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Reershemium, et al., Initial Validation of a Soil-Based Mass-Balance Approach for Empirical Monitoring of Enhanced Rock Weathering Rates, Environmental Science & Technology (2023)

Literature Review Series

Authored by Wil Burns, Co-Director, Institute for Carbon Removal Law & Policy, American University

There is growing interest in enhanced rock weathering (ERW) as a potentially important component of a carbon dioxide removal portfolio. Recent studies project that large-scale application of pulverized silicate rocks, such as olivine, basalt, or wollastonite, to croplands could effectuate atmospheric carbon dioxide removal of 0.5-4 gigatons annually by 2100. However, one of the most imposing barriers to scaling ERW as a climate response mechanism is the difficulty of monitoring and verifying carbon sequestration.

A new study in the journal Environmental Science & Technology tries to help address this issue. The study introduces a new tool to monitor and verify ERW sequestration. The approach seeks to measure differences in concentrations of ERW feedstock pre- and post-weathering by comparing concentrations of mineral-bound metal cations before and after feedstock deployment. More specifically, the approach, which is referred to as “TiCAT,” seeks to estimate the total loss of cations from the solid phase of soil samples vis-à-vis a titanium tracer. The researchers contend that this mass-balance approach can help us estimate the time-integrated amount of weathering of a silicate mineral, basalt in this study, within a given soil profile. The TiCAT approach was initially assessed through a laboratory mesocosm experiment that measured the concentration of reaction products in soils and leachate solution pools.

The researchers concluded that the TiCAT process accurately estimated initial CDR within the standard error or means of results from the more conventional method used to calculate weathering and initial CDR in mesocosm experiments. This suggests that “it can yield an accurate and robust estimate of initial CDR in enhanced weathering systems.” This could be a significant breakthrough, because prior methods of estimating ERW, especially those reliant on measuring quantities and transport of weathering reaction products, pose barriers to scaling given their time and labor intensiveness. Moreover, this approach could “directly integrate into existing agronomic practices,” as samples from the uppermost layer of soils are routinely taken for nutrient and soil pH analysis.

However, the authors of the study also proffer a number of caveats in terms of their findings, including the following:

    • The estimates from this approach are only an initial value, subject to potential leakage of initially captured carbon as it’s transported an alkalinity and dissolved inorganic carbon from soil to the oceans;
    • The variable lag time between feedstock dissolution and the capture of carbon dioxide needs to be taken into account in accurately assessing CDR;
    • As a next step, it needs to be established the approach can scale weathering rates from discrete sampling points to larger systems;
    • There may be site-specific conditions in some settings that would preclude accurate use of this approach, such as areas with high levels of physical erosion or where feedstocks with chemical conditions similar to those in the soils are applied.

As Mercer recently noted, robust monitoring, reporting and verification (MRV) of greenhouse gas removal approaches is a “market shaper” that can address a market failure that may preclude scaling of many options. Moreover, it’s critical to engender public acceptance and trust. While not as sexy as images of the construction of new CDR facilities, research of this nature needs to be front and center in our consideration.

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Review of Lefebvre, et al., Biomass residue to carbon dioxide removal: quantifying the global impact of biochar

Literature Review Series

Authored by Wil Burns, Co-Director, Institute for Carbon Removal Law & Policy, American University

To date, the vast majority of purchases of durable carbon dioxide removal have been for biochar, a process that can transform biogenic carbon dioxide into a far more stable form via the process of pyrolysis. Pyrolysis is a thermal process that, in the absence of oxygen, can deconstruct bio-polymers into, among other things, biochar, a charcoal-like substance that can securely store carbon for hundreds to thousands of years when applied to soil. Conversion of biomass to biochar can sequester 50% of initial carbon, compared to 3% associated with burning, or less than 10-20% after 5-10 years from biological decomposition.

Image Credit: Lefebvre, et al. The above image is a graphical abstract.

A number of studies in recent years have suggested that biochar potential could be much greater in the future, perhaps in the range of 3.5 GtCO2/yr., or up to 350 Gt during this century. However, to date, studies have focused on global or regional aggregate estimates. In a recently published study, researchers led by David Lefebvre of the University of British Columbia sought to extend these analyses by assessing the potential of biochar sequestration in each of 155 countries. The study restricts itself to the assessment of sequestration potential associated with biomass residue feedstocks in the contexts of agriculture, forestry wood residues, animal manure, and wastewater biosolids. The study also presumes that 30% of residues are left in the fields in the interest of maintaining long-term soil health.

Among the conclusions of the study are the following:

  • Four countries, all characterized by large populations, land areas, and agricultural sectors have the greatest potential, including:
    • China: 468 Mt CO2e/yr.
    • United States: 398 Mt CO2e/yr.
    • Brazil: 303 Mt CO2e/yr.
    • India: 225 Mt CO2e/yr.
  • North America and South America are characterized by a large number of countries with biochar sequestration potential of 25 Mt CO2e/yr., with bands of relatively low potential across North Africa into the Middle East, with low potential in portions of Europe and southern Africa.
  • 28 countries have the potential to sequester more than 10% of their CO2 emissions with biochar, with the largest number in Europe
  • The “conservative approach” of the study (including assessment of only recalcitrant carbon with permanence factors based on national averages) yielded an estimated carbon dioxide removal potential of 6.23% of total greenhouse gas emissions of the 155 countries in the study.

Notably, the researchers observed that its estimates didn’t take into account a number of potentially compelling co-benefits, such as potentially reducing emissions of methane and nitrous oxide, enhancement of crop yields and displacement of fossil fuels. Any effort to assess the potential costs and benefits of biochar deployment in individual countries, as well as globally, will require a more granular assessment of these factors, suggesting one potential research tributary flowing from this study.

Overall, this study could prove extremely helpful in helping to operationalize biochar programs nationally, and regionally, moving forward. It suggests that biochar could play an important role in the carbon dioxide removal portfolio of many countries.