GHG Inventory Reveals Shifting Emission Intensity in US Corn Production

Greenhouse gas (GHG) inventories are critical for policy development to reduce emissions and mitigate climate change. While inventories have been compiled for the last 20 years in the United States, there has been limited investigation of metrics surrounding the emissions data. Our objective was to evaluate emissions intensity of corn production from 1980 to 2015 as an alternative metric to explore emission patterns. Corn production has steadily increased over the past few decades based on the USDA National Agricultural Statistics Survey, but there is limited information about trends in the GHG intensity of production. In order to quantify GHG emissions, an inventory was conducted using the DayCent ecosystem model with land use and management data for croplands based on the USDA National Resources Inventory, USDA Conservation Effects Assessment Project, USDA Agriculture Resource Management Surveys and other datasets. The two main sources of emissions in croplands are soil N₂O and changes in soil organic C pools. The latter can serve as a source or sink of emissions, and from our analysis, soil organic C has been a sink for atmospheric CO₂ on lands used for corn production from 1980 through 2015, ranging from 6.1 to 30.0 MMT CO₂ eq. yr^-1. This sink was strongest in the 1980s and early 1990s, but declined in the mid to late 1990s, before increasing again in the late 2000s. In contrast, soil N₂O emissions have steadily increased across the time series and are considerably larger than the CO₂ sink, ranging from 44.0 to 72.3 MMT CO₂ eq. yr^-1. Emissions intensity has varied from 106.6 to 276.1 Kg CO₂ eq. per metric tonne of yield for corn production. The highest intensity was in the mid to late 1990s when the soil C sink was the weakest. Furthermore, even though N₂O emissions increased over the time series, the emissions intensity declined in the late 2000s through 2015 as yields continued to increase. While emissions data demonstrate the overall importance N₂O emissions in croplands, the emissions intensity metric provides further insight into the potential role of a soil C sink to reduce the emissions intensity of corn production.