Gas Transport and Emission Dynamics in Soil Systems

Gas movement and exchange between soil and atmosphere are governed by a combination of physical transport processes and biological transformation of trace gases. Molecular diffusion and advection through interconnected pore networks determine the rates at which oxygen enters and carbon dioxide, nitrous oxide and methane exit the soil profile. Soil texture, structure and moisture content set the geometry and continuity of air‐filled pores, while tortuosity and pore‐size distribution influence the effective diffusivity of gases. Microbial processes such as nitrification, denitrification and methanogenesis occur within microhabitats defined by local redox conditions, water saturation and substrate availability. Root respiration further contributes to CO₂ production and can modulate oxygen gradients near the rhizosphere. Seasonal wetting and drying cycles, compaction or tillage practices alter pore connectivity, shifting the balance between aerobic and anaerobic zones and thus changing greenhouse-gas fluxes. Advances in non‐invasive imaging, automated chamber monitoring and predictive pore‐scale modelling now allow a more quantitative understanding of the links between soil physical state, microbial activity and gas flux dynamics. Insights into these interconnections are essential for refining greenhouse-gas budgets, improving emission mitigation strategies in agriculture and predicting soil responses to climate variability.

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Technical terms

Gas diffusivity: A measure of the rate at which gas molecules spread through the soil pore network, influenced by tortuosity, porosity and moisture.

Pore connectivity: The extent to which air-filled or water-filled pores form continuous pathways facilitating transport of gases or liquids.

Tortuosity: The degree of twisting or complexity of pore pathways, which slows the effective movement of gases relative to a straight channel.

Knudsen diffusion: A diffusion regime occurring when gas molecule collisions with pore walls dominate over intermolecular collisions, typically in very fine pores.

Denitrification: A microbial process converting nitrate to gaseous nitrogen species (NO, N₂O, N₂) under low-oxygen conditions, a key source of soil N₂O emissions.

Water-filled pore space (WFPS): The fraction of soil pore volume occupied by water, which determines oxygen availability and zones of anaerobic microbial activity.

References

1. Water‐ and air‐filled pore networks and transport parameters under drying and wetting processes. Vadose Zone Journal (2022).
2. Soil-Gas Diffusivity-Based Characterization of Variably Saturated Agricultural Topsoils. Water (2022).
3. Impact of Pore Geometry and Water Saturation on Gas Effective Diffusion Coefficient in Soil. Applied Sciences (2018).