Legumes and livestock in no-till crop rotations : Effects on nitrous oxide emissions, carbon sequestration, yield, and wheat protein content

Summary

Context: Crop rotation is seen as a ‘Climate-Smart Agriculture’ practice, but there are knowledge gaps around their climate impacts. This is the first direct measurement of nitrous oxide emissions (N₂O) from cropland soils in South Africa. Objective: Assess the production performance, soil greenhouse gas emissions, and soil carbon sequestration of different crop rotations. Methods: Continuous measurement over one year of direct soil N₂O and methane fluxes and analysis of 20 years of historical data on soil carbon sequestration, yields, fertiliser applications, and wheat (Triticum aestivum) protein content. Results and conclusions: Rotations that contained legumes and livestock produced higher wheat (3.5–3.6 vs 3.1 t ha⁻¹ year⁻¹) and canola yields (1.5–1.8 vs 1.3 t ha⁻¹ year⁻¹) with superior wheat protein contents, while the cash crop only system's protein content decreased by 0.085 absolute % points annually (compared to 0.01–0.05 %). The results suggest a strong crop rotation legacy effect on the accumulation and availability of nitrogen in the soil profile, for both crop growth and N₂O production, where systems which integrated legumes and livestock vs. cash crops only had 0.31–0.42 vs. 0.14 kg N₂O-N ha⁻¹ year⁻¹. All systems showed a significant increase of soil organic carbon of 0.24–0.30 Mg C ha⁻¹ year⁻¹ over the 20-year period. Significance: Legumes and livestock incorporation in crop rotations interact with nitrogen management. Most N₂O emissions occurred after precipitation in the otherwise dry summer, making reduction difficult as minimal management activities occur over this fallow period. A significant challenge in designing N₂O mitigation strategies is the lack of existing N₂O flux datasets needed to develop specific, regional emission factors.