Interaction between enhanced weathering and sources of organic carbon: effects on reactivity of metal slags and inorganic carbon dynamics

The optimization of enhanced mineral weathering as a carbon dioxide removal technology is a topic of recent research, also for agricultural land in Europe. The combination of organic amendments with silicates is one strategy to optimize the carbon dioxide removal potential of enhanced weathering. In the C-Farms project the focus is on the added value of combining enhanced weathering through basalt or metal slags with the application of C-rich materials in agricultural soils. Co-application of inorganic (silicate rocks or metal slags) with organic (biochar, digestates or compost) amendments revealed synergistic effects whereby organic amendments may increase the rates of silicate weathering. These interaction effects are studied in soil mesocosms studies and controlled soil leaching trials.

In one of the experiments without using soil as a matrix, the direct interaction between weathering and organic C in mixtures of metal slags from stainless steel production and different carbon-rich materials was studied. Mixtures of 80% v/v carbon-rich materials and 20 % v/v metal slags were amended with mineral fertilizer, moistened and incubated for 8 weeks at 20°C. Different types of biochar were compared with other organic amendments like hydrochar, compost, manure-based hydrochar, and dried digestate. CO₂ flux was measured three times per week. After the incubation period, the mixtures were analysed for chemical composition, including pH, mineral N and both organic and inorganic C content.

Negative CO₂ fluxes were observed in most of the mixtures during incubation, confirming the reactivity of the stainless steel slags and their potential for carbon dioxide removal. Some organic materials had a lower biological stability, resulting in higher CO₂ fluxes than for organic amendments with a higher biological stability. Although some types of manure-based organic matter had high positive CO₂ fluxes due to the lower biological stability when incubated as a pure material, the combination of the unstable organic matter with metal slags resulted in negative CO₂ fluxes, indicating that metal slags counteracted the CO₂ emissions from the organic amendment. This may be related to the high pH and acid-buffering capacity of the metal slags, resulting in a pH of the mixture beyond the optimal pH range for biological activity.

By mixing metal slags and manure-based products with a lower biological stability and thus a higher CO₂ release, the C capture as inorganic C by the metal slags may be increased. This may indicate a higher potential for enhanced weathering than when only metal slags are applied in the soil, but this should be confirmed in ongoing soil-based experiments. The added value for enhanced weathering of testing the direct interaction between pure metal slags and organic amendments will be discussed.