Core Ideas Preferential flow is prevalent in clay soil under both frozen and thawed conditions. Preferential flow dominates the infiltration regime under frozen soil conditions in silt loam. Subsurface placement of fertilizer can limit subsurface nutrient leaching. Subsurface placement is particularly effective in soil with abundant preferential flow. Subsurface placement is recommended for fall fertilizer application. Agricultural runoff containing P and N from drainage tiles contributes to nutrient loading in waterways, leading to downstream eutrophication. Recent studies suggest that nutrient losses through tile drains can be reduced if nutrients are applied in the subsurface. This study explored interactions between nutrient supply and infiltrating water during a simulated nongrowing season using a laboratory experiment to understand how water and nutrients move through partially frozen and unfrozen soil and if fertilizer placement influences NO3− and dissolved reactive P (DRP) leaching. Intact silt loam and clay soil monoliths (28 by 30 by 30 cm) were fertilized with P and N via subsurface placement or surface broadcast and subjected to simulated rainfall under unfrozen (10°C) and partially frozen (∼0°C) conditions. Conservative tracers (Br−, Cl−, and D2O) applied to characterize subsurface flow paths throughout a subset of events indicated that matrix flow dominated in unfrozen silt loam soil. However, preferential flow paths dominated in unfrozen clay and in both soil types under partially frozen conditions, transporting applied nutrients while minimizing contact with the soil matrix. The subsurface placement of inorganic fertilizer relative to surface broadcast reduced both NO3− (by 26.85 kg ha−1 [23%] in silt loam and 65.73 kg ha−1 [61%] in clay) and DRP losses (by 2.33 kg ha−1 [60%] in silt loam and 4.25 kg ha−1 [64%] in clay). This study demonstrates the advantage of subsurface placement of fertilizer in the reduction of nutrient leaching by limiting the interaction of the nutrient supply with preferential flow pathways.

Preferential flowpaths transport phosphorus (P) to agricultural tile drains. However, if and to what extent this may vary with soil texture, moisture conditions, and P placement is poorly understood. This study investigated (a) interactions between soil texture, antecedent moisture conditions, and the relative contributions of matrix and preferential flow and (b) associated P distributions through the soil profile when fertilizers were applied to the surface or subsurface. Brilliant blue dye was used to stain subsurface flowpaths in clay and silt loam plots during simulated rainfall events under wet and dry conditions. Fertilizer P was applied to the surface or via subsurface placement to plots of different soil texture and moisture condition. Photographs of dye stains were analysed to classify the flow patterns as matrix dominated or macropore dominated, and soils within plots were analysed for their water‐extractable P (WEP) content. Preferential flow occurred under all soil texture and moisture conditions. Dye penetrated deeper into clay soils via macropores and had lower interaction with the soil matrix, compared with silt loam soil. Moisture conditions influenced preferential flowpaths in clay, with dry clay having deeper infiltration (92 ± 7.6 cm) and less dye–matrix interaction than wet clay (77 ± 4.7 cm). Depth of staining did not differ between wet (56 ± 7.2 cm) and dry (50 ± 6.6 cm) silt loam, nor did dominant flowpaths. WEP distribution in the top 10 cm of the soil profile differed with fertilizer placement, but no differences in soil WEP were observed at depth. These results demonstrate that large rainfall events following drought conditions in clay soil may be prone to rapid P transport to tile drains due to increased preferential flow, whereas flow in silt loams is less affected by antecedent moisture. Subsurface placement of fertilizer may minimize the risk of subsurface P transport, particularily in clay.