As we count down to the 4th International Conference on Carbon Dioxide Removal in Milano, we are hosting a series of discussions on the research that will be shaping our sessions this June! 
This research synthesizes four years 
of laboratory 
studies exploring the combined use of Enhanced Rock Weathering 
and Biochar 
for terrestrial Carbon Dioxide Removal 
. The authors show that co-applications 
and rock-enhanced biochars can improve carbon sequestration efficiency 
, soil properties 
, and nutrient dynamics 
through synergistic effects , while also identifying important trade-offs 
related to rock type , soil conditions , and long-term field performance 
that must be addressed for large-scale deployment 
.
Full Abstract: Greetings from the lab: A synthesis from four years of research on combining enhanced rock weathering and biochar
Authors: Maria-Elena Vorrath, Johannes Meyer zu Drewer, Reinaldy Poetra, Nikolas Hagemann, Thorben Amann, Tobias Linke, Cierra Aldrich, Janine Börker, Maria Ansari, Mikita Maslouski, Mathilde Hagens, Josha N. Becker, Annette Eschenbach, Lennard Stoeck, Lara Feiertag, Sedrik Nauenburg, Ivo Nauenburg, Marvin Scherzinger, Theresa Siegmund, Dirk Eifler, Jens Hartmann, Claudia Kammann
Enhanced Rock Weathering (ERW) and biochar can be combined in co-applications or as rock-enhanced (RE-)biochars produced by co-pyrolysis of biomass and rock powder. Both strategies offer promising avenues for terrestrial carbon dioxide removal (CDR), leveraging synergistic effects while minimizing land-use competition. Here, we synthesize recent laboratory findings from controlled weathering experiments. A range of silicate-rich rocks (basanite, dunite), industrial waste products (concrete, steel slag) and biochars (wood, straw, soybean meal) were applied pure, in co-application, and as RE-biochar to soils differing in properties and origin (e.g. sandy; loamy; clayey oxisol). Experiments conducted over 27 and 75 weeks were used to evaluate inorganic and organic pathways for CDR, nutrient fluxes, and the impact on soil organic carbon under different pCO2 levels. Key findings include: • Biomass doping with dissolved magnesium increases carbon conversion efficiency during co-pyrolysis 7%.
• The co-application of ERW and biochar provides the highest CDR per mass due to additive effects.
• In specific cases, co-pyrolysis produces up to 16.1% higher synergistic CDR compared to pure applications due to increased stability of organic carbon in biochar.
• Biochar addition improves water drainage and prevents waterlogging, thus facilitating alkalinity export and prolonging weathering
• Biochar does not delay cation fluxes or decrease the leaching of weathering products.
• Application of pure limestone and concrete increases the retention of dissolved organic carbon (DOC) in the soil.
• Co-pyrolysis of dunite accelerates its weathering through thermal activation of serpentine minerals.
• Minor effects of biochar on the weathering of concrete, dunite, and basanite enable straightforward calculation of geogenic and biogenic contributions to dissolved cations and additional CO2 sequestration.
• For all soil amendments, soils with low cation exchange capacity and soil organic carbon content generally provide the highest inorganic CDR and experience faster cation leaching and low rates of secondary mineral formation.
Trade-offs and uncertainties include:
• In some rock types (concrete, steel slag, basanite), weathering is suppressed following co-pyrolysis.
• Strong DOC leaching is observed when steel slag is applied pure or with biochar.
• Poorly matched rock powder and soil texture risks waterlogging.
• Limited understanding of long-term soil dynamics, microbial responses, and field-scale performance.
Overall, the spatial stacking of CDR offers a circular, resource-efficient approach to CDR, with potential co-benefits for soil properties and agricultural productivity. Based on our data, we recommend to use concrete, dunite, and basanite in single or co-application with biochar, while steel slag needs to be treated with caution. We emphasize the importance of multi-year field trials to optimize practice and policy.
Can soil ever be a reliably engineered carbon sink given the complexity of long-term interactions shown here?
