UC Davis

Efforts to build soil organic carbon (SOC) in global croplands are rapidly expanding. Evidence suggests that long-term increases in SOC can lead to improved crop yield and reduced nitrogen (N) fertilizer and water requirements, two important sustainability challenges. However, increases in SOC may also trigger higher soil nitrous oxide (N2O) emissions due to changes in labile soil carbon and N pools, among other soil functions. Using wheat as a case study in a controlled environment, we tested the hypothesis that increasing SOC will improve yields and the efficiency of water and N fertilizer use, but this will come at the cost of higher soil N2O emissions. Mesocosms were constructed using intact soil cores (30 cm diameter by 25 cm depth) sampled from two treatments in a long-term experiment that differed in SOC following 20 years of conservation soil management, labeled as SOClow and SOChigh for this study. Wheat was grown in each soil at three different N fertilizer levels (0, 100, and 200 kg N ha-1) under drip irrigation in the greenhouse. Soil N2O emissions were measured using the closed chamber methodology. Results indicate that SOChigh did not increase wheat grain yield, thus water and nitrogen use efficiencies were similar at the different N levels. Yet, soil N2O emissions significantly increased by 25-112% under SOChigh conditions, which represents a tradeoff for climate change mitigation. While enhancing SOC storage in croplands is likely to bring well-documented crop and environmental benefits, these results suggest that changes in soil N2O emissions should also be considered to determine the magnitude of net GHG emission reductions.