Geo-mineralogical characterisation of Mars simulant MMS-1 and appraisal of substrate physico-chemical properties and crop performance obtained with variable green compost amendment rates

The high pH of green compost (8.25) did not allow reduction of pH of R70C30 to sub-neutral level required for proper nutrient availability.

“However, the high pH (8.25) did not allow to reduce the pH of the mixture R70C30 to the sub-neutral level required to ensure the proper nutrient availability (Caporale et al., 2020).”

MMS-1 is a coarse, alkaline material (pH 8.86) consisting of plagioclases, amorphous components, and zeolite, containing K, Ca, Mg, Fe, but lacking organic C, N, P, fine particles with colloidal properties, and suitable water retention.

“The MMS-1 is a coarse, alkaline material (pH 8.86), consisting of a mixture of plagioclases, amorphous components, and zeolite, containing some essential plant nutrients (e.g., K, Ca, Mg, and Fe), but lacking organic C, N, P, fine particles with colloidal properties and a suitable water retention (Caporale et al., 2020).”

The green compost used showed a low C/N ratio, implying good N availability, and a good aromatic moiety, guaranteeing stability of organic matter and gradual mineral release over time.

“The green compost (Vivai Gardea, Villafranca di Verona, Italy), from the natural, slow microbial decomposition of pruning waste and grass cuttings (3 months in controlled conditions), showed a low C/N ratio, implying a good N availability, and a good aromatic moiety, guaranteeing the stability of the organic matter and a gradual mineral release over time (Caporale et al., 2020).”

Despite Mars regolith simulant MMS-1 containing several essential nutrients for plant growth, it is not able to fulfil the plant requirements.

“Despite Mars regolith simulant MMS-1 contains several essential nutrients for plant growth (Caporale et al., 2020), it is not able to fulfil the plant requirements.”

Amendment with green compost concomitantly resolves the disposal issue of organic effluents in future manned missions to Mars.

“concomitantly resolving the disposal issue of organic effluents in future manned missions to Mars”

Caporale et al. supposed that the consistent bioavailable pool of Na in MMS-1, together with alkaline pH and absence of biological fertility, could have induced salt stress in plants grown in pure simulant substrate.

“Caporale et al. [37] supposed that the consistent bioavailable pool of Na in MMS-1, together with an alkaline pH and the absence of biological fertility, could have induced a salt stress in plants grown in pure simulant substrate.”

The higher SO4 accumulation can be explained by increasing bioavailability of the anion with increasing compost rate in the growth substrate.

“which can be explained by the increasing bioavailability of the anion with the increasing rate of compost in the growth substrate [37]”

Compost addition to the simulant proportionally increased the amount of water retained by the substrate and enhanced more macropore and micropore domains.

“the compost addition to the simulant proportionally increased the amount of water retained by the substrate and enhanced more macropore and micropore domains [37]”

The high accumulation of PO4, K and Cl in plants cultivated in 100% compost, and Mg and Ca in plants cultivated in 100% simulant, is mostly explained by the abundance of bioavailable fractions of these ions in the mixtures.

“The high accumulation of PO4, K and Cl in plants cultivated in 100% compost, and Mg and Ca in plants cultivated in 100% simulant, is mostly explained by the abundance of bioavailable fractions of these ions in the mixtures.”

The presence of aromatic moieties and stable humified organic compounds in the compost may have stimulated the production of polyphenolic compounds in lettuce foliar biomass.

“The relevant presence of aromatic moieties and, hence, of stable and humified organic compounds in the compost, evidenced by Caporale et al. [37] through infrared spectroscopy and thermogravimetric analysis, may have stimulated the production of polyphenolic compounds in lettuce foliar biomass [64,65].”

MMS-1 is a coarse-textured alkaline mineral substrate mainly composed of plagioclase and amorphous material with accessory minerals including zeolite, hematite and smectite clays.

“They found that MMS-1 is a coarse-textured alkaline mineral substrate mainly composed of plagioclase and amorphous material with accessory minerals, including zeolite, hematite and smectite clays.”

MMS-1 lacks organic matter, N, P and S.

“it lacks organic matter, N, P and S”

Organic matter, N, P and S can be supplied through compost amendment, which enhances the main physical, chemical and hydraulic properties of the plant growth substrate.

“which can be only supplied through a compost amendment, which, in turn, enhances the main physical, chemical and hydraulic properties of the plant growth substrate”

Although MMS-1 is rich in Al oxides, Salanova plants did not show Al phytotoxic effect, since Al is poorly soluble and poorly bioavailable in sub-alkaline growth substrates, while it exerts phytotoxicity at highly acidic pHs.

“although MMS-1 was found to be very rich in Al oxides [37], Salanova plants did not show any Al phytotoxic effect, since this element is poorly soluble and poorly bioavailable in sub-alkaline growth substrates such as those used in the experiment, whilst it exerts phytotoxicity at highly acidic pHs, with soluble cations undergoing acid hydrolysis [50]”

MMS-1 can be a source of nutrients such as Ca, Fe, Mg and K.

“Although MMS-1 can be a source of nutrients (i.e., Ca, Fe, Mg, K)”

In 100% MMS-1 substrate, water-soluble K, Cl, NO3, PO4 and SO4 concentrations were less than 4% of the concentrations in 100% compost substrate.

“the concentrations of water-soluble K, Cl, NO3, PO4 and SO4 in the 100% MMS-1 substrate were less than 4% of the concentrations of the same nutrients in the 100% compost substrate”

Amendment with green compost enhanced the physicochemical and hydraulic properties of the alkaline and nutrient-poor Mars simulant.

“the amendment with green compost enhanced the physicochemical and hydraulic properties of the alkaline and nutrient-poor Mars simulant”

MMS-1 amended with compost had enhanced physical and hydraulic properties compared to pure simulant, which may have positively influenced crop performance.

“Besides a higher content of nutrient, MMS-1 amended with compost also had enhanced physical (bulk density and pore-size distribution) and hydraulic (water holding capacity and retention) properties compared to the pure simulant, which may have positively influenced the crop performance [37].”

In 100% MMS-1 substrate, Mg and Ca were 13% and 17% less than in 100% compost, respectively.

“while Mg and Ca were 13% and 17% less, respectively”

The mineralogical and physico-chemical properties of the four MMS-1:compost mixtures are as reported in Caporale et al. [37].

“The mineralogical and physico-chemical properties of both mineral and organic substrates of the four mixtures are reported in Caporale et al.’s [37] study.”

MMS-1 is an alkaline (pH 8.86) and coarse textured substrate, lacking adequate water holding capacity and occurrence of fine particles exerting colloidal properties.

“It is an alkaline (pH 8.86) and coarse textured substrate, lacking adequate water holding capacity and occurrence of fine particles exerting colloidal properties.”

The compost comprised a significant aromatic moiety, ensuring a good stability of the organic matter which can slowly release vital plant nutrients over time.

“At the same time, it comprised a significant aromatic moiety, ensuring a good stability of the organic matter which can slowly release vital plant nutrients over time.”

MMS-1 mainly consists of plagioclases in mixture with amorphous materials and zeolite, containing essential plant nutrients such as K, Ca, Mg and Fe, but lacking organic C, N, P and S.

“Mojave Mars regolith Simulant MMS-1 mainly consists of plagioclases in mixture with amorphous materials and zeolite, containing essential plant nutrients such as K, Ca, Mg and Fe, but lacking of organic C, N, P and S (Caporale et al. 2020;Duri et al. 2020).”

The compost had a low C/N ratio, implying a good availability of N for microorganisms and plant roots.

“This compost, previously characterized by Caporale et al. (2020), had a low C/N ratio, implying a good availability of N for microorganisms and plant roots.”

The compost's alkaline pH (8.25) did not allow to adjust the pH of MMS-1/compost mixtures to a sub-neutral range required to ensure the highest nutrient availability.

“However, its alkaline pH (8.25) did not allow to adjust the pH of MMS-1/compost mixtures to a sub-neutral range (Caporale et al. 2020), required to ensure the highest nutrient availability.”

The amendment with green compost enhances the physicochemical and hydraulic properties of alkaline and nutrient-poor Mars simulants, and makes the regolith a suitable substrate for cultivation of lettuce.

“Previous results showed that the amendment with green compost enhances the physicochemical and hydraulic properties of alkaline and nutrient-poor Mars simulants, and makes the regolith a suitable substrate for cultivation of lettuce (Caporale et al. 2020).”

As stated by Caporale et al. (2020), MMS-1 shows numerous critical features for plant health, such as high alkalinity and Na bioavailability, poor physical structure, predominance of macro vs. micropores, and low water holding capacity.

“Furthermore, as stated by Caporale et al. (2020), it shows numerous critical features for plant health, such as high alkalinity and Na bioavailability, poor physical structure, predominance of macro vs. micropores, and low water holding capacity.”

Simulants contain non-negligible amounts of sodium (Lunar: 2.8% of Na2O; Martian: 2.4% from Na-plagioclases) and potentially toxic elements (e.g., Cr as Cr2O3), which may induce salt or other abiotic stresses in rhizosphere-competent microorganisms and plants.

“Simulants also contain non-negligible amounts of sodium (Lunar: 2.8% of Na 2 O; Martian: 2.4% from Na-plagioclases) and potentially toxic elements (e.g., Cr as Cr 2 O 3 ), which may induce salt or other abiotic stresses in rhizosphere-competent microorganisms and plants (Caporale et al., 2020).”

The only data regarding EC in Martian simulants indicate low values of 0.2-0.3 dS m-1, suggesting no adverse effect of salinity on plant growth.

“The only data regarding EC in Martian simulants indicate low values of 0.2-0.3dS m -1 (Gilrain et al., 1999;Caporale et al., 2020), suggesting no adverse effect of salinity on plant growth.”

In terms of water and nutrient transport processes, all the mixtures were acceptable as growth media in a hydroponic cultivation system.

“In terms of water and nutrient transport processes, all the mixtures were acceptable as growth media in a hydroponic cultivation system, where a timely water supply is guaranteed.”

The addition of green compost to the MMS-1 simulant affects the physical and hydrological properties of the mixture.

“This study showed how the addition of green compost to the MMS-1 simulant affects the physical and hydrological properties of the mixture.”

The addition of compost to pure simulant proportionally increased the maximum amount of water retained.

“The addition of compost to pure simulant proportionally increased the maximum amount of water retained.”

If the objective is to manage lettuce irrigation to minimize irrigation frequency, the 70:30 mixture was the most promising substrate due to its hydraulic properties.

“However, if the objective is to manage lettuce irrigation to minimize irrigation frequency, the 70:30 mixture was the most promising substrate due to its hydraulic properties.”

Based on its hydraulic properties, the 70:30 mixture was the best substrate, as the percentage increase in large pores (diameter, 50-120 μm) was more significant in this mixture than in the 30:70 one.

“Based on its hydraulic properties, the 70:30 mixture was best substrate, as the percentage increase in large pores (diameter, 50-120 μm) was more significant in this mixture than in the 30:70 one.”

Caporale et al. (2020) found discrepancies between the measured bulk chemistry of the MMS-1 simulant and that provided by the producer.

“The study by Caporale et al. (2020) found discrepancies between the measured bulk chemistry of the MMS-1 simulant and that provided by the producer.”

Several simulant properties-such as an alkaline pH, a high availability of sodium, the predominance of macro- vs. micropores, and a scant water holding capacity-can influence plant growth, health, and vigor in extra-terrestrial farming.

“For extra-terrestrial farming, it is wellestablished that several simulant properties-such as an alkaline pH, a high availability of sodium, the predominance of macro-vs.micropores, and a scant water holding capacity-can influence plant growth, health, and vigor (Wamelink et al., 2014;Caporale et al., 2020).”

Organic amendment lowered the pH of the alkaline simulant, enhanced its cation exchange capacity, organic C and N levels, and the availability of macro- and micronutrients.

“It also lowered the pH of the alkaline simulant, enhanced its cation exchange capacity, organic C and N levels, and the availability of macro-and micronutrients.”

Although MMS-1 was a source of nutrients for lettuce, it did not supply organic matter, N, and S, and provided very scant amounts of P.

“Although it was a source of nutrients for lettuce, it did not supply organic matter, N, and S, and provided very scant amounts of P.”

Organic amendment improved the physical properties of the simulant (such as bulk density and water holding capacity).

“As reported above in Section 2.3, organic amendment improved the physical properties of the simulant (such as bulk density and water holding capacity).”

MMS-1 was found to be a coarse-textured alkaline substrate mostly composed of plagioclase, amorphous material, and (to a lesser degree) zeolite, hematite, and smectites.

“MMS-1 was found to be a coarse-textured alkaline substrate mostly composed of plagioclase, amorphous material, and (to a lesser degree) of zeolite, hematite, and smectites.”

Bulk density progressively decreased with increasing rates of compost in the mixture, ranging from 1.39 g cm-3 (pure simulant) to 0.60 g cm-3 (pure compost).

“As expected, the bulk density progressively decreased with increasing rates of compost in the mixture, ranging from 1.39 g cm -3 (pure simulant) to 0.60 g cm -3 (pure compost).”

Compost addition to the pure simulant exerted a more significant effect on the macropore region than on the micropore domain.

“The authors also showed that compost addition to the pure simulant exerted a more significant effect on the macropore region than on the micropore domain.”

The red Salanova lettuce grown in the 30:70 simulant:compost mixture showed the best crop performance, photosynthetic activity, intrinsic water use efficiency, and quality traits (mineral, carotenoid, and phenolic contents).

“The red Salanova ® lettuce grown in the 30:70 mixture showed the best crop performance, photosynthetic activity, intrinsic water use efficiency, and quality traits (mineral, carotenoid, and phenolic contents).”

Water held in macropores exceeding 120 μm in diameter was not directly beneficial for root water uptake in lettuce, and even caused root asphyxia.

“Water held in macropores exceeding 120 μm in diameter was not directly beneficial for root water uptake in lettuce (the plant considered in the study), and even caused root asphyxia.”