Effects of Soil Bulk Density on Gas Transport Parameters and Pore‐Network Properties across a Sandy Field Site

Abstract

The gas diffusion coefficient, air permeability, and their interrelations with air-filled porosity are essential for characterization of diffusive and convective transport of gases in soils. Variations in soil bulk density can affect water retention, air-filled pore space, and pore-network connectivity and tortuosity and, thereby, control gas diffusion and air permeability. Considering 86 undisturbed core samples with variable bulk densities that were extracted on a 15 by 15 m grid from the top layer of a sandy field, the effects of soil bulk density on gas transport parameters and the soil water characteristic were investigated. Interactions with soil organic matter, sand, and clay fractions were also examined. To evaluate bulk density effects, two constitutive parameters were derived from each of the three measured relationships. The Campbell pore-size distribution index (b) and the air-entry matric potential (y ae ) were derived from the soil water characteristic; the diffusive percolation threshold (e DPT ), the air-filled porosity where gas diffusivity ceases to almost zero because of interconnected water films creating isolated-inactive air content, and a pore-network connectivity index (A 2 ) were derived from the gas diffusivity curve, and the analogous parameters convective percolation threshold (e CPT ) and convective pore-network connectivity index (B 2 ) from the air permeability curve. All six parameters showed significant negative correlations with bulk density. To further account for the effects of both bulk density and macroporosity in parametric gas transport models, a diffusive-analog macroporosity-dependent model (DAMP) for gas diffusivity and a generalized Kawamoto et al. model (GK) for air permeability, which yielded improved predictive capabilities when compared with previous models, were developed. Both new models apply a reference point of prediction at −100 cm H 2 O matric potential (macroporosity drained), corresponding to the point where analysis of pore-network tortuosity (T) and equivalent pore diameter for gas transport (d g ) showed diminishing effects of water blockage on gas transport in the sandy soil.

Soil‐Gas Diffusivity and Soil‐Moisture effects on N2O Emissions from Intact Pasture Soils

“…For example, Hamamoto et al (2011) found ε p values ranged from 0.004 to 0.034 for intact soils: while Arthur et al (2013) reported values of 0.002 to 0.044 m 3 m −3 for sieved repacked soils, with soil ρ b values from 1.18 to 1.44 Mg m −3 , over a matric potential range of ‐3 to ‐100 kPa. Similarly, Masís‐Meléndez et al (2015) found that intact sandy soil cores had ε p values that varied from 0.02 to 0.10 when ψ ranged from ‐2.9 to ‐98 kPa over soil ρ b values ranging from 1.29 to 1.58 Mg m −3 .…”

Effects of Sand Compaction and Mixing on Pore Structure and the Unsaturated Soil Hydraulic Properties

“…Soil compaction alters pore spaces and consequently affects the bulk density, porosity, and soil hydraulic properties. Many studies over the years have shown that soil compaction and related surface sealing problems at the field scale lead to reductions in (i) infiltration and recharge rates, (ii) soil aeration and the gas diffusivity, (iii) the efficiency of fertilizer use, (iv) seedling emergence, and (v) the growth and distribution of plant roots (e.g., Bradford et al, 1987; Richard et al, 2001a, 2001b; Hadas, 2004; Wall and Heiskanen, 2009; Glab and Kopec, 2009; Vahyala et al, 2013; Kuncoro et al, 2014; Gregorich et al, 2014; Masís‐Meléndez et al, 2015; Berli et al, 2015; Sela et al, 2015). Compaction and the related processes of pore clogging and soil deformation were comprehensively reviewed by Assouline (2006a) and Keller et al (2013).…”

Soil–air phase characteristics: Response to texture, density, and land use in Greenland and Denmark

“…The fitted connectivity-tortuosity parameter, X (Figure 3c), significantly correlated to ρ b (r = −.81 *** ); furthermore, a linear regression explained 79% of the variation (Figure 4a). Several other studies (Deepagoda et al, 2011a;Masís-Meléndez et al, 2015) found similar linear correlations between the total porosity or derivations of the latter and X; given the direct linear dependence of Φ on ρ b , those correlations can be considered as equivalent. Soils showing high X exhibited a more tortuous pore network (Deepagoda et al, 2012), which basically could be traced back to an increase in the amount of parallel and well-connected pores with increasing bulk density, according to Poulsen et al (2001).…”