Soil organic carbon trends in US Midwest continuous maize and maize–soybean systems over 20 years

Efforts have focused on managing agricultural lands to optimize ecosystem health and provide key ecosystem services, including soil carbon (C) sequestration, yield stability, and climate resilience. These efforts often rely on adopting climate‐smart practices, including no‐till agriculture, crop diversification, cover cropping, and the application of compost and manure. However, there is limited understanding of the effects of long‐term management on soil C sequestration and climate mitigation in row‐crop production systems of the US Corn Belt. Using 20‐year data (2001–2020) from the AmeriFlux and the Long‐Term Agroecosystem Research network sites in Nebraska, we examined the total soil organic carbon (SOC) stocks and change under different management practices at two field‐scale sites in eastern Nebraska. Both sites were under irrigation, but one site represented continuous maize ( Zea mays L.; CM) and the other site represented maize and soybean [ Glycine max (L.)] (MS) rotation throughout the study period. Evaluation of the changes in SOC stocks using direct soil measurements showed that long‐term agricultural management had minimal effect on SOC stocks under irrigation, compared to the baseline (2001) level. Statistical analysis revealed no significant effect of management practices on SOC stocks ( p < 0.05). In fact, the eddy covariance method shows a small SOC loss, although not significant ( p < 0.05). Additionally, analysis of the SOC change using three methods showed large variation in SOC stocks between and within sites, indicating that sources of uncertainties associated with different methods need to be quantified for accurate assessment of SOC stocks at scales. Further analysis of the effects of crop rotation indicated that CM had the highest SOC stocks, compared to MS rotation. Our results show that despite two decades of conservation‐tillage and no‐tillage practices in highly productive CM and MS cropping systems, there were no significant changes in SOC. These differences were primarily driven by the direct feedback between rates of biomass production, post‐harvest residue retention, biological activity, and SOC formation and stabilization.