Grazing Effects on Nitrous Oxide Flux in an Integrated Crop-Livestock System

Abstract

Integrated crop-livestock (ICL) systems can improve profitability, production efficiencies, and reduce environmental degradation. Among the many environmental metrics important for assessing the sustainability of ICL systems, nitrous oxide (N2O) flux occupies a central role given its effect on net greenhouse gas balance of agricultural practices. Unfortunately, there is limited understanding of ICL system effects on N2O flux, particularly in North America. Therefore, we measured N2O flux from ICL and non-ICL practices near Mandan, ND USA using static chamber methodology over a 3-yr period (Oct 2016 - Oct 2019). Treatments included grazed and ungrazed cropland and grassland on a site with gently rolling topography and fertile Haplustoll soils. Grazing was done by yearling cattle in the fall of each year (0.41-0.76 ha steer-1). Maximum N2O flux from cropland generally occurred in March and April when soil moisture was abundant and air temperatures exceeded 0 °C (i.e., during spring snow melt). Nitrous oxide flux was over eight times greater under cropland compared to grassland (Mean = 33.7 vs. 4.1 μg N2O-N m-2 h-1, respectively). However, N2O flux was not affected by grazing in either production system. Within cropland, N2O flux was greater in corn (Zea mays L.) and spring wheat (Triticum aestivum L.) phases of the rotation (Mean = 40.5 μg N2O-N m-2 h-1) compared to the soybean (Glycine max L.) and cover crop phases (Mean = 20 μg N2O-N m-2 h-1). Stepwise regression found N2O flux to be weakly related to volumetric water content when the soil was not frozen (r2 < 0.2). Study outcomes suggest ICL systems including grasslands or grass-dominant cover crops will be most effective at mitigating N2O flux.