Evolution of soil organic matter pools during Martian regolith terraforming, with a focus on organo-Fe (oxyhydr)oxide interactions.

The poor fertility of Martian regolith, due to its lack of organic matter (OM) and nitrogen (N), limits its suitability as a plant substrate. While compost amendment enhances short-term fertility, the mechanisms underlying long-term OM stabilization, particularly through interactions with iron (Fe) minerals, remain poorly understood. This study explores OM fractionation and Fe mineral transformations in Mojave Mars Simulant (MMS-1), both pure (R100) and amended with compost (R70C30), across two consecutive cropping cycles (potato followed by Vicia faba). Following Vicia faba cultivation, total C increased 12-fold in R70C30 (18.8 g kg-1) compared to R100 (1.6 g kg-1), with a 140 % increase in amended and 90 % in pure regolith relative to post-potato levels. Both particulate organic matter (POM) and mineral-associated organic matter (MAOM) also increased substantially: POM-C rose 7-fold, while MAOM-C increased by 947 %, suggesting the formation of organo-mineral complexes. MAOM also exhibited a 447 % rise in total N and the lowest C/N ratio (∼9), consistent with more microbially processed and stabilized OM. Fe speciation via Fe K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) revealed compost-driven enrichment of ferrihydrite and hematite, with distinct mineral profiles across POM and MAOM fractions. EXAFS further identified lepidocrocite and magnetite, phases undetected by XANES, highlighting the complementary role of reactive and crystalline Fe minerals in stabilizing OM in mineral matrices. These findings underscore the potential of organic amendments and leguminous crops to promote biologically functional, nutrient-rich substrates from Martian regolith simulants, offering critical insights for in situ resource utilization in space agriculture.