Is biomass yield increase a potential indicator for CDR performance of enhanced weathering?

Enhanced weathering (EW) of alkaline feedstocks such as natural rocks or industrial by-products is a promising approach to remove CO₂ from the atmosphere. Finely ground feedstock, containing silicate and/or carbonate minerals, is spread on fields. When rain and CO₂ form carbonic acid, it dissolves these minerals, which releases base cations and nutrients into the soil and also increases soil pH. The beneficial effects of rock dusts like lime on yields have been appreciated by farmers for centuries. For CDR projects, crop yield increase is a welcome co-benefit, while the main desired effect is that the cations from the dissolving rock accompany bicarbonate ions in the leachate waters, which can be measured as an increase in total alkalinity (TA). This carbon transport is essentially a way of capturing carbon from the air, a climate mitigation approach that is currently challenged by the high cost of MRV. When assessing the above-ground biomass and leachate data from our 2023/2024 greenhouse experiment, we observed a significant correlation between the two. A significant increase in TA export of the rock-dust-treated pots almost always coincided with a significant increase in biomass. However, our data does not yet allow us to determine what is the cause and what is the effect. To fill this knowledge gap, we published a call for proposals in October in which we offered our sample archive to laboratories for further investigation. This research is underway, and we expect to see some exciting first results at our Carbon Drawdown Initiative symposium in June 2026. Because crop yield increase is an early and easy-to-assess metric, it would be valuable to understand any relationship between feedstock weathering, CDR effect, and biomass yield. If the correlation shown here also holds in field experiments, yield response could potentially become a low-cost way to assess EW sites and treatments. Another potential application could be the identification of heterogeneity clusters in fields using aerial or remote sensing plant growth analysis, which could guide sampling locations and derived MRV extrapolations when verifying carbon credits from larger-scale deployments. Both approaches could potentially reduce overall MRV costs, making this a worthwhile topic for further exploration. A word of caution when reading this paper: the observations presented here are based on our specific set of German soils and feedstock selection. We used only one plant species (English Ryegrass), and the experiments were carried out in a highly controlled greenhouse environment with constant irrigation. Further research is necessary to investigate how these results may transfer to other ERW setups with different soils, feedstocks, and plants in outdoor settings.