UC Merced

Pedogenic iron (Fe) oxides are important soil minerals that contribute to carbon (C) storage and stability in weathering soils around the world. Soil organic matter (SOM) associated with pedogenic Fe can persist in the soil systems for varying amount of time depending on Fe oxide crystallinity and concentration, chemical composition of SOM, and soil aggregation status. However, questions remain as to can the relationship between pedogenic Fe oxides and soil C storage be generalized in global scale, or how Fe oxide distribution in the soil profile responds to long-term processes of soil development and/or short-term exposure to changes in environmental conditions. Therefore, in this dissertation I addressed four research questions regarding the role of Fe oxides on soil carbon storage, distribution, and composition. First, I synthesized data from 15 representative studies conducted in soil systems with different weathering intensity to reveal the general relationships between pedogenic Fe oxides and soil development, and implications for soil C storage in variety of environments globally. Second, I evaluated the associations of Fe in different extractable pools of soil with C in distinct aggregate size classes along depth profiles in a landscape scale to evaluate the crystallinity of Fe oxides on distribution of SOM within soil aggregates. Third, I further determined the relationship of pedogenic Fe oxide crystallinity and chemical composition of C that is physically protected in soil aggregates. Finally, I evaluated the effects of Fe oxides concentrations on chemical vs. physical stabilization of SOM using sorption and desorption experiments with and without ultrasonication to determine the capacity of Fe oxides to selectively stabilize different organic C functional groups. My results show that Fe oxide concentrations and its crystallinity reflect soil weathering intensity and demonstrates a close relationship with soil C storage under a variety of soil types, including weakly weathered soils. At the landscape scale, I also found that pedogenic Fe oxides and amount of physically occluded SOM were closely related, where Fe oxides with varying crystallinity was distributed differently along soil aggregate size range. Preferential retention of different organic functional groups in pedogenic Fe oxides containing fraction was also observed. My results contribute to the growing body of literature that is highlighting the role of Fe oxides on soil C storage and stabilization in systems at varying stages of soil development (weathering environments) globally.