Initial Validation of a Soil-Based Mass-Balance Approach for Empirical Monitoring of Enhanced Rock Weathering Rates

Abstract

Enhanced rock weathering (ERW) is a promising scalable and cost-effective carbon dioxide removal (CDR) strategy with significant environmental and agronomic co-benefits. A major barrier to large-scale implementation of ERW is a robust monitoring, reporting, and verification (MRV) framework. To successfully quantify the amount of carbon dioxide removed by ERW, MRV must be accurate, precise, and cost-effective. Here, we outline a mass-balance-based method in which analysis of the chemical composition of soil samples is used to track in situ silicate rock weathering. We show that signal-to-noise issues of in situ soil analysis can be mitigated by using isotope-dilution mass spectrometry to reduce analytical error. We implement a proofof-concept experiment demonstrating the method in controlled mesocosms. In our experiment, a basalt rock feedstock is added to soil columns containing the cereal crop Sorghum bicolor at a rate equivalent to 50 t ha −1 . Using our approach, we calculate rock weathering corresponding to an average initial CDR value of 1.44 ± 0.27 tCO 2 eq ha −1 from our experiments after 235 days, within error of an independent estimate calculated using conventional elemental budgeting of reaction products. Our method provides a robust time-integrated estimate of initial CDR, to feed into models that track and validate large-scale carbon removal through ERW.

Enhanced weathering in the US Corn Belt delivers carbon removal with agronomic benefits

“…Our soil-based analysis has the advantage of providing a time-integrated quantification of the loss of reactive mineral cations in forward weathering reactions that drive carbon removal in the field ( 14 ). It avoids the need to quantify the spatial and temporal variability in water movement that transports soluble EW CDR reaction products (alkalinity, cations, and dissolved inorganic carbon).…”

“…Soil samples were treated with ammonium acetate to leach the non-mineral-bound exchangeable fraction and subsequently prepared for metal analysis using isotope dilution (ID-) ICP-MS (inductively coupled plasma mass spectrometry) at the Yale Metal Geochemistry Center ( 14 ). Soil pH was measured following previously described procedures ( 53 ), and soil exchangeable acidity was measured using a soil-buffer equilibration method ( 54 ) with substitution of 0.1M sodium phosphate buffer for the highly toxic Shoemaker-McLean-Pratt buffer reagent ( 55 ).…”

“…We quantify weathering mass loss of major divalent cations from the added basalt grains using straightforward cation accounting in soils relative to immobile trace element concentration (e.g., refs. 13 and 14 ). Temporary (reversible) retention of weathered cations on soil exchangeable sites prior to drainage discharge ( 15 – 17 ), and downstream changes in CDR efficiency ( 18 ), are not directly accounted for.…”

On error, uncertainty, and assumptions in calculating carbon dioxide removal rates by enhanced rock weathering in Kantola et al., 2023

“…However, we have several concerns about the method-developed largely by the research team of the for-profit EW supplier Eion Carbon-used to quantify CDR from EW in this study.Our hope is that highlighting these concerns will help facilitate progress in tracking carbon fluxes in enhanced weathering deployments.Soil-based mass balance approaches have a long history of providing key insights into weathering rates (e.g., Brimhall & Dietrich, 1987;Chadwick et al, 1990). We fully support the development of new soilbased mass balance approaches to track EW and indeed are actively pursuing research on the topic (Beerling et al, 2023;Reershemius et al, 2023). Given the large variety in soil and feedstock compositions, it is likely that no single method will be applicable or feasible in every setting.…”

“…Soil-based mass balance approaches have a long history of providing key insights into weathering rates (e.g., Brimhall & Dietrich, 1987;Chadwick et al, 1990). We fully support the development of new soilbased mass balance approaches to track EW and indeed are actively pursuing research on the topic (Beerling et al, 2023;Reershemius et al, 2023). Given the large variety in soil and feedstock compositions, it is likely that no single method will be applicable or feasible in every setting.…”