Elsevier

Advances in Space Research

Volume 46, Issue 10, 15 November 2010, Pages 1257-1265
Advances in Space Research

Martian base agriculture: The effect of low gravity on water flow, nutrient cycles, and microbial biomass dynamics

https://doi.org/10.1016/j.asr.2010.07.012Get rights and content

Abstract

The latest advances in bioregenerative strategies for long-term life support in extraterrestrial outposts such as on Mars have indicated soil-based cropping as an effective approach for waste decomposition, carbon sequestration, oxygen production, and water biofiltration as compared to hydroponics and aeroponics cropping. However, it is still unknown if cropping using soil systems could be sustainable in a Martian greenhouse under a gravity of 0.38g. The most challenging aspects are linked to the gravity-induced soil water flow; because water is crucial in driving nutrient and oxygen transport in both liquid and gaseous phases, a gravitational acceleration lower than g = 9.806 m s−2 could lead to suffocation of microorganisms and roots, with concomitant emissions of toxic gases. The effect of Martian gravity on soil processes was investigated using a highly mechanistic model previously tested for terrestrial crops that couples soil hydraulics and nutrient biogeochemistry. Net leaching of NO3- solute, gaseous fluxes of NH3, CO2, N2O, NO and N2, depth concentrations of O2, CO2 and dissolved organic carbon (DOC), and pH in the root zone were calculated for a bioregenerative cropping unit under gravitational acceleration of Earth and for its homologous on Mars, but under 0.38g. The two cropping units were treated with the same fertilizer type and rate, and with the same irrigation regime, but under different initial soil moisture content. Martian gravity reduced water and solute leaching by about 90% compared to Earth. This higher water holding capacity in soil under Martian gravity led to moisture content and nutrient concentrations that favoured the metabolism of various microbial functional groups, whose density increased by 5–10% on Mars as compared to Earth. Denitrification rates became substantially more important than on Earth and ultimately resulted in 60%, 200% and 1200% higher emissions of NO, N2O and N2 gases, respectively. Similarly, O2 and DOC were consumed more rapidly in the Martian soil and resulted in about 10% increase in CO2 emissions. More generally, Martian cropping would require 90% less water for irrigation than on Earth, being therefore favourable for water recycling treatment; in addition, a substantially lower nutrient supply from external sources such as fertilizers would not compromise nutrient delivery to soil microorganisms, but would reduce the large N gas emissions observed in this study.

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 NO3- 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 NO3- 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

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