In this talk I will explore how soil–plant feedbacks, following the ‘‘reset’’ of natural succession caused by mining activities, influence the restoration of degraded ecosystems. Across a 14-year restoration chronosequence in central Brazil, spontaneous plant colonization and an unexpected accumulation of soil organic matter were observed following the incorporation of iron-rich biosolids (sewage sludge) into exposed regoliths. Isotopic analysis revealed that soil carbon accumulation was triggered by the formation of iron-coordinated organic complexes, stabilized into physically protected (occluded) soil fractions. The strength of weak ties, an analogy borrowed from the study of social networks, is evident on the restoration trajectory, determined by subtle connections between ecological and biogeochemical process occurring above and below ground.
A fast formation of microaggregates was observed shortly after the application of biosolids, characterized by an association between pyrophosphate-extractable iron and plant-derived organic matter. Microbially-formed iron-coordinated organic complexes served as nuclei for macroaggregate formation, which drastically increased soil carbon content (~140 Mg C/ha). The highest accumulation of soil carbon was observed during the period of highest plant diversity (>30 species; years 3–6), declining significantly with the exclusion of native species by invasive grasses (years 9–14), which caused a steady decline in the concentration of available soil nutrients. In contrast with previous but unsuccessful attempts to establish plant communities in mined sites through nutrient application alone, iron-mediated stabilization of vegetation inputs favored the regeneration of a barren stable state that had persisted for over five decades. Coupled organic matter and iron “fertilization,” combined with management of invasive species, has the possibility to enhance terrestrial carbon sequestration and accelerate the restoration of degraded lands, while addressing important challenges associated with urban waste disposal.