Abstract

Ferric Iron (Fe) minerals that lack long-range crystal order or are highly substituted with ions or organic matter are critically important for a range of sorption and electron transfer reactions in soils. These short-range-ordered (SRO) Fe mineral phases are extremely dynamic in terms of reactivity, structure and function in soils. In fact, these particles are more susceptible to alteration of their mineral structure than any other solid phase constituents commonly present in environmental systems. The dynamics of iron cycling are critically dependent on changes in the oxygen content or redox state of the soil, which in turn depends largely on the availability of labile, reduced carbon (organic matter). This talk will review some of our recent findings from laboratory and field experiments from a variety of settings, including the Luquillo and Calhoun Critical Zone Observatories, long-term rice paddy experiments, and model mineral-microbe systems. We find that the SRO Fe phases are extremely plastic in terms of atomic structure, undergoing rapid (hrs to days) atom exchange with aqueous Fe2+(aq) when present under anoxic conditions, but can exhibit relative high-stability in terms of mineral composition when co-precipitated with stabilizing ions or organic matter as long of physico-chemical conditions are maintained. Exposure to dynamic redox fluctuations does not appear to confer a singular influence on iron mineral composition (i.e., it does not by itself promote either increases or decreases in iron crystallinity), but rather increases the likelihood for changes in iron mineral structure that are determined by biogeochemical and physico-chemical conditions of system, such as precipitation/dissolution rates, the presence of foreign ions or organic compounds that can sorb or co-precipitate with the iron phases, or microbial community composition/activity. Understanding of these interactions is a critical aspect necessary to predict the fate of a range of elements in soils, in particular under conditions of changing external forcings (i.e., climate).

Citation

Thompson, Aaron (2018): Short-range-ordered (SRO) iron minerals are dynamic, emergent representations of recent environmental conditions in soils . American Geophysical Union 2018 Fall Meeting, Washington, DC, 10-14 Dec 2018.

This Paper/Book acknowledges NSF CZO grant support.