Insights into Enhanced Rock Weathering Field Experiments


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Measuring Enhanced Rock Weathering Signals

In this post, our new Research Lead Dr. Philipp Swoboda will provide us with some insights into the challenges of working with Enhanced Rock Weathering and specifically the many ways of measuring the amount of carbon sequestered through the rock dissolution.

Enhanced Rock Weathering (ERW) of silicate minerals releases essential elements and sequesters CO₂ dissolved in rainwater as inorganic carbon (principally bicarbonate (HCO3)), which is transferred and stored in the oceans for >100.000 years. Another ERW route that sequesters about half the CO₂ is the subsequent formation of carbonate minerals (carbonation). So, to quantify CO₂ sequestration through ERW, changes in dissolved inorganic carbon (DIC), total inorganic carbon (TIC, solid phase), and mineral released elements (solution and solid phase) must be calculated – what is so challenging about this? Well, it’s all about the actual measuring.

Measurement through Dissolved Inorganic Carbon (DIC)

Let’s start with DIC and the elements in the solution. Overall, we try to measure a small (additional) signal of a process that naturally and constantly happens in the soil – mineral weathering. Figure 1 shows a soil profile including two magnified circles. In the first magnified circle (pink) mineral weathering and some of its major products are shown: these include HCO3 and cations like Ca₂+, Mg₂+, K+, and Na+, which leach and diffuse in various directions into the soil. The second magnified circle (yellow) shows the main and heterogeneously aggregated constituents of the soil matrix: minerals (~45%), water (~25%), air (~25%), and organic matter (~5%) – and the added rock powder particles that constitute only a very tiny fraction compared to the already present soil minerals.

A soil profile showing mineral weathering and some of its major products and major soil constituents

Now consider a high rock powder dosage of 50t/ha – a practically questionable but probably necessary amount to attain a measurement signal. This corresponds to 5 kg of rock powder per m². Assuming that (for ideal tropical conditions) 4% of that rock powder, 200g, would dissolve per year, less than a gram of rock powder per day. The resulting daily milli- and microgram flux of weathering products must be tracked within several tons of highly heterogeneous soil via pore or leachate water samples, which are point measurements that only capture a small part of the overall water flux.

As already indicated in the beginning, the challenge hereby is that the natural flux of soil weathering products is in the range of milli- and micrograms and that the variations of this natural flux alone can be higher than the (additional) weathering fluxes from ERW. A major reason for these natural weathering flux variations is the spatiotemporal variation in rainfall and temperature interacting with the highly heterogeneous soil structure.

An unresolved issue hereby is also how exactly to handle/analyze the soil water samples in order to be representative, as the partial pressure of CO₂ in the soil substantially differs from the atmosphere (where the analysis is done), thereby influencing the amount of dissolved CO₂ and thus the amount of DIC. On the other side, an additional problem for cations like Mg₂+ as weathering proxies is that they are not only transported in various directions but are also taken up by plant roots, attached to organic matter, or reprecipitated on existing mineral surfaces – thereby obscuring actual weathering rates [1].

Measurement through Total Inorganic Carbon (TIC)

The other ERW route of carbonation is typically quantified through changes in TIC[2] and additional differentiations of pedogenic (soil-formed) and lithogenic (inherited) carbonates [3]. Measuring carbonation is equally challenging as carbonates can form over the whole depth of the soil profile. More specifically, carbonation depends upon pH and the activities of carbonate species and (mostly) Ca₂+/Mg₂+, which can substantially differ within the heterogenous macro- and micro aggregates that make up the soil. As the typical amount of soil analyzed for carbonate content is only some grams, the samples – as in the case of the soil solution – might not capture a representative signal.

Overall, there are other and/or combined ways to tackle measurement challenges, such as changes in isotope composition[4], alkalinity [5], plant analysis [6], weathering bags, and soil respiration[7].

The benefits of deploying ERW in the tropics

First, silicate dissolution is favored in strongly weathered soils combined with higher temperatures and high precipitation rates. Second, these strongly weathered soils typically have low reserves of weatherable silicate minerals, implying that the natural background “weathering” noise is lower, and it is thus more likely to get a signal – both in the solid phase and the solution. Third, even after decades of liming, these soils often do not contain carbonates, meaning that the second and less efficient route of ERW via carbonation can likely be ruled out (personal communication with Prof. Antonio Azevedo). Nevertheless, there are also unresolved issues in tropical soils, such as the fate of the carbonate species when the pH is very low, which might lead to partial degassing of CO₂ again.

Importantly, not getting a signal does not necessarily mean that ERW doesn’t work – in fact, most prior experiments show it does work – it just implies that a robust quantification is challenging. In the future, we likely have to employ as many parameters and replicates as possible and juxtapose the resulting data through various models to incrementally approach the ERW signal in the haystack.

If you like to learn more about the monitoring, reporting, and verification of Enhanced Rock Weathering in the tropics or want to start your first enhanced weathering project with us, get in touch!