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

摘要

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.