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 study explores how combining Biochar and Enhanced Weathering (EW) can increase carbon removal while improving soil health and agricultural productivity. 
By testing these approaches in Germany and Kenya, the researchers found that co-application can enhance soil carbon storage, reduce N₂O emissions and nitrate leaching, and significantly boost crop yields, highlighting the potential of stacked CDR strategies to deliver both climate and farming benefits. 
Full Abstract: Combining Biochar with Silicate Rock Weathering: Can we achieve technical and agricultural synergies to increase CDR? - Experimental results of the PyMiCCS consortium within the German CDRterra research initiative.
Authors: Claudia Kammann, Susanne Hamburger, Johannes Meyer zu Drewer, Maria Ansari, Carolyn M. Görres, Joscha N. Becker, Annette Eschenbach, Maria-Elena Vorrath, Jens Hartmann, Nikolas Hagemann
To optimize CDR potential per area of land potential synergies of combining methods needs to better understood. The PyMiCCS consortium of the German (CDRterra program) investigates the synergies of combining biochar and enhanced weathering (EW) of silicate rock powder (RP) as soil amendments. Biochar can improve agronomic properties and stable soil-organic carbon storage, while weathering of RP binds CO₂ in alkalinity (bicarbonate & carbonate ions) with neutral/positive effects in soils. Biomass feedstock, impregnated with RP pre-pyrolysis (= rock-enhanced (RE-)biochar), and mixed biochar+RP was used to test potential CDR amplification by following mechanisms: (1) Alkali and Alkaline Earth Metals (AAEMs) in RP can increase the stable biochar-C yield (Yc) in RE-biochar; RE-biochar can (2) increase plant and microbial biomass, aggregate stability or labile-C retention for additional soil-C stock increases, and (3) reduce N2O emissions and nitrate leaching. Plant-soil-environmental effects were tested by amending a temperate sandy soil under near-field conditions in Germany and a tropical weathered, P-deficient soil in Kenya.
Results on technical CDR synergies:
• First quantification of alkalinity production by biochar (relevant for MRV)
• Thermal treatment of basanite-RP did not increase alkalinity production
• Co-application and co-pyrolysis increase alkalinity production and possibly silica leaching
• Co-pyrolysis of biomass with volcanic silicate rock (up to 750 °C) did not increase carbon conversion efficiency (Yc) and biochar stability, since AAEMs embedded in crystalline structures remain largely unreactive during pyrolysis
• Materials with more free AAEMs (concrete, slag) had the desired effect on Yc and PyC stability (see Vorrath et al.)
• Some classical methods for biochar analysis did not work on RE-biochars due to fundamentally different thermal and reflective properties of the rock fraction
Results on soil, plant and environmental CDR-increasing synergies:
• Soil only: No effects of RP
• Biochar-containing amendments improved soil properties, including stable SOC pools
• Positive effects were partly more pronounced following co-application compared to RE-biochar application
• Plant-growth, Germany: again, no effects of RP
• Pure biochar- or co-application with RP significantly increased yields over four cropping seasons
• RE-biochar increased yields less often, depending on the cultivation set
• Biochar-containing treatments significantly reduced nitrate leaching and N2O emissions
• Plant growth, Kenya: Increased P supply and availability in Oxisol with biochar-containing amendments.
• Amendments improved total-factor-productivity by 3-153%.
• Maize yields of 200% of the national average (Kenya) were achieved with 50% less fertilizer
Our results indicates that CDR-amplifying effects can be achieved (i) technically, but not with basanite RP (see Vorrath); and (ii) agronomically (by SOC-relevant amplification mechanisms).
Can combining Biochar and Enhanced Weathering unlock greater carbon removal while simultaneously improving agricultural productivity? 
