Future space missions to Mars will depend on the development of bioregenerative life support systems. Mars regolith contains most of the nutrients needed for plant growth, but not organic matter (OM). Although Mars simulants have been deeply characterized and tested as growing media, no data are available about their possible modification occurring during terraforming, including the interaction of exogeneous OM with iron (Fe) oxides, particularly abundant in Mars regolith. The aim of this study was to investigate the mineral transformation and the OM evolution occurring in the early stages of the terraforming process. Potato was grown for 99 days on Mojave Mars Simulant MMS-1, alone (R100) and mixed with a compost 70:30 v:v (R70C30), and on a fluvial sand, alone (S100) and mixed with compost (S70C30), for comparison. Bulk (BK) and potato tubero/rhizo-sphere (RH) soils were fractionated to obtain particulate OM (POM) and mineral-associated OM (MAOM). Bulk samples and corresponding fractions were characterized for total nitrogen and organic carbon (C) and analyzed by Fe K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy. Organic C increased by 10 and 25 times in S70C30 and R70C30, respectively, compared to S100 and R100. Most of the organic C accumulated in the POM fraction of both growing substrates, while its content in the MAOM was 3 times higher in R70C30 than in S70C30. No significant differences between BK and RH were found. Finally, ferrihydrite mediated exogenous OM stabilization in regolith-based substrates, while Fe(III)-OM complexes were detected exclusively in sand-based growing media. Understanding mechanisms and testing potential sustainable practices for creating Mars regolith similar to terrestrial soil will be fundamental to sustain food crop production on Mars.

The identification of iron (Fe) forms throughout a sediment sequence was investigated by X-ray Absorption Near Edge Spectroscopy (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) at the Fe K-edge, paired with Raman micro-spectroscopy. The contribution of different organic and inorganic Fe-bearing compounds was quantified by Linear Combination Fitting (LCF) carried out on both XANES and EXAFS spectra. Fe-XANES showed that the Fe(II)/Fe(III) ratio of different Fe-bearing minerals in sediments can be quantified with reasonable accuracy. The main Fe species detected were ferrihydrite, goethite, hematite, clay minerals (smectite, illite, nontronite), and Fe(III)-organic matter (Fe(III)-OM). A more accurate quantification of ferrihydrite was possible with LCF conducted on Fe-EXAFS spectra. With the exception of hematite, the concentration of these mineral species does not have a clear trend with depth, probably because water infiltration caused continuous Fe reduction and oxidation cycles in these sediments. From an analytical perspective, Fe oxide compounds can be difficult to identify or distinguish unless multiple techniques are used. X-ray diffraction (XRD; previous work) and Raman spectroscopy turn out to be not particularly useful in identifying ferrihydrite, while they are best suited for a broad mineralogical analysis that requires integrative spectral studies for an accurate Fe speciation. In detail, XANES and EXAFS allowed for the detection of Fe-bearing clay minerals and a more refined identification of Fe species, including Fe(III)-OM. Thermal analysis was useful to confirm some mineralogical components observed using both XRD (data previously published) and Raman spectroscopy (e.g., goethite, todorokite). In conclusion, this study underlines how a multi-technique approach is required to investigate peculiar environments such as karst pedosequences.

Paddy soils experience long-term redox alternations affecting the interactions between the biogeochemical cycling of iron (Fe) and carbon (C). Although the higher soil organic matter (SOM) accumulation rates in paddy topsoils with respect to non-paddy soils is generally assumed to be due to limited mineralization under anoxic soil conditions resulting from frequent field flooding, there is growing evidence questioning this assumption. Moreover, differences in particle aggregation and SOM turnover are likely to both affect and be affected by the trajectory of Fe mineral evolution/crystallinity with redox fluctuations. We hypothesized that redox cycling in paddy soils will affect the particle aggregation, the distribution and mineralogy Fe (hydr)oxides between aggregate size fractions, and consequently the mechanisms of SOM stabilization. In particular, we expect finer aggregate and particle size classes to have a higher proportion of short-range ordered (SRO) Fe oxides with respect to larger aggregates under paddy management, compared to non-paddy management, and that paddy management can result in lower amounts of Fe(hydr)oxides in the topsoil with respect to non-paddy soils.   We tested these hypotheses by evaluating mineralogical changes, and the distribution of Fe species and organic C between different aggregate and particle-size fractions in topsoil (eluvial) and subsoil (illuvial) horizons of soils under long-term paddy (P) horizons (Arp1, Arp2, Arpd, Brd1, Brd2) and non-paddy (NP) horizons (Ap1, Ap2, Bgw) in NW Italy. Soil aggregates (microaggregates: <200 μm, free silt: (53-2 μm), free clay: <2 μm, and, after sonication, fine sand, silt and clay within microaggregates) have been obtained frombulk soils using an aggregate and particle size physical fractionation method. After fractionation, Fe phases were evaluated by selective extraction procedures, X-ray diffraction (XRD) and Fe K-edge extended X-ray fine structure (Fe EXAFS) spectroscopy (Elettra XAFS beamline). Our results indicate: (1) a depletion in the contents of ferrihydrite in the P topsoil horizons with respect to NP, though redox cycling favoured an increase in ferrihydrite in the P subsoil, possibly due to Fe(II) translocation from topsoil to subsoil, with consequent ferrihydrite precipitation and aggregates formation; (2) more crystalline Fe mineral phases were associated with intra-aggregate clay fraction in the P topsoil. In the clay fraction in the Brd2 subsoil horizon magnetite was observed. In the NP soil the illuvial horizons were not characterized by a significant increase in ferrihydrite. Our hypothesis that finer aggregate and particle size classes have a higher proportion of SRO Fe oxides with respect to larger aggregates under P management, with respect to NP management, was confirmed; (3) more organic C was associated with the fine fraction in P with respect to NP suggesting that redox cycling enhances the chemical stabilization of mineral-associated SOM. These findings focused on localized Fe dynamics and biogeochemical coupling with SOM, suggesting that redox-driven changes in aggregate-size classes distribution were also linked to the differences in organic C and Fe stocks in these two agro-ecosystems.

The decline of soil organic matter (SOM) as well as the depletion of micro and macronutrients are among the most serious threats facing many agricultural soils of the world. Consequently, in many countries, soil amendments are increasingly originating from a wide range of organic wastes, as a win-win strategy to mitigate global warming while increasing soil fertility. Digestate, a by-product of the anaerobic digestion generally characterized by both high biological stability and content of nutrients (including nitrogen – N –), is often used as a fertilizer and/or organic amendment. However, a safe and functional application of digestate in agriculture requires an in-depth understanding of its possible impacts on the native SOM, on carbon sequestration mechanisms, as well as on soil microbial communities.The aim of this work was to test the influence of solid vs. liquid digestates on both SOM accrual and the distribution of SOM in particulate organic matter (POM) and mineral associated organic matter (MAOM) pools.The experiment was set up as a randomized complete block design with two factors, namely solid and liquid digestate, with 3 replicates. A control treatment (no digestate), two depths (0-15 and 15-30 cm) and a digestate application at a rate of 340 kg of potentially available N ha-1 yr-1 were considered. Soil samples were collected few days following the amendment (t0), and after 3 (t3) and 6 (t6) months. All samples (54) were characterized for pH, EC, bulk density, total organic C (TOC), total N (TN), texture and major and trace elements. Moreover, the abundance of key genes related to the N cycle was assessed by quantitative polymerase chain reaction (PCR). Finally, POM and MAOM fractions were isolated and characterized by elemental analysis (CHNS).Preliminary data suggest that, after 6 months from the amendment, soil organic C (SOC) stock in the first 15 cm of depth was not affected by digestate application, whereas a significant influence was observed for TN, especially when liquid digestate was added. On the opposite, the application of digestate had a significant influence on both SOC and TN stock at 15-30 cm of depth. Moreover, while the distribution of SOC between MAOM and POM was exclusively affected by the application of digestate, independently of depth, the distribution of TN between SOM pools was influenced by both the treatment and the time. In particular, the time factor seems to play an important role on TN concentration in the MAOM. As regards the abundance of bacterial genes, the solid digestate conferred an increase to those for nitrification (amoA), denitrification (nirK, nosZ) and total bacteria (16S), particularly at 15-30 cm of depth. The effect reached its maximum after three months from the application and decreased near control levels after six months. AcknowledgementsSM and CZ thank Cattolica Agricola soc. agr. a r.l. and Iniziative Biometano s.p.a. for allowing the experimental activity.