Microgravity Oxygen Diffusion and Water Retention Measurements in Unsaturated Porous Media aboard the International Space Station

Abstract

Water distribution patterns in pore spaces of particulate porous media directly control the resulting diffusion pathway for air required for biological activity (e.g., plant root respiration). Motivated by the potential of using plants for future life-support systems in space, the question arises whether luid behavior in porous media is signiicantly altered under microgravity (»10 −6 g earth ) conditions. Altered luid phase distribution has been suspected in the onset of hypoxia in plant root modules under microgravity, yet the exact mechanisms remain uncertain. The Optimization of Root Zone Substrates (ORZS) experiment was the irst to directly measure porous-media water retention and oxygen diffusion parameters under prolonged microgravity conditions (suficient for equilibration of the luid phases). Porous-ceramic aggregates tested included 1-to 2-mm Tur face, 0.25 -to 1-mm Proile, and a 50:50 mixture of both.Each porous medium had three replicates in a nine-cell array. The experiment used sealed dual-chamber diffusion cells controlled by an automated measurement system with water-content adjustment. Sensors measured matric potentials and water input or withdrawal in the media, along with oxygen concentrations dynamics in the gas-illed chambers conining the medium. The effective oxygen-diffusion coeficients were determined from temporal variations in measured oxygen-concentrations itted to a Fickian-type relationship for the dual-chamber geometry. Ground-based determinations of matric potential and diffusion coeficients as a function of air-illed porosity were compared to microgravity data. Results pointed to enhanced hysteresis in the oxygen-diffusion dependency on bulk air-illed porosity in microgravity indicative of altered water-distribution patterns relative to Earth-based measurements. During drying we observed fundamentally different diffusivities in Proile and Mix attributed to nonuniform water distributions forming under drying conditions dominated by capillary and viscous forces in the absence of a hydrostatic force not observed on Earth. Water-retention parameters were not signiicantly different from Earth-based parameters, although gas diffusion parameters were signiicantly different for iner particle-sized media. The apparent reduction in the volume-averaged diffusive transport in microgravity may require adjustment in plant-growth system management protocols and model development for reliable response prediction of microgravity porous-medium systems.Predicting distribution and transport of luids in porous media under reduced conditions is vitally important for optimal design and management of root-zone hydration systems for plant-based extraterrestrial life support. Vigorous plant growth requires the favorable sustenance through luxes of water, nutrients and gas exchanges by selection of an optimum (or least-restricting) root-zone substrate and maintenance of optimum water content. he challenge is to maximize the inversely related processes of gas andWe present oxygen diffusion and water retention ...

Enhancing the mechanical and hydraulic properties of coarse quartz sand using a water‐soluble hydrogel based on bacterial alginate for novel application in agricultural contexts

“…The stability of soil aggregates plays a key role on the soil storage and movement of water (i.e., hydraulic conductivity), soil aeration, soil erosion, soil microbiological activity, and root growth of plants (Amézketa, 1999; Carminati et al., 2008; Carminati & Flühler, 2009; Carminati et al., 2012; Galloway et al., 2018; Heinse et al., 2015; Kramer & Boyer, 1995; Le Bissonnais, 2016). In this study, we used coarse quartz sand to quantify the real effect of the Ca‐alginate hydrogel on the aggregate stability of soil.…”

Review and Evaluation of Root Respiration and of Natural and Agricultural Processes of Soil Aeration

“…Moldrup et al (2001), for example, showed almost linear relationships between air content and air permeability or O 2 diffusivity in an undisturbed soil. These small (close to 1) exponents may also suggest preferential air flow and O 2 diffusion in a small fraction of the air‐filled pore system (a linear relationship between gas diffusivity and air content was observed also under microgravity conditions; Heinse et al, 2015). The exponents were higher for a sieved soil, which also had higher relative (to air‐dry soil) O 2 diffusivity than air permeability, contradicting the aforementioned explanation.…”

“…Dependence of soil's effective gas diffusivity ( D s ) on the volumetric air content (θ a ) according to various published models: Bu, from Buckingham (1904), Pe, from Penman (1940), Ma, from Marshall (1959), Mi, from Millington (1959), MQ, from Millington and Quirk (1961), Tr, from Troeh et al (1982), FM, from Friedman and Mualem (1994), PMQ, from Moldrup et al (1997), TPM and WLR, from Moldrup et al (2004), and Th, from Thorbjørn et al (2008). The depicted lines describe the models fitted (i.e., not using generic parameter sets) to measurements in an aggregated porous medium, assuming a diffusivity in free air, D 0 , of 1.64 m 2 d −1 (after Heinse et al, 2015) and soil porosity of 0.4. Fitted parameter δ was taken to be 5.5 for PMQ, 0.8 for Th, and 1.6 for TPM, WLR, MQ, and Tr; fraction of nonconducting gaseous phase ξ = 0.03 for models MQ, Th, and TR.…”

Design of a Module for Cultivation of Tuberous Plants in Microgravity: The ESA Project “Precursor of Food Production Unit” (PFPU)

“…In container-grown plants, the features of the pot-substrate system (i.e., size and shape of the pot and type, volume, and structure of the substrate) strongly influence the nutrient solution holding capacity, determining the mineral and water availability for plant uptake and affecting the rate of root development and the root system architecture (Jones et al, 2012;Heinse et al, 2015). Particularly, small pots and cohesive substrates can ultimately result in root restriction, impeding the proper plant growth, with stronger effects on tuberous and taproot plants.…”