Martian base agriculture: The effect of low gravity on water flow, nutrient cycles, and microbial biomass dynamics
Introduction
Mars represents the ultimate challenge for agriculture experimentation and technology aimed at supporting long-term life support of humans in extraterrestrial outposts. In the hypothesis that Mars can be inhabited by a six-people crew by 2050 (Yamashita et al., 2006) or earlier 2030 (Schrope, 2010), soil-based agriculture is being considered as a bioregenerative strategy to recycle water, produce food, sequester carbon dioxide (CO2) while producing oxygen (O2), and decomposing organic wastes (e.g., Salisbury, 1992, Yamashita et al., 2006). However, Mars gravitational acceleration (0.38g with g = 9.806 m s−2), air pressure (less than 1% the Earth’s one), atmosphere composition (about 95.3% CO2, 2.7% dinitrogen (N2), 1.6% argon, and only 0.13% O2), extremely cold and variable temperature (ranging between about −150 and +20 °C), and low solar radiations (approximately 40% less than on Earth) represent limiting conditions for agriculture as it is practiced on Earth.
Hydroponics and aeroponics techniques, which use nutrient solutions in a bath and as aerosols, respectively, were investigated in the past decades (e.g., Hoagland and Arnon, 1950) and have been proven successful to sustain crop production for relatively long time periods. Because hydroponics and aeroponics do not use soil, they are highly advantageous in terms of weight-reduction on spacecrafts or orbiting space stations, in addition to the fact that they can be highly automated to control nutrient and water dosage. The most important cropping experiments in microgravity were performed in the early 90s onboard the MIR Soviet station using porous media in phytotron cells, where a capillary-driven water delivery system was designed to feed plants. The choice of using a porous matrix rather than a hydroponic or aeroponic system was motivated by the possibility to entrap water by capillary forces within a medium in contrast to having free water in the absence of gravity. Besides the use of porous material to constrain the free movement of water, Nelson et al. (2008) have further indicated that Earth-like soil media would carry more beneficial effects on a Martian station. In fact, soil microbial communities can metabolize most compounds of potential toxicity and mineralize organic matter whereas hydroponic and aeroponic systems lose these capabilities because of the absence of microorganisms. Additionally, soil systems offer a sustainable waste recycling strategy to recover water and nutrients through systems such as constructed wetlands, bio- and phyto-filtration systems (Silverstone et al., 2003; Nelson et al., 2008), and composting (Finstein et al., 1999a, Finstein et al., 1999b, Kanazawa et al., 2008, Wheeler, 2003). Overall, soil-based cropping and composting involve natural processes that offer compactness, low energy demand, near-ambient reactor temperatures and pressure, reliability, forgiveness of operational errors or neglect (Finstein et al., 1999a, Nelson et al., 2008). Large uncertainties for the success of Martian base agriculture, however, lie in the response of plant growth and nutrient cycling to low gravity, in the potential use of Martian soils as an in-situ resource and in the way human by-products and inedible composted biomass would interact with off-planet soil materials.
In the perspective of practicing soil-based cropping in a Martian greenhouse, the atmosphere pressure, composition, and temperature, as well as convection in the greenhouse environment can presumably be controlled with a narrow range of uncertainty, whereas gravity cannot be controlled. Hence, the most important aspect requiring a careful analysis is how low gravity could affect soil physical and biogeochemical processes (e.g., Monje et al., 2003, Silverstone et al., 2003, Porterfield, 2002). Because of Mars’ lower gravitational acceleration than the Earth’s one, convective gas mixing would be weaker and could easily be suppressed by viscous flows (Yamashita et al., 2006). Therefore, gas convection around plant leaves could substantially decrease compared to a situation on Earth, thereby reducing the evaporation rate (Monje et al., 2003, Hirai and Kitaya, 2009) with possible repercussions on O2 and CO2 exchange rates. Alteration of gas and nutrient exchange rates could be the cause of the uneven and unhealthy plant growth experimentally observed by Hoson et al. (2000) in phytotron chambers under microgravity.
In the soil root zone, adequate supply of water, nutrients and O2 is required for plants and microorganisms’ metabolism, and consequently for organic matter mineralization (e.g., Salisbury, 1992, Monje et al., 2003). The rate at which nutrients become available to roots and microorganisms is principally determined by the soil texture and structure (e.g., porosity), the soil hydraulic properties (e.g., permeability), the soil moisture content, and the water flow rate within the medium. Water, solutes and gases move through the soil by advective and diffusive flows but whereas diffusion is not susceptible to gravity (i.e., it can be described as temperature-dependent Brownian motion) advection on Mars would be substantially different as compared to advection on Earth, and would result in a net change in nutrient dispersion and transport rate at the Darcy’s scale. It has been observed that a lower gravity results in lower infiltration rate and longer water and solute residence time (e.g., Jones and Or, 1999, Heinse et al., 2007), but it is not clear whether this would hinder, maintain or facilitate nutrient accessibility for plants and soil microorganisms. From a theoretical viewpoint, pore wetting in reduced gravity would occur with a higher tendency for the water to distribute on the pore surface, thus potentially creating a liquid film on the surface of soil solid particles that could isolate air pockets (Jones and Or, 1999, Monje et al., 2003, Heinse et al., 2007). These air pockets would presumably reduce significantly the soil permeability and could entrap soluble nutrients and gaseous species that would consequently not be available to roots and microorganisms at the rates they would on Earth. In addition, experiments in microgravity have demonstrated that a transition in moisture content would dislocate particles by buoyancy, thus affecting the soil hydraulic properties in a dynamical way (Jones and Or, 1999).
For an effective and safe soil-based agriculture on Mars, we aim to address two aspects: the first is the effect of 0.38g gravity on water flow, and the second is the feedback that water flow may exert on the nutrient and biomass dynamics. Whereas concerns have been expressed on these aspects in qualitative terms and on water dynamics only (Monje et al., 2003, Silverstone et al., 2003, Porterfield, 2002, Heinse et al., 2007), we propose here an approach to quantitatively analyze the implications of low gravity on the coupled dynamics of soil hydraulics and biogeochemistry as a whole. To this end, a highly mechanistic model, TOUGHREACT-N (Maggi et al., 2008), was calibrated on experimental data from Earth and was used to predict the effects of Martian low gravity on the small-scale interplays between physical and biogeochemical feedbacks in a bioregenerative soil unit. Attention is devoted to water flow, rates of microbially-mediated biogeochemical reactions, and gaseous and leaching losses of nitrogen (N) and carbon (C) species. Finally, we discuss the implications of such findings for the short and long-term run of a hypothetical soil-based cropping system on Mars.
Section snippets
Bioregenerative crop unit
Several functioning schemes and technical designs have been proposed for space applications of bioregenerative cropping units (e.g., Hossner et al., 1991, Bingham et al., 2000), including recycling of water and nutrients to reduce system costs and to preserve water. Here, we have schematized a bioregenerative unit as an isolated chamber that receives water and nutrient from a recirculation system (Fig. 1) with the N-fertilizer supplied as in the same N-equivalent dose as in typical
Calibration on terrestrial data
TOUGHREACT-N was calibrated using data from the Earth tomato field (Section 2.1) for a one-dimensional 60 cm long soil column with a spatial resolution of 1.25 cm. Fertilization was modeled in TOUGHREACT-N as an upscaled uniform concentration of 96 g NH3 m−2 (0.12 mol L−1 N) in the top 10 cm of the soil column on day 0. Irrigation consisted of one 24 h event at a rate of 8.64 L H2O m−2 d−1 on day 9 followed by a second 96 h irrigation event at a rate of 34.6 L H2O m−2 d−1 from day 10 to day 14. Evaporation
Discussion
The differences in biogeochemical dynamics observed in the Martian and terrestrial root zones were exclusively dictated by the effect of low Martian gravity (0.38g) on water advection and diffusion with respect to the gravity on Earth (1g), which conditioned nutrient transport and delivery to microorganisms. Two main features arise from this study in relation to nutrient cycling and microbial biomass dynamics.
First, we observed that the temporal pattern of concentrations was very similar
Conclusions
We have analysed the effects of low gravity on the hydraulic and biogeochemical processes taking place in soils for potential applications in Martian base agriculture. Mars was considered as an example where low-gravity agriculture could raise important and challenging aspects in that concern with the soil, water, nutrient and biomass interactions. Using a mechanistic model of soil hydraulics and biogeochemistry under low gravity, we have given evidence that (i) water infiltration rate
References (39)
- et al.
Microgravity effects on water supply and substrate properties in porous matrix root support systems
Acta Astronautica
(2000) - et al.
Numerical simulation of organic carbon, nitrate, and nitrogen isotope behavior during denitrification in a riparian zone
J. Hydrol.
(2004) - et al.
The first “space” vegetables have been grown in the “SVET” greenhouse using controlled environmental conditions
Acta Astronautica
(1993) - et al.
Space agriculture task force g, space agriculture for habitation on Mars with hyper-thermophilic aerobic composting bacteria
Adv. Space Res.
(2008) - et al.
Farming in space: environmental and biophysical concerns
Adv. Space Res.
(2003) - et al.
Integration of lessons from recent research for ‘‘Earth to Mars’’ life support systems
Adv. Space Res.
(2008) Some challenges in designing a lunar, martian, or microgravity CELLS
Acta Astronautica
(1992)- et al.
Development and research program for a soil-based bioregenerative agriculture system to feed a four person crew at a Mars base
Adv. Space Res.
(2003) Carbon balance in bioregenerative life support systems: some effects of system closure, waste management, and crop harvest index
Adv. Space Res.
(2003)- et al.
Role of nitrifier denitrification in the production of nitrous oxide
Soil Biol. Biochem.
(2001)
TOUGHREACT – A simulation program for non-isothermal multiphase reactive geochemical transport in variably saturated geologic media: applications to geothermal injectivity and CO2 geological sequestration
Comput. Geosci.
Kinetics of nitrite oxidation by Nitrobacter winogradskyi
Biochem. J.
Biological oxidation of nitric oxide in a humisol
Biol. Fertil. Soils
Composting on Mars or the Moon: I. Comparative evaluation of process design alternatives
Life Support Biosph. Sci.
Composting on Mars or the Moon: II. Temperature feedback control with top-wise introduction of waste material and air
Life Support Biosph. Sci.
Aqueous and gaseous nitrogen losses induced by fertilizer application
J. Geophys. Res.
Measurements and modeling of variable gravity effects on water distribution and flow in unsaturated porous media
Vadose Zone J.
Effects of gravity on transpiration of plant leaves, interdisciplinary transport phenomena
Ann. N.Y. Acad. Sci.
The Water–Culture Method for Growing Plants Without Soil, Circular 347
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