Measuring and modeling soil carbon sequestration under diverse cropping systems in the semiarid prairies of western Canada

Tillage and crop residue management can have large impacts on soil organic carbon (SOC) sequestration, as well as additional feedbacks on crop productivity and agricultural sustainability. The objectives of this study were to (i) evaluate the effects of summer fallow, legume green manure, pea or canola in rotation with spring wheat on crop yields, biomass production and SOC dynamics (0–15 cm) from six long-term cropping systems under semiarid conditions in western Canada, (ii) calibrate and validate three process-based models (DNDC, DayCent and DSSAT) using experimental data, and (iii) investigate the long-term impacts of tillage (no-tillage (NT) vs. conventional tillage (CT)) and straw residue (returned vs. removal) on SOC changes from diverse rotation systems. Over the experimental period from 1987 to 2015, the average wheat yields in Wheat (W)-Canola (C)–W-Pea (P) and legume green manure (LGM)-W-W rotations increased by 33.5 and 10.8%, respectively, compared to the continuous wheat (ContW) cropping systems. Higher SOC sequestration rates were found under ContW, LGM-W-W and W–C–W–P cropping systems mainly due to higher residue C input and larger nitrogen (N) inputs/fixation when compared to fallow-wheat systems. All three models performed "fair" to "good" in simulating crop yield, biomass and SOC content over time across all treatments. DSSAT had the lowest errors of prediction for yields (e.g., normalized RMSE (nRMSE) ≤ 27.2%, index of agreement (d) ≥ 0.79), while DNDC had better estimates of SOC content (e.g., nRMSE ≤ 4.5%, d ≥ 0.74). Long-term scenario analysis demonstrated that the mean annual changes in SOC across rotations based on model ensemble ranged from 0.03 Mg C ha−1 y−1 under CT with straw removal to 0.31 Mg C ha−1 y−1 under NT with straw returned. Predicted SOC sequestration potential of NT was relatively small compared to crop straw return for all rotation systems. This study suggests that diverse crop rotations which include canola and legume crops have the potential to store more SOC than traditional wheat-based cropping (e.g., fallow-wheat, ContW) for the semiarid prairies of Canada. These diverse rotations are already becoming more common and can help to enhance crop yields and maximize SOC sequestration when they are coupled with NT and residue management.