Soil-Gas Diffusivity Modelling in Aggregated Soils

Abstract:

Anthropogenic nitrous oxide (N₂O) emissions are primarily derived from fertilizer use and animal excreta. Soil modelling of N₂O movement within aggregated agricultural soils is confounded by large differences in scale between macro- and micro-pore regions having marked effects on gas diffusion, specifically conceptualized as the soil-gas diffusivity (D_p/D_o), where D_p and D_o are gas diffusion coefficients for a given gas in porous medium and free air, respectively. However, simple linear or power law (or combinations thereof) functions are generally not capable of representing D_p/D_o behaviour, especially within well-aggregated agricultural soils, a consequence of preferential gas movement through two distinct pore regions (inter- and intra-aggregate pores). A Two-Region Model previously developed for parameterising D_p/D_o in repacked aggregated soils was tested using four soils and four plant growth media, varying in texture and structure. The Two-Region model out-performed previous models by clearly distinguishing tortuosity effects on gas movement with respect to density and textural variations. Four fitting parameters used varied with respect to soil density, and a weighting parameter clearly distinguished the boundary between two regions (Intra-aggregate and Inter-aggregate) of structured soils. Adapting the Two-Region model to determine D_p/D_o in non-aggregated soils improves our understanding of gas exchange between soil and atmosphere. This allows the effects of land-use management practices to be better understood in terms of both O₂ diffusion and the ensuing anaerobic-aerobic processes that affect N₂O production. The adapted model is therefore a useful numerical tool to predict soil gas diffusivity and N₂O emissions from agricultural soils.