Beyond Earth: Designing Root Zone Environments for

Special Section: Soil Architecture and Function

ScoƩ B. Jones* Dani Or Robert Heinse Markus Tuller

Extending Earth life beyond our planet requires understanding of how biological species such as plants and microbes would grow under reduced gravity. For porous medium-based roo ng environments altera on of the physical proper es may be the key to successful plant growth in addi on to applica on of appropriate water management approaches. Sco B. Jones, Dep. Plants, Soils and Climate, Utah State Univ.; Dani Or, Dep. Environmental Sciences, Swiss Federal Ins tute of Technology (ETH), Zürich; Robert Heinse, Dep. Plant, Soil and Entomological Sciences, Univ. of Idaho; Markus Tuller, Dep. Soil, Water and Environmental Science, The Univ. of Arizona. *Corresponding author (sco . [email protected]). Vadose Zone J. doi:10.2136/vzj2011.0081 Received 6 July 2011.

© Soil Science Society of America 5585 Guilford Rd., Madison, WI 53711 USA.

All rights reserved. No part of this periodical may be reproduced or transmi ed in any form or by any means, electronic or mechanical, including photocopying, recording, or any informa on storage and retrieval system, without permission in wri ng from the publisher.

Beyond Earth: Designing Root Zone Environments for Reduced Gravity CondiƟons Fluid management in plant root zones is cri cal for long dura on space missions including lunar- or Mar an-based missions, but key aspects of design and delivery of fluids under these condi ons are poorly understood due to limited experimental opportuni es. We review theore cal and experimental concepts for advancing understanding of fluidporous media interac ons to improve design and management of plant-based life support systems for reduced gravity environments. In situ u liza on of na ve lunar and Mar an granular materials for plant-growth media requires reliable characteriza on of media physical and hydraulic proper es and processes. A key aspect is the enhanced effects of capillarity in reduced gravity resul ng in an array of micro- and macroscale changes in fluid phase organiza on rela ve to condi ons on Earth that may affect mass fluxes to plant roots and poten ally result in excess water and hypoxia. Increasing the medium par cle diameter above 1 mm and narrowing the distribu on of par cles, and thus pore sizes, may counter reduced gravity effects. Approaches used in previous microgravity systems involving sensor-based ac ve water management assuming prescribed op mal set points (i.e., water poten al) may fail in reduced gravity due to dynamic pore space altera ons arising from air- or liquid-phase entrapment and root growth in a restricted volume that may alter the porous medium characteris cs on which water management is o en based. For example, about a 10% reduc on in volumetric pore space was observed following rice (Oryza saƟva L.) root growth, which could change a well-aerated root zone into an anoxic environment if not accounted for. Numerical modeling of plant transpira on and irriga on using volumetrically controlled water content under different gravity environments revealed similar hydraulic responses in fine-textured porous media typically unsuitable for plant growth in greenhouses. Volumetric water content–based management of plant root environments appears to be a safer approach than other methods discussed here. Abbrevia ons: ISRU, in situ resource u liza on; WLR, Water-induced Linear Reduc on.

Materials mined on the surface of the Moon or Mars are candidate media for in situ resource utilization (ISRU), which is aimed at using native materials for construction, energy generation, and resource recovery in space. The alternative of bringing resources to an extraterrestrial environment is prohibitively costly and challenged by stringent mass and volume payload restrictions of spacecraft. When it comes to plans for growing food in longterm habitats, native materials could serve as plant growth media to provide root anchoring as well as storage and delivery of water, oxygen, and nutrients. However, reduced gravity forces present a challenge for balancing the critical supply of both liquids and nutrients in addition to facilitating exchange of gasses with plant roots. These two opposing processes are most easily managed through control of the porous medium (growth medium or soil) water content. Water is an extremely limited and critical resource, and alternative plant growth approaches including hydroponics, aeroponics, and thin fi lm methods have been proposed to conserve and minimize the water requirement (Bugbee and Salisbury, 1989). Plant water use estimates under artificial lighting (860 μmol m−2 d−1) can reach 1 cm d−1 (Wheeler, 2006). While these methods proved capable of growing more food with less water due to higher planting densities and better resource management, the added tradeoffs include higher maintenance, more complex technology and infrastructure, and difficulty in expanding to large-scale crop production (Bugbee and Salisbury, 1989; Hossner et al., 1991; Monje et al., 2003; Salisbury, 1992; Silverstone et al., 2003, 2005; Wheeler, 2003). In addition, the potential for failure is greater due to the enhanced power requirement and required controls on pH and root oxygen supply via pumping. The discussion here regarding the design of root zone environments for reduced gravity will be limited to porous media, where Martian or lunar gravity is utilized to passively manage water similar to practices on Earth. For the microgravity environment on orbiting spacecraft , active www.VadoseZoneJournal.org

management of the porous medium water becomes necessary with varied approaches presented elsewhere (Heinse et al., 2005a; Hoehn et al., 2000; Jones et al., 2002; Scovazzo et al., 2001). Past microgravity research has focused on a few limited applications of liquid transfer at low matric potentials within coarse-grained porous materials such as zeolite, arcillite, floral foam, agar gel, or perlite (Hoehn et al., 2000; Ivanova and Dandolov, 1992b; Levine et al., 1998; Morrow et al., 1993; Morrow et al., 1994; Podolsky and Mashinsky, 1994). Jones and Or (1999) pointed to two reasons for deficiencies in media selection and characterization: (i) the “desirable” high liquid fluxes obtained at low matric potential ensure a commensurate reduction in the rates of gas transfer (e.g., oxygen) at the high water contents, and (ii) most of the results were not cast in general and transferable forms and thus are of limited use for general inferences. The objectives were to review key relationships between porous medium pore-size based on available particle size information to provide meaningful relationships for plant growth media that address challenges of design and construction in plant growth facilities for lunar or Martian gravity environments. The focus is on lunar (0.16 g) and Martian (0.38 g) gravity levels with occasional reference to understanding gained in microgravity aboard parabolic aircraft or orbiting spacecraft. We base design considerations on equilibrium hydraulic properties and provide examples of temporal changes in hydraulic properties occurring from differences in gravity force and from the growth of plant roots. We point to limited experimental results highlighting anticipated reduced gravity response with respect to root zone water management. Indeed, much of the discussion is focused on water as it controls most of the other processes, such as nutrient and gas transport, and in turn mediates root and microbial growth rates. We also allude to the advantages and limitations of managing the rooting environment either through measured matric potential or volumetric water content.

Martian design considerations (Jones et al., 2009). Early questions about the applicability of Earth-based models to describe fluid behavior in porous media under microgravity arose after a number of microgravity flight experiments (Jones and Or, 1999). Equal buoyancy of fluids in microgravity hinted at difficulties predicting liquid and gas fluxes based on conventional models. However, observations from parabolic fl ight experiments (Heinse et al., 2007) revealed the impact of reduced gravity is manifest at the mesoscale (cluster of pores) rather than at the level of a single pore. Reduced gravity was found to accentuate the role of pre-wetted surfaces for transport, enhancing liquid- or gas-phase entrapment, and affecting macroscopic imbibition and drainage processes and sample scale hydraulic properties (Or et al., 2009). Ultimately, the particle-size distribution and structure determine the water retention properties of a porous medium. Figure 1 illustrates the measured particle-size distribution of lunar regolith with a mean size of 0.072 mm (Carrier, 2003). This silt- and sandsized surface covering of 1- to 2-m thickness was formed over millennia, largely through micrometeorite strikes. Also shown is the particle-size distribution of Martian dune sand estimated from wind trajectory calculations, with a mean particle size of 0.5 mm and standard deviation of 0.1 mm (Edgett and Christensen, 1991). Lastly, depicted is a commonly used calcined clay for plant growth media in microgravity (aboard orbiting spacecraft), referred to as Turface (1–2 mm). One of the objectives of this paper is to address the importance of the particle-size distribution and how it affects pore size, and thus the energy (i.e., matric potential or suction) required to manage water in plant root growth modules in reduced gravity environments. The concept of porous medium design was explored by Jones and Or (1998), who employed hydraulic and gas retention and transport models to parameterize the hydraulic characteristics of a porous medium based on prescribed boundary and flux requirements. The approach was effective in designing a sandy soil that was subsequently constructed from sieved particles.

6 Recent Progress with Reduced

Gravity Plant ProducƟon Systems

The 21st century brought renewed interest in space exploration, with NASA and other space agencies exploring plant production systems for space. Results from a July 2000 workshop highlighted the need to develop greater understanding of water, air, and solute transport in unsaturated porous media under conditions of microgravity (Steinberg et al., 2002). With plans to grow food during long-term space exploration, a number of facilities were developed and tested under microgravity conditions on orbiting spacecraft (Bingham et al., 2002; Hoehn et al., 2003; Morrow et al., 1993), and planetary systems were proposed for lunar and Martian exploration (MacCallum et al., 2000). Guidelines for selecting plant growth media for microgravity environments were also addressed (Hoehn et al., 2003; Jones and Or, 1998; Scovazzo et al., 2001), but much less emphasis has been placed on lunar or

Fig. 1. Particle-size distribution (sample size, n = 350) of lunar regolith (Carrier, 2003), estimated Martian dune sand (Edgett and Christensen, 1991) and microgravity plant growth media, Turface (Heinse et al., 2005b; Monje et al., 2003; Steinberg et al., 2005).

www.VadoseZoneJournal.org

The interpretation of required particle size came from estimated pore sizes. This led to an iterative procedure where a scaling parameter was adjusted to match the physical “particle pack” to subsequent measured water retention for comparison to the original specification. The Arya and Paris model (Arya and Paris, 1981) was used to describe the pore (r)–particle (R) relationship in that study. Here we ignore the separation of the porous medium into different size ranges and assume the scaling parameter, β, is equal to 1 for simplicity. With these simplifications, the reduced relationship is given in terms of the system porosity, φ, (Jones and Or, 1998) ⎡ 2 ⎛ φ ⎞⎤ 0.5 ⎟⎟⎥ r = R ⎢ ⎜⎜ ⎢ 3 ⎜⎝1−φ⎠⎟⎟⎥ ⎣ ⎦

[1]

Additional particle–pore relationships for scaling water retention to particle-size distribution can be found in more recent studies (Arya et al., 1999, 2008; Chan and Govindaraju, 2004; Vaz et al., 2005; Deepagoda et al., unpublished data, 2012). For estimation of the relationship between porous medium particle size and the resulting energy or matric potential, the capillary rise equation is used, written as:

h=

2σcos γ ρ w gr

[2]

where h is the height of capillary rise, r is the radius of a capillary tube, σ is the surface tension of the liquid, γ is the contact angle of the solid–liquid interface, ρ w is the density of liquid, and g is the acceleration due to gravity. This then leads to describing the porous medium water retention, which is the key relationship which has been tested and ultimately modeled in reduced gravity. The volumetric water content (or effective saturation, Θ) is expressed as a function of capillary pressure head h (vanGenuchten, 1980), which is negative for unsaturated conditions. The function is written ⎡ θ −θ r ⎢ 1 Θ= =⎢ θ s −θ r ⎢1 + (α h )n ⎣⎢

⎜⎛

estimated pore-size distribution was obtained (Eq. [1]), followed by the associated capillary potentials. These distributions and associated matric potentials were used to generate porous medium water retention curve data, which were expressed by fitting Eq. [3] parameters listed in Table 1, assuming draining conditions. The physical meaning of α is inversely related to air entry pressure of the substrate, and n is related to the pore-size distribution. Saturated hydraulic conductivities are given for typical values of sand (Mars dune sand) and silt loam (lunar regolith) using the UNSODA database (Leij et al., 1999). Heinse et al. (2007, 2005b) observed some gravitational effects on water retention in porous media samples during parabolic flight but found no impact on saturated hydraulic conductivity (Ks) of densely packed tubes. A comparison of water retention at 1 g and microgravity revealed differences in fitted water retention model parameters under repeated wetting and draining cycles, pointing to lower air entry values in microgravity. However, these results may have been influenced by the hyper-gravity phase accompanying parabolic flight. Space shuttle experiments providing sustained microgravity revealed no significant difference in plant transpiration, photosynthesis or dry biomass production compared to porous medium-based plant growth experiments on Earth (Monje et al., 2005; Stutte et al., 2005). A recent review of microgravity research suggests that fluid displacement patterns become unstable and enhance phase entrapment in the absence of gravity, thereby modifying macroscopic transport properties (Or et al., 2009). Although effects of microgravity on fluid distribution and retention appear to produce subtle differences in phase distribution relative to earth’s environment, such minute differences may have significant impacts on macroscopic transport properties controlling mass fluxes, especially gases. One of the most critical aspects for plant growth media in reduced gravity is the maintenance of adequate gas exchange (i.e., O2 and CO2) (Hoehn et al., 2000). There is general agreement that 10 to

1 ⎞⎟

⎤ ⎜⎜⎝1− n⎠⎟⎟ ⎥ ⎥ ⎥ ⎦⎥

[3]

where θ r and θ s are the residual and saturated water contents, respectively, and α and n are shape parameters typically fit to measured data.

Table 1. Measured or approximate porous-medium hydraulic parameters for the van Genuchten (1980) water retention model and the saturated hydraulic conductivity. Turface and Profi le are aggregated media with dual pore characteristics. Only the inter-aggregate pore hydraulic system parameters are given here. Draining water retention parameters α Media

The foregoing relationships allow us to estimate lunar and Martian hydraulic properties based on particle-size distributions shown in Fig. 1, providing insights for water management simulations. Under equilibrium conditions, the water retention curve is related to the height of capillary rise (Eq. [2]) characterized by reduced water content as a function of height above the bottom of a porous medium sample. Using the particle-size distributions for lunar regolith and Martian dune sand shown in Fig. 1, the

n

cm−1

θs 3 ——cm cm−3 ——

cm d−1 41,000

θr

Ks

Turface† (1 g)

0.19

0.34

0.73

Mars dune sand (0.38 g)

0.0304 10

0.05

0.4

500‡

Lunar regolith (0.16 g)

0.0066

1.6

0.05

0.45

30‡

Profi le (1 g)

0.086

5.1

0.36

0.71

Profi le with 40-d roots (1 g)

0.055

3.1

0.36

0.60

† Heinse et al. (2007). ‡ Leij et al. (1999).


5.6

20% air-fi lled porosity is desirable (Bunt, 1988). Schjønning et al. (2003) suggested 2% as a lower threshold for soil gas diff usivity in coarse-textured porous media. Critical levels of air-fi lled porosity are related to gas percolation threshold values associated with the initiation of gas transport. The diff usion of gas in porous media, Dp, is generally scaled by the diff usion in air, Do, expressed as a function of air-fi lled pore space, ε, which can be estimated with empirical models such as the Water-induced Linear Reduction (WLR) function of Moldrup et al. (2000): Dp Do

=

ε 2.5 φ

[4]

where φ is the total porosity of the porous medium of a mono-sized pore systems. From known water content, the relative gas diffusion coefficient is estimated from Eq. [4]; i.e., ε = φ − θ. For dual-porosity systems such as are common in stable aggregate porous media used in microgravity, one must appropriately account for the dual pore system as described by Jones et al. (2003). The exchange of gasses within the root zone is primarily a function of the air-filled porosity and gas concentration gradients developing as a result of root and microbial respiration, for example, driving CO2 outward and O2 inward (Glinski and Stepniewski, 1985). Gas exchange is an important aspect of plant growth, especially in reduced gravity environments, where the balance of air and water is crucial, yet difficult to manage (Bingham et al., 2000). Elaborate systems have been developed for managing and monitoring plant and root zone gas exchange in microgravity (Ivanova et al., 2005). In addition to root contributions to gas exchange, microbial activity is also an important aspect to be considered for the design of plant growth media. Life on lunar or Martian outposts will be controlled by microbial populations much as it is on Earth, including processes of pathogenesis, contamination, spoilage, decay, and biogeochemical cycling (Alexander et al., 1989; Stotzky, 1989). Although the full range of reduced gravity effects on microbial growth and function in porous media is beyond the scope of this discussion, certain aspects concerning the microscale diff usional environment may be deduced from foregoing analysis. Microbial communities are stimulated and play a prominent role in the plant rhizosphere, facilitating various biogeochemical processes (Hanson et al., 2000; Hopmans, 2006; Oertli, 1996). In a recent review by Or et al. (2009) micro- and macroscale effects of fluid phase configurations on gaseous and nutrient fluxes to microbial communities in soils have been discussed. Specifically, a macroscopic formulation of the interplay between enhanced gaseous diff usion and concurrent decrease in nutrient diff usion pathways with decreasing water content (Schjønning et al., 2003; Skopp et al., 1990) also leads to an “optimal” water content that maximizes microbial activity at a plant root zone scale. Additionally, periodic convective nutrient supply with irrigation may significantly modify the diff usion fields in which microbial life functions. An interesting and yet unexplored area, is the impact of minute changes in fluid

configuration under reduced gravity on microbial diversity and coexistence in the container medium. Fragmentation of aquatic environments was cited as a primary contributor to promotion and maintenance of microbial diversity found in soils on Earth (Dechesne et al., 2008; Long and Or, 2005, 2007). The increased role of capillarity under reduced gravity and conflicting evidence regarding formation of larger, yet poorly connected water clusters (Heinse et al., 2007) might create radically different aquatic fragmentation patterns that would alter microbial diversity essential for proper functioning of the plant root zone (e.g., see http://www.sdl.usu.edu/programs/orzs). More recent modeling work has advanced efforts to understand impacts of reduced gravity on the hydrodynamic and biogeochemical processes associated with soil. Maggi and Pallud (2010a,b) employed the TOUGHREACT-N numerical code to model microbially mediated biogeochemical reactions and resulting gaseous byproducts and nitrogen and carbon leaching. While these modeling efforts are driven by a powerful toolbox of complex reactions, the unknowns regarding physical and chemical conditions beyond Earth obscure the critical initial and boundary conditions needed for rigorously modeling lunar and Martian soils. Furthermore, interpretation of their modeling results is difficult given the lack of experimental data from reduced gravity for comparison. Maggi and Pallud (2010b) found 90% lower water and nutrient leaching in reduced gravity compared to Earth. This difference resulted in increased nitrogen gas emissions relative to Earth. A casual observer may assume these effects are a result of reduced gravity when, in fact these are the result of higher water content giving rise to reducing conditions, which comes from the reduced gravity force in the Richards equation. In other words, comparison of excess watering regimes for different gravity levels naturally leads to higher water contents in reduced gravity (Heinse et al., 2009). Perhaps a more ideal comparison would be to compare identical water contents (i.e., managed electronically via sensor control) resulting in what should be similar modeling outcomes in terms of leaching and nitrogen emissions (more on this later). The only gravity-dependent numerical model component we are aware of is the gravity term in the Richards equation, which affects water flow and distribution. Additional gravity-dependent effects may yet need to be incorporated for adequate modeling of extraterrestrial soil processes. It is clear that more interdisciplinary collaboration is needed to take advantage of and to synergize our collective understandings of plant, soil, hydraulic, biogeochemical, micrometeorological, and biological processes.

6 New Insights for Root-Pore

and Microbial Impacts on Porous Media

Questions regarding the impact of plant root and microbial growth on root zone environments have not been addressed. Some of the major considerations relate to how operational parameters might


be impacted by temporal shifts in pore-size morphology as roots and microbial communities invade. We highlight these issues with a novel root growth experiment and Earth-based observations of microbial behavior. The root growth experiment stems from the idea of designing a porous medium water management system for plant growth with established management set points that evolve to critical levels as roots displace pore space, creating water or oxygen deprivation.

Impact of Plant Root Growth on Water RetenƟon An automated water retention measurement system capable of growing rice in a saturated aggregated porous medium was developed to facilitate nightly (i.e., outside the photoperiod) measurements of water retention to observe the impact of plant root growth on pore space morphology with time (Fig. 2). Saturated conditions were critical to measure the full range of water retention, and rice was ideal because of its ability to develop arenchyma facilitating oxygen supply to roots. A 15.4-cm-diameter PVC cap was used as a container to hold 1000 cm 3 of plant growth medium, resulting in a 5.3-cm rooting depth. A 6.25-mm-thick ceramic plate (surface area = 189 cm 2) was glued into the bottom of the PVC cap (confi gured like a Buchner funnel) to facilitate suction-induced drainage and rewetting of the porous medium. A variant of Turface referred to as Profi le (i.e., calcined clay derived from the same source material of diameter 0.25– 0.85 mm) was thoroughly rinsed and subsequently packed to a bulk density of 0.67 g cm−3. With a particle density of 2.5 g cm−3, the total porosity was approximately 0.74, with 50% of that being inter-aggregate (between aggregates) porosity, while the other 0.37 cm 3 cm−3 consisted of intra-aggregate (within aggregates) porosity. The inter-aggregate pore space was monitored and evaluated for temporal changes due to root growth. A KD Scientific (Model KD250) syringe pump with four 140-mL syringes in parallel (560-mL capacity) was used to automatically add or remove water stepwise through the porous ceramic plate. Each increment (60 mL or less) of water withdrawal or infusion was constrained by both time and pressure limits (i.e., −50 cm to +5 cm) moderating removal and additions. Average output from four Sensotec pressure transducers (Model 060-A37302) connected to four porous stainless-steel cups (10-μm pore size) installed in the growth medium 2 cm above the ceramic plate were used to infer soil matric potential referenced at the ceramic plate level. Four additional Sensotec pressure transducers connected to the syringe tubing measured pressure in the water being infused and withdrawn through the ceramic plate, thereby constraining syringe movement to within preprogrammed and acceptable pressure limits. Control of the syringe pump, lighting and Marriotte watering–disconnect valve in addition to monitoring of the pressure transducers was performed using a Campbell Scientific CR-1000 datalogger. Six Super Dwarf rice plants (mutant selection from Shiokari) were grown in the porous medium during a 10-h photoperiod under a high-pressure

Fig. 2. Automated substrate water characteristic (SWC) measurement system for nightly assessment of matric potential (via Tensiometers)– volumetric water content relationship (via syringe pump). Dwarf rice was grown in 0.25- to 0.85-mm Profile with slow release fertilizer. Water was maintained at saturation during the day using a constant head (Marriotte) reservoir. A valve disconnected the supply during overnight SWC measurements.

sodium vapor light (600 μmol m−2 s−1) between 16 June and 3 Aug. 2009 at laboratory temperature (27 ± 3°C). Slow release fertilizer (Nutricote T-270) was mixed with the porous medium at 15 g L−1 before seeding. Between 0800 h and 1800 h, lights were on and saturated conditions were maintained in the root zone using a large Marriotte bottle fi lled with tap water. Between 1800 h and 0800 h, lights were turned off and water retention measurements were performed. Two sets of retention parameters are given in Table 1 for Profi le based on measurements without roots and later with roots growing in the Profi le porous medium. Figure 3 illustrates root-induced dynamic displacement of macropore space occurring as dwarf rice roots grow within the porous medium (Profi le) over a 6-wk period. In this experiment the root zone was purposely minimized to a 1000-cm3 volume, creating a confi ned space for root growth under a high light source. Figure 3a shows how the water retention curves evolved with time moving from 71% saturation down to 60% after 42 d, a pore space reduction of 11%. At the end of the sixth week 65 g of fresh roots were extracted from the root zone, translating to approximately 9% of the total pore space (17% of inter-aggregate pores) assuming a density of 1 g cm−3. Trapped air could explain the difference between the two estimates for pore space displacement. The two extreme retention curves associated with Days 1 and 42 are shown in Fig. 3b. The large difference between the two curves is evidence that root intrusion can significantly alter the porous medium hydraulic properties. For example, assuming a system operating at a fixed matric potential of −8-cm capillary pressure head, there is a shift in volumetric water content, which root growth could cause, based on the wetting water retention curve. We also used the draining retention curves (typical measurement approach) to fit the water retention parameters shown in Table 1. However, the real problem is not the exact amount of volume displacement, rather the consequence of the displacement on water management. For further illustration, we discuss several potential management approaches later.


Fig. 4. Minute changes in hydration status (expressed as matric potential) of rough surfaces affecting bacterial colony expansion rates. A comparison between measured Pseudomonas putida colony expansion rates (data from Dechesne et al., 2008) and predicted rates (numerical simulations and analytical expressions) for a narrow range of matric potential values. Error bars indicate one standard deviation; simulated expansion rates depicted by lines with shaded area representing one standard deviation (Wang and Or, 2010).

Fig. 3. Substrate water retention curves in Profile (0.25–1 mm) plant growth medium showing (a) inter-aggregate pore space displacement by rice roots and (b) shifting water contents related to matric potential–based set points. Each curve represents an average of three nightly measurements; i.e., the curve at Day 42 is the average of Days 41, 42, and 43. Each curve was scaled to the residual water content of 0.37 assuming syringe-induced drainage of inter-aggregate pores was complete each night. Each drainage–imbibition process began under saturated conditions (+ pressure) and ended near 0.

Impact of Microbial Growth on Porous Medium ProperƟes The competing effects of gaseous and nutrient diffusion on microbial activity in partially saturated porous media have been described in the landmark study of Skopp et al. (1990) focusing primarily on sample-scale considerations. The extension of design considerations presented in previous sections concerning plant function are readily extendable to microbial function at the root zone scale. A more challenging question pertains to potential effects of porescale variations in fluid configurations under reduced gravity on microbial aquatic habitats and thus on the function of individual microbial colonies. Such changes may affect microscale nutrient diffusion pathways and dispersion rates—both factors are known to shape diversity of microbial life in porous media. In the absence of definitive tests in variable gravity we may illustrate the extreme sensitivity of microbial dispersion rates (i.e., resource interception

potential) to minute alterations in aquatic habitat shape and connectivity as illustrated in Fig. 4 for expansion of flagellated Pseudomonas putida KT2440 wild-type colonies initiated from single cells and grown on rough ceramic surfaces (Dechesne et al., 2008). The computations result from a simple biophysical model for a motility idealized roughness network model for grain surfaces. The matric potential is associated with water content through Eq. [3], and therefore these relationships are interchangeable. The significant sensitivity of colony expansion on matric potential may add a design constraint whenever microbial life beneficial to rhizosphere function is considered. Th is is especially true where control of microbial expansion is critical and the necessary water and nutrient supply to plant roots can be managed simultaneously. Generally, impacts on porous medium hydraulic properties as a result of microbial growth include cell accumulation leading to clogging (Baveye and Valocchi, 1989; Martin, 2010), capsule (polymer) production (Vandevivere and Baveye, 1992), gas generation (Seki et al., 1998), and chemical property modification of liquid or solid phases (Rockhold et al., 2002). In summary, both root and microbial growth can impact the porous medium pore space, leading to altered hydraulic properties. These impacts are generally the result of pore-space displacement by biomass production yielding smaller pores and reduced porosity leading to modified water retention and transport. These impacts can be significant and should be considered in the design of porous medium plant growth facilities.

Sample-Scale ConsideraƟons To illustrate the impact of reduced gravity on the water retention characteristic, Jones et al. (2005) scaled the Earth-based curve to lunar and Martian gravity, demonstrating the expected capillary


rise in a coarse-textured Minnesota lunar simulant. In a subsequent paper, Jones et al. (2009) took a different approach, scaling the water retention curves that would theoretically result in a similar water content distribution for a 10-cm-tall container at each of the three gravity levels. Here we take yet another approach, looking at native extraterrestrial materials and expressing their hypothesized properties based on the limited information shown in Fig. 1. A simplified approximation of the substrate water retention curves for lunar regolith and Martian dune sand is based on the previous discussion and is shown in Fig. 5. The resulting 1 g curves were then scaled by the gravitational forces on Mars and on the Moon. Using principles of similarity, Miller and Miller (1955) demonstrated linear scaling of the water retention curve as a function of physical parameters including gravity force. Considering the hydraulic parameters used to describe water retention, the van Genuchten (1980) model parameter α was scaled by the gravity force. Since α (cm−1) is related to the inverse of the air-entry pressure, the scaling was done by multiplying α by the ratio of Earth’s gravity force to that of Martian or lunar gravity. Hydraulic parameters are listed in Table 1 for the scaled lunar and Martian porous media. As seen in Fig. 5, there is a sixfold difference between the equilibrium retention on Earth and under lunar gravity, where water in lunar regolith could theoretically be retained at one-half saturation 4 m above a free water table. This has unfavorable implications for utilizing gravitational force alone for drainage of growth media. Excessively tall columns would be required to achieve adequate root zone aeration while providing only a small fraction of column height usable for root growth and requiring disproportionate water, nutrient, and soil resources.

Fig. 5. Hypothetical equilibrium capillary rise in Martian dune sand and lunar regolith at 1-g and under their respective gravitational fields. For lunar regolith, the height at which 50% relative water content is attained would occur approximately 70 cm above the bottom of a regolith-filled column on earth, while on the moon equal water content would occur at a 400 cm column height.

Evidence of Enhanced Air- or Liquid-Phase Entrapment under Reduced Gravity Or et al. (2009) have shown theoretically that under the reduced gravity in parabolic flight the flow regime and fluid displacement front morphology may exhibit strong sensitivity to the velocity of water injection and the mean pore size in the system. Results from those experiments are shown in Fig. 6, in which water was withdrawn from 30-cm 3 porous medium–filled syringes at constant rates of 15 and 30 cm 3 min−1, revealing increased liquid-phase entrapment in zero gravity relative to 1 g. Liquid “pore” volume extraction was measured until an optical air-bubble sensor triggered air-phase passage at the bottom (air inlet at the top) of the satiated column during zero gravity and in repeated experiments under 1 g. We interpret the results in Fig. 6 as partial confirmation of the onset of an unstable front (fingered pattern) with decreasing restraining force of gravity and with diminishing capillary forces (larger particles) (Chau and Or, 2006; Meheust et al., 2002). The differential effect vanishes for small particles, where capillary forces restrain viscous forces (or dominate under 1 g withdrawal). The response illustrated in Fig. 6 for lunar or Martian gravity will likely be scaled between the 1-g and 0-g curves. This suggests particle diameters on the order of 0.5 to 3 mm may exhibit transitory hydrodynamic properties in reduced gravity.

Fig. 6. Water entrapment in a 30-mL column under earth gravity (1 g) and microgravity during water withdrawal (drainage) experiments. Liquid withdrawal rates from the bottom were 15 mL min−1 for particle radii of 0.313 and 0.338 mm and 30 mL min−1 for particle radii of 0.563, 0.75, 0.8, and 1.5 mm. An automatic bubble detector at the bottom of the column indicated the onset of hydraulic discontinuity marking the end of a withdrawal experiment (Or et al., 2009).

Numerical SimulaƟons of Water DistribuƟon and Management As previously discussed, the critical process likely limiting plant growth in past microgravity studies was gas exchange within the root zone. This point is illustrated using numerical simulations of water distribution in a 15-cm-tall sample of Profi le whose initially uniform water content was redistributed over 24 h at 1, 0.38, 0.16, and 0 g. The resulting volumetric water contents and relative gas diff usivities as a function of sample height are shown in Fig. 7. The critical gas diffusivity of 0.02 is shown to be satisfied by all gravity levels except 1 g in Fig. 7b. This assumes a vertical


diff usion of gas and that the increased water content at sample bottom is the constraint for diff usion (note that gas diff usion may be adequate for microbial and root respiration closer to the surface of the porous medium where water content is reduced). Furthermore, plant root extraction could further modify the distribution, especially under high-intensity evapotranspiration and depending on the distribution and density of roots. Assuming only gravity-induced water distribution, Fig. 7 therefore suggests gas percolation thresholds in reduced gravity will be smaller than on Earth. Therefore, gas diff usion measurements made on Earth may not necessarily represent gas transport by diffusion and resulting ε values under reduced gravity conditions where the Bond number (ratio of gravity to capillary force) is reduced. One of the little-studied and yet potentially crucial aspects of managing plant root environments in reduced gravity is associated with the concept of maximizing crop yield while conserving space using extremely compact root zones.

Management by SucƟon Control Figure 3b illustrates how a suction-induced control set point for matric potential in a root zone environment could lead to altered volumetric water content control as the water retention curves evolve with root intrusion. The basic premise for this type of control is that the wetting retention curve serves as the expected minimum water content response for a given matric potential set point (i.e., rewetted media will reduce water content until the wetting curve is reached). In this case, as the inter-aggregate space is displaced by roots the result is a reduction in volumetric water content associated with any given matric potential (leftward sliding water retention curve). While such a shift could potentially increase gas diffusivity, there is a commensurate reduction in liquid flux with associated reduction in unsaturated hydraulic conductivity. In addition, these systems typically require powered infrastructure to maintain negative pressure in water supply–removal lines. A long-term or frequent loss of power could be devastating for a crop in such systems; therefore, alternative methods for water management should be considered.

Management by Volume Control Early attempts to manage water in the Svet Greenhouse on board the Russian Space Station Mir were based on a heat pulse sensor that was calibrated to volumetric water content on Earth (Podolsky and Mashinsky, 1994). Functionally, reduced sensor readings led to small injections of water to a pair of hydrophilic accumulator tubes from which water would disperse into the coarse-textured porous medium Balkanine (Ivanova and Dandolov, 1992a; Ivanova et al., 1993). However, gravitational force differences led to difficulties in managing the water due to the well-known sensitivity of heat pulse sensors to contact resistance, which varies with packing density, water distribution, and other factors. Bingham et al. (1996) added an array of 16 heat pulse sensors to the Svet system, improving management of water with more sensors but still with a considerable margin of error in accurately resolving volumetric water content.

Fig. 7. (a) Numerically modeled volumetric water content redistribution in Profile (0.25–1 mm) 24 h after inducing a uniform initial water content of 0.5 (i.e., no redistribution at 0 g). (b) Computed Water-induced Linear Reduction (WLR) model–based gas diffusion coefficients (Eq. [4] as a function of gravitational force and intra-aggregate pore space (i.e., 0.37 < θv < 0.74). The arrow indicates a minimum gas diffusion threshold of 2%.

A significant challenge for many water content probes is sensitivity to root intrusion, which for heat pulse probes means a possible shift in thermal properties and contact resistance with time. Such effects may result in target water content management shifts similar to those shown in Fig. 3b. More accurate water content measurement systems (e.g., time domain reflectometry, capacitance-based) are generally excluded from consideration for space flight applications due to safety hazards such as electromagnetic emissions. Other methods for water content–based management are of interest and should be explored to provide the optimal management configuration available. Numerical simulation of a 15-cm-deep root zone comprised of lunar regolith was performed using HYDRUS-1D (Simunek et al., 2008). Water flow and wheat (Triticum aestivum L.) root water uptake were simulated for 100 d with daily changes in plant transpiration (evaporation was assumed to be zero given the likelihood of an evaporation barrier). The van Genuchten hydraulic model was employed, excluding effects of hysteresis. The upper


boundary was atmospheric with a surface layer, while the bottom boundary was a seepage face. Transpiration of a wheat crop was simulated by assuming a linear increase from 0 cm d−1 at Day 1 to 1 cm d−1 at Day 60, remaining constant until Day 90 when it subsequently dropped to 0.5 cm d−1 by Day 100. The irrigation regime was manually imposed in the model as a periodic upper boundary surface flux (i.e., 3 cm applied over 24 h; 0.25 cm h−1) based on the cumulative root water uptake resulting in increased volumetric water content. The relative fractional root distribution went from a value of 1 at the surface down to 0 roots at the bottom of the root zone. The initial volumetric water content was set at 0.25 throughout the soil profi le. Four locations within the root zone were monitored, 0, 5, 10, and 15 cm. With the assumption that a more accurate monitoring method will be available for ground-based systems, we demonstrate the capability of managing water volumetrically in the fi ne-textured lunar regolith. The HYDRUS-1D simulation was based on numerical modeling of periodic irrigation and plant transpiration processes illustrated in Fig. 8 using hydraulic parameters shown in Table 1 (α for 1 g = 0.04 cm−1). Transpiration of a wheat crop was simulated by assuming a linear increase from 0 at Day 1 to 1 cm d−1 at Day 60, remaining constant until Day 90 when it subsequently dropped to 0.5 cm d−1 by Day 100. Each time Θ dropped below 0.2 cm 3 cm−3 , 3 cm of water was applied over 24 h to avoid local saturation at the surface of the 15-cm deep root zone. The point of this exercise is to illustrate that whether the simulation occurred at 1 or 0 g, the results were essentially the same hydraulically and with proper management there is no difference in the porous medium water content regime impacting microbial activity and plant root growth.

6 Key Issues and Outlook Particulate porous media (i.e., soil) have been the economical choice for large-scale plant growth systems on Earth and will likely be used in future lunar and Martian habitats in large part because of their limited high-tech resource requirements compared to advanced plant growth facilities such as hydroponics or thin fi lm techniques. Crushing and segregating native rock for construction and other purposes already plays a part of NASA’s plan for in situ resource utilization suggesting varied particle sizes may be available. Here we have presented some ideas about how the gas percolation threshold might be modified in reduced gravity largely coupled to changes in water content, distribution, and pore networks. We considered the influence of reduced gravitational force and increases in particle sizes for lunar and Martian applications as well as root intrusion and microbial colony development. Biomass production from either activity can lead to reduced pore size and decreased porosity. Consequences of a shift in gas percolation threshold from reduction in pore space (i.e., falling below the threshold) may lead to reduced plant growth or complete system failure.

Fig. 8. Hypothetical sensor-controlled volumetric water content management of lunar regolith (Table 1) showing periodic irrigation (3-cm depth) based on a threshold of 0.2 cm3 cm−3. Despite the finetextured porous medium in a shallow 15-cm-deep layer, air-filled pore space is maintained in excess of 25% of the total pore space.

Hydrodynamic and hydrostatic observations along with numerical modeling within both microgravity and Earth’s gravitational fields point to the dominance of capillarity for sub-millimeter-sized particles. Contrary to popular opinion, fine-textured porous media could be used to grow plants in containers using volumetric water content-based management. Th is is based on measurements and modeling that produced similar outcomes in terms of both water retention and fluid transport processes, regardless of gravity force. On the other hand, matric potential–based management of porous medium water content (i.e., via suction maintained in porous medium) provides the advantage of transportability to any reduced gravity level yielding similar management outcomes. Precautions for matric potential–based management lie in the possibility of shifting water retention characteristics resulting from dense root growth or fluid-phase entrapment. However, if the safety provided by gravity force-drained systems (e.g., greenhouse production) is preferred, porous medium mean particle size must be increased as a function of reducing g force. Both matric potential controls through maintaining constant suction or sensor-based volumetric water content control approaches have been suggested and used successfully in the past. Neither approach is problem-free, suggesting the need for innovative and novel approaches to root zone design and management strategies for future long-term extraterrestrial plant growth systems.

Acknowledgments

The authors gratefully acknowledge funding from NASA-JSC grants NAG 9-1284 and NAG 9-1399 and support from the Utah Agricultural Experiment Station (UAES), Utah State University, Logan, UT and approved as journal paper number 8347. We express appreciation to Ricardo Estevez and Bill Mace for programing, data collection, and technical assistance with experimental portions of this work.


References Alexander, D.B., D.A. Zuberer, and D.H. Hubbell. 1989. Microbiological considera ons for lunar-derived soils. p. 245–255. In D.W. Ming and D.L. Henninger (ed.) Lunar base agriculture: Soils for plant growth. ASA, CSSA, and SSSA, Madison, WI. Arya, L.M., D.C. Bowman, B.B. Thapa, and D.K. Cassel. 2008. Scaling soil water characteris cs of golf course and athle c field sands from par cle-size distribu on. Soil Sci. Soc. Am. J. 72:25–32. doi:10.2136/sssaj2006.0232 Arya, L.M., F.J. Leij, M.Th. van Genuchten, and P.J. Shouse. 1999. Scaling parameter to predict the soil water characteris c from par clesize distribu on data. Soil Sci. Soc. Am. J. 63:510–519. doi:10.2136/ sssaj1999.03615995006300030013x Arya, L.M., and J.F. Paris. 1981. A physicoempirical model to predict the soil moisture characteris c from par cle-size distribu on and bulk density data. Soil Sci. Soc. Am. J. 45:1023–1030. doi:10.2136/ sssaj1981.03615995004500060004x Baveye, P., and A. Valocchi. 1989. An evalua on of mathema cal models of the transport of biologically reac ng solutes in saturated soils and aquifers. Water Resour. Res. 25:1413–1421. doi:10.1029/WR025i006p01413 Bingham, G.E., S.B. Jones, D. Or, I.G. Podolski, M.A. Levinskikh, V.N. Sytchov, T. Ivanova, P. Kostov, S. Sapunova, I. Dandolov, D.B. Bubenheim, and G. Jahns. 2000. Microgravity effects on water supply and substrate proper es in porous matrix root support systems. Acta Astronaut. 47:839–848. doi:10.1016/S0094-5765(00)00116-8 Bingham, G.E., S.B. Jones, I. Podolsky, and B.S. Yendler. 1996. Porous substrate water rela ons observed during the greenhouse—II flight experiment (Mir Space Sta on, 1995). SAE Technical Paper 961547. SAE, Warrendale, PA. Bingham, G.E., I.G. Podolsky, T.S. Topham, and J.M. Mulholland. 2002. Lada: The ISS plant substrate microgravity testbed. SAE Technical Paper 2002-012388. SAE, Warrendale, PA. Bugbee, B.G., and F.B. Salisbury. 1989. Controlled environment crop produc on. p. 107–130. In D.W. Ming and D.L. Henninger (ed.) Lunar base agriculture: Soils for plant growth. ASA, CSSA, and SSSA, Madison, WI. Bunt, A.C. 1988. Physical aspects. In Media and mixes for container-grown plants. 2nd ed. Unwin Hyman, London. Carrier, D.W. 2003. Par cle size distribu on of lunar soil. J. Geotech. Geoenviron. Eng. 129:956–959. doi:10.1061/(ASCE)1090-0241(2003)129:10(956) Chan, T.P., and R.S. Govindaraju. 2004. Es ma ng soil water reten on curve from par cle-size distribu on data based on polydisperse sphere systems. Vadose Zone J. 3:1443–1454. Chau, J.F., and D. Or. 2006. Linking drainage front morphology with gaseous diffusion in unsaturated porous media: A la ce Boltzmann study. Phys. Rev. E Stat. Nonlin. So Ma er Phys. 74:056304. doi:10.1103/ PhysRevE.74.056304 Dechesne, A., D. Or, G. Gulez, and B.F. Smets. 2008. The porous surface model, a novel experimental system for online quan ta ve observa on of microbial processes under unsaturated condi ons. Appl. Environ. Microbiol. 74:5195–5200. doi:10.1128/AEM.00313-08 Edge , K.S., and P.R. Christensen. 1991. The par cle size of Mar an aeolian dunes. J. Geophys. Res. 96:22765–22776. doi:10.1029/91JE02412 Glinski, J., and W. Stepniewski. 1985. Soil aera on and its role for plants. CRC Press, Boca Raton, FL. Hanson, P.J., N.T. Edwards, C.T. Garten, and J.A. Andrews. 2000. Separa ng root and soil microbial contribu ons to soil respira on: A review of methods and observa ons. Biogeochemistry 48:115–146. doi:10.1023/A:1006244819642 Heinse, R., S.B. Jones, S.L. Steinberg, M. Tuller, and D. Or. 2007. Measurements and modeling of variable gravity effects on water distribu on and flow in unsaturated porous media. Vadose Zone J. 6:713–724. doi:10.2136/ vzj2006.0105 Heinse, R., S.B. Jones, S.D. Humphries, R.W. Mace, S.L. Steinberg, M. Tuller, R. Newman, and D. Or. 2005a. Measurement of porous media water reten on during parabolic flight induced microgravity. SAE Technical Paper 2005-012950. SAE, Warrendale, PA. Heinse, R., S.D. Humphries, R.W. Mace, S.B. Jones, S.L. Steinberg, M. Tuller, R. Newman, and D. Or. 2005b. Measurement of porous media hydraulic proper es during parabolic flight induced microgravity. SAE Technical Paper 2005-01-2950. SAE, Warrendale, PA. Heinse, R., S.B. Jones, M. Tuller, G.E. Bingham, I. Podolskiy, and D. Or. 2009. Providing op mal root zone fluxes: Challenges of capillary-driven hystere c water distribu ons in microgravity. SAE Technical Paper 2009-01-2360. SAE, Warrendale, PA. Hoehn, A., P. Scovazzo, J. Clawson, T. Geissinger, W. Kalinowski, and J. Pineau. 2003. Design, tes ng and opera on of porous media for dehumidifica on and nutrient delivery in microgravity plant growth systems. SAE Technical Paper 2003-01-2614. SAE, Warrendale, PA. Hoehn, A., P. Scovazzo, L. Stodieck, J. Clawson, W. Kalinowski, A. Rakow, D. Simmons, A.G. Heyenga, and M.H. Kliss. 2000. Microgravity root zone hydra on systems. SAE Technical Paper 2000-01-2510. SAE, Warrendale, PA.

Hopmans, J.W. 2006. Plant water and nutrient uptake in soil-root systems. 5.1. Rhizosphere models. p. 495–96. In Handbook of methods used in rhizosphere research. Swiss Federal Research Ins tute, WSL, Birmensdorf. Hossner, L.R., D.W. Ming, D.L. Henninger, and E.R. Allen. 1991. Lunar outpost agriculture. Endeavour 15:79–85. doi:10.1016/S0160-9327(05)80009-2 Ivanova, T.N., Y.A. Bercovich, A.L. Mashinsky, and G.I. Meleshko. 1993. The first “space” vegetables have been grown in the “SVET” greenhouse using controlled environmental condi ons. Acta Astronaut. 29:639–644. doi:10.1016/0094-5765(93)90082-8 Ivanova, T.N., and I.W. Dandolov. 1992a. Dynamics of the controlled environment condi ons in “SVET” greenhouse in flight. C.R. Acad. Bulg. Sci. 45:33–35. Ivanova, T.N., and I.W. Dandolov. 1992b. Moistening of the substrate in microgravity. Microgravity Sci. Technol. 3:151–155. Inanova, T.N., S.M. Sapunova, P.T. Kostov, and I.I. Ilieva. 2005. Recent advances in the development of the SVET Space Greenhouse Experiment. p. 722– 727. In S. Kurnaz et al. (ed.) Proc. of the 2nd Int. Conf. on Recent Advances in Space Technologies (RAST 2005). Vol. IEE Catalog Number 05EX1011. IEE, Istanbul, Turkey. Jones, S.B., B. Bugbee, R. Heinse, D. Or, and G.E. Bingham. 2009. Porous plant growth media design considera ons for lunar and mar an habitats. SAE. Technical Paper 2009-01-2361. SAE, Warrendale, PA. Jones, S.B., R. Heinse, G.E. Bingham, and D. Or. 2005. Modeling and design of op mal growth media from plant-based gas and liquid fluxes. SAE Technical Paper 2005-01-2949. SAE, Warrendale, PA. Jones, S.B., and D. Or. 1998. Design of porous media for op mal gas and liquid fluxes to plant roots. Soil Sci. Soc. Am. J. 62:563–573. doi:10.2136/ sssaj1998.03615995006200030002x Jones, S.B., and D. Or. 1999. Microgravity effects on water flow and distribu on in unsaturated porous media: Analysis of flight experiments. Water Resour. Res. 35:929–942. doi:10.1029/1998WR900091 Jones, S.B., D. Or, and G.E. Bingham. 2003. Gas diffusion measurement and modeling in coarse-textured porous media. Vadose Zone J. 2:602–610. Jones, S.B., D. Or, G.E. Bingham, and R.C. Morrow. 2002. ORZS: Op miza on of root zone substrates for microgravity. SAE Technical Paper 2002-01-2380. SAE, Warrendale, PA. Leij, F.J., W.J. Alves, M.Th. van Genuchten, and J.R. Williams. 1999. The UNSODA Unsaturated Soil Hydraulic Database. p. 1269–1281. In M.Th. van Genuchten et al. (ed.) Proc. Int. Workshop Characteriza on and Measurement of the Hydraulic Proper es of Unsaturated Porous Media, Riverside, CA. Levine, H., B. Wells, K. Anderson, W. Aiastuch, J. Moyer, W. Kno , and G. Etheridge. 1998. Microgravity plant nutrient experiment MPNE-01 flight report. Kennedy Space Center, FL. Long, T., and D. Or. 2005. Aqua c habitats and diffusion constraints affec ng microbial coexistence in unsaturated porous media. Water Resour. Res. 41:W08408. doi:10.1029/2004WR003796 Long, T., and D. Or. 2007. Microbial growth on par ally saturated rough surfaces: Simula ons in idealized roughness networks. Water Resour. Res. 43:W02409. doi:10.1029/2005WR004781 MacCallum, T.K., J.E. Poynter, and C.P. McKay. 2000. Mars greenhouse experiment module, an experiment to grow flowers on mars. h p://www. lpi.usra.edu/mee ngs/robomars/pdf/6219.pdf (accessed 1 Feb. 2011). Maggi, F., and C. Pallud. 2010a. Space agriculture in micro- and hypo-gravity: A compara ve study of soil hydraulics and biogeochemistry in a cropping unit on Earth, Mars, the Moon and the space sta on. Planet. Space Sci. 58:1996–2007. doi:10.1016/j.pss.2010.09.025 Maggi, F., and C. Pallud. 2010b. Mar an base agriculture: The effect of low gravity on water flow, nutrient cycles, and microbial biomass dynamics. Adv. Space Res. 46:1257–1265. doi:10.1016/j.asr.2010.07.012 Mar n, T. 2010. Comparison of bioclogging effects in saturated porous media within one- and two-dimensional flow systems. Ecol. Eng. 36:839–849. doi:10.1016/j.ecoleng.2010.04.001 Meheust, Y., G. Lovoll, K.J. Maloy, and J. Schmi buhl. 2002. Interface scaling in a two-dimensional porous medium under combined viscous, gravity, and capillary effects. Phys. Rev. E 66:051603. doi:10.1103/PhysRevE.66.051603. Miller, E.E., and R.D. Miller. 1955. Theory of capillary flow: I. Prac cal implica ons. Soil Sci. Soc. Am. J. 19:267–271. doi:10.2136/ sssaj1955.03615995001900030005x Moldrup, P., T. Olesen, J. Gamst, P. Schjonning, T. Yamaguchi, and D.E. Rolston. 2000. Predic ng the gas diffusion coefficient in repacked soil: Waterinduced linear reduc on model. Soil Sci. Soc. Am. J. 64:1588–1594. doi:10.2136/sssaj2000.6451588x Monje, O., G. Stu e, and D. Chapman. 2005. Microgravity does not alter plant stand gas exchange of wheat at moderate light levels and satura ng CO2 concentra on. Planta 222:336–345. doi:10.1007/s00425-005-1529-1 Monje, O., G.W. Stu e, G.D. Goins, D.M. Porterfield, and G.E. Bingham. 2003. Farming in space: Environmental and biophysical concerns. Adv. Space Res. 31:151–167. doi:10.1016/S0273-1177(02)00751-2 Morrow, R.C., R.J. Bula, T.W. Tibbi s, and W.R. Dinauer. 1994. The ASTROCULTURE flight experiment series, valida ng technologies for growing plants in space. Adv. Space Res. 14:29–37. doi:10.1016/0273-1177(94)90276-3


Morrow, R.C., W.R. Dinauer, R.J. Bula, and T.W. Tibbi s. 1993. The ASTROCULTURE-1 flight experiment: Pressure control of the WCSAR porous tube nutrient delivery system. SAE Technical Paper 932282. SAE, Warrendale, PA. Oertli, J.J. 1996. Transport of water in the rhizosphere and in roots. p. 616. In Y. Waisel et al. (ed.) Plant roots: The hidden half. 2nd ed. Marcel Dekker, New York. Or, D., M. Tuller, and S.B. Jones. 2009. Liquid behavior in par ally saturated porous media under variable gravity. Soil Sci. Soc. Am. J. 73:341–350. doi:10.2136/sssaj2008.0046 Podolsky, I., and A. Mashinsky. 1994. Peculiari es of moisture transfer in capillary-porous soil subs tutes during space flight. Adv. Space Res. 14:39– 46. doi:10.1016/0273-1177(94)90277-1 Rockhold, M.L., R.R. Yarwood, M.R. Niemet, P.J. Bo omley, and J.S. Selker. 2002. Considera ons for modeling bacterial-induced changes in hydraulic proper es of variably saturated porous media. Adv. Water Resour. 25:477– 495. doi:10.1016/S0309-1708(02)00023-4 Salisbury, F.B. 1992. Some challenges in designing a lunar, mar an, or microgravity CELSS. Acta Astronaut. 27:211–217. doi:10.1016/00945765(92)90200-3 Schjønning, P., I.K. Thomsen, P. Moldrup, and B.T. Christensen. 2003. Linking soil microbial ac vity to water- and air-phase contents and diffusivi es. Soil Sci. Soc. Am. J. 67:156–165. doi:10.2136/sssaj2003.0156 Scovazzo, P., T.H. Illangasekare, A. Hoehn, and P. Todd. 2001. Modeling of two-phase flow in membranes and porous media in microgravity as applied to plant irriga on in space. Water Resour. Res. 37:1231–1243. doi:10.1029/2000WR900311 Seki, K., T. Miyazaki, and M. Nakano. 1998. Effects of microorganisms on hydraulic conduc vity decrease in infiltra on. Eur. J. Soil Sci. 49:231–236. doi:10.1046/j.1365-2389.1998.00152.x Silverstone, S., M. Nelson, A. Alling, and J.P. Allen. 2003. Development and research program for a soil-based bioregenera ve agriculture system to feed a four person crew at a Mars base. 31:69–75. Silverstone, S., M. Nelson, A. Alling, and J.P. Allen. 2005. Soil and crop management experiments in the laboratory biosphere: An analogue system for the Mars on Earth facility. Adv. Space Res. 35:1544–1551. doi:10.1016/j. asr.2005.06.008 Simunek, J., M. Sejna, H. Saito, M. Sakai, and M.Th. van Genuchten. 2008. The HYDRUS-1D so ware package for simula ng the one-dimensional

movement of water, heat, and mul ple solutes in variably-saturated media. Verson 4.0. Univ. of California, Riverside. Skopp, J., M.D. Jawson, and J.W. Doran. 1990. Steady-state aerobic microbial ac vity as a func on of soil water content. Soil Sci. Soc. Am. J. 54:1619–1625. doi:10.2136/sssaj1990.03615995005400060018x Steinberg, S., G.J. Kluitenberg, S.B. Jones, N.E. Daidzic, L.N. Reddi, M. Xiao, M. Tuller, R.M. Newman, D. Or, and J.I.D. Alexander. 2005. Physical and hydraulic proper es of baked ceramic aggregates used for pant growth medium. J. Am. Soc. Hor c. Sci. 130:767–774. Steinberg, S.L., D. Ming, and D. Henninger. 2002. Plant produc on systems for microgravity: Cri cal issues in water, air and solute transport through unsaturated porous media. h p://ston.jsc.nasa.gov/collec ons/TRS/_techrep/ TM-2002-210774.pdf (accessed 1 Feb. 2011). NASA/JSC, Hannover, MD. Stotzky, G. 1989. Microorganisms and the growth of higher plants in lunarderived soils. p. 131–137. In D.W. Ming and D.L. Henninger (ed.) Lunar base agriculture: Soils for plant growth. ASA, CSSA, and SSSA, Madison, WI. Stu e, G., O. Monje, G. Goins, and B. Tripathy. 2005. Microgravity effects on thylakoid, single leaf, and whole canopy photosynthesis of dwarf wheat. Planta 223:46–56. doi:10.1007/s00425-005-0066-2 Vandevivere, P., and P. Baveye. 1992. Effect of bacterial extracellular polymers on the saturated hydraulic conduc vity of sand columns. Appl. Environ. Microbiol. 58:1690–1698. van Genuchten, M.Th. 1980. A closed-form equa on for predic ng the hydraulic conduc vity of unsaturated soils. Soil Sci. Soc. Am. J. 44:892–898. doi:10.2136/sssaj1980.03615995004400050002x Vaz, C.M.P., M. de Freitas Iossi, J. de Mendonca Naime, A. Macedo, J.M. Reichert, D.J. Reinert, and M. Cooper. 2005. Valida on of the Arya and Paris water reten on model for Brazilian soils. Soil Sci. Soc. Am. J. 69:577–583. doi:10.2136/sssaj2004.0104 Wang, G., and D. Or. 2010. Aqueous films limit bacterial cell mo lity and colony expansion on par ally saturated rough surfaces. Environ. Microbiol. 12:1363–1373. doi:10.1111/j.1462-2920.2010.02180.x Wheeler, R. 2006. Potato and human explora on of space: Some observa ons from NASA-sponsored controlled environment studies. Potato Res. 49:67– 90. doi:10.1007/s11540-006-9003-4 Wheeler, R.M. 2003. Carbon balance in bioregenera ve life support systems: Some effects of system closure, waste management, and crop harvest index. Adv. Space Res. 31:169–175. doi:10.1016/S0273-1177(02)00742-1
