Subsoil acidity causes long delays in inorganic carbon sequestration by Enhanced Weathering
Pith reviewed 2026-07-03 02:09 UTC · model grok-4.3
The pith
Subsoil acidity delays Enhanced Weathering carbon sequestration by 30-100 years even after topsoil liming.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Using century-long agricultural liming of formerly acidic heathland as a proxy for Enhanced Weathering, this study provides empirical evidence of subsoil-imposed delays. Below such limed terrain, we observed a downward-progressing front of topsoil-produced alkalinity that still requires 30-100 years to penetrate the approximately 5 m thick acidic sandy unsaturated zone and reach the groundwater table. Subsoil acidity thus may cause beyond-reasonable delays, prohibiting EW as a viable short-term carbon capture strategy even on topsoils made non-acidic by preceding liming.
What carries the argument
The downward-progressing alkalinity front observed beneath century-long limed sites, used as a proxy to measure the time required for EW-produced alkalinity to traverse acidic subsoil to the water table.
If this is right
- Subsoil acidity imposes delays on inorganic carbon sequestration by EW that exceed those caused by topsoil acidity alone.
- EW cannot serve as a viable short-term carbon capture method on sites with acidic subsoils even after topsoil liming.
- Planning any EW scheme requires assessment of stored acidic cations throughout the top- and subsoil.
- The rate and composition of infiltrating water determine how long the sequestration delay lasts.
Where Pith is reading between the lines
- EW site selection should incorporate full soil-profile acidity mapping to forecast realistic sequestration timelines.
- Regions with deep acidic subsoils may need alternative carbon removal approaches until subsoil conditions are addressed.
- Long-term monitoring of actual EW applications in sandy acidic profiles could directly test the proxy-based delay estimate.
Load-bearing premise
The alkalinity front created by century-long liming will continue moving downward at a comparable rate and composition through the entire 5 m acidic sandy unsaturated zone.
What would settle it
Direct field measurement at comparable limed sites showing the alkalinity front reaching the groundwater table in substantially less than 30 years.
Figures
read the original abstract
While a looming atmospheric CO$_2$ overshoot calls for immediate carbon sequestration, delays associated to Enhanced Weathering (EW) carbon dioxide removal are being investigated. Topsoil acidity is already known to delay EW carbon sequestration, but subsoil acidity remains underexplored. Using century-long agricultural liming of formerly acidic heathland as a proxy for EW, this study provides empirical evidence of subsoil-imposed delays. Below such limed terrain, we observed a downward-progressing front of topsoil-produced alkalinity that still requires 30-100 years to penetrate the approximately 5 m thick acidic sandy unsaturated zone and reach the groundwater table. Subsoil acidity thus may cause beyond-reasonable delays, prohibiting EW as a viable short-term carbon capture strategy even on topsoils made non-acidic by preceding liming. When planning EW schemes, the amounts of stored acidic cations in top- and subsoil, as well as the rate and composition of infiltrating water, controlling the duration of the delay, require careful assessment.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript uses century-long agricultural liming of formerly acidic heathland as a proxy for enhanced weathering (EW). It reports an observed downward-progressing alkalinity front below limed terrain that is stated to still require 30-100 years to traverse the remaining ~5 m acidic sandy unsaturated zone and reach the groundwater table. The central claim is that subsoil acidity can therefore impose delays that make EW unsuitable as a short-term carbon sequestration strategy, even after topsoil neutralization by liming; the authors recommend assessing stored acidic cations and infiltrating water properties when planning EW.
Significance. If the proxy observations and extrapolation hold, the result would add a subsoil-specific constraint to EW deployment timelines, complementing known topsoil acidity effects. The empirical proxy approach (real-world liming history rather than fitted parameters or purely synthetic models) is a methodological strength that grounds the delay estimate in observable field data.
major comments (2)
- [abstract] Abstract, proxy-use paragraph: the 30-100 year delay estimate for the remaining ~5 m is presented without explicit data on current front position, century-scale average advance velocity, or uncertainty on that velocity. The central claim that subsoil acidity prohibits short-term EW therefore rests on an unquantified extrapolation whose robustness cannot be evaluated from the given information.
- [abstract] Abstract and proxy description: no transport model, sensitivity analysis, or accounting is supplied for depth-dependent changes in cation-exchange capacity, pH buffering capacity, or water flux that could decelerate or accelerate the alkalinity front through the full unsaturated zone. If the front slows, the delay lengthens beyond the stated range; this assumption is load-bearing for the prohibition on short-term viability.
Simulated Author's Rebuttal
We thank the referee for the constructive comments and for recognizing the strength of our empirical proxy approach. We address each major comment below.
read point-by-point responses
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Referee: [abstract] Abstract, proxy-use paragraph: the 30-100 year delay estimate for the remaining ~5 m is presented without explicit data on current front position, century-scale average advance velocity, or uncertainty on that velocity. The central claim that subsoil acidity prohibits short-term EW therefore rests on an unquantified extrapolation whose robustness cannot be evaluated from the given information.
Authors: We agree that the abstract would benefit from explicit quantification to support the delay range. The full manuscript reports the observed alkalinity front position from soil sampling and derives the century-scale average advance velocity from the known start of liming and the current front depth; uncertainties are estimated from replicate profiles and historical records. In the revision we will add these values and their derivation to the abstract so that the basis of the 30-100 year extrapolation is transparent. revision: yes
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Referee: [abstract] Abstract and proxy description: no transport model, sensitivity analysis, or accounting is supplied for depth-dependent changes in cation-exchange capacity, pH buffering capacity, or water flux that could decelerate or accelerate the alkalinity front through the full unsaturated zone. If the front slows, the delay lengthens beyond the stated range; this assumption is load-bearing for the prohibition on short-term viability.
Authors: The study is intentionally empirical and uses the observed front progression under actual liming conditions rather than a calibrated transport model; this is the methodological choice highlighted as a strength in the referee summary. We nevertheless recognize that depth-dependent changes in soil properties could alter front velocity. In revision we will add a dedicated paragraph in the discussion that compiles available depth profiles of CEC and pH buffering capacity from the study site, provides a qualitative assessment of how variations in these parameters and in water flux would affect the extrapolation, and states the linear-assumption limitation explicitly. revision: partial
Circularity Check
No circularity: empirical proxy observation with no derivation reducing to inputs
full rationale
The paper presents field observations of an alkalinity front in a limed proxy site as direct evidence for subsoil delays in EW. No equations, fitted parameters, or self-citations are invoked to derive a 'prediction' that collapses back to the input data by construction. The central claim rests on measured proxy behavior extrapolated to EW scenarios, which is an empirical inference rather than a self-referential derivation. The skeptic concern about front velocity assumptions is a question of evidence strength, not circularity under the defined patterns.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Century-long agricultural liming of formerly acidic heathland serves as a valid proxy for the alkalinity front movement expected under enhanced weathering.
Reference graph
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