Modeling agricultural reactive nitrogen emissions with soil carbon amendments using an enhanced version of Fertilizer Emissions Scenario Tool for CMAQ (FEST-C)

Inefficient management and overuse of fertilizers makes agriculture the leading contributor to reactive nitrogen emissions in the United States, imposing a range of adverse impacts on air quality, health, and climate. Reactive nitrogen emitted from agricultural soils includes air pollutants nitrous acid (HONO), nitric oxide (NO), and ammonia (NH3), which contribute to health-damaging tropospheric ozone and particulate matter, and the potent greenhouse gas nitrous oxide (N2O). High spatiotemporal variability and complex influences of soil properties, climate conditions, and farming practices complicate efforts to control these emissions, which are often neglected by policymakers. Biochar is carbon-rich material produced from biomass pyrolysis under oxygen-limited conditions. Novel materials produced from methane pyrolysis are also being developed. Applying carbonaceous soil amendments has gained considerable attention because of its ability to influence nitrogen emissions and crop yields. However, those impacts vary widely in various agricultural regions, from positive to negative and even neutral, depending on the properties of biochar and its interaction with soil properties, farming practices, and climate conditions. Previous studies either relied on field measurements or field-scale agroecosystem models that were inadequate to characterize conditions across U.S. agricultural lands.
In this study, we incorporate biochar algorithms into an agroecosystem model Fertilizer Emissions Scenario Tool for CMAQ (FEST-C) to investigate how nitrogen emissions vary with soil carbon amendments. FEST-C has a number of input data generation tools with built-in databases that enable simulation across U.S. agricultural lands. Our previous work (Luo et al., ES&T, 2022) enhanced FEST-C by updating its nitrogen schemes to estimate reactive nitrogen emissions in a consistent and mechanistic manner. We further enhance the functions of FEST-C to simulate soil carbon amendments by adapting the algorithms developed by Lychuk et al. These algorithms assume that the essence of biochar is organic matter and its additions to soil could change the soil carbon pool and the bulk density. Second, the high surface area and charge density of biochar could increase the soil pH and cation exchange capacity. To evaluate the FEST-C biochar model performance, we compare its estimates of the impacts of biochar with those observed in published field studies in the United States. We then perform scenario analyses with biochar application rates of 5 and 20 ton/ha across U.S. agricultural lands. Our results demonstrate that for most agricultural regions, high-dose applications of biochar could mitigate reactive nitrogen emissions while low-dose applications could stimulate them. The net impacts of biochar amendments on nitrogen emissions depend mostly on how they influence nitrification rates. We are also exploring the potential impacts of soil carbon amendments derived from pyrolysis of methane rather than biomass.