Nitrogen-based fertilizers play a critical role in modern agriculture, yet they are not without drawbacks. Runoff from fertilized fields causes coastal dead zones and fouled waterways, and nitrogen gases released from the soil contribute to increased radiative forcing, stratospheric ozone loss, and nitrogen deposition.
Nitrous oxide (N2O) and nitric oxide (NO) emissions are among the gases released from fertilized soils. Over broad spatial scales, scientists rely on emission factors to estimate the proportion of applied fertilizer that might be emitted as nitrous or nitric oxide. This top-down approach has been adopted by the Intergovernmental Panel on Climate Change (IPCC). However, the IPCC default emission factors for N2O and NO are coarse predictors, at best. Estimates of N2O emissions at the local scale can yield values many times greater than those estimated through top-down approaches. For increased insight, scientists rely on a conceptual ecosystem model, known as the hole-in-the-pipe (HIP) model, to predict N2O and NO emissions.
Soil moisture regulates these soil gas emissions, and the variable plays a central role in the HIP model. Still, experimental tests of nitrogen emissions as a function of soil moisture are lacking in the scientific literature. Hall et al. attempted to bridge this knowledge gap by explicitly examining N2O and NO emissions in response to varying soil moisture. Their experimental approach tested the built-in assumptions of the HIP model and its theoretical foundation that NO emissions decrease as N2O emissions increase.
The authors incubated two soils (mollisols) from corn and soybean fields in Iowa across seven soil moisture treatments. They applied nitrogen fertilizer to mimic the fall or early spring fertilizer applications in Midwestern farmlands and tracked the gas exchange between the soils and the atmosphere for 50 days following the addition.
The study revealed that fertilizers increased the rate of nitrogen emissions by up to 3 orders of magnitude in wet soils, although the rate did not increase linearly with soil moisture. Emissions peaked at different times for each soil type, although both soil types returned to baseline emissions after 35 days. Counter to the theory behind the HIP model, the soil continued to leak nitrogen gases at moisture levels between field capacity and saturation. This was particularly true for N2O, whose emissions significantly exceeded NO.
The authors found that N2O emissions were substantially higher than NO emissions at all moisture levels; however, there was no trade-off between N2O and NO production as predicted by the HIP model. Furthermore, the N2O emissions were much higher than the IPCC default emission factor values.
The study casts new light on the importance of soil moisture for regulating nitrogen emissions and highlights the role that soil wet spots play in agricultural gas emission. It also reveals critical discrepancies between nitrogen emission estimates conducted at local scales and those estimated at the global scale. Such differences suggest N2O emission estimates in the Midwest region of the United States may be underestimated. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2018JG004629, 2018)