Iron-Carbon Associations In Tropical Soils Of The Luquillo Critical Zone

Much of the uncertainty in the biogeochemical behavior of soil carbon (C) in humid tropical ecosystems derives from an incomplete understanding of soil C stabilization processes. Long-term soil C stability is traditionally attributed to organomineral interactions, however, the 2:1 phyllosilicate clays often associated with temperate organomineral complexation are largely absent in humid tropical soils due to extensive weathering. In contrast, these soils contain a spectrum of iron- and aluminum-bearing minerals, exhibiting a broad range of crystallinity, surface area and surface charge, and susceptible to frequent reduction-oxidation (redox) oscillations. This dissertation investigates the composition, distribution, and function of iron-mediated organomineral associations across a range of spatial scales within the Luquillo Critical Zone Observatory (LCZO). Underlain by contrasting lithologies, the LCZO is characterized by highly-weathered, volcaniclastic Oxisols or quartz diorite-derived Inceptisols, producing an experimental gradient of iron content and speciation. To characterize the interactions between inherently heterogeneous soil C and often amorphous mineralogy, this dissertation paired high-resolution analytical techniques and inorganic selective dissolution experiments. We found low-crystallinity, short-range-order (SRO) iron and crystalline iron phases exert control on distinct reservoirs of soil C across both soil types. Notably, organomineral associations were responsible for accumulation of a subset of soil C, rather than driving trends in total soil C. Examination of solid-phase speciation across soil types revealed evidence for unique mineral matrix architecture in each soil. SRO FeIII-oxhydroxide phases in Oxisol soils were also found to be resistant to laboratory reduction events, suggesting that these phases are immune to redox-induced dissolution and may provide a long-term C stabilization mechanism. Investigation of iron-associated C at the molecular scale revealed preferential complexation of distinct C compounds has occurred at mineral interfaces of varying crystallinity and reactivity, suggesting that the array of association mechanisms described may be fractionating soil C. This work demonstrates that iron-mediated organomineral association serves as a reactive filter for soil C across spatial and temporal scales, which may impact both the quantity and identity of C cycling through the critical zone.