Silicic acid and oxidizable carbon movement in a Walla Walla silt loam as related to long-term residue and nitrogen applications Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/n296x2912

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  • Intensive cultivation of Walla Walla silt loams (coarse silty, mixed, mesic, typic Haploxeroll) in eastern Oregon and Washington during the past 50 years, has decreased internal soil drainage in the 15 to 40 cm layer. However, no measurements have been made to determine the causes and corrective management for this internal drainage problem. Gradual acidification of the plow layer, partially as a result of long-term use of nitrogen fertilizers in a summer-fallow management system may possibly have contributed to release of soil constituents that contribute to cementation of a high strength soil layer of variable thickness found at depths from 15 to 40 cm. Therefore, the potential for release, and possible interactions, of silicic acid, organic carbon, and major cations from the plow layer and subsequent movement through or desposition in layers below were evaluated. The test soils came from selected plots subjected to residue and nitrogen addition treatments for a period of 50 years. Samples were taken from the four upper 15-cm layers of a Walla Walla silt loam, placed in a system of laboratory columns, and leached with distilled water. The columns were designed to simulate a leaching soil profile and to facilitate continuous collection of leachate for analysis. Results indicate that soluble silica concentrations in Walla Walla soils are high in comparison to soils of the United States found outside of the Pacific Northwest. When no silica enters a layer (the 0 to 15-cm layer) silicic acid concentration in the leachate increases as original soil pH decreases. When the input fluid contained silicic acid, as in all layers below 15 cm, there was a smaller trend for increasing silicic acid concentration with decreasing soil pH. Silicic acid release from the 0 to 15-cm layer was decreased 24% by addition of large amounts of organic material, and 32% by the addition of lime compared to other treatments with lower soil pH values. All management systems showed a net release of silicic acid from the 0 to 15-cm layer and a net gain in each 15 cm layer below to a depth of 60 cm. Maximum release was 4.3 x 10⁻² mol H₄S10₄/m of liquid depth and maximum gain was 2.7 x 10⁻² mol H₄SiO₄/m of liquid depth, both in the plot treated with 45 kg N/hayr. Silicic acid concentration and transfer were each correlated (R² = 0.99 and 0.87, respectively) with both soil pH and long-term carbon additions to the soil surface. Soluble carbon also gave a release and movement response with a maximum net release of 1.3 x 10 ⁻¹ mo1 C/m of liquid depth and a maximum gain of 7.2 x 10 ⁻² mol C/m of liquid depth. However, there were no significant correlations of either soluble organic carbon or major cations with silicic acid release or retention. Silicic acid solubility increases with increasing soil acidity. More silicic acid leaves the plow layer thereby increasing the potential for adsorption or deposition in lower layers. Desorption, movement and sorption of silicic acid may affect such soil physical and chemical properties as hydraulic conductivity and cementation. Because silicic acid in soil solutions, and associated movement, is susceptible to changes in soil management, silica solubility effects on soil physical properties deserves more attention.
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