The gas-diffusivity-based Buckingham tortuosity factor from pF 1 to 6.91 as a soil structure fingerprint

Subsurface migration of greenhouse gases and other gaseous phase contaminants is predominantly controlled by soil gas diffusion coefficient (Dp) and its variations with soil-air content (ε) and soil moisture (or potential) status. Buckingham (1904) stated that Dp is proportional to ε X with X characterizing the tortuosity and connectivity of air-filled pore space. In the Buckingham model, as well as many subsequent power-law models, the tortuosity factor, X, is assumed to be constant for a given soil irrespective of its soil potential status. This study shows a marked variation of X with soil matric potential (ψ) given by pF [= log (-ψ, cm H2O)] ranging from 1 to 6.91 for a range of structureless and, in particular, aggregated/structured soils. The X-pF function for aggregated soils showed a monotonic decrease with increasing pF up to pF 3 near which inter-aggregate pores are completely drained, and an increase from pF 3 to pF 6.91 with the draining of intraaggregate pores. Two X(pF) expressions are proposed in order to independently describe the interaggregate tortuosity viz X, (for pF ≤ 3) and intra-aggregate tortuosity, X*, (for 3 ≤ pF ≤ 6.91), These expressions not only yielded better predictions of Dp but also provided useful soil structure fingerprints across a wide range of soil types.