ABSTRACT
Hydric soils are described in Chapter 2 as soils that form under anaerobic conditions that develop while the soils are inundated or saturated near their surface. These soils can form under a variety of hydrologic regimes that include nearly continuous saturation (swamps and marshes), short duration flooding (riparian systems), and periodic saturation by
CONTENTS
Introduction ................................................................................................................................. 105 Oxidation and Reduction Basics ............................................................................................... 106
Alternate Electron Acceptors ................................................................................................ 107 Principal Reducing Reactions in Hydric Soils ................................................................... 108 Factors Leading to Reduction in Soils ................................................................................. 109
Quantifying Redox Reactions in Soils ...................................................................................... 110 Thermodynamic Principles ................................................................................................... 110 Eh/pH Phase Diagrams ........................................................................................................ 111 Reliability of Phase Diagrams for Field Use ....................................................................... 112
Mixed Redox Couples ....................................................................................................... 112 Reaction Kinetics ............................................................................................................... 113
The Concept of pe ................................................................................................................... 113 Measuring Reduction in Soils ................................................................................................... 114
Chemical Analyses ................................................................................................................. 114 Dyes .......................................................................................................................................... 114 IRIS Tubes ................................................................................................................................ 115 Redox Potential Measurements ............................................................................................ 115
Correcting Field Voltages to the Standard Hydrogen Electrode ................................ 116 Temporal Variability in Redox Potential ........................................................................ 117 Spatial Variability in Redox Processes ............................................................................ 119 Interpreting Redox Potential Changes in Nature ......................................................... 120
pH Changes in Reduced Soils .............................................................................................. 120 Elemental Redox Cycles, Transformations, and Plant Nutrient Pools ................................ 120
Carbon ...................................................................................................................................... 121 Nitrogen ................................................................................................................................... 123 Phosphorus .............................................................................................................................. 125 Sulfur ........................................................................................................................................ 128
Summary ...................................................................................................................................... 129 References .................................................................................................................................... 130
groundwater. The most significant effect of excess water is the isolation of the soil from the atmosphere and the slowing of O2 from entering the soil. The blockage of atmospheric O2 induces biological and chemical processes that change the soil from an aerobic and oxidized state to an anaerobic and reduced state. This shift in the aeration status of the soil allows chemical reactions to occur that develop the common characteristics of hydric soils such as the accumulation of organic carbon in A horizons, gray-colored subsoil horizons, and production of gases such as H2S and CH4. The creation of anaerobic conditions requires adaptations in plants if they are to survive in the anaerobic hydric soils. In addition, redox reactions in wetland soils help regulate environmental quality, impacting release and sequestration of greenhouse gases, nutrient pollution of surface water, and mobilization of potentially toxic trace elements to groundwater.
