Soil Organic Carbon (SOC) Fraction Dynamics Influenced by the Enhanced Rock Weathering with Blast Furnace Slag and Mine Tailings

Enhanced rock weathering (ERW) is a promising carbon dioxide removal (CDR) technology, capable of removing up to 95 t CO₂ ha^-1 yr^-1 from the atmosphere. In recent research, not only natural rocks but also industrial by-products, such as blast furnace slag (BFS) and mine tailings (MT), have been found to exhibit ERW potential, highlighting the need for comprehensive evaluations of their effects. While the CDR potential of ERW is mainly assessed by the increase in soil or leachate inorganic carbon, its impact on soil organic carbon (SOC) dynamics remains underexplored. To address this gap, we conducted a 290-day greenhouse experiment to evaluate the effects of BFS and MT, representative industrial by-products, on SOC dynamics. The experiment consisted of a factorial design with three replicates, with or without plants (alfalfa, Medicago sativa L.), and applying BFS and MT at a rate of 60 t ha^-1. SOC dynamics were analyzed through temporal changes in SOC fractions, including free particulate organic carbon (fPOC), occluded particulate organic carbon (oPOC), and mineral-associated organic carbon (MAOC). Additionally, we measured soil pH, available nutrients (NH₄⁺, NO₃^-. and P₂O₅), microbial biomass carbon (MBC), and microbial activities (hydrolase and oxidase). Results showed decreased total SOC in BFS and MT after 290 days. The SOC fraction dynamics showed distinct temporal dynamics. The fPOC content declined rapidly within 122 days and continued to decrease slowly thereafter. In contrast, oPOC and MAOC showed minimal or statistically insignificant changes over time. Microbial parameters, including MBC and enzyme activities, significantly increased in response to BFS and MT applications. These results indicate that the high levels of (3127.3 mg kg^-1), Na (4.0 mg kg^-1), and Mg (12.6 mg kg^-1) contained in BFS and MT stimulated microbial activity, thereby promoting the decomposition of labile SOC fraction (fPOC). Despite the SOC loss, BFS and MT significantly enhanced above- and below-ground plant biomass carbon. This augmented plant growth suggests the potential for increased carbon (e.g., plant residues) input to the soil, which may counterbalance the reduced SOC. These results highlight a complex interplay between ERW materials, soil microbes, and plant growth. To further explore these interactions, we plan to use synchrotron micro-CT to investigate the spatial arrangements between ERW materials, soil carbon, and microbial activity.