Enhanced DNDC modeling of the effects of tillage inversion, residue burial, and manure management on crop productivity and soil carbon in Quebec

Tillage-induced soil inversion and buried residues are often overlooked in agroecosystem models, particularly when combined with manure applications. This may lead models to overestimate soil organic carbon (SOC) losses from moldboard plowing, biasing carbon accounting relative to other tillage practices. This study improved the DeNitrification DeComposition (DNDC) model to better represent moldboard and chisel plowing, with and without, manure on SOC dynamics at depth. DNDC was enhanced by incorporating soil inversion under moldboard plowing and allocating residues to deeper layers where decomposition slows. Field studies included a barley monoculture and barley-perennial forage rotation on silty clay soil at Normandin, Quebec, and a corn-soybean-wheat rotation on a sandy loam soil at Université Laval, Quebec. At Normandin DNDC reasonably simulated barley grain and residue yields (NRMSE 18.4–47.4%) but had difficulties with interannual forage variability, likely due to winter-kill. Simulated SOC aligned with observations across fertilizer and tillage treatments (NRMSE 2.8–14.8%). DNDC captured observed trends, with manure plus barley‑forage increasing SOC at deeper depths under moldboard plowing, while chisel plowing retained more SOC near the surface. At Université Laval, DNDC effectively simulated yields (NRMSE 8.3–27.1%) and crop residues (NRMSE 8.8–29.6%), and captured surface SOC differences among treatments, with minimum tillage increasing surface SOC and moldboard plowing increasing total SOC through residue burial. DNDC showed limited sensitivity to management in deeper layers, reflecting limitations in simulating dissolved organic carbon transport. Overall, findings emphasized the importance of explicitly modelling residue burial and soil inversion as models increasingly support regional assessments and carbon credit programs.