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The soil gas diffusion coefficient ([D.sub.p]) and its dependency on soil air content, [epsilon], and tortuosity--connectivity of the air-filled pore networks control the transport and fate of gaseous-phase contaminants in variably saturated soil. The bimodality in pore size distribution of structured soil often yields a variation of [D.sub.p] with [epsilon] in the intraaggregate pore region that is distinctly different from that in the interaggregate region. Data imply a highly nonlinear behavior of soil gas diffusion [D.sub.p]([epsilon])/[D.sub.o] (where [D.sub.o] is the gas diffusion coefficient in free air), in the interaggregate region of aggregated soils similar to that of structureless soils with a unimodal pore size distribution, probably due to diffusion-limiting effects by connected water films at low [epsilon]. In contrast, for the intraaggregate region, we show that the impedance factor [F.sup.*] (= [D.sub.p]/[epsilon][D.sub.o]) and tortuosity factor T [=[(1/[F.sup.*]).sup.1/2]] are approximately constant for most soil media. We suggest a typically well-defined separation between the two pore regions at the minimum for the pore connectivity factor [X.sup.*] [= log([D.sub.p]/[D.sub.o])/log([epsilon])], at which point the interaggregate pores are devoid of water while the intraaggregate pore region is water saturated. Based on this, a hierarchical two independent region (TIR) [D.sub.p]/[D.sub.o] model was developed by applying a cumulative series of Buckingham--Currie power-law functions, F[[epsilon].sup.X]. A nonlinear, water-content-dependent expression for F best described the measured [D.sub.p]/[D.sub.o] in the interaggregate region, while constant F (around 0.5) and X (around 1) generally sufficed for the intraaggregate region. The TIR model better predicted gas diffusivities for both aggregate fractions and highly structured soils across the entire range of moisture conditions with RMSE reduced by two to five times compared with traditional predictive [D.sub.p]([epsilon])/[D.sub.o] models. Abbreviations: AIC, Akaikc Information Criterion; TIR, two independent region; WLR, water-induced linear reduction. doi:10.2136/sssaj2009.0055

The transport of gaseous compounds in soil takes place by gas diffusion, advection, and dispersion. Gas transport processes are influenced by the soil-gas diffusion coefficient ([D.sub.p]), air permeability ([k.sub.a]) and soil-gas dispersion coefficient ([D.sub.H]), respectively. Of three gas transport parameters, [D.sub.H] is the least understood, especially how it is correlated to soil type, moisture conditions, and other transport parameters (i.e., [D.sub.p] and [k.sub.a]). In this study, a unified measurement system (UMS) that enables sequential measurement of [D.sub.p], [k.sub.a], and [D.sub.H] on the same soil core was developed. The experimental sequence is based on a two-chamber measurement of [D.sub.H] and [k.sub.a], followed by a one-chamber measurement of [D.sub.p]. Gaseous oxygen concentration and air pressure sensors are located in inlet and outlet chambers as well as at multiple points along the soil column. Using different particle-size fractions of non-aggregated (Toyoura sand) and aggregated (Nishi-Tokyo loam) soils, the effects of soil structure, particle (aggregate) size, and column scale (5-cm i.d. and 30-cm or 60-cm length) on the three gas transport parameters were investigated. For both soils, [D.sub.H] linearly increased with increasing pore-air velocity. For Toyoura sand, gas dispersivity ([lambda] = [D.sub.H]/[u.sub.O]) decreased with increasing soil-air content, while for NishiTokyo loam, gas dispersivity decreased with increasing soil-air content to a minimum value when inter-aggregate pores were drained and increased again when the pores inside the soil aggregates started to act as tortuous air-filled pathways. In the arterial pore region (corresponding to the total pore volume for Narita sand and the inter-aggregate pore volume for Nishi-Tokyo loam), a linear relation between tortuosity of the air-filled pore network (T, calculated from [D.sub.p]) and the gas dispersivity ([lambda]) was observed. Abbreviations: APR, Arterial Pore Region; UMS, Unified Measurement System; WLR, Water-induced Linear Reduction. doi: 10.2136/sssaj2009.0012

Water repellency (WR) of soils can induce hydrological problems such as reduced water infiltration, enhanced surface runoff and erosion, and the forming of preferential flow patterns in soil. Although soil organic matter (SOM) may cause both soil aggregation and a hydrophobic-material-coating of aggregates, little is known about WR in aggregated soils. We investigated the degree of WR as functions of volumetric water content (0) and pF [= log ([psi]; soil-water potential)] for sieved fractions of a volcanic ash soil samples from different depths with varying soil organic carbon (SOC) contents of between 1.1 and 12.3%. Water repellency was determined by the molarity of ethanol droplet (MED) test. Water repellency was observed in the samples with SOC [greater than or equal to] 4.6%, and the effects of sample pretreatments (pressure chamber desorption, air-drying at 20[degrees]C, and oven-drying at 60[degrees]C) on the degree of WR were small. The degree of WR varied greatly with both SOC content, [theta], and pE Peaks of WR were observed when the water retained in intra-aggregate pores was drained to a moderate extent with the corresponding pF values located in a relatively narrow range from 3.2 to 3.6. This indicates that the hydrophobicity of high-SOC aggregate surfaces might be enhanced the most at a specific soil-water potential. Examining relations between water repellency parameters, the integrated areas below the WR-[theta] and WR-pF curves were useful indexes for characterizing WR, and linear relationships between the integrated areas and both SOC and water contents at maximum repellency were found. Abbreviations: MED, molarity of ethanol droplet; SOC, soil organic carbon; SOM, soil organic matter; WR, water repellency; VG, van Genuchten.

Growing plants in extraterrestrial environments, for example on a space station or in a future lunar or Martian outpost, is a challenge that has attracted increasing interest over the last few decades. Most of the essential plant needs for optimal growth (air, water, and nutrient supply, and mechanical support) are closely linked with the basic physical properties of the growth media. Diffusion is the main process whereby oxygen and nutrients are supplied to plant roots, and gas and solute diffusivity are the key parameters controlling the diffusive movement of oxygen and nutrients in the root zone. As one among several essential aspects of optimal porous media design for plant growth, this study presents a diffusion-based characterization of four commercial, aggregated growth media. To account for the observed large percolation threshold for gas diffusivity in the selected media, an inactive pore and density corrected (IPDC) model was developed and excellently described measured gas diffusivity in both inter- and intraaggregate pore regions. A strong relation (r² = 0.98) between percolation threshold for gas diffusivity and mean particle (aggregate) diameter was found and suggested to be used in future design models. Also, critical windows of diffusivity (CWD) was defined identifying the air content range where gas diffusivity (hence, oxygen supply) and solute diffusivity or the analogous electrical conductivity (hence, nutrient supply) are above pre-defined, critical minimum values. Assuming different critical values for gas diffusivity under terrestrial and Martian conditions, the four growth media were compared and it was found that one medium did not fulfill the pre-set criteria. Overall, the analyses suggested that particle (aggregate) sizes below 0.25 and above 5 mm should likely be avoided when designing safe plant growth media for space. The CWD concept was also applied to a natural volcanic ash soil (Nishi-Tokyo, Japan), and the natural soil was found competitive or better than the tested commercial growth media. This could bear large perspectives for Martian outpost missions, since NASA has found that Martian dust/soil mostly resembles volcanic ash soil among terrestrial materials. [ABSTRACT FROM AUTHOR]