Net Global Warming Potential Index Rather Than Soil Carbon Stock Change Could Provide Better Understanding of the Carbon Balance in Soil Systems

This study was conducted to determine the soil organic carbon (SOC) stock change factor for green manure crops that was developed by the Intergovernmental Panel on Climate Change (IPCC) Tier 2 method and compare this with the net global warming potential (GWP) index that is used to evaluate the contribution of green manuring to global warming. Four treatments were barley (Hordeum vulgare L.; B), hairy vetch (Vicia villosa R.; HV), a barley/hairy vetch mixture (BHV) and a conventional treatment (C). The aboveground biomass of green manure crops was incorporated into the soil on 25 May 2018, 26 April 2019, 29 April 2020, 30 April 2021 and 2 May 2022. Maize (Zea mays L.) was transplanted as the subsequent crop after the incorporation of green manures. SOC stock decreased with green manures, even though carbon input with green manures, including B, HV and BHV, was greater than that with C. The mean value of the SOC stock change factor for green manure crops, including B, HV and BHV was 0.627 and was significantly lower than that of the C. However, the net GWP also decreased with the incorporation of green manure crops, and the mean value of the relative net GWP index across B, HV and BHV was 0.853. These conflicting results were caused by different estimation methods between annual SOC change (△SOC) and net GWP. The estimation of SOC stock change using △SOC suggested by the IPCC method may overestimate the contribution of green manure crops to global warming. The net GWP method with comprehensive input and output of carbon in the soil system could provide a better understanding of the carbon balance in soil systems. In the current study, the comparison of △SOC and net GWP was conducted for at one site of upland soil for 5 years. Therefore, further research on estimating the effect of green manure crops on net GWP in various types of soil for longer years should be conducted.