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Wetland Soils

Rhizosphere Iron (III) Deposition and Reduction in a Juncus effusus L.-Dominated Wetland

^a George Mason Univ., Fairfax, VA 22030, Present address: U.S. Geological Survey, Reston, VA 20142
^b American Type Culture Collection, Manassas, VA 20110
^c Smithsonian Environmental Research Center, Edgewater, MD 21037-0028

^* Corresponding author (jweiss{at}usgs.gov)

Iron (III) plaque forms on the roots of wetland plants from the reaction of Fe(II) with O₂ released by roots. Recent laboratory studies have shown that Fe plaque is more rapidly reduced than non-rhizosphere Fe(III) oxides. The goals of the current study were to determine in situ rates of: (i) Fe(III) reduction of root plaque and soil Fe(III) oxides, (ii) root Fe(III) deposition, and (iii) root and soil organic matter decomposition. Iron (III) reduction was investigated using a novel buried-bag technique in which roots and soil were buried in heat-sealed membrane bags (Versapor 200 membrane, pore size = 0.2 µm) in late fall following plant senescence. Bags were retrieved at monthly intervals for 1 yr to assess changes in total C and Fe mass, Fe mineralogy, Fe(II)/Fe(III) ratio, and the abundances of Fe(II)-oxidizing bacteria (FeOB) and Fe(III)-reducing bacteria (FeRB). The soil C and Fe pools did not change significantly throughout the year, but root C and total root Fe mass decreased by 40 and 70%, respectively. When total Fe losses were adjusted for changes in the ratio of Fe(II)/Fe(III), over 95% of the Fe(III) in the plaque was reduced during the 12-mo study, at a peak rate of 0.6 mg Fe(III) g dry weight^–1 d^–1 (gdw^–1 d^–1). These estimates exceed the crude estimate of Fe(III) accumulation [0.3 mg Fe(III) g dry weight^–1 d^–1] on bare-root plants that were transplanted into the wetland for a growing season. We concluded that root plaque has the potential to be reduced as rapidly as it is deposited under field conditions.

Abbreviations: BPDS, bathophenanthroline disulfonate • diH₂O, deionized water • DO, dissolved oxygen • FeOB, Fe(II)-oxidizing bacteria • FeRB, Fe(III)-reducing bacteria • MPN, most probable number