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

Abstract

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.