Tracking soil organic carbon trajectories: depth and fractions matter for carbon monitoring in US midwestern long-term no-tilled systems

This is the authors' abstract. We don't add key points for this paper.

Understanding how soil organic carbon (SOC) changes over time and depth in agricultural systems is critical for informing soil health stewardship and carbon monitoring in SOC-focused programs. Yet, knowledge is limited on how SOC trajectories vary across depth and among different fractions and soil health metrics—particulate organic carbon (POC), mineral-associated organic carbon (MAOC), permanganate oxidizable carbon (POXC), and mineralizable carbon (MINC). We analyzed archived soils from three long-term Midwest trials: two in Nebraska comparing residue removal and retention in continuous corn over 10 and 19 years, and one at the Great Lakes Bioenergy Research Center (GLBRC) comparing continuous corn and perennial switchgrass after 9 years. At the 19-year Nebraska experiment, increasing plant inputs through residue retention enhanced SOC, particularly in surface soils. At both Nebraska experiments, POXC more closely tracked total SOC than MAOC, POC, or MINC. At GLBRC, surface-level SOC losses occurred under corn, whereas switchgrass exhibited whole-profile SOC losses. Across sites, MAOC showed minimal change, MINC often declined, and POC responses were variable, while POXC was the most reliable indicator of SOC gains and losses. Extending sampling beyond surface soils revealed larger changes, highlighting that surface-focused measurements alone can misrepresent SOC dynamics. Overall, long-term monitoring across multiple SOC fractions and depths is essential for accurately assessing SOC change under different management practices. Our findings provide actionable insights for regenerative agroecosystems and highlight the potential use of POXC as a soil health metric for informing management decisions and SOC-focused programs.