<p>Agricultural soils have great potential to sequester carbon, mitigating climate change while enhancing soil health. Subsoil layers are particularly promising for long-term carbon storage due to their lower carbon density and slower carbon turnover compared to topsoil. The reduced subsoil carbon density primarily results from limited carbon inputs at depth, while slower turnover is driven by 1) stronger physiochemical constraints on microbial decomposition, and 2) limited availability of high-quality, energy-rich substrates. These factors underscore the opportunities to target management practices that either increase carbon inputs to subsoil layers or reduce carbon turnover rates to enhance subsoil carbon sequestration. Advancing this field requires understanding the vertical distribution of carbon input quality and quantity, as well as the processes driven vertical carbon transport within soil profiles. Additionally, it is critical to elucidate how substrate properties (e.g., energy and nutrient content) and vertical environmental constraints (e.g., hydrothermal regimes and oxygen availability) influence microbial efficiency. Addressing these knowledge gaps will enable the design of effective management practices, unlocking the full potential of whole-profile carbon sequestration in agricultural systems.</p>