Impacts of Carbonaceous Soil Amendments on Agricultural Nitrogen Emissions across U.S. Croplands

Fertilized soils are a leading source of reactive nitrogen (Nr) emissions, including nitric oxide (NO) and ammonia (NH3), which contribute to fine particulate matter and ozone air pollution, and nitrous oxide (N2O), a potent greenhouse gas. Applying biochar or other carbonaceous substances to soils could influence crop yields and carbon sequestration and mitigate Nr emissions. However, those impacts vary substantially with soil properties, climate conditions, and the properties of the carbonaceous materials. Since impacts on NO, NH3, and N2O may differ, there may be trade-offs or co-benefits for air quality and climate. However, most previous studies have focused on individual pollutants rather than integrated assessments.

We develop an integrated framework that consists of an enhanced process-based biogeochemical model, the Fertilizer Emission Scenario Tool for CMAQ (FEST-C), to predict Nr emissions; a reduced form air quality model, Air Pollution Emission Experiments and Policy Analysis (APEEP), to associate emissions with monetized health impacts; and the social cost of N2O to value climate impacts. Our linking of FEST-C and APEEP enables us to quantify the impacts of agricultural emissions in a spatially explicit manner. We enhance the nitrogen schemes in FEST-C by including NO and N2O from nitrification and NO from denitrification, adapting mechanisms from DayCent. We also adapt biochar algorithms, developed by Lychuk et al. in the Environmental Policy Integrated Climate (EPIC) model, for use in FEST-C to examine the effects of biochar amendments on Nr emissions. These algorithms treat biochar as organic matter and account for the variations of soil cation exchange capacity, pH, organic carbon, and bulk density after amendments. Additionally, we supplement FEST-C’s built-in farming practices database by adjusting its default crop demand-driven fertilizer database to account for fertilizer sales data from The U.S. Geological Survey. We first run the integrated assessment framework to value the impacts of U.S. agricultural Nr emissions on air quality and climate. We then perform scenario simulations using carbonaceous amendments with diverse properties to explore the variations of Nr emissions and identify where these amendments could achieve the greatest net benefits for air quality and climate.