The “Carbdown Model” of carbon fluxes and pools in the soil (enhanced weathering for CDR)
The principal idea of enhanced weathering (EW) as a climate solution is to mix rock dusts into the soil to reroute a fraction of the natural carbon cycle through trapping CO2 into the leachate water in the form of bicarbonates which are ultimately stored in the oceans. It is a nature-based method that effectively copies Earth’s time-tested natural method to control its atmospheric CO₂ concentration through natural rock weathering.
However, weathering of added rock dusts in the complex soil environment needs more time and research to be fully understood. This could delay the required large-scale deployment of EW as a climate solution. In an effort to circumvent the vast complexities of the interactions in the soil, we came up with the hypothesis that the CDR-relevant changes in the carbon cycle could potentially be monitored in a simplified way by looking at the soil “CO2 efflux“ (=CO2 gas fluxes out of the soil) and the CO₂ concentration of soil gas (soil pCO₂). We thereby expect to see EW signals more quickly when monitoring CO2 (gasses move fast and CO₂ sensors are very sensitive) than other, slower monitoring approaches such as solid-phase dissolution measurements or aqueous alkalinity measurements as both need certain amounts of rock dissolution and more time before their CDR signal can be distinguished from the natural background noise.
If we succeed in picking up a weathering signal from CO₂ measurements, this could potentially become an early indicator for CDR, e.g. in large-scale experiments with lots of rock/soil combinations. And it might even be a basic component of monitoring, reporting and verification (MRV) of carbon dioxide removal (CDR) in the future.
Our reasoning is based on a carbon-focused thought-model of the processes that occur in the soil after adding rock amendments for EW. Figure 1 shows a representation of the carbon fluxes and pools in a natural plant-soil ecosystem, flanked by carbon reservoirs of the atmosphere above and the groundwater below which both have - compared to a single pot in our experiment - unlimited storage and/or supply capacity for carbon. Through photosynthesis plants absorb atmospheric CO2 to form their aboveground biomass and their subsurface roots. A plant’s metabolism also produces CO2 which is partly respired by its roots into the soil air. When (parts of) the plants die they become part of the dead organic carbon pool within the soil that upon decay is once again transformed into CO2. This CO2 released into the soil by root respiration and organic material decay enters the atmosphere as soil CO2 efflux or dissolves in soil pore water and becomes dissolved inorganic carbon (DIC) that leaches into the groundwater.
By adding rock dust to the soil we want to enhance the formation of bicarbonate in the leachate water (dark blue arrow) which pulls carbon from the shown carbon cycle down into the ground, thus lowering the available carbon for the rest of the cycle and effectively moving it away from the atmosphere.
Our idea is that we should be able to get a basic understanding of the carbon-related effects of adding rock dust to a soil if we successfully monitor the changes (compared to the respective control which is the same soil without rock dust amendment) in CO₂ efflux, alkalinity flux in the leachate (blue arrows), soil gas pCO₂, soil carbon pools and biomass whilst most other parameters remain the same for both treatment and control - as is the case in our greenhouse. Ideally a set of metrics will emerge from CO2 measurements which can give an early indication of successful, delayed, non existent or even failed CDR (=a carbon source was created instead of a carbon sink) through enhanced weathering before slower signals such as leachate alkalinity or rock dissolution will show this.
Our elaborate greenhouse experiment setup and sampling strategy should allow us to track changes in carbon cycling over short time scales (weeks/months) in hundreds of experiments. We combine 24/7 monitoring of both soil pCO2 and CO2 efflux using electronic sensors with high frequency lab data of leachate water, biomass and various soil derived data.
In the first instance, we are mainly interested in the carbon fluxes into and out of the soil column (blue arrows in figure 1). Because in a simplistic view one would expect that the CO2 efflux goes down after treatment (less CO₂ emitted to the atmosphere) whilst the carbon transport as bicarbonates in the leachate goes up. Of course, it turns out to be more complicated than that…
A closer look at the carbon fluxes in soil with and without rock dust
The main process pumping carbon into the soil is plants taking up CO₂ from the atmosphere for photosynthesis. Carbon then enters the soil mainly via two pathways:
Through the plant's internal carbon transport downward where it is used to grow roots and it is respired as CO₂ into the soil (this produces carbonic acid after dissolving in water (H2O+CO₂⇆H2CO3), which weathers the soil’s mineral components and releases nutrients for the plant to take up).
Through dead organic material (e.g. biomass from old roots, leaves, plant residues and other dead organisms) which contributes to the soil's solid carbon storage pools.
Both paths already carry carbon away from the atmosphere (good!), but the storage is not permanent (less than centuries). The dead biomass is decomposed by microbes, a process which releases CO₂ into the soil (mineralisation of organic carbon). Together with the roots’ CO₂ respiration, this increases the soil's CO₂ gas pool which is ultimately released upwards to the atmosphere as a soil CO₂ efflux (driven by Fick’s 1st law).
The desired carbon capture driven by rock dust amendment mainly happens in the form of bicarbonate ions in leachate water (i.e. downward carbon export from the soil, potentially accompanied by DOC and POC which are less relevant for the intended permanent carbon storage effect due to low storage permanence between decades to a century). These bicarbonate ions are the products of the chemical reactions that occur when added rock dust minerals are dissolved by carbonic acid.
In natural ecosystems, the carbon cycle is largely carbon-neutral in a climate sense (a steady-state apart from annual/seasonal changes when undisturbed), circulating carbon between soil and atmosphere whilst only some of it is lost as dissolved species in leachate. This natural cycle serves as our benchmark and is represented by what we measure in our untreated control experiments.
Introducing rock dusts as a CDR technology will create two effects:
The rock dust is weathered by carbonic acid (=formed through the combination of water and CO₂ from air and soil gas) which produces cations that will eventually end up in the leachate, leaving the soil system together with the bicarbonate ions they counterbalance (= the desired effect). This might take a while (months-years-decades?) to fully materialize due to temporary immobilization of cations (and anions) as they move down in the soil column, see cartion park model. The bicarbonate/alkalinity signal is a slow CDR signature that lags behind the rock dust dissolution, but it is a rather reliable one as it directly measures the CO2 that leaves the soil system captured as bicarbonates.
The cations released by the rock weathering reactions may also act as fertilizer for plant growth, change the soil pH and impact soil biotic processes. This effect of cation release due to rock dust dissolution could lead to increased yields (Beerling et al. 2024) and may occur faster than the increase of cation content in the leachate water.
From our initial observations it appears that rock dust amendments indeed alter multiple carbon fluxes in our pot experiments linked to leachate alkalinity and soil CO2 gas, although not at the same rate and pace. Plant growth and biotic activity in the soil is often boosted, at least for some time after the rock dust application. We saw an increase in biomass after rock dust amendments in our experiment and similar observations have been reported in the review paper of Swoboda et al. 2022, section 4. These biological processes can both increase CO2 respiration into the soil, which in turn likely boosts the weathering reactions due to higher availability of CO₂.
Our experiments’ carbon flux data
In our latest working paper we took a look at the preliminary 2023 results from our main greenhouse experiment. The data presented are based on 36 treatments and 17 controls with 4 replicas each, i.e. in total covering 212 of our 400 lysimeters, monitored by 635,608 automated flux measurements and 3,486 manual on-site leachate titrations over a timespan of 11 months in 2023.
Read on in our working paper (where also most of this blogpost was taken from).