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Carrying capacity as constraint for maximum efficient CDRin agricultural soils
K.UlrichMayer1✉Phone++778-875-2875Emailuli@terradot.earth
SergioA.Bea1
DanyangSu1
JenniferSoong1
JennyMills1
ShawnG.Benner1
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Terradot Soil IncOctober 82025San FranciscoCAUSAK. Ulrich Mayer*,1, Sergio A. Bea1, Danyang Su1, Jennifer Soong1, Jenny Mills1, and Shawn G. Benner1
1Terradot Soil Inc., San Francisco, CA, USA
October 8, 2025
*Corresponding Author: uli@terradot.earth + + 778-875-2875
Abstract
Enhanced Rock Weathering (ERW), in which crushed basaltic rocks are spread on croplands, has emerged as a promising carbon dioxide removal (CDR) approach to mitigate climate change impacts. Important known constraints on weathering rates include temperature, humidity, and feedstock grain size. However, the quantitative prediction and optimization of CDR is currently limited by uncertainty in the processes and rates governing weathering and export. Here, we propose to evaluate the product of effective groundwater recharge and dissolved inorganic carbon (DIC) concentrations as a measure of CDR. Since maximum DIC concentrations in pore water are controlled by soil gas PCO2 and achievable Ca and Mg concentrations from weathering, we define this CDR flux as the “carrying capacity”. We consider the onset of precipitation of secondary Ca-carbonate minerals in solution due to the accumulation of ERW reaction products in the shallow soil pore water as the upper limit for the effective generation of CDR. Our results therefore present values of “maximum efficient CDR”, yielding CDR export to groundwater in the absence of carbonate mineral precipitation, as a function of effective groundwater recharge. Extending the carrying capacity concept to global croplands highlights the potential importance of groundwater recharge in determining regions with highest ERW potential. Given the simplifying assumptions in our assessment, we estimate a global CDR potential of 0.15 and 0.85 Gt CO2 yr− 1. Our results indicate that regions with high groundwater recharge and feedstocks rich in leachable Mg provide the highest potential for efficient CDR generation, assuming feedstock dissolution is not limiting. Our analysis does not account for CDR losses in the near field or far-field zone, for example due to nitrification or the release of stored acidity, but also omits soil exchange and other reactions that may restrict calcite precipitation and thus lead to higher maximum efficient CDR.
Keywords:
Enhanced Rock Weathering
Carbon Dioxide Removal
Climate Change
Negative Emission Technologies
Carrying Capacity
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1. Introduction
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Negative emissions technologies (NETs), in which CO2 is actively removed from the atmosphere, are increasingly considered essential to achieve the goal of limiting global warming to 1.5°C above present day temperatures. Enhanced Rock Weathering (ERW) on croplands, has emerged as a promising carbon dioxide removal (CDR) strategy. The concept behind ERW is to accelerate weathering of silicate minerals, often applied as ground basaltic rock on cropland soils. The weathering of this feedstock generates alkalinity and increases base cation (primarily Ca and Mg) concentrations and pH in pore water, and in the process sequesters CO2 in the form of dissolved bicarbonate. For CDR to be realized, these dissolved cations and the associated bicarbonate species must be exported downwards from the shallow soil to the underlying groundwater and then to rivers and finally the ocean for durable storage1,2. Enhanced weathering is typically facilitated by spreading crushed, highly-weatherable, mafic or ultra-mafic rocks (e.g. basalt) on farm fields3. ERW mimics, but enhances, natural geological CO2 sequestration, but is intended to occur at a human-relevant timescale3,4. Field trials3,5–7 and modeling projections8–10 have suggested that CDR on the order of Gt CO2 yr− 1 can be achieved with ERW on a global scale. A recent contribution11 estimated that the global CDR potential related to ERW likely ranges between 0.2 and 0.7 Gt CO2 yr− 1; however, individual estimates vary widely from values below 0 to values far exceeding 5 Gt CO2 yr− 1, leaving substantial uncertainty11.The documentation and prediction of CDR in agricultural soils during ERW remains a challenge due to complexity of physical and biogeochemical processes in soils. Rates of infiltration, evaporation and transpiration, autotrophic and heterotrophic soil respiration, fertilizer application and nitrification, root solute uptake and exudation, as well as adsorption and exchange reactions complicate quantification and prediction of field-scale ERW12. In-situ weathering rates in the soil environment are highly variable owing to the complex nature of the biogeochemical reactions in the soil matrix and their interactions with the applied rock materials12. Transport limitations affected by soil texture and structure are a major constraint on achievable reaction rates12–14. Together these interacting processes make it difficult to predict or quantify actual CDR produced by ERW.
Fig. 1
Conceptual diagram of CDR due to ERW of basalt. Infiltration of meteoric water is reduced by evaporation and transpiration, the remainder yielding groundwater recharge. Soil respiration provides the main source for DIC, which will either be emitted to the atmosphere or exported towards groundwater carried by recharge. Ca and Mg released from ERW will be counterbalanced by bicarbonate and possibly other anions or may be adsorbed or taken up by plants. DIC export to groundwater in response to ERW can be considered a measure of CDR.
Our approach is to determine the maximum CDR that can theoretically be obtained with ERW within the constraint of Ca-carbonate mineral precipitation. In this context, we define CDR under quasi-steady state conditions as the DIC flux (
) that is transferred from the soil towards groundwater, reducing emissions of CO2 produced by soil respiration to the atmosphere (Fig. 1). Since we are interested in identifying maximum CDR, we neglect any potential loss terms in the overlying soil profile or downgradient, which in practice will reduce the realized CDR. We also omit (non-acidic) cation exchange reactions that could provide a sink for Ca, effectively leading to an increase in maximum achievable CDR by increasing total base cation concentrations, if weathering is sufficiently fast for the solution to approach equilibrium with respect to Ca-carbonates.
This carbon flux can be estimated based on the product of the effective groundwater recharge rate (
) and the dissolved inorganic carbon (DIC) concentrations carried by this water flux, facilitated by the release of cations from weathering, predominately Ca and Mg.
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The effective recharge rate is defined here as the water flux below the soil zone affected by evapotranspiration (Fig. 1). Importantly, the DIC concentration that can be carried by this water is not unlimited. The upper limit of DIC will be modulated by carbonate mineral solubility, which is in turn controlled by pore water pH, soil gas pCO2 and cation concentrations in the pore water. Based on these considerations, we coin the term “carrying capacity”, which is defined here by the recharge flux carrying the maximum possible DIC concentration based on thermodynamic and other geochemical constraints. The concept of “carrying capacity” has previously been applied to rivers to define their capacity to export alkalinity from ERW interventions to the ocean15,16 but is equally applicable to DIC export from a soil profile towards groundwater.In this study, we consider a range of soil gas PCO2 values and calcite solubility as the limiting factor for dissolved Ca-concentrations. In addition, we consider a range of Mg-concentrations, since obvious solubility controls for Mg under relevant pH conditions are not apparent. The formation of secondary Mg-carbonates is unlikely in agricultural soils due to kinetic constraints for magnesite precipitation and the high solubility of other Mg-carbonates such as hydromagnesite and nesquehonite17–19. On the other hand, calcite or aragonite have modest kinetic constraints and tend to precipitate at relatively low Ca-concentrations.
We acknowledge that maximum achievable CDR may be higher than CDR defined by
above, if carbonate mineral precipitation is occurring. However, if carbonate mineral precipitation does occur, the efficiency of ERW in generating CDR declines by 50%, since calcite precipitation removes Ca and carbonate at a 1:1 ratio from solution, while Ca in solution can balance two bicarbonate ions20. Accordingly, carbonate mineral precipitation does not provide a true upper bound for CDR but rather represents an important efficiency threshold. Our analysis based on
focuses on the quantification of the maximum achievable CDR without forming carbonate minerals within the soil profile, which we define as “maximum efficient CDR”.
The specific objectives of this contribution are as follows:
Quantify maximum efficient CDR values for a range of plausible pore water compositions, as a function of effective groundwater recharge rates, assuming that carbonate mineral precipitation does not occur in the soil profile
Using global, spatially distributed, groundwater recharge rates, average annual surface temperatures, and the global distribution of croplands and grasslands, create maps of global maximum efficient CDR that identify regions with high CDR export capacity and provide estimates of maximum global CDR potential
2. Methods
To determine maximum efficient CDR dictated by DIC carrying capacity, it is necessary to identify the range of values and global distribution of groundwater recharge. In addition, maximum achievable DIC concentrations in soil pore water must be constrained for a range of conditions. To determine the global distribution of maximum efficient CDR for defined geochemical conditions, temperature-dependence and land use must be taken into consideration.
2.1. Range and global distribution of effective groundwater recharge rates
Groundwater recharge can vary widely in natural systems. Much work has been done on quantifying and mapping the distribution of groundwater recharge on a global scale21–23, generally yielding similar results. Mohan et al.22 provide the global distribution of average groundwater recharge over an extended period of time (1981–2014), which we use to define
in our analysis to determine the global distribution of
(see Supporting Information, Figure S.1). Groundwater recharge rates generally vary between 0 and 1,000 mm yr− 1 and the estimated global average groundwater recharge rate is 134 mm yr− 1 22.
2.2. Range and global distribution of average annual surface temperatures
We included the influence of temperature on carbonate mineral phase solubility when determining the global distribution of
. We used the global distribution of average annual surface temperatures [°C] based on the average of daytime and nighttime temperatures from the MOD21C2 dataset from 2024-01-01 to 2024-12-3124 (see Supporting Information, Figure S.2).
2.3. Constraints on maximum DIC concentrations in agricultural soils subjected to ERW
To determine maximum achievable DIC concentrations, we consider a simplified geochemical system, assuming that Ca and Mg are the dominant cations and bicarbonate is the dominant anion. In the context of ERW projects, Na and K are commonly considered relatively minor contributors and are neglected here4,25. Since we are aiming at determining maximum efficient CDR, we assume that other anion concentrations, including NO3−, SO42− and Cl− are small relative to bicarbonate. For this system, the pore water composition, including DIC, alkalinity and pH, can be determined for a fixed PCO2 and given Ca and Mg concentrations.
Soil CO₂ concentrations (PCO2) in agricultural fields exhibit substantial vertical and temporal variability influenced by tillage practices, residue decomposition, soil type and soil moisture. For example, Yonemura et al.26 observed PCO2 concentrations ranging from ~ 5,000 to 23,000 ppmv at 50 cm depth. Lockhart et al.27 observed PCO2 levels ranging between 7,000 and 22,000 ppmv in a temperate semi-arid region of the United States. In our analysis, we consider soil PCO2 concentrations of 5,000, 10,000 or 20,000 ppmv, to represent the range of observed values.
To constrain dissolved Ca-concentrations, we assume that calcite provides a solubility control on Ca. Buckingham and Henderson28 performed geochemical speciation calculations on pore waters affected by either carbonate rock (aglime) or basalt amendments and identified saturated to slightly supersaturated conditions with respect to calcite, supporting that calcite serves as an effective solubility control for Ca. Slightly supersaturated conditions with respect to calcite can be explained by inhibition of its precipitation due to the presence of various inhibitors (e.g. organic molecules, PO43−). For example, Reddy29 found that calcite formation was inhibited in the presence of organic substances. In our analysis, we consider pore water in equilibrium with calcite and pore water with a saturation index (SI) of either 0.5 or 1.0 with respect to calcite as a constraint for Ca-concentrations in solution.
For Mg, effective solubility controls do not exist in agricultural soils, as discussed above, and maximum Mg-concentrations will effectively be controlled by release rates from feedstock. Observations of dissolved Mg-concentrations in ERW field trials or lab experiments are currently sparse. Measured Mg-concentrations in leachate from ERW mesocosm experiments performed by Vienne et al.30 were on the order of 1 mM. Amann et al.14 performed lab experiments with olivine-bearing dunite, producing Mg-concentrations reaching on average up to 5 mM in surficial soils, although concentrations declined with depth. Information on Mg pore water composition is also available for sites regularly subjected to aglime amendments with Mg-concentrations ranging from 8–24 mg L− 1 (0.3-1 mM) 31,32. Based on these observations, our analysis explores a range of observed Mg concentrations from 0 to 5 mM Mg (0-122 mg L− 1) to evaluate sensitivity of maximum efficient CDR towards this parameter.
Based on these constraints we perform a series of speciation calculations to determine DIC and the associated pore water compositions. We consider a suite of combinations for our selected controlling parameters (PCO2 = 5,000, 10,000 or 20,000 ppmv, calcite SI = 0, 0.5 or 1.0, Mg-concentrations = 0, 1, 2, 3, 4 or 5 mM, resulting in 54 simulation cases to constrain the range of maximum achievable DIC. Simulations were performed using phreeqci, version 3.7.3.1596833 using the wateg4f database. The full simulation grid is depicted in the Supporting Information (Figure S.3). Temperature was initially set to 25°C. Subsequently, additional speciation calculations were performed covering the range of global average annual surface temperatures for the same simulation grid (Figure S.3). Only temperatures above freezing were considered.
2.4. Global distribution of croplands and grasslands
Determining CDR on a global scale is only meaningful for regions with active agriculture. Dynamic World provides a near real time land cover dataset with high spatial resolution34. The Dynamic World dataset distinguishes between cropland and grassland. We extracted both cropland and grassland coverage for 2024 (see Supporting Information, Figure S.4). The total land area covered by croplands is 13.4 million km2 (or roughly 9 percent of the global terrestrial surface). The total land area covered by both cropland and grassland is 17.8 million km2 (corresponding to approximately 12% of the global terrestrial surface).
2.5. Determination of maximum efficient CDR and its global distribution
Based on the constraints outlined above, we proceed to determine the range of maximum efficient CDR, using DIC concentrations from the 54 scenarios (Figure S.3). We first compute maximum efficient CDR for T = 25°C as a function of effective groundwater recharge values ranging from 0-1000 mm y− 1. We then proceed to compute the global distributions of maximum efficient CDR as a function of local effective groundwater recharge values (Figure S.1) and local surface temperatures (Figure S.2) for the geochemical conditions considered. We apply filters of either cropland or cropland + grassland coverage (Figure S.4), when plotting the global distribution of maximum efficient CDR. As a final step, we spatially integrate these data for the various scenarios to obtain a range of maximum globally achievable CDR without precipitation of carbonate minerals in units of Gt CO2 yr− 1.
3. Results
3.1. Sensitivity of soil pore water compositions to pCO2, calcite SI, and Mg-concentrations
Figure 2 summarizes simulated DIC concentrations for selected cases at T = 25°C, covering the range of geochemical conditions explored. DIC concentrations across the simulation range from 120 to 697 mg L− 1 (expressed as CO2 for CDR calculations in tn CO2 ha− 1 yr− 1) (Fig. 2, Table S.1a-c). The results indicate that increasing soil gas PCO2 from 5,000 to 20,000 ppmv increases DIC in soil pore water between 12% and 79%, given other parameters remain unchanged, indicating a modest sensitivity towards PCO2. Changes in PCO2 have the largest impact at low Mg concentrations (Tables S.1s-c). Similarly, increasing calcite SI from 0.0 to 1.0 increases DIC in soil pore water between 17% and 125%, with the most significant impact seen in the absence of Mg. DIC is most sensitive to Mg-concentrations, with DIC increasing between 48% and 275% across the 0 to 5 mM range of the simulations, with the highest sensitivity seen at low pCO2 values and calcite at saturation. Overall, the variability of DIC over the range of all geochemical conditions considered is relatively modest, limited to a factor of 5.8.
Fig. 2
Calculated DIC-concentrations for selected simulation cases at T = 25°C. Full water chemistry (PCO2, pH, Ca, Mg, DIC, alkalinity) for all simulation cases is provided in the Supporting Information in tabular form (Table S.1)
The impact of increased Mg-concentrations on maximum achievable DIC is modulated by declines in Ca-concentrations with increasing Mg-concentrations. With increasing Mg concentrations, DIC and pH increase, therefore reducing Ca, constrained by calcite solubility. This impact is most significant for lower PCO2 values and calcite SIs (see Figure S.5).
3.2. Maximum efficient CDR as a function of groundwater recharge rates at T = 25°C
Figure 3 presents the values of maximum efficient CDR export towards groundwater for the various scenarios for groundwater recharge values ranging from 0 mm yr− 1 to 1,000 mm yr− 1 at T = 25°C. As discussed above, groundwater recharge is here defined as the annual water flux across a control plane below the root zone, unaffected by evaporation and transpiration processes. Full results are provided in the Supporting Information in tabulated form (Table S.2).
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This analysis illustrates the linear dependence of maximum efficient CDR on groundwater recharge. For selected effective groundwater recharge rates of 100, 200 and 500 mm yr− 1, we obtain ranges of maximum efficient CDR of 0.12–0.70, 0.24–1.39 and 0.60–3.46 tn CO2 ha− 1 yr− 1, respectively, based on the full range of geochemical conditions considered in our analysis (see Table S2a-c). If we focus on the simulations with 1 mM Mg (24 mg L− 1), which coincide most favorably with commonly observed Mg-concentrations in soil pore water30–32, we obtain ranges of maximum efficient CDR of 0.16–0.51, 0.32–1.01 and 0.81–2.54 tn CO2 ha− 1 yr− 1, respectively, for the same subset of effective recharge rates (see Tables S2a-c). To provide context, average annual recharge rates for farmland in Brazil ranged from 315 ± 76–324 ± 78 mm yr− 1 for measured precipitation rates of 1194–1247 mm yr− 1, respectively35,36. For very wet climates, groundwater recharge may exceed these values36,37. On the other hand, in arid and semi-arid climates, groundwater recharge is often restricted to a few mm per year36,38,39.Fig. 3
Maximum efficient CDR [tn CO2 ha− 1 yr− 1] for T = 25°C calculated as the product of groundwater recharge
and pore water DIC as a function of groundwater recharge rates [mm yr− 1] for a range of pCO2 values, calcite SIs (0–1) and Mg concentrations in pore water (0 mM – 5 mM in 1 mM increments). Numerical results for all simulation cases are provided in tabular form in the Supporting Information (Table S.2).
Using the spatially distributed groundwater recharge rates reported by Mohan et al.22 (Figure S.1) and applying filters for cropland and cropland + grassland (Figure S.4), we calculated the areal frequency distribution of recharge rates (Figure S.6). These frequency distributions show that > 70% of cropland and grassland are in regions with groundwater recharge rates of less than 100 mm yr− 1. These data also show that groundwater recharge rates above 500 mm yr− 1 are very rare globally. Areas covered by cropland with recharge rates between 100 and 500 mm yr− 1 amount to ≈ 25% for cropland and ≈ 30% for grassland + cropland (Table S.6). Taking into consideration that maximum efficient CDR for a recharge rate of 100 mm yr− 1 or less is relatively low, these results suggest that ERW can be most effectively applied on ≈ 25% of the total cropland.
3.3. Temperature dependence of DIC in pore water
Figure 4 depicts the temperature dependence of DIC concentrations in pore water for the simulation cases with PCO2 = 10,000 ppmv and calcite SI = 0.5 for the full range of Mg-concentrations considered. Results indicate substantially higher DIC concentrations for colder conditions due to variations of equilibrium constants and declining solubility of calcite with increasing temperature. For example, for Simulation 26 with [Mg] = 1 mM, DIC concentrations decline by 40% with increasing temperatures over the range of temperatures considered. Between 5°C and 25°C, DIC concentrations decrease by ≈ 26% for this case, illustrating the impact of average annual surface temperature on DIC in pore water, if subjected to the geochemical constraints.
Fig. 4
Calculated DIC for select simulation cases as a function of temperature for a soil gas pCO2 of 10,000ppm and calcite SI = 0.5. Mg concentrations range from 0 to 5 mM (1 mM increments).
3.4. Global map of maximum efficient CDR for select pore water conditions
Figure 5 depicts the global distribution of maximum efficient CDR constrained by the distribution of groundwater recharge, under the assumption that pore water concentrations obtained from Simulation 26 can be achieved (pCO2 = 10,000 ppm, calcite SI = 0.5, Mg concentrations of 1 mmol L− 1 (24 mg L− 1). This map takes into consideration local recharge values (Figure S.1), the distribution of average annual surface temperature (Figure S.2) and DIC-concentrations as a function of temperature (Fig. 4). Data are only displayed for the terrestrial surface covered by croplands (Figure S.4). For comparison, we also plotted the results for the same geochemical conditions for cropland and grassland (see Supporting Information, Figure S.7), and for the geochemical conditions covering the lower and upper DIC concentrations considered in our analysis (Figures S.8 and S.9, Simulations 1 and 54, respectively).
Figure 5 illustrates regional differences in achievable maximum efficient CDR, reflecting areas of relatively high and low effective groundwater recharge. In North America, elevated potential CDR carrying capacity is found in the central corn belt region, while lower groundwater recharge rates in much of western North America result in lower potential CDR. In South America, high rates of groundwater recharge in south-central Brazil result in elevated potential CDR carrying capacity. In Europe, highest values are found along the northern coast region, while southern Europe generally exhibits lower carrying capacity. In Asia areas around the Gulf of Bengal have very high potential CDR carrying capacity while much of the croplands of India exhibit more modest carrying capacity. In Africa, the Rift Valley region is elevated, while croplands immediately south of the Sahara exhibit lower carrying capacity. CDR carrying capacity of croplands in Indonesia is generally elevated while values are lower in Australia.
Spatial integration of these data across the globe indicates that that global maximum efficient CDR that can be achieved based on the given geochemical conditions (pCO2 = 10,000 ppm, calcite SI = 0.5, [Mg] = 1mM) amounts to 0.34 Gt CO2 yr− 1, if only cropland is taken into consideration (Table S.3b in Supporting Information). If both cropland and grassland are considered, global maximum efficient CDR increases to 0.54 Gt CO2 yr− 1 (Table S.4b in Supporting Information). Expanding this analysis over the entire range of geochemical conditions considered suggests that the range of global maximum efficient CDR covers 0.15–0.85 Gt CO2 yr− 1, if only cropland is considered and 0.24–1.36 Gt CO2 ha− 1 yr− 1, if both cropland and grassland are considered (see Tables S.3 and S.4). These values are generally consistent with recent findings by Buma et al.11.
Fig. 5
Global map for maximum efficient CDR on cropland as a function of effective groundwater recharge for pore water chemistry and average annual surface temperature determined based on simulation 2 (pCO2 = 10,000ppm, calcite SI = 0.5, Mg concentration 1 mmol L− 1 (24 mg L− 1).
For Simulation 26 (pCO2 = 10,000 ppm, calcite SI = 0.5, [Mg] = 1 mM), further analysis indicates that 13.6% of cropland have the potential of producing a maximum efficient CDR of 0.5 tn CO2 ha− 1 yr− 1 or greater, 2.6% of cropland has a potential to produce a maximum efficient CDR exceeding 1 tn CO2 ha− 1 yr− 1, while only 0.2% of cropland has a potential to produce a maximum efficient CDR exceeding 2 tn CO2 ha− 1 yr− 1 (Table S.3b). These percentages increase substantially when geochemical conditions enable higher DIC concentrations. For example, for the case with PCO2 = 20,000 ppmv, calcite SI = 1.0 and [Mg] = 5 mM, 42.8% of cropland has the potential to generate maximum efficient CDR of 0.5 tn CO2 ha− 1 yr− 1 or greater. For the same scenario, 19.5% of cropland has the potential to reach or exceed 1 tn CO2 ha− 1 yr− 1, while 5.2% has the potential for a maximum efficient CDR equal or greater than 2 tn CO2 ha− 1 yr− 1 (Table S.3c). Tables S.3 and S.4 provide these percentages along with absolute areas of cropland and cropland + grassland associated with the corresponding maximum efficient CDR values for all geochemical conditions considered in our analysis.
4. Discussion
The analysis performed here provides estimates for maximum efficient CDR (given the caveats of our approach noted above) for a range of pore water conditions in agricultural soils and groundwater recharge conditions. Results show the important role of the coupled constraints of DIC concentrations and effective groundwater recharge as a potentially important limiting factor for CDR.
The results indicate that the relatively low solubility of calcite in soil environments may provide an important upper limit on efficient CDR export to groundwater. We acknowledge that calcite precipitation in soils can act as a sink for inorganic carbon, contributing to CDR in addition to DIC export to groundwater, yielding CDR values that are higher than the maximum efficient CDR quantified here. Under natural, background, weathering conditions, calcite precipitation is most commonly observed in arid and semi-arid regions, due to low water availability and the accumulation of reaction products in solution. Our results show, due to low recharge rates, maximum efficient CDR in these regions is quite low and that achieving viable CDR values would require the occurrence of substantial carbonate formation. These considerations suggest that arid and semi-arid regions may be less suitable for the deployment of ERW. It is much less likely that carbonate mineral precipitation occurs in regions with high precipitation and high effective groundwater recharge. Under these conditions, feedstock dissolution kinetics are more likely to limit Ca and Mg concentrations in the soil pore water.
The largest uncertainty in our analysis is associated with dissolved Mg concentrations. The absence of effective solubility controls on Mg suggests that using feedstocks containing large fractions of labile Mg-rich minerals, such as olivine, may enable substantially higher CDR export rates. The relatively wide range of Mg concentrations (0–5 mM) considered in our analysis can also be viewed as accounting for the impact of Ca exchange with other cations within soils that lower dissolved Ca concentrations and supply other base cations to solution, lowering the potential for Ca-carbonate mineral precipitation.
It is important to recognize that the CDR values presented here are representative of maximum achievable values under the assumption that secondary carbonate minerals do not precipitate. Any loss terms and inefficiencies due to processes in the overlying soil profile or along the flowpath from the soil profile through aquifers and streams to the ocean are not considered. In the soil profile, a reduction of actual CDR can be due to various biogeochemical processes and farming practices, including fertilizer application and nitrification, release of stored acidity from the soil matrix, uptake of Ca and Mg by the plant root system, as well as root exudation and release of organic acids. Losses of Ca and Mg due to acid exchange reactions and root solute uptake can theoretically be compensated by enhanced feedstock dissolution, if rates are sufficient. However, any processes that lead to the addition of anions to the soil water solution (nitrification, release of phosphate from fertilizer, release of organic acids) will inevitably reduce achievable CDR, since Ca and Mg are partially used to counterbalance these anions. This is most relevant for anions that do not protonate at circum-neutral pH, such as nitrate and sulphate. Similarly, any losses along the flowpath, for example, carbonate mineral precipitation in streams and CO2 degassing from surface water are not considered in our analysis. We also do not consider the effect of soil structure and heterogeneities which may lead to the infiltration of relative fresh precipitation water along preferential flowpaths, therefore not effectively contributing to the carrying capacity. In addition, our analysis is based on annual averages and does not take into consideration major precipitation and recharge events possibly carrying relatively fresh precipitation water, diurnal and seasonal fluctuations, as well as the crop cycle. Furthermore, our analysis is based on total DIC export towards groundwater and does not correct for DIC export under Business-As-Usual (BAU) conditions. Finally, we assume that basalt dissolution is not rate limiting across environments.
Nevertheless, the concept of carrying capacity and maximum efficient CDR based on carbonate mineral solubility shows promise in delineating areas that would provide maximum achievable CDR export and provides an alternative method to estimate global potential for CDR on croplands and grasslands. Our results suggest that effective groundwater recharge is limited to less than 100 mm yr− 1 for more than 70% of croplands and grasslands. In these regions, it will be difficult to achieve maximum efficient CDR values via groundwater discharge greater than 0.5 tn CO2 ha− 1 yr− 1. On the other hand, maximum efficient CDR of more than 1 tn CO2 ha− 1 yr− 1 may be possible in some regions with higher annual average groundwater recharge.
Additional Information:
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Data Availability
The datasets generated and analyzed during this study are available from the corresponding author upon request.
Competing Interest Statement:
K.U.M, S.A.B, D.S, J.M, J.S and S.G.B are employed by and hold minority equity stakes in the CDR company Terradot Soil Inc.
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Funding:
Funding for this work was provided by Terradot Soil Inc.
Electronic Supplementary Material
Below is the link to the electronic supplementary material
Declarations
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Competing Interests
K.U.M, S.A.B, D.S, J.M, J.S and S.G.B are employed by and hold minority equity stakes in the CDR company Terradot Soil Inc.
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A
Author Contribution
Conceptualization, K.U.M., J.M. and S.G.B; methodology, K.U.M., J.M. and S.G.B.; formal analysis, K.U.M., S.A.B., D.S.; interpretation, K.U.M; writing—original draft preparation, K.U.M.; writing—review and editing, K.U.M, J.S., J.M and S.G.B; visualization, S.A.B., D.S., and K.U.M.; supervision, K.U.M.; project administration, J.S. and J.M. All authors have read and agreed to the published version of the manuscript.