Preferential Movement of Oxygen-18-Labeled Water and Nitrogen-15-Labeled Urea through Macropores in a Nicollet Soil
Studies were conducted to evaluate the possibility that preferential movement of water through soil macropores was an important factor in previously observed losses of surface-applied urea and nitrate from Iowa soils. Granules of 15N-labeled urea were applied to the surfaces of undisturbed soil columns at their water-holding capacities, and then 18O-labeled water was applied to simulate a 25-mm rainfall event. Effluent from the bottoms of the columns was incrementally collected and analyzed. Soil from the columns was removed in layers 24 h after initiation of the rainfall, and each layer was analyzed to determine depths of movement of labeled water and N. Significant amounts of labeled water and urea were present in the first increment of effluent from some columns. Labeled water and urea moved much deeper and were more dispersed in all soil columns than would be expected in absence of preferential movement of water through macropores. The results suggest that preferential movement of water through macropores is more important as a factor affecting the movement and fate of nitrogen in Iowa soils than generally has been recognized. A better understanding of where and when this preferential movement occurs and how it influences leaching of N and other solutes in soils is needed to advance efforts to identify management practices that reduce losses of N and other chemicals from agricultural soils to the environment.
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