Risk of CO2 Emissions from Caliche Under Ground-Mounted
Solar Power Installations
05/30/2011
Damon Turney
Vasilis Fthenakis
Introduction
Recent measurements of CO2 flux from desert soils find high rates of sequestration,
on an order of magnitude comparable to young forests(Emmerich 2003; Jasoni, Smith et al.
2005; Stone 2008; Wohlfahrt, Fenstermaker et al. 2008; Xie, Li et al. 2009; Thomas and
Hoon 2010; Allen 2011). The sequestered carbon is believed to be deposited as caliche,
which is a calcium carbonate material buried within a few meters of the desert soil surface.
Disturbance of this sequestration process by ground-mounted solar power installations
could slow this sequestration. Other studies suggest a risk for release of the buried caliche
(Emmerich 2003; Jasoni, Smith et al. 2005; Mielnick, Dugas et al. 2005; Allen 2011), giving
the potential to double or triple the carbon emissions resulting from the installation of
solar power. The topic generally is in a state of uncertainty (Stone 2008; Schlesinger,
Belnap et al. 2009). This increased footprint is a significant concern because deep cuts in
anthropogenic CO2 emissions are necessary to stabilize or reduce atmospheric greenhouse
gas concentrations(Hansen, Sato et al. 2008; Matthews and Caldeira 2008). Leading studies
suggest emissions targets below 10% of current emission rates, which means that the CO2
emission rate of solar power must be understood and reduced.
Caliche Spatial Density and Variability
(Schlesinger 1982) used the prior measurements of the Soil Conservation Service
(SCS 1974) in the top 1.0 meter in Arizona to correlate the density (kg/m2) of carbonate to
precipitation and elevation. Schlessinger found the frequency distribution as shown in
Figure 1, and correlations with precipitation and elevation shown in Table 1.
Figure 1. From Schlessinger (1982)
Table 1. From Schlessinger (1982)
The data used by Schlessinger were generated by a soil sampling auger or tube of
diameter ~2 inches. Redundant measurements were made in the same soil type at different
locations. Average inorganic carbon density was ~25 kg m-2 for the different soil types,
with spatial variability typically ~50%. Another early rigorous study of caliche came from
southern New Mexico (Grossman 1995) where again 25 kg m-2 of caliche was found, on
average, in areas excluding limestone. Limestone can increase the density of carbonate by
many fold. The density of organic carbon was 10 times smaller than inorganic carbon, due
to desert conditions. Accumulation rates of caliche were estimated at 0.1 to 1.4 g m-2 yr-1 .
More recently (Wang, Li et al. 2010) found that ~50% or more of the inorganic
carbon is located below 1 meter depth, as shown in Figure 2 below. This is important to
keep in mind with respect to other studies that do not probe below 1 meter depth.
Figure 2. Data from (Wang, Li et al. 2010)
(Guo, Amundson et al. 2006) collected prior soil measurements from STATSGO
(1992 or earlier) and other measurements from the SCS (now called the National Resource
Conservation Service, NRCS) to make a national map of soil organic carbon (SOC)
concentration and soil inorganic carbon (SIC) concentration. Soils down to 2.0 meter depth
were included in the analysis. The resulting maps are shown in Figures 2 and 3 below. The
spatial density of the measurements was roughly 5 km2, i.e., much larger than the cm2 scale
used by Schlessinger (1982) and {Grossman, 1995 #31}. However, the spatial averages of
Schlessinger’s data and Grossman’s data match those of Guo et al. for Arizona and New
Mexico respectively.
Figure 2. Map of soil organic carbon (SOC), from Guo et al. (2006)
Figure 3. Map of soil inorganic carbon (SIC), from (Guo, Amundson et al. 2006)
(Feng, Endo et al. 2002) measured inorganic carbon in the top meter of 17 different
types of desert in China. They found regressions of soil inorganic carbon vs
evapotranspiration (r2 coefficient of 0.75 with slope 0.01 kg m-2 mmH20-1, on average) and
annual precipitation (r2 coefficient of 0.20 with slope -0.2 kg m-2 mmH20-1, on average).
Mechanistic Model of Caliche Formation and Stability
Standard textbooks on soil chemistry describe the formation of caliche (Sposito
1989; Stumm and Morgan 1996) (Chadwick and Graham 2000). Isotopic field
measurements found that caliche forms slowly, at 0.1 to 1 g C m-2 yr-1 (Schlesinger 1985),
and is often stable for millennia (Pendall, Harden et al. 1994). (Serrano-Ortiz, Roland et al.
2010) describe how caves could be growing the caliche and storing large amounts of CO2.
The description below is from (Goddard, Mikhailova et al. 2009):
Calcite precipitation vs. dissolution in soils is dependent on a number of
environmental and geologic factors, including temperature, concentrations
of dissolved Ca2+ and CO2 in the soil pores, and the alkalinity and pH of the
soil solution (Sposito 1989; Chadwick and Graham 2000) (Stumm and
Morgan 1996; Ming 2002)
Ca2+(aq) +2HCO−(aq) ↔ CaCO3(s) + H2O(l) + CO2(g) [1]
In general, the precipitation of calcite (as well as other carbonates) in soils is
favored by increasing Ca2+ concentration, decreasing CO2 concentration,
increasing pH, and increasing alkalinity (Sposito 1989; Stumm and Morgan
1996) (Chadwick and Graham 2000). Therefore, because elevated
atmospheric CO2 concentration is of increasing concern, terrestrial C
sequestration strategies based on carbonate storage require conditions that
favor and maintain calcite and other carbonate precipitation associated with
minimal leaching and soil erosion.
Figure 4. Conceptual model of the chronological formation of caliche in gravelly (top) and
loamy (bottom) soils, from (Gile 1966). The soils are older from left to right. Caliche is
depicted as black. The K terminology refers to the soil horizon where caliche is typically
found.
(Marion, Schlesinger et al. 1985) produced a computer model to predict caliche formation
(Jimenez and Lal 2006) makes a very good review of physical mechanisms of how carbon is
sequestered into soils, as described below:
The SIC occurs in carbonate minerals in two forms, i.e., calcium carbonate (CaCO3)
and dolomite (MgCO3). In tropical highly weathered acid-soils the amount of SIC is
not considerable because most of the carbonates present in the parent material have
been dissolved. The total carbonate pool in world soils ranges from 222 to 245 Pg C
(0–30 cm), and from 695 to 748 Pg C for the upper 100 cm (Batjes, 1996), values
similar to those obtained by Schlesinger (1982) and Sombroek et al.
(1993). Values of SIC for different soil types in Latin America are indicated in
Table 8.
The formation of secondary carbonates occurs through the reaction of carbonic acid
(H2CO3) with Ca2+ andMg2+ (Lal and Kimble, 2000; Raymond and Cole, 2003).
Most of the carbonate that precipitates in soils and forms calcic or pretrocalcic
horizons
is the result of aerosolic deposits of carbonate through dusts, or primary carbonates
in the rock or sediment that are dissolved and re-deposited. Some studies have
reported that much of the Ca2+ is originated from carbonate in dry dust or from
Ca2+ dissolved precipitation that came from carbonate dust, thus indicating that
direct involvement of atmospheric CO2 is negligible (Gile and Grossman, 1979;
Chadwick and Capo, 1993). The SIC sequestration may constitute a significant
pathway of Csequestration in arid and semi-arid regions (Lal et al., 2000).
The formation of secondary carbonates also occurs in soils of the humid regions.
The rate of sequestration is very low as to have a considerable effect in climate
change mitigation. For example, Schlesinger (1997) estimated that the magnitude of
SIC flux with the atmosphere is low at about 0.023 Pg C yr−1. However,
the process can be important at larger temporal scales. Monger and Gallegos (2000)
and Nordt et al. (2000) argue that the formation of secondary carbonates is an
important mechanism of SIC sequestration to mitigate climate change. How to
distinguish primary from secondary carbonates is rather complicated
(Rabenhorst et al., 1984). There is not only a lack of studies on the role of SIC in C
sequestration but also the mechanisms are the least understood in the global process
(Lal, 2003). Leaching of bicarbonates into the groundwater is a major mechanism of
SIC sequestration.The rate of C sequestration by this mechanism may be 0.25 to
1.0 Mg C ha−1 yr−1. There is no agreement on the rate of C sequestration through
formation of secondary carbonates and it is still a matter of discussion. Some argue
that the rate is slow (3 to 5 g C m−2 yr−1) and of little significance (Schlesinger,
1997). Others, however, support the idea that rate of sequestration of atmospheric C
may be much higher with a maximum rate of 0.114 to 0.124 Mg C ha-1 yr-1 (Table
9) (Monger and Gallegos, 2000; Nordt et al., 2000). For example, the formation
of secondary carbonates can be accentuated by biotic activity because of high
concentration of CO2 in the soil air (e.g., root growth, termites) (Monger and
Gallegos, 2000). An important mechanism of SIC sequestration is through leaching
of carbonates in irrigated soils, if the irrigation water is not already
saturated (Nordt et al. 2000). Reconstruction of carbonate fluxes in soil formed in
strongly calcareous parent material over geological time periods suggests that this
mechanism could account
for upward of 1 Mg ha−1 yr−1 of SIC.
Plant Root Spatial Density and Variability
The architecture of root systems affect the formation and distribution of caliche. In
desert environments the root architecture is shaped by competition for water. Shallow
roots have first access to rain water, while deeper roots have more consistent access to
water during droughts. As shown in Figure 5 below from (Jackson, Canadell et al. 1996),
deserts root architectures have high density and run deep compared to other vegetation
types, a comparison that is emphasized by the fact that deserts have relatively low
aboveground biomass and primary production. Figure 6, also from Jackson et al., shows the
distribution of roots for shrubs, trees, and grasses. Shrubs dominate desert landscapes.
(Schwinning and Hooten 2009) make a good review of root distribution in the
Mojave desert. Generally, desert plants are water limited, and therfore try to capture as
much water as possible by pushing roots wide and deep. Figure 7 shows a plan view of the
root architecture from (Brisson and Reynolds 1994)
Figure 5. A collection of field measurements of root mass density, from a review of the
literature by (Jackson, Canadell et al. 1996) is shown for many different biomes. The
symbols correspond to individual publications in the literature.
Root architecture in creosote desert plants. From (Brisson and Reynolds 1994)
Disturbance and Loss of Caliche or Soil Inorganic Carbon
Disturbance to desert lands from solar power plants produces a risk for release of
significant CO2 from destabilization of the caliche. This risk is highly uncertain because the
chemical mechanisms of formation and destruction of caliche are poorly understood, as
mentioned in {Jimenez, 2006 #14}. Additionally, field measurements of the response of
caliche to solar power plant disturbance are not reported in the literature, although several
studies have been done regarding disturbance from agriculture. (Wu, Guo et al. 2009)
provided field measurements, and found that agriculture increased soil organic carbon but
decreased soil inorganic carbon. (Li, Li et al. 2010) give similar results. (Conant, Klopatek et
al. 2000; Conant, Dalla-Betta et al. 2004; Zhang, Chen et al. 2009; Zhang, Chen et al. 2010)
explain how changes to soil moisture and precipitation can change net ecosystem
respiration as measured by CO2 fluxes.
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