arid soils, may comprise an important carbon sink. Carbon dioxide, CO2, originating from the atmosphere and exhaled by tree roots into the soil, may be hydrated by soil water within the unsaturated zone (USZ) of semi-arid soils to produce the carbonic acid (H2CO3) solutes HCO3- bicarbonate and H+ Hydrogen ion. This H+ may then dissolve relict soil CaCO3 carbonate (calcite), to release Ca+2 calcium cations and more HCO3- bicarbonate. When conditions allow, one mole of Ca+2 and two moles of HCO3- combine to precipitate 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 not sequester significant amounts of present day atmospheric CO2. The reasons given were that they originate in part from the pre-existing limestone; and that for every mole of calcite precipitated, one mole of CO2 may be liberated to the atmosphere. It was argued that only if the Ca+2 cation is derived from a non-carbonate source can sequestration be assumed. We have tested these assumptions under field conditions at two semi-arid sites in Israel. We found that bicarbonate, originating from root exhalation, is depleted and is incorporated within the USZ as carbonates precipitate. Thus, a net sequestration 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 than pre-existing calcite. Forestation can therefore augment pedogenic carbonate formation. By extrapolating our data globally, we suggest that worldwide semi-arid forests (existing and to be planted) may sequester 5-20% of the current annual anthropogenic increase of atmospheric carbon dioxide as pedogenic carbonate.

📄 Content

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Sequestration of atmospheric carbon dioxide as inorganic carbon in the unsaturated zone under semi-arid forests
Israel Carmi1, Joel Kronfeld1, Murray Moinester2

1Department of Geosciences, Tel Aviv University, 69978 Tel Aviv, Israel, carmiisr@post.tau.ac.il, joel.kronfeld@gmail.com
2School of Physics and Astronomy, Tel Aviv University, 69978 Tel Aviv, Israel, murray.moinester@gmail.com

Abstract Inorganic carbon, in the form of allogenic (transported) and pedogenic (soil) carbonates in semi-arid soils, may comprise an important carbon sink. Carbon dioxide, CO2, originating from the atmosphere and exhaled by tree roots into the soil, may be hydrated by soil water within the unsaturated zone (USZ) of semi-arid soils to produce the carbonic acid (H2CO3) solutes HCO3- bicarbonate and H+ Hydrogen ion. This H+ may then dissolve relict soil CaCO3 carbonate (calcite), to release Ca+2 calcium cations and more HCO3- bicarbonate. When conditions allow, one mole of Ca+2 and two moles of HCO3- combine to precipitate 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 not sequester significant amounts of present day atmospheric CO2. The reasons given were that they originate in part from the pre- existing limestone; and that for every mole of calcite precipitated, one mole of CO2 may be liberated to the atmosphere. It was argued that only if the Ca+2 cation is derived from a non- carbonate source can sequestration be assumed. We have tested these assumptions under field conditions at two semi-arid sites in Israel. We found that bicarbonate, originating from root exhalation, is depleted and is incorporated within the USZ as carbonates precipitate. Thus, a net sequestration 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 than pre-existing calcite. Forestation can therefore augment pedogenic carbonate formation. By extrapolating our data globally, we suggest that worldwide semi-arid forests (existing and to be planted) may sequester 5-20% of the current annual anthropogenic increase of atmospheric carbon dioxide as pedogenic carbonate.

1. Introduction Since the Industrial Revolution, the carbon dioxide concentration in the atmosphere has risen from approximately 280 ppmv (parts per million by volume) to approximately 405 ppmv at present (Craig and Keeling, 1963; ProOxygen, 2017). The present global atmospheric CO2 reservoir of ~3150 billion tons has been recently increasing annually by ~20 billion tons. This increase has occurred predominantly through the burning of fossil fuels, and secondarily through the processes of deforestation and desertification and forest fires. This is releasing carbon that had been previously taken out of the atmosphere in prior geological eras, particularly during the Carboniferous Period, when the great coal deposits were laid down 2

(Berner and Kothavala, 2001; Rothman, 2001, 2002; Bergman et al., 2004; Franks et al., 2014). The CO2 had been stored primarily as a chemically reduced form of plant-based organic material that had been converted to coal or petroleum. These fuels are now being oxidized and the carbon is being returned to the atmosphere as carbon dioxide. Considering the goal of limiting global warming, large climate engineering projects have been proposed; for example, carbon dioxide removal (CDR) and solar radiation management (SRM) (Linnér and Wibeck (2015)). The effectiveness, cost and risk of these and other proposals are currently being investigated. Some of these methods should indeed be employed to retard the increasing atmospheric CO2 concentration. Another way being considered is to once again utilize plant photosynthesis to abstract atmospheric CO2, and store it as organic carbon in trees. Thus, planting trees has been proposed as being effective in sequestering atmospheric CO2, both as Above-ground Biomass Carbon (ABC) as well as in the roots (Johnson and Colburn, 2010; Kell, 2012; Tans and Wallace, 1999; Watson et al., 2000). The carbon observed in the above ground plant mass, the leaf litter, and organics disseminated in the soils (soil organic matter), comprise organic carbon. Forestation can store large amounts of organic carbon. For example, almost 4 billion tons of ABC has been stored since 2003 by tree planting in northern China. This equals almost 25% of its annual fossil fuel emissions (Liu et al., 2015). Ancillary benefits also accrue. Among these are soil stabilization, reduced erosion and runoff, improved soil structure and quality, as well as reduced soil biogenic nitric oxide (NO) emissions (Gelfand et al., 2009). However, all may not be so sanguine, considering that trees have a finite lifetime. Except for exceptional species, they sh

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