UBC Theses and Dissertations
Reactive transport modelling of greenhouse gas cycling and emissions from macroporous agricultural soils Jia, Mengqi
Agricultural lands are important anthropogenic sources of greenhouse gases (GHG) and play a vital role in affecting global climate change. GHGs (i.e., CO₂, N₂O and CH₄) are all produced (or consumed) as a result of microbial processes; however, the size of the concentrations and fluxes depends heavily on soil structure characteristics. Macroporous agricultural soils have a multitude of pore domains (e.g., macropores, soil matrix, soil aggregates) leading to varying degrees of fluid flow, solute and gas transport. Consequently, a spectrum of biogeochemical C-N processes may occur in macroporous soils that collectively, and coupled with flow and mass transport, determine GHG cycling. In this work, various modeling approaches were developed to assess the mechanisms of GHG production, consumption, transport and emissions, and interpret the simulation results in the context of observations at a field site in Ontario, Canada, featuring macroporous agricultural soil. The modeling approaches include a classical uniform reactive transport model, a dual-permeability reactive transport model with an emphasis on non-equilibrium gas transport and exchange, a discrete macropore model, and a more complex hybrid multi-domain model. Simulation results from all modeling approaches suggest that spatial distributions (e.g., hotspots) of GHG emissions can be attributed to differences in soil characteristics, soil nutrient supply, and organic C availability, whereas short- (hot moments) and long-term (seasonal) temporal variations are strongly affected by environmental factors including seasonal temperature and in-season acute precipitation events. In addition, results showed that all modeling approaches were successful in reproducing observed spatial and long-term temporal variations in pore gas GHG concentrations and fluxes; however, the hybrid multi-domain model clearly improved the simulation of hot moment N₂O emissions related to acute precipitation events compared to other approaches, which highlights the potential of improving the simulation of N₂O hot moments through accounting for three interconnected subdomains. The strengths and weaknesses of modeling approaches for simulating GHG cycling and emissions from macroporous agricultural soil were compared and evaluated. All modeling approaches showed promise for future studies with the aim to develop a more complete understanding of how complex soil structure affects the spatiotemporal variations of GHG cycling from macroporous agricultural soils.
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