The biogeochemical implications of iron transformations during soil development in Hawaii

Abstract:

Soil iron (Fe) serves three broad categorical functions in terrestrial ecosystems: (Role 1) a structural component of the soil matrix and as a cementing bridge in soil aggregates; (Role 2) a sorbent retaining P, C and trace elements; and (Role 3) a host to electron-transfer reactions including acting as a terminal electron acceptor during anaerobic soil respiration. The dynamics of iron cycling are defined by the timescales required to shift the amount and/or composition of iron participating in these roles. The degree of crystallinity of Fe phases has great significance because it is typically inversely related surface area and by extension adsorptive (Role 2) and electron transfer (Role 3) reactivity. As soils age, Fe phases with higher crystallinity predominate consistent with their overall higher thermodynamic stability. Thus, soil aging leads to a progressive loss of C and P storage capacity. Changes in crystallinity are driven by dissolution/re-precipitation reactions associated with repeated wet/dry or oxic/anoxic cycles, but the process is non-linear. In drier climates, these cycles promote crystalline Fe phases; whereas in more humid climates they can be protected. However, excessive moisture leads to anoxia and fundamental changes in the microbial metabolisms that drive soil respiration. Anoxia leads to redox changes that produce their own pulsing effects on Fe mineral evolution via dissolution/re-precipitation cycles (Role 3), which in turn promote soil structural changes (Role 1) and changes in sorption capacity (Role 2). For instance, we find they can mobilize organic-rich colloids and contribute to the formation of deep C reservoirs. Furthermore our empirical observations suggest the same reactions influence P mobilization or re-partitioning from mineral to organic adsorbents. In this presentation we present evidence of Fe transformations and their implications for terrestrial ecosystems by considering a series of climo- and chrono-sequences in Hawaii.