Saturday, 15 July 2006

Effects of Organic Matter on New Redoximorphic Feature Formation Under Induced Hydric Soil Conditions.

Adam Gray and Martin C. Rabenhorst. Univ of Maryland, Dept. NRSL, 1112 HJ Patterson Hall, College Park, MD 20742

In soils with impeded internal drainage, organic matter provides a critical energy source for soil microorganisms that is necessary for the reduction of iron during the development of redoximorphic (hydromorphic) features. Organic matter properties such as lignin content and C:N ratio influence the microbial degradation processes, and as a result, can affect the nature of redox features formed in wet soils. In this study, the effects of fresh organic matter additions on soil redox feature formation was evaluated in undisturbed well-drained soil cores (mesocosms) using a time series experiment. Five different types of organic materials, (oak leaves - Quercus rubra; maple leaves - Acer rubrum; timothy hay -Phleum pratense; wheat straw - Triticum; and sawdust - mixed hardwoods) were evaluated by uniformly mixing each of the five organic matter types within the upper 10 cm of soil using a rate of 1.7kg C/m2 (approximately 60 grams per 15 cm diameter mesocosm). In addition, two of the organic materials (oak leaves and wheat straw) were added to separate mesocosms to further evaluate the effect of the mixing method. These two materials were added to separate mesocosms either by placing the material on the soil surface or by concentrating the organic material into deeper zones within the soils Mesocosms were subjected to simulated wetland hydrology consisting of five wet-dry cycles. Each cycle consisted of an eight week period of saturation when the water table was at or near the soil surface followed by an eight week drained period when the soil was permitted to dry out. At the completion of each 16-week cycle, a block of mesocosms was dissected and all newly formed redox features were carefully described (quantity, size, color). Soil redox potentials (Eh), pH, and soil temperature were monitored throughout the experiment. The five organic materials had C:N ratios of 44:1 (timothy hay), 49:1 (maple leaves), 68:1 (oak leaves), 82:1 (wheat straw), and 189:1 (sawdust). Mesocosms had pH values that mainly ranged between 5.8 and 6.5. Plotted values of Eh and pH showed that periods of reduction and oxidation corresponded well with wet and dry periods. Using the technical standard of the National Technical Committee on Hydric Soils to define oxidizing and reducing conditions (similar to rH of the World Resource Base for Soil Resources) redox potentials rise into the oxidizing zone during the dry period and fall into the reducing zone during the saturated period. Mesocosms have developed noteworthy hydromorphic features. The most prominent features are iron concentrations that have formed in macropores that were created either by redox electrodes or pH sampling. Most other features are very fine and faint. Trends show that the iron oxide features are increasing with time. Redoximorphic features were least evident in cores that had been mixed and were most evident in cores that had organic materials concentrated in deeper zones. Use of different types of organic materials did not appear to significantly affect size, color or abundance of redoximorphic features observed. A comprehensive data set and statistical analysis will be presented.

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