Effects of Soil Oxidation-reduction Conditions on Internal Oxygen Transport, Root Aeration, and Growth of Wetland Plants

Characterization of hydric soils and the relationship between soil oxidation-reduction processes and wetland plant distribution are critical to the identification and delineation of wetlands and to our understanding of soil processes and plant functioning in wetland ecosystems. However, the information on the relationship between flood response of wetland plants and reducing soil conditions is limited. We have examined the influence of intensity and capacity of soil reduction on internal oxygen transport, rhizosphere oxygenation, nutrient uptake, root and shoot growth, and survival of several wetland species. Whereas the study species displayed a wide range of responses, intense soil reduction below -200 mV adversely affected growth and biomass accumulation in the majority of these species. It is clear that high oxygen demand in soil resulting from intense reduction influences oxygen transport and release to the rhizosphere. In addition, root elongation and shoot growth are profoundly influenced by the intensity and capacity of soil reduction. 1 Pezeshki, Professor of Biology, Department of Biology, University of Memphis, Memphis 38152, TN; DeLaune, Professor, Wetland Biogeochemistry Institute, Louisiana State University, Baton Rouge, LA 70803. Citation for proceedings: Holland, Marjorie M.; Warren, Melvin L.; Stanturf, John A., eds. 2002. Proceedings of a conference on sustainability of wetlands and water resources: how well can riverine wetlands continue to support society into the 21st century? Gen. Tech. Rep. SRS-50. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station. 191 p. INTRODUCTION Numerous reviews and book chapters on plant responses to soil waterlogging are available that provide detailed literature synthesis on this topic (Armstrong and others 1994; Drew 1990, 1997; Hook and Crawford 1978; Jackson and others 1991; Kozlowski 1984a, 1984b, 1997; Pezeshki 1994; Vartapetian and Jackson 1997). Despite this wealth of literature, close examinations reveal that in most studies the status of soil oxidation-reduction conditions has not been reported. Although some researchers have reported oxygen concentrations in the root medium, such measure in wetland soil does not provide adequate information to allow evaluation of the intensity of soil reduction (DeLaune and Pezeshki 1991). This point is important because in wetland systems most plants are well adapted to endure soil-oxygen deficiency but may differ in ability to withstand certain levels of intense soil-reducing conditions. In a typical flooded wetland soil, plants respond to the soil physicochemical changes. These responses may lead to a wide range of plant-stress symptoms. Although various plant responses to flooded soil conditions have been addressed in numerous publications, little information can be found on the relationship between wetland plant functions and the two aspects of soil redox potential—the intensity and the capacity of reduction. The reduction of the inorganic redox systems in a flooded soil may be characterized in intensity or capacity terms. The intensity factor determines the relative ease of reduction whereas the capacity factor refers to the amount of the redox system undergoing reduction (DeLaune and Pezeshki 1991). From a plant physiological-ecology standpoint, there are many uses of interpretations of redox processes in soils; one example is that the knowledge of the soil redox potential represents an indication of the oxidation-reduction status of various soil compounds. For example, a redox potential of zero mV indicates that oxygen and nitrate are not likely to be present and that the bioreducible iron and manganese compounds are in a reduced state. At this same potential, however, sulfate is stable in the soil with no sulfide being formed, which is toxic to plants. A redox potential of +400 mV indicates that oxygen may be present even though there may be excess water in the soil (DeLaune and Pezeshki 1991). Thus, the primary objective of the present paper is to summarize and assess evidences on the significance of the intensity and capacity of reduction in soils to wetland plant functioning. We will emphasize the relationships between soil flooding, reduced soil conditions (low soil redox potential, Eh), the components of the soil oxidation-reduction system, namely the intensity and the capacity of reduction, and their influence on internal oxygen transport, rhizosphere oxygenation, root and shoot growth, and survival of wetland species. Soil Oxidation-Reduction Potential (Eh) Soil flooding initiates a chain of reactions leading to reduced soil conditions (low soil redox potential, Eh). These reactions include physical, chemical, and biological processes that have significant implications for wetland plant functioning, survival, and productivity (see Gambrell and others 1991, Gambrell and Patrick 1978, Ponnamperuma 1984). Physical processes include restriction of soil-atmospheric gas exchange and depletion of soil oxygen needed for root respiration. Once flooded, the limited supply of oxygen in floodwater is depleted by roots, soil microorganisms, and soil reductants rapidly (Ponnamperuma 1972). The depletion of oxygen results in a series of chemical changes in soil including accumulation of CO2, methane, N2, and H2 (Ponnamperuma 1984). The processes that follow include denitrification, reduction of iron, manganese and sulfate, and