How Robust Is the Rotation Effect in Semi-Arid Systems?.

Cropping system experiments have demonstrated that, in conjunction with no-till management, cropping system intensification (reducing fallow frequency) can increase annualized grain yields and soil organic carbon (SOC), which are key metrics of profitability and soil health. However, these effects have not been assessed on working farms or across a range of environmental and management factors that can influence SOC and annualized grain yields. We evaluated SOC (0-10 and 0-20 cm depths) from 96 dryland no-till fields, and 5-year crop yield and fertilizer input histories from 46 working farms. Each level of cropping intensity, ranging from winter wheat-fallow to 4-year continuous rotations, was represented across a potential evapotranspiration (PET) gradient from southeastern Colorado to northwestern Nebraska. Annualized grain yield increased significantly with cropping intensity, with PET and nitrogen (N) fertilizer use as covariates. Surprisingly, total N application did not differ cropping system intesity. Five-year annualized grain yields were unrelated to SOC, although longer-term crop rotation intensity was a significant predictor of SOC with PET, years in no-till, soil clay content, and slope position as covariates. SOC in continuous rotations was 3 Mg/ha higher to 20 cm depth than wheat-fallow and medium-intensity rotations. These results suggest cropping system intensification is a robust practice for enhancing annualized grain yield, but requires greater fertilizer use to achieve maximum benefit. Additionally, cropping intensity is a significant predictor of SOC amidst environmental and management variability.