Organo-Mineral Interactions from Field to Molecular Scale

Soil organic carbon (SOC) represents the largest carbon (C) stock on the Earth’s surface and plays a pivotal role in regulating the global C cycle. A clear understanding of the mechanisms controlling the persistence of C in soil organic matter (SOM) across different ecosystems has never been more needed. This thesis is focused on the chemical reactions regulating organo-mineral interactions (sorption, precipitation, aggregation), aiming to understand the effects of different environmental conditions and land uses on fundamental ecological mechanisms that control the fate of essential elements, metals, and, most importantly, the cycling and storage of C. The investigated ecosystems included a variety of natural soils (broadleaved and coniferous forest soils, grassland soils), technosols and agricultural soils amended with biochar and organic fertilizers. Using elemental and thermal analyses, changes in the quantity and quality of physically-fractionated SOM pools characterized by different mechanisms of protection from decomposition have been examined. A common mechanism dominates SOM dynamics; microbial by-products, largely independent of ecosystem type, govern SOM accumulation in the mineral-associated organic matter. In a second step, organo-mineral interactions in soils under different land uses, widely differing in their SOM content, were studied by a double approach: (i) a size fractionation by wet sieving after sonication, to isolate the particulate organic matter (POM, >20 µm) from the organo-mineral complex (OMC, <20 µm), and (ii) a characterization of the OMC by sequential extractions with different chemicals, each one disrupting a specific kind of bond between SOM and active mineral surfaces. The main pool of the OMC occurred by far in the final residue remaining after removing all the active mineral components causing insolubility, suggesting that the biochemical evolution from plant-derived material towards insoluble microbial forms is a relevant path for SOM stabilization. Iron (Fe) (hydr)oxide minerals have been suggested as an important phase for the stabilization of SOC. Using state-of the-art synchrotron-based techniques, numerous studies focused on model metal (hydr)oxide systems (pure Fe (hydr)oxides) and Fe(III) complexation with different types of organic matter, including dissolved organic matter (DOM), peats, humic substances, small organic acids. Fe speciation in soils is highly dependent on environmental conditions and chemical interactions with SOM. However, it is less clear how stabilization mechanisms in natural systems (rather than model systems) protect SOM from decomposition under different land uses. Thus, chemical interactions between soil Fe species and physically-fractionated SOM pools have been investigated. By sorption/desorption experiments in which Fe was added to the system, the mechanisms controlling Fe(III)-mediated OC stabilization illustrated that carbohydrates are associated to Fe(III) oxides by surface complexation, possibly by an inner sphere ligand exchange mechanism. The irreversible nature of this complexation was confirmed by the reaction with Fe(III), where increasing added Fe(III) correlated to a decrease in organic C desorbed. These results demonstrated that the binding of labile SOM compounds to Fe(III) contributes to its preservation, and that the mechanisms involved (flocculation vs. coating) depend on the size fractions. Moreover, the products of these batch experiments were analyzed by Fe K-edge extended X-ray absorption fine structure (EXAFS). This approach helps reveal the mechanisms by which SOM pools can control Fe(III) speciation and to elucidate how both Fe(III)-OM complexes and Fe(III) polymerization can affect SOM reactivity. In order to provide additional information about the environmental factors that control transformation rates and products, Fe EXAFS has been applied to the fine silt/clay and fine sand fractions obtained from a variety of natural soils and agricultural soils amended with biochar alone and combined with organic fertilizers. The role of SOM amount and quality on the hindrance of Fe hydrolysis, crystallization and transformation processes has been unraveled. Considered the research gap between model and natural systems in understanding the possible SOM stabilization mechanisms as a function of ecosystems, physical fractionation methods coupled with synchrotron-based characterization provide unique information relevant to regulation of global terrestrial SOC cycling.

Una chiara comprensione dei meccanismi che controllano la persistenza del carbonio (C) nella sostanza organica del suolo (SOM) in diversi ecosistemi non è mai stata più necessaria, a causa delle crescenti preoccupazioni sui cambiamenti climatici. Il principale obiettivo di questa tesi è di studiare l’impatto dell'uso del suolo sulla dinamica della SOM e sul funzionamento degli ecosistemi. È stata studiata la distribuzione SOM tra le frazioni del suolo caratterizzate da diversi meccanismi di protezione in una varietà di suoli in diversi ecosistemi. Gli ecosistemi presi in considerazione includono una varietà di suoli naturali (suoli di latifoglie e conifere, suoli di prati), tecnosuoli e suoli agricoli ammendati con biochar e fertilizzanti organici. La caratterizzazione termogravimetrica delle frazioni della SOM ha rivelato un meccanismo comune nelle dinamiche di trasformazione della SOM, in cui i sottoprodotti microbici, largamente indipendenti dal tipo di ecosistema, regolano l'accumulo di SOM sotto forma di frazione di sostanza organica associata ai minerali. Inoltre, estrazioni sequenziali hanno rivelato che il pool principale dei complessi organo-minerali è rappresentato dal residuo finale, suggerendo che l'evoluzione biochimica dei residui vegetali verso forme microbiche insolubili è un percorso rilevante per la stabilizzazione della SOM. Per identificare le interazioni chimiche tra le specie di ferro (Fe) ed i pools di SOM ottenuti mediante frazionamento fisico, due frazioni della SOM sono state estratte da tre suoli con diversa destinazione d’uso e testati in esperimenti di adsorbimento / desorbimento in cui Fe(III) è stato aggiunto a ciascun sistema. I meccanismi di adsorbimento e stabilizzazione del C organico promossi dal Fe(III) hanno dimostrato che i carboidrati sono associati agli ossidi di Fe(III). La spettroscopia Fe K-edge extended X-ray absorption fine structure (EXAFS) è stata applicata alle frazioni complessate al Fe(III) ed alle frazioni di limo/argilla e sabbia fine ottenute da un suolo agricolo ammendato con biochar, da solo o in combinazione con fertilizzanti organici, svelando il ruolo della quantità e della qualità di SOM sull'inibizione dei processi di idrolisi, cristallizzazione e trasformazione del Fe. I metodi di frazionamento fisico, associati all’utilizzo di sofisticate tecniche spettroscopiche, forniscono informazioni uniche rilevanti per la regolazione del ciclo del C organico terrestre globale.