Does co-deployment of enhanced weathering and biochar affect soil organic matter stabilization?

Terrestrial enhanced weathering (EW) involves the application of crushed silicate-rich rock on soil which can then sequester inorganic CO₂ by forming bicarbonates and carbonates. However, more recently, the effects of EW on soil organic matter (SOM) cycling have been gaining increasingly more attention. An increase in soil pH and the release of nutrients from silicate minerals might activate microbes to decompose more SOM, thereby possibly offsetting CO₂ removed via the inorganic pathway. On the other hand, EW might promote SOM stabilization mechanisms that could increase the lifetime of carbon bound in SOM and strongly boost CO₂ removal by EW. There are two major pathways of SOM stabilization in soils. Firstly, EW can increase formation of mineral-associated organic matter (MAOM) by forming highly reactive secondary minerals such as (hydr)oxides and other clay-sized minerals that can chemically bind SOM. Secondly, the formation of stable aggregates can physically protect SOM from microbial decomposition. While these stabilization mechanisms could greatly elevate the potential of EW, they have received little attention compared to the inorganic CO₂ removal pathway of EW. Studies that focused on SOM stabilization mechanisms by EW have been showing contrasting, possibly context-depending effects. Therefore, more experimental data is needed to unravel the complex network of interactions between EW and SOM stabilization.

Co-deployment of EW with biochar (biomass stabilized via pyrolysis) could promote the CO₂ removal efficiency even further. To date, studies that combined EW with biochar application remain scarce, limiting our understanding on how the two technologies interact. Biochar application can stimulate, slow down or exhibit neutral effects on SOM decomposition. Given the potential for co-deployment of EW and biochar in agricultural soils, there is a need for understanding interactions between EW, biochar and SOM.

In a mesocosm experiment we mixed basalt and poplar wood biochar with a sandy loam soil in a full factorial design. We quantified SOM stabilized in aggregates and via mineral association after 9 and 21 months. Aggregates were separated by size via wet sieving and their carbon content was quantified via loss on ignition (4h at 550 °C). Subsequently, the smallest size fraction (<50 µm) was used to target different organo-mineral associations via sequential extractions to determine MAOM formation. Results will be shown at the conference.