The concept of terrestrial enhanced weathering (EW) is relatively simple - speed up the natural CO2 removal potential of mineral weathering (see our science page for more on weathering) by spreading crushed rock material on farmland. However, when it comes to measuring and quantifying CO2 removal in the field, the process is anything but simple! Both weathering and CO2 removal are influenced by a myriad of factors including local soil characteristics, local climatic and hydrological conditions, plant growth and soil microbial activity (Calabrese et al., 2022). Not to mention common farming practices such as tilling and fertilisation. The result is that CO2 removal varies from field to field, farm to farm and region to region.
To reliably estimate carbon dioxide removal (CDR) on a large scale, these factors should first be well-understood and accounted for through direct in-field measurement (Almaraz et al., 2022). At this early stage, our measurement approach is designed to produce the scientific data that will ultimately lay the foundation for scalable monitoring reporting and verification of CDR.
Since 2021, Silicate has been carrying out field trials on commercial farmland, working with farmers to integrate our deployment and monitoring with their usual farming schedules and practices. We divide each of our trial fields into control and treatment sections. In the treatment section milled returned concrete is spread at a rate determined by the farmer (typically 8-10 tonnes per hectare), while in the control section no material is applied. This division allows us to assess the CDR effect the concrete has compared with ‘business as usual’ farming. We designate sampling locations within each of these sections, returning on a weekly basis to take measurements across the growing season – tracking the progress of weathering and CDR.
There are three primary ways to go about measuring carbon removal through enhanced weathering and we do all of them! Each approach has its merits and limitations, but if all three converge on the same story, then we’re well on our way to the holy grail of verified CO2 removal.
Cation loss from soils
The most well-known and widely adopted approach to CDR measurement in the field is regular soil chemical analysis. Shortly after the concrete amendment is applied to our fields, the total concentration of cations (primarily calcium) in the soil is measured. After a few months, new samples are taken from the same location and any loss of cations is attributed to weathering. It is crucial to note that cation loss is a measure of weathering rather than CO2 removal. In agricultural settings, fertiliser derived strong acids, such as nitric acid, can preferentially weather the concrete, resulting in a loss of cations from the soil amendment, but no carbon removal (Hartmann et al., 2013). For this reason, other tests are needed to verify CDR.
Soil water chemistry is the most reliable indication that atmospheric carbon is being removed. Bicarbonate ions (HCO3-) - the molecules in which carbon is captured and stored - are produced when concrete is weathered by carbonic acid. These bicarbonate ions are water soluble and are washed into rivers from the soils where they are formed, eventually ending up in the ocean (Andrews and Taylor, 2019). An increase in both bicarbonate ions and calcium cations (2:1 ratio) in our treatment sites is a great indication that weathering is resulting in carbon removal. At our field sites, we have installed soil-water samplers at each of the designated sampling locations. We collect these samples on a weekly basis – comparing the chemical composition at our control and treatment sites.
Gas flux measurements are another promising method of indirectly measuring carbon removal in the field. In all soils, there is a natural movement of carbon dioxide, a 'flux’, from the soil to the atmosphere (Brevik, 2012). If our concrete amendment is doing its job, the flux of CO2 from the soil should be reduced as weathering proceeds and carbon is captured. We measure the CO2 flux from our treatment sites and compare with the ‘business as usual’ flux from our control sites. Simultaneous measurement in the control and treatment are essential as CO2 fluxes are sensitive to the time of day, weather conditions and soil disturbance (Ray et al., 2020).
Mass balance (does it all add up?)
The gold standard for quantifying carbon removal is to find agreement between these three independent measurements. If the CO2 uptake predicted by weathering measurements in soil samples can be reconciled with an equivalent increase in bicarbonate in the soil waters and decrease in CO2 flux from soil, then we can be confident in our carbon removal efforts. To work towards this goal, our research is focused on rigorous monitoring of small heavily instrumented research plots. We believe that detailed, plot-scale research carried out across a range of environmental conditions will allow us to first understand the processes governing carbon removal before developing a more scalable protocol for carbon removal measurement. Ultimately, we hope that the data generated by our, and other, enhanced weathering research trials will inform the development of a computer model that can accurately estimate carbon removal across different soil types, cropping systems and climatic conditions, reducing the reliance on in-field measurement, and helping us scale EW to climate-relevant levels of CDR.