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Observations on Preferential Flow and Horizontal Transport of Nitrogen Fertilizer in the Unsaturated Zone

U.S. Geological Survey, 301 W. Lexington, Room 227, Federal Building, Independence, MO 64050.

^* Corresponding author (wilkison{at}usgs.gov).

ABSTRACT

A study site underlain by a claypan soil was instrumented to examine the transport of fertilizer nitrogen (N) under corn (Zea mays L.) cultivation. The study was designed to examine N transport within the unsaturated zone and in intedlow (the saturated flow of water on top of the claypan). A ¹⁵N-labeled fertilizer (labeled N), bromide (Br), and chloride (Cl) were used as field tracers. Rapid or prolonged infiltration events allowed water and dissolved solutes to perch on the claypan for brief periods. However, a well-developed network of preferential flow paths quickly diverted water and solutes through the claypan and into the underlying glacial till aquifer. Excess fertilizer N in the unsaturated zone supplied a continuous, but declining input of N to ground water for a period of 15 mo after a single fertilizer application. Calculated solute velocities through the claypan matrix (6.4 x 10^–6 cm s^–1) were similar to horizontal transport rates along the claypan (3.5 to 7.3 x 10^–6 cm s^–1) but much slower than infiltration rates determined for preferential flow paths (1.67 x 10^–3 cm s^–1). These flow paths accounted for 35% of the transport. A seasonally variable, dual mode of transport (matrix and preferential flow) prevented the daypan from being an effective barrier to vertical transport. Simulations of selected field observations, conducted using the variably saturated two-dimensional flow and transport model, VS2DT, confirmed the presence of a dual flow regime in the claypan.

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