Integrated modeling of reactive nitrogen emissions from U.S. fertilized soils with carbon amendments

Agricultural emissions of reactive nitrogen (Nr) are a major concern in air quality management and climate change mitigation. Due to a lack of efficient management and overuse of fertilizer, agriculture has become the largest source of Nr emission in the U.S., including air pollutants nitrogen oxides (NOx), ammonia (NH3), and the potent greenhouse gas nitrous oxide (N2O). Managing nitrogen in agricultural soils is thus necessary to mitigate Nr emissions and their associated negative impacts on air quality and climate. Soil carbon amendment is a carbon-rich material produced from biomass or methane under an oxygen-limited environment. Applying carbonaceous amendments to soils has been proposed as a mitigation strategy due to its potential to reduce Nr emissions and increase crop yields. However, the complex interactions between Nr emissions and carbon amendments are spatially variable, influenced by soil conditions, farming practices, climate conditions, and the properties of carbon amendments. Consequently, the impacts of carbon amendments on Nr emissions range from positive to negative or neutral. Furthermore, the contributions of NOx and NH3 to particulate matter (PM) and ozone (O3) pollution are spatiotemporally heterogeneous, while long-lifetime N2O exerts radiative forcing regardless of its origins. Besides emission variations, assessing how carbon amendments affect the air quality and climate impacts associated with Nr emissions is also needed when evaluating the effectiveness of the carbon amendments. Unfortunately, there has been a lack of an integrated study considering the impacts across Nr species and their associated impacts.
In this study, we use an integrated assessment framework of Nr emissions developed by our previous work to predict the Nr emissions from U.S. fertilizer soils in a mechanistic and consistent way and value their multiple impacts in a spatially explicit manner. This assessment framework consists of an agroecosystem model Fertilizer Emission Scenario Tool for CMAQ (FEST-C) to predict Nr emissions, and a reduced-form air quality model Air Pollution Emission Experiments and Policy Analysis (APEEP) and the social cost of non-carbon greenhouse gases to estimate the marginal damages and total damages of associated impacts of Nr on air quality and climate, respectively. Biochar is chosen as the representative of carbon amendments as their relatively well-documented interactions with nitrogen and carbon cycling in soils. We further improve the integrated framework by incorporating the biochar algorithms developed by Lychuk et al. into FEST-C. To evaluate the model performance of the FEST-C biochar model, we compare the relative variations of Nr emissions with biochar amendments observed in U.S. field studies. Sensitivity analysis is utilized to identify which biochar properties influence Nr emissions most. A national-scale analysis is then performed to investigate how Nr emissions and associated impacts vary with biochar amendments by applying the same biochar from Lychuk et al. to U.S. fertilized soils with various application rates.
Our results indicate the FEST-C biochar model tends to capture the observed sign of emissions variations but predict smaller changes than are observed. Sensitivity analysis shows that Nr emissions are sensitive to the organic carbon fraction of biochar and the application rates. For most agricultural regions, high-dose biochar applications could reduce reactive Nr emissions while low-dose applications could increase them. The impact of biochar additions on nitrogen emissions depends on how they influence nitrification rates. We are also exploring how other carbonaceous soil amendments, such as the pyrolysis products of methane, influence nitrogen emissions. Our integrated assessment of soil carbon amendments could inform policymakers and farmers where and how to apply these materials to generate benefits in reducing Nr emissions to improve air quality and mitigate climate change.