Preferential leaching of surface-applied nitrogen fertilizers in Iowa soils

{dollar}sp{lcub}15{rcub}{dollar}N-tracer studies were conducted to acquire a better understanding of the fate of urea fertilizer during the first few weeks after surface applications to Iowa soils. Labeled urea was surface-applied to microplots and soil samples were collected from these microplots at various times after application to determine recoveries of labeled N.; Results showed that significant amounts (often more than half) of N from labeled urea were lost from the surface 1-m layers of soil by eight weeks after application. Contrary to initial expectations, ammonia volatilization did not appear to be the major mechanism responsible for these losses. Distributions of labeled N in soil profiles indicated a rapid downward movement of this N during rainfall events, which were frequent during the years the studies were conducted. These distributions indicated degrees of dispersion of labeled N that could be best explained by preferential movement of water and this N through soil macropores. The importance of such preferential movement was demonstrated by applying {dollar}sp{lcub}18{rcub}{dollar}O-labeled water to columns of undisturbed soil and observing the movement of this water. The importance of preferential movement also was demonstrated in studies in which {dollar}sp{lcub}15{rcub}{dollar}N-labeled nitrate fertilizer was surface-applied to microplots. In microplots treated with labeled nitrate, as in microplots treated with labeled urea, distributions of labeled nitrate and soil-derived nitrate indicated that labeled nitrate was leached more rapidly during rainfall than was soil-derived nitrate. This selective loss of surface-applied N can be explained by the tendency for this N to be located with mobile water in macropores and the tendency for soil-derived N to be located in immobile water between macropores.; Overall, these findings suggest that preferential leaching through soil macropores is an important fate of surface-applied N fertilizers under conditions frequently found in the spring in Iowa. Because leaching of fertilizer N from agricultural soils results in economic losses for crop producers and often causes degradation of groundwater supplies, selecting N management practices to minimize preferential leaching could have substantial economic and environmental benefits.