Colorado State University and USDA scientists identify gap in US greenhouse gas reporting

Nitrous oxide, the greenhouse gas that contributes to the depletion of protective ozone in the upper atmosphere, can be reduced through best-management practices without impacting crop yields, suggest researchers in the USA.

A recent study conducted at Colorado State University, with partners from the Agricultural Research Service at the US Department of Agriculture, examined the impacts of accounting for nitrous oxide emission pulses triggered by melting of snow and thawing of surface soil layers during spring in the USA.  

Cropland and grazing lands are the primary human-caused source of nitrous oxide, which is generated by the nitrification of fertilizers that are critical to food and crop production. Slow-release fertilizers that release nitrogen when the crop needs it can help reduce these emissions, but these fertilizers cost more to use, so they need to be incentivized to be more feasible for agriculturalists.

The study, called A gap in nitrous oxide emissions reporting complicates long term climate and published in the Proceedings of the National Academy of Sciences, described how gas measurements from research sites in the USA and Canada were used to enhance the DayCent model and examined implications for the GHG inventory and mitigation efforts.

CSU collaborates with ARS to combine measurements of nitrous oxide emissions with the DayCent ecosystem model to account for how land management practices interact with soil and plant dynamics to control emissions.  

Simulations are conducted for most of the agricultural lands across the USA, and then the estimates for nitrous oxide and other soil greenhouse gases (methane and soil carbon sequestration) are included in the national GHG inventory compiled by the US Environmental Protection Agency. The resulting inventory is reported to the United Nations Framework Convention in Climate Change.  

“Including spring thaw-related N2O pulses increased emissions from this source up to 16% at the national level,” said study co-author Stephen Ogle, a professor in the Department of Ecosystem Science & Sustainability and senior research scientist at the Natural Resource Ecology Lab.

“Soil nitrous oxide is considered a key GHG source category, and including the previously under-recognized spring thaw process improves the accuracy of agricultural sector emission estimates,” said Ogle, who also serves as the technical lead for this GHG source in the national inventory.

“At the global scale, agriculture is responsible for about a fourth of human-related GHG emissions, whereas in the US this sector’s share is close to 10%,” said study lead author Stephen Del Grosso, a soil scientist with USDA Agricultural Research Service and CSU affiliate who collaborates with Ogle to compile GHG inventories for US-EPA and USDA.  

“Although spring thaw induced N2O pulses occur over a short time period, they sometimes contribute half or more of annual emissions in northern agro-ecosystems,” Del Grosso said.

Researchers found that the original version of DayCent substantially under-estimated spring thaw related N2O emissions when comparing model outputs with high quality N2O observations from research sites in Ontario and Manitoba.

They then showed that accounting for how snowpack depth and severity of the winter freeze improved model performance at both the plot level and when comparing model generated emissions for the Northern Great Plains/Corn Belt region of the United States with regional emissions inferred from atmospheric N2O data.  

“It’s crucial to compare model outputs with observations from diverse landscapes at different spatial and temporal scales to expose model weaknesses and spur model development,” Del Grosso said. “Combining models with various ground based and remotely sensed measurements helps to reduce uncertainties and improve estimates of GHG emission as well as nutrient losses from agro-ecosystems.”

The findings have implications for the Paris agreement to limit global warming to less than 2°C, Ogle said.

Including spring thaw induced emissions means that baseline nitrous oxide levels from the United States are higher than previously thought. Consequently, more aggressive mitigation is required to accomplish climate goals to reduce emissions from this key source.

“Fortunately, we can reduce net greenhouse gas fluxes from agriculture by up to 50% using best management practices without a decrease in crop production,” said study co-author William Parton, professor emeritus with the Natural Resource Ecology Lab, who originally developed the model in the 1980s and 1990s.

In addition, practices to reduce GHG emissions can also increase plant nutrient and water use efficiencies and decrease leaching of nutrients that contribute to eutrophication of aquatic systems such the Gulf of Mexico, Parton said.