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

soil and rates of radial oxygen loss. Radial O2 loss is in turn influenced by plant activity (Bedford et al., 1991; Iron (III) plaque forms on the roots of wetland plants from the Kuehn and Suberkropp, 1998) and morphological charreaction of Fe(II) with O2 released by roots. Recent laboratory studies acteristics such as suberized and lignified roots (Armhave shown that Fe plaque is more rapidly reduced than non-rhizostrong et al., 2000). Under anaerobic conditions, the sphere 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) amorphous Fe(III) oxides in root plaque serve as an exoxides, (ii) root Fe(III) deposition, and (iii) root and soil organic cellent substrate for FeRB that mediate Fe(III) reduction matter decomposition. Iron (III) reduction was investigated using a in freshwater environments (Lovley, 2000). Iron(III) novel buried-bag technique in which roots and soil were buried in reducing bacteria have been successfully enriched from heat-sealed membrane bags (Versapor 200 membrane, pore size the rhizosphere (King and Garey, 1999) where they 0.2 m) in late fall following plant senescence. Bags were retrieved can account for up to 12% of all bacterial cells (Weiss at monthly intervals for 1 yr to assess changes in total C and Fe mass, et al., 2003). Fe mineralogy, Fe(II)/Fe(III) ratio, and the abundances of Fe(II)The juxtaposition of oxic and anoxic conditions in the oxidizing bacteria (FeOB) and Fe(III)-reducing bacteria (FeRB). The rhizosphere, separated either temporally or spatially, soil C and Fe pools did not change significantly throughout the year, results in a rhizosphere Fe cycle in which Fe plaque is 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 alternately deposited and then reduced. Previous studies Fe(II)/Fe(III), over 95% of the Fe(III) in the plaque was reduced have reported a higher Fe(III) reduction potential in during the 12-mo study, at a peak rate of 0.6 mg Fe(III) g dry weight 1 the rhizosphere Fe(III) pool than the non-rhizosphere d 1 (gdw 1 d 1). These estimates exceed the crude estimate of Fe(III) soil pool. For example, we recently reported that 75% accumulation [0.3 mg Fe(III) g dry weight 1 d 1] on bare-root plants of the Fe plaque is reduced in 10 d vs. 40% of the soil that were transplanted into the wetland for a growing season. We Fe(III) oxide pool (Weiss et al., 2004). Results from anconcluded that root plaque has the potential to be reduced as rapidly other short-term (7 d) experiment examining rates of as it is deposited under field conditions. Fe(III) reduction in salt marshes support the idea of more rapid Fe(III) reduction in rhizosphere than nonrhizophere soils (Gribsholt et al., 2003). However, both I deposits known as Fe plaque are comof these studies presented relatively short-term rates monly observed on the roots of wetland and aquatic determined in a laboratory environment. To our knowlplants (Mendelssohn et al., 1995). Iron plaque forms when edge, the kinetics of Fe(III) reduction have not been preO2 leaking from plant roots reacts with Fe(II) produced viously investigated in situ over periods of time 1 wk. in surrounding anaerobic soils (Armstrong, 1964). As a Furthermore, although researchers have reported obresult, the presence of vegetation dramatically alters subservations suggesting rapid rates of Fe(II) oxidation in surface biogeochemistry by concentrating large amounts the rhizosphere (Roden and Wetzel, 1996), no data set of solid-phase Fe(III) in the rhizosphere (Kostka et al., exists for in situ rates of Fe plaque accumulation. 2002; Ratering and Schnell, 2000). This Fe(III) pool is The goals of the current study were to quantify: (i) dominated by amorphous Fe(III) oxides (Taylor et al., Fe(III) reduction rates in the rhizosphere and non-rhi1984; Batty et al., 2000; Weiss et al., 2004), formed either zosphere soil following fall senescence, (ii) net Fe plaque by abiotic oxidation or FeOB (Weiss et al., 2003). In deposition on selected wetland plants during a growing laboratory studies, FeOB isolated from the wetland rhiseason, and (iii) decomposition rates of roots and soil zosphere were found to mediate between 18 and 90% organic matter. Based on our previous findings, we hyof Fe(II) oxidation (Neubauer et al., 2002; Sobolev and pothesized that the rhizosphere Fe pool would be more Roden, 2001), suggesting that they could play an impordynamic than the non-rhizosphere Fe pool, resulting in tant role in the formation of Fe plaque. significant amounts of plaque deposition and reduction The rhizosphere can vary between oxic and anoxic over an annual cycle. depending on microbial or chemical O2 demand in the MATERIALS AND METHODS J.V. Weiss, George Mason Univ., Fairfax, VA 22030; Present address: For the purposes of this study the term “rhizosphere” is U.S. Geological Survey, Reston, VA 20142; D. Emerson, American defined as the root surface and its associated Fe plaque. This Type Culture Collection, Manassas, VA 20110; J.P. Megonigal, Smithoperational definition is conservative because it is likely that sonian Environmental Research Center, Edgewater, MD 21037-0028. radial O2 loss and Fe(II) oxidation penetrate to some extent Received 3 Jan. 2005. *Corresponding author ([email protected]). beyond the boundary of the Fe plaque. The advantage of this Published in Soil Sci. Soc. Am. J. 69:1861–1870 (2005).