Multi‐Domain Reactive Transport Modeling of GHG Emissions From Macroporous Agricultural Soils With a Focus on N <sub>2</sub> O Hotspots and Hot Moments
Mengqi Jia, David R. Lapen, Danyang Su, K. Ulrich Mayer
Abstract
Abstract Agricultural soils significantly contribute to greenhouse gas (GHG) emissions, influencing global climate change through natural microbial processes and human activities. Soil structure plays a crucial role in regulating these processes, especially in driving the formation of GHG hotspots and hot moments. We developed a new multi‐domain reactive transport model to assess the mechanisms governing GHG emissions from macroporous agricultural soils, focusing on N 2 O hotspots and hot moments. The multi‐domain model adequately reproduced observed soil moisture, pore gas O 2 and GHG levels, and short‐ and long‐term fluxes at an experimental site in eastern Ontario, Canada, outperforming other modeling approaches (i.e., uniform and dual porosity/permeability models). Building on the calibrated model, we focused on investigating the development of N 2 O hotspot and hot moments. While consistent with conventional assumptions that N 2 O production is concentrated in organic soils and subsoils, our results further reveal the fine‐scale spatiotemporal variability of hotspots driven by localized variations in moisture, anaerobic conditions, and nutrient and organic carbon availability. N 2 O hot moments occurring during periods of heavy rainfall were closely linked with N 2 O hotspots in organic soils. A combination of sensitivity analysis and structural equation modeling suggests that the formation of N 2 O hotspots and hot moments is strongly influenced by physical and geochemical parameters, such as immobile matrix radius, gas saturation within the mobile matrix, and the ratio of N 2 O production to consumption. While aligning with field data, the model's complexity poses challenges, including non‐uniqueness due to uncertain inputs, subdomain characterization, and gas/solute transfer between the subdomains. Nevertheless, the multi‐domain model provides valuable insights that can guide future research into how complex soil structural characteristics influence GHG cycling and emissions from agricultural soils.