Changes in soil N2O emissions and nitrogen use efficiency following long-term soil carbon storage: Evidence from a mesocosm experiment

Abstract

Policy and market incentives are rapidly expanding to promote soil organic carbon (SOC) sequestration in global croplands. Evidence suggests that long-term increases in SOC can influence both crop yield and nitrogen (N) fertilizer requirements, with the potential to help address two important sustainability challenges. However, increases in SOC may also trigger higher soil nitrous oxide (N₂O) emissions, which would represent an important tradeoff for climate change mitigation. We tested the hypothesis that long-term increases in SOC are associated with higher crop yields and fertilizer N use efficiency (NUE), but at the cost of higher N₂O emissions. Wheat was grown in two soils (SOC_low and SOC_high) under three N fertilizer rates (0, 100, and 200 kg N ha⁻¹) in a mesocosm experiment. Soils were obtained (0–25 cm) from a 22-yr field experiment on no-till and cover cropping in California. Results indicate that total biomass and grain yield were higher for SOC_low than SOC_high at 100 kg N ha⁻¹ but not the other N levels. Crop N uptake was also 28% greater for SOC_low at 200 kg N ha⁻¹, resulting in higher overall NUE. Soil N₂O emissions increased for SOC_high by 25–112% compared to SOC_low, likely due to long-term changes in labile C and N pools, microbial activity, and soil structure influencing porosity and gas diffusion. While there are well-documented crop and environmental benefits from enhancing SOC in agricultural soils, results from this study suggest that changes in soil N₂O emissions should be considered to accurately determine net GHG emission reductions.