Agricultural Emissions of Reactive Nitrogen Gases From Constrained Simulations Using the Community Land Model

Reactive nitrogen (Nr) gases (NH₃, N₂O, and NO) emitted from agricultural fields constitute a substantial portion of the Nr flux from land to the atmosphere. Here, we coupled the Flows of Agricultural Nitrogen (FANv3) with a modified version of the Community Land Model (CLM5.1) to produce a model (CLM-FANv3) that simulates the emissions of NH₃, NO, and N₂O. CLM-FANv3 is developed under the strong observational constraints of measurements conducted in mesocosms from a US Corn-Belt agricultural site where corn is planted subject to different fertilizer amounts and soil types. Combining measurements and models allows us to integrate disparate measurements of the mesocosm to produce more complete insights into the dynamic flow of nitrogen within the soil system. The new nitrogen dynamics in CLM-FANv3, as constrained by the measurements, are different than CLM5.1 with much-enhanced nitrification (2.5×), enhanced uptake of nitrate in plants (5×), increased nitrate leaching (an order of magnitude larger), and decreased denitrification (about half) at the mesocosm site. NH₃ emissions are comparable to denitrification to N₂, a significant term neglected in most Earth System Models. The emission factors for NO and N₂O in CLM-FANv3 (0.8%, 1.9%) are in reasonable agreement with the measured emission factor (1.7%, 1.4%), a substantial improvement over the CLM5.1 coupled with FANv2 model (0.2%, 3.5%). The strong measurement constraints have important implications for simulating expected future changes in Nr emissions. Differences in nitrogen cycling also impact projected amounts of carbon stored in agricultural soils over the long term.