Agricultural management strategies to actively promote subsoil carbon storage

Agricultural subsoils (> 0.2 m depth) are a vital carbon (C) sink, offering significant stable, long-term C storage due to their undisturbed and protective soil physicochemical properties and low microbial activity. Full of untapped potential, the subsoil is a critically important C reservoir for global C sequestration that has been underutilised and overlooked both by policymakers and researchers, resulting in significant research gaps with wide-reaching impacts on regional and global C modelling. However, subsoil environments often present inherent challenges for biological C inputs, including increased compaction, acidity, oxygen deficiency, and nutrient limitations that constrain root development and microbial activity. Recent advances in agricultural management have identified several key strategies to promote subsoil C sequestration, including mechanical interventions (e.g., deep tillage, straw burial), chemical amendments (e.g., biochar, mineral additions), biological approaches (e.g., deep-rooting crops and forage species), and land use transitions (e.g., cropland conversion to grassland or agroforestry systems). Here, we critically examine the current evidence for different subsoil C storage and protection strategies, focusing on their mechanisms, efficacy, and practical implications for agricultural systems. We identify key research gaps and the balance between the co-benefits (e.g., improved soil structure, reduced nutrient leaching at depth) and negative impacts (e.g., positive priming, agrichemical binding) of increasing subsoil C with these aforementioned strategies. The barriers to successful implementation of these methods are discussed, recognising the socioeconomic constraints that without policy incentives may limit their adoption outside of larger, financially stable farm enterprises. We emphasise that overcoming subsoil constraints through targeted breeding programs and integrated management approaches is essential for maximizing biological C inputs to deeper soil layers. Finally, we recommend that future research must prioritise large-scale, longitudinal studies that can comprehensively assess the ecological, economic, and agronomic implications of subsoil C management interventions. In addition, these need to consider combining physical, chemical, and biological subsoil C sequestration strategies, to maximise the benefits, rather than investigating them in isolation. • Subsoils offer significant potential for stable, long-term C sequestration in agriculture. • Deep straw burial increases subsoil C while improving soil structure and fertility. • Deep-rooting crops deliver C to subsoils through multiple biological pathways. • Chemical amendments show variable success in protecting C from microbial degradation. • Combining biophysical and chemical approaches may maximise subsoil C storage.