If I use legumes and do not apply any additional fertilizers, why are soil/fertilizer emissions positive?


Soil emissions include background emissions as well as nitrogen application- related emissions. Background emission always occurs also if no nitrogen is added, due to mineralization of soil organic matter. Also, the below-ground biomass of your crop adds nitrogen to the soil, which is mineralized and triggers emissions. The highest nitrogen fraction for below-ground biomass can be found for nitrogen-fixing forage and is lowest for crops such as rice, sorghum or millet. The soil-related emissions are assessed using not the IPCC guidelines, but the widely accepted Bowman model.