Timescales for perturbing the ecosystem function of soil iron

Aaron Thompson1
1 University of Georgia

Invited Talk 11 in Weathering and chemical processes as keys to ecosystem functioning

15.07.2014, 10:00-10:30, H20

The biogeochemical cycles of the major plant nutrients (nitrogen, phosphorus and potassium) are tightly linked to ecosystem function at short timescales, perhaps days, weeks or seasons. At only slightly longer timescales, ecosystem function also hinges on the framework elements in soils (Si, Al, Fe, and Si), which are predominately present as minerals. Of these elements, Fe has the greatest potential for a shift in ecosystem function because it behaves very differently under anoxic and oxic conditions. Soil iron serves three broad categorical roles in terrestrial ecosystem function: (1) a structural role as part of the soil matrix; (2) a sorbent role for the retention of nutrients, carbon and trace elements; and (3) a role in electron-transfer reactions. The dynamics of iron cycling are defined by the timescales required to shift the amount and/or composition of iron participating in these roles. Our recent work on soils from the Luquillo Critical Zone Observatory in Puerto Rico, USA illustrate how shifts in redox conditions alter the functional composition of iron minerals on week to year timescales. First we show that Fe-reduction rates in these soils are increased dramatically—from ~ 3 mmol FeII kg-1 soil d-1 to > 45 mmol FeII kg-1 soil d-1—when high amplitude (0 - 21% O2) redox oscillations are imposed with a ratio of time under oxic to anoxic conditions of 1:6. This increase was accompanied by a concomitant increase in the FeII concentration plateaus from 10 to a maximum of 180 mmol FeII kg-1 soil, which is similar to the amount of Fe extractable by citrate-ascorbate—a chemical extraction known to target short-range-ordered (SRO) Fe phases.  In addition, we probed the susceptibility of soil Fe phases toward FeII-facilitated atom exchange using 57Fe2+(aq). In 28 days, the aqueous and soil iron pools both moved ~7% toward the isotopic equilibrium. After 28-days, Mössbauer spectroscopy suggests the incorporated 57Fe label re-crystallized as short-range-ordered (SRO) FeIII-oxyhydroxides. We project the entire SRO Fe pool could be completely exchanged within a year of continuous exposure to Fe2+(aq). While, these soils do not remain continuously anoxic for a full year, even intermittent anoxia could result in turnover of the SRO Fe pool within a decade via atom exchange. Thus, the timescales for alteration of soil iron composition are strongly dependent on the degree of redox variability; in highly redox dynamic soils, iron may be as tightly coupled (temporally) to ecosystem function as the major plant nutrients.

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Letzte Änderung 08.04.2014