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Title: Short-Term Reducing Conditions Decreases Soil Aggregation

Author
item DE-CAMPOS, A. - UFG, BRASIL
item Iliasson, Amrax
item Huang, Chi Hua

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/31/2008
Publication Date: 2/19/2009
Citation: De-Campos, A.B., Mamedov, A.I., Huang, C. 2009. Short-Term Reducing Conditions Decreases Soil Aggregation. Soil Science Society of America Journal. 73:550-559.

Interpretive Summary: Upland soils in the Midwestern US often undergo reducing conditions when soils are temporally flooded during the spring and remain water saturated for days, weeks, or even months. Effects of the prolonged saturation on chemical, biological, and physical properties of these soils are still not well understood and should vary according to the soil type and environmental conditions. The temporary reducing condition influences soil chemical and physico-chemical condition. Thus reducing condition may also affect nutrient status and aggregate stability of the soils, which are important issues regarding to plant growth and water quality. The objective of this study was to determine how redox changes during periods of saturation of upland soils will impact nutrients cycling and soil aggregate stability. We assumed that soil aggregate stability would decrease under reducing conditions. The hypothesis was tested on three cultivated and three forest soil using biogeochemical reactor designed to continuous measurements of redox potential (Eh), pH, and CO2 during anaerobic incubation (from 1h to 14 day). After each incubation period the soil solution analyzed for metals and dissolved organic carbon. Aggregate stability of samples were determined using high energy moisture characteristic methods, where the destructive force used to break down aggregates was controlled wetting rate. In general, redox potential and aggregate stability decreased with increasing incubation time for all six soils. There was significantly change in pH, electrolyte concentration (EC), metal concentrations (Fe, Mn, Na, K, Ca, Mg), and DOC due to the reducing condition. Cultivated soils were less stable than uncultivated soils for the same incubation period. Soils under stronger reducing conditions were less stable. All noticed changes depended on the initial stability of the soil after the incubation treatments and the soil organic matter content. This study showed that under reducing conditions, changes in concentrations of redox sensitive metals, organic matter decomposition, and EC of the soil can affect soil structural or aggregate stability and should be considered in chemical transport processes and water quality studies.

Technical Abstract: Upland soils in Midwestern US are often ponded during the spring for days or weeks and may undergo reducing state. Short-term reducing conditions change the chemistry of the soil and that may affect soil aggregation. The objective of this paper was to determine how changes in the redox status of the soil can impact soil aggregation during short-term flooding conditions. A biogeochemical reactor was built to achieve reducing conditions. Six different soils, three cultivated and three uncultivated, with different organic carbon contents were incubated (1:1.5 soil:water ratio) in the reactor for 1 hour, 1, 3, 7 and 14 days. After each treatment, the soil solution was collected and analyzed for metals and dissolved organic carbon (DOC). A High Energy Moisture Characteristic (HEMC) procedure was used to determine aggregate stability of the incubated soil samples. A strong correlation was found between changes in redox potential (Eh) and aggregate stability. As Eh decreased aggregate stability decreased. The changes in redox sensitive elements (Mn and Fe), alkaline metals (Ca, Mg, K), and DOC under reducing conditions were found to also contribute to decrease aggregate stability. Aggregate stabilities of cultivated soils were more sensitive to reducing conditions than those of uncultivated ones. The decrease in aggregate stability reached up to 21% in the cultivated soils and might be irreversible under field conditions. Short-term reducing conditions may have detrimental effects on soil structure and water quality due to changes in soil aggregation and chemistry of soil solution.