Cropping system, site and topographic impacts on deep soil carbon dynamics in no-till dryland agroecosystems

Abstract

Long-term research of no-till management in the US Great Plains has shown that increasing cropping intensity can potentially increase soil organic carbon (SOC) and crop yields compared to traditional winter wheat (Triticum aestivum L.)- fallow management systems. However, due to varying climate and topographical factors, SOC accrual rates may change with time and soil depth. We sampled SOC in a long-term experiment 36 years after its establishment. The study is located across three sites in eastern Colorado, and it characterizes a gradient of potential evapotranspiration (PET) with multiple slope positions at each site. Thus, the objectives of this study are to understand 1) the effects of different crop rotations and cropping system intensification on SOC after 36 years in the surface soil in a no-till, dryland system., and 2) how climate and topography influence SOC and SIC dynamics in deeper layers (> 20 cm) and potentially interact with management. The cropping rotations examined were wheat-fallow, wheat-corn (Zea mays L.)-fallow, continuous summer cropping and a grass strip to represent the Conservation Reserve Program, all planted across three sites with similar annual precipitation but increasing PET and a slight slope gradient. We found cropping intensity, slope position, soil depth, and site (PET) all independently impacted SOC and SIC concentration (g kg-1) and stocks (Mg ha-1). Aside from the perennial GRASS treatment that consistently had higher SOC to depth, the management effect was seen most pronounced in the surface layers of the soil, but beyond 20 cm, SOC and SIC were influenced more by site and slope. As previously seen, the toe slope accumulated the most SOC in the surface layers; however, it did not persist in the deeper layers where the summit and side slope positions accumulated more. The findings of this research contribute to addressing the information gap surrounding deep SOC and SIC dynamics and their interactions with climate and management of no-till in dryland agroecosystems. We revealed that while the surface soil SOC responds to intensification, deeper SOC and SIC layers show a more complex interplay between climatic and topographic factors. These insights are crucial for developing sustainable agricultural practices and enhancing carbon sequestration to inform climate change mitigation strategies in the Great Plains.