Biogeochemistry of Fe and Mn in a
Seasonally Reduced Riparian Soil

John Baham¹, Rio Roland¹, Stephen M. Griffith², and Georg Grathoff³

^Abstract

INTRODUCTION

Agricultural buffer strips are increasingly employed as a "best management practice" to improve the quality of water draining from agricultural lands. The seasonal reduction of Fe- and Mn-oxides in poorly drained riparian buffer strips is an important process in Willamette Valley agricultural soils. These biogeochemical processes are linked to:

Biotic and Abiotic Denitrification of Excess NO₃
Control of P Solubility/Mobility
Reductive Dechlorination via Reduced Fe-Mineral Phases
Flux of Fe from the Reduced Soil Ecosystem
Mineralization and Global C Cycle

Little is known regarding the temporal and spatial nature of Fe and Mn dynamics despite the importance of these terrestrial ecosystems to water quality.

OBJECTIVES

Evaluate the temporal and spatial biogeochemistry of Fe and Mn in the following soil pools: 1) FINE-GRAIN, 2) NODULE, and 3) SOIL SOLUTION.

Evaluate the formation of known minerals using soil solution data collected from diffusion samplers.

1) DYNAMICS OF THE FINE-GRAIN Fe POOL

Fe(II) production is derived from microbially driven reductive dissolution of silt and clay sized oxides and correlated in time w.r.t. the soil redox potential.
Annual reductive potential is 320 mol e m^-2y^-1, based on a steady state biomass production rate of 1kg C m^-2y^-1.
Multiple cycles of Fe reduction and oxidation may occur during the flooded period, therefore a conservative lower estimate for electrons consumed during reductive dissolution is approximately 20 mol e m^-2y^-1 (6% of the annual ecosystem electron flow).

riparian.jpg (145320 bytes)

Fe(II) production = Fe²⁺_(aq) + Fe(II)_ex + Fe(II)-minerals, where Fe²⁺_(aq)
is less than 10% with respect to the other two forms.

2) NODULE POOL

Genesis of Fe/Mn soil nodules is related to seasonal redox cycles.
Framboidial morphology suggests "active" modes of formation, while rounded forms may be due to a smoothing of the nodule by clay pressure faces.
Nodules are enriched in redox active species relative to the fine-grain oxide pool.
Greatest nodule concentration is found at lower boundary (45 cm) of a perched water table.
Nodules, as a result of their low surface area, serve as a sink for Fe and Mn under current pedogenic conditions.

3) SOIL SOLUTION CHEMISTRY

Soil solution was collected via membrane pore water diffusion chamber samplers (PEEPERS, 0.2um) equipped with Pt electrodes.
Reduction is greatest in the upper 20 cm of the soil, as evidenced by the soluble concentrations/activities of Fe(II), Mn, SO₄, and e.
Soil solutions are both under- and over-saturated with respect to a suite of probable Fe containing minerals.

wpe13.jpg (34278 bytes) wpe14.jpg (126726 bytes)

CONCLUSIONS

A significant fraction (10-20%) of the Fe and Mn in the fine grain mineral oxide pool experiences reductive dissolution and reoxidation on an annual basis.

Dynamic changes in the mineralogy may include the formation of Siderite and a "green rust" during the reduced phase of the annual cycle, followed by the oxidation to Lepidocrocite and/or Ferrihydrite under oxic conditions.

¹Crop and Soil Science, 3017 ALS Bldg. Oregon State University, Corvallis, OR 97331-7306 United States
John.Baham@orst.edu
²USDA-ARS, 3450 SW Campus Way, Corvallis, OR 97331-7102 United States
³Geology, Portland State University P.O. Box 751, Portland, OR 97207-0751 United States

Biogeochemistry of Fe and Mn in a Seasonally Reduced Riparian Soil

Abstract