Unravelling the Role of Triclinic Birnessite in Soil Carbon and Phosphorus Biogeochemistry in Soils

The world's soils are a critical reservoir for carbon (C) and nutrients, vital for life as we know it. Their ability to store and cycle these elements hinges on complex interactions, often mediated by overlooked minerals such as birnessite (MnO2·nH2O). Birnessite is found in natural environments either as triclinic birnessite (TcBi) or hexagonal birnessite (HBi). However, compared to HBi, TcBi has received limited attention. Although present in natural systems, the role of TcBi in mediating soil organic carbon (SOC) and nutrient dynamics remains poorly understood, creating a critical knowledge gap in the involvement of birnessite in soil C and nutrient interactions. Triclinic birnessite and HBi are also interconvertible under certain conditions, hence the critical importance of addressing this knowledge gap. Therefore, this thesis investigated the role of TcBi in mediating SOC transformation and stabilization, and its impact on P transformation and speciation in natural systems. This study first investigated the role of TcBi in the sorption and molecular transformation of vermicompost-derived dissolved organic carbon (DOC) under soil-relevant pH and temperature conditions typical of temperate and semi-arid soils. DOC adsorption increased at pH 4, 50 °C, reaching approximately 2.5 times the level at 25 °C, but declined significantly at pH 8, 50 °C. Spectroscopic evidence revealed TcBi-mediated increases in DOC aromaticity, with the highest levels at pH 4, 50 °C. O-alkyl C was detected only in sorbed fractions at pH 4, indicating that TcBi promoted esterification or etherification under acidic conditions. Manganese K-edge Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy showed that DOC-TcBi interactions led to the emergence of new mineral phases, including HBi, manganite, and ramsdellite. The formation of HBi was pH-dependent, being favoured under acidic conditions. Manganite was favoured at 50 °C, and ramsdellite emerged only at pH 8, 25 °C. Triclinic birnessite showed greater stability under alkaline conditions and at 25 °C. These findings offer key insights into the role of TcBi in organic C interactions, retention, and associated mineral transformations under environmentally relevant conditions. The second and third investigations within this study employed Mn K-edge X-ray Absorption Near Edge Structure (XANES) and EXAFS spectroscopy to assess the speciation and transformation of TcBi over a 35-day reaction period in DOC and soil systems. The DOC comprised reaction systems with microbial growth suppression (+MS) and without microbial growth suppression (-MS). In both systems, birnessite formation was favoured at pH 4, but declined significantly in the -MS system by day 35 of the reaction, and to a lesser degree in the +MS system. While the complete loss of TcBi occasionally occurred under various conditions, it was more stable under alkaline conditions in the +MS system. The main transformation products across reaction systems include HBi, lithiophorite, Mn(III) oxy(hydr)oxides, and Mn(III) phosphate. Lithiophorite was favoured at pH 4, suggesting a likely pathway involving HBi, while small amounts of Mn(III) oxy(hydr)oxides were detected across pH conditions. The study also showed a likely microbial involvement in forming Mn(III) phosphate, as it was only observed in the -MS system. The stability and transformation of TcBi was assessed under alkaline soil conditions, where TcBi exhibits greater stability. These assessments focused on its behaviour within particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) fractions. The study showed that TcBi was more stable at a higher Mn:C ratio, with the minimal formation of other mineral phases, while the formation of HBi was favoured at lower Mn:C ratio. Triclinic birnessite showed a decline of ~31% in both soil fractions, resulting in the formation of other mineral phases, such as manganite, lithiophorite, and Mn(III) phosphate, particularly in the MAOC fraction. Notably, manganite was observed in both soil fractions, while bixbyite and lithiophorite were only observed in the POC and MAOC fractions, respectively. These findings offer key insights into the biogeochemical stability of TcBi driven by organo-mineral interactions and provide broader implications for SOC interactions. The fourth study reports the role of TcBi in mediating molecular transformations of DOC and SOC over 35 and 90 days, respectively. In the DOC system, biotic and abiotic contributions were assigned to -MS and +MS reaction systems, respectively. The study showed higher organic C accumulation in the MAOC fractions and higher chemical lability. Based on C 1s Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy and Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS), the study showed TcBi mediated phenol C and quinone C redox cycling, with the reverse generation of phenol from quinone possible in -MS systems, a mechanism attributed to microbial conversion. The FT-ICR-MS results showed that TcBi enhanced the aromaticity indices of reacted DOC, especially under acidic conditions, accompanied by the depletion of labile carbon compounds and increased polyphenolic constituents. Carbon 1 s NEXAFS revealed a decline in aromatic C within POC and MAOC between 35 and 90 days. Despite higher microbial abundance in TcBi-treated soils, CO2 emission was 82-86% less than recorded in unreacted soils over the 90-day incubation period. These findings show that TcBi can mediate the molecular transformations crucial to the stabilisation of SOC whilst offering new insights into microbial involvement in the soil C continuum. Organo-mineral interactions impact phosphorus (P) cycling and speciation in soils and natural systems, resulting in either the fixation of P into stable mineral forms or the oxidative transformation of organic P compounds. A fundamental knowledge gap exists regarding the role of TcBi in P transformation and speciation in natural environments. To address this, the fifth study investigated TcBi-induced P transformations and speciation in dissolved and soil organic matter. Phosphorus K-edge X-ray Absorption Near Edge Structure (XANES) spectroscopy revealed the formation of Mn(II/III) phosphate and preferential phosphate adsorption under acidic conditions, whereas calcium phosphate dominated under alkaline pH. Under slightly alkaline soil conditions, birnessite-adsorbed P was the primary form, with calcium phosphate contributing less than 8.5% despite high dissolved Ca²⁺ concentrations. Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS) further showed that TcBi facilitated the oxidative transformation of dissolved organic P into higher molecular weight, more aromatic compounds. These findings demonstrate that TcBi promotes the oxidative stabilization of organic P and may lower P fixation in Mn-rich soil environments. This research reveals the dynamic role of TcBi in redox-mediated organo-mineral interactions, providing critical insights into C and P cycling in soils, which has far-reaching implications for soil health and environmental management.<p></p>