Formation of organo-Fe (oxyhydr)oxide interactions during the first stages of Martian regolith terraforming: a step forward

Mars regolith lacks organic matter (OM), although potentially contains most of the essential nutrients for plant growth. This deficiency of OM and of few important nutrients (like nitrogen -N-) and the underperformance as a growth substrate can be overcome adding exogenous OM (e.g., compost). However, little is known about the potential interaction between exogeneous OM and minerals, including iron (Fe) oxides, with time, which is fundamental to improve physical and chemical features of Martian simulants. This study aims at tracking the evolution of organo-mineral interactions, mineral transformation, and OM turnover after the early stages of terraforming. Potato was grown on the Mojave Mars Simulant MMS-1, alone (R100) and mixed with a commercial compost 70:30 v:v (R70C30), for 99 days in greenhouse. After this first experiment, the same substrates were kept to age for 1 year and then used again in greenhouse as a growing media for fava bean plants (Vicia faba L.) cv. Sfardella. At the sampling, organic matter in bulk (BK) and rhizo-sphere (RH) soils was fractionated, obtaining particulate (POM) and mineral associated OM (MAOM); both pools were characterized for total N and organic carbon (OC), total element concentration (ICP-OES) and by Fe K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). After fava bean growing cycle (i.e., 106 days from sowing to the ripening of fava bean seeds), most of the OC accumulated in the POM fraction, which was 20× higher in R70C30 compared to R100; however, comparing these data with those obtained during the previous vegetative cycle of potato cultivation, differences were not significant. In the MAOM fraction, OC in R70C30 was 12× higher than in R100. If this finding on one side confirmed the trend already observed during the first experiment, where an increase of organic C content in MAOM of 4× was found, on the other side it clearly underlines how the process of exogenous OM stabilization by minerals increased significantly in the medium-to-long term. No significant differences between BK and RH were found. Finally, ferrihydrite was confirmed to mediate exogenous OM stabilization in regolith-based substrates. Consequently, revealing Fe species involved in the formation of organo-mineral interactions over a range of time scales will help to identify the main critical aspects and future challenges related to sustainable space farming using in-situ Martian resources.