Preferential flowpaths and fertilizer placement influence subsurface P transport across soil textures and seasonal conditions
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Agricultural tile drains are a source of phosphorus (P) contributing to eutrophication. Preferential flowpaths can rapidly transport P to tile drains, but their activation in different soil textures and under variable seasonal conditions (antecedent moisture conditions and presence of soil frost) is not well understood. Subsurface placement of fertilizer has been proposed as a management practice to reduce P loss, compared to surface applications. However, how subsurface placement reduces P loss is not well understood. The goal of this thesis is to relate subsurface flowpaths and fertilizer placement to identify source and transport mechanisms controlling P movement to tile drains, across soil textures and seasonal conditions. A lab experiment was done on intact soil monoliths (clay, silt loam) to investigate interactions between fertilizer placement, subsurface flowpaths, and soil frost. Conservative water tracers (Brˉ, Clˉ and D₂O) applied through successive events identified matrix flow as the dominant flowpath in unfrozen silt loam, while preferential flow dominated in unfrozen clay and in both soil types under partially frozen conditions. Subsurface placement of fertilizer reduced dissolved reactive P losses by 60% in silt loam and 64% in clay over the simulated non-growing season compared to surface broadcast applications. A field study used blue dye as a tracer to investigate subsurface flowpaths in clay and silt loam plots under wet and dry conditions. Dye stain patterns were analyzed to determine the relative importance of matrix and macropore flow. Soil samples were collected to determine soil P distribution post-fertilization. Preferential flow occurred under all soil texture and moisture conditions. Dry clay soil showed the deepest staining (92 ± 7.6 cm), followed by wet clay (77 ± 4.7 cm). In silt loam soil, depth of staining did not differ between wet (56 ± 7.2 cm) and dry (50 ± 6.6 cm) conditions. Soil water-extractable P distribution varied with fertilizer application in the top 10 cm of the soil profile, but did not differ at depth. Together, the results of this research suggest subsurface placement is a suitable practice for minimizing subsurface nutrient loss, by reducing contact between the nutrient supply and preferential flowpaths, particularly in clay soils prone to preferential flow. This work provides an improved understanding of subsurface flowpaths carrying P to tile drains, and more broadly, solute transport through preferential flowpaths.
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Kirsten Grant (2018). Preferential flowpaths and fertilizer placement influence subsurface P transport across soil textures and seasonal conditions. UWSpace. http://hdl.handle.net/10012/14248