In-stream Nitrogen Processing and Dilution in an Agricultural Stream Network

Monday, 15 December 2014
Kara Prior1, Adam S Ward1, Caroline A. Davis1, Amy J Burgin2, Terry Loecke2, Diego A Riveros-Iregui3, Steven A Thomas2 and Martin A St. Clair4, (1)University of Iowa, Iowa City, IA, United States, (2)University of Nebraska Lincoln, Lincoln, NE, United States, (3)University of North Carolina at Chapel Hill, Chapel Hill, NC, United States, (4)Coe College, Cedar Rapids, IA, United States
The interaction of agricultural fertilizer use and extremes in drought and flood conditions in 2012-2013 set up conditions for a natural experiment on watershed-scale nutrient dynamics. The region-wide drought in 2012 left surface soils disconnected from stream networks and restricted nutrient use by crops, resulting in an unusually large nitrogen pool in soil columns through the winter. When wet conditions returned to the Midwest in 2013, the unused fertilizer was mobilized, resulting in a six-week period of extremely high in-stream nutrient concentrations. This study analyses three synoptic samples from the Iowa-Cedar River Basin in 2013 to quantify patterns in nitrogen dynamics. We use multiple conservative ions as tracers to estimate dilution by lateral inflows. We also estimate nutrient spiraling metrics by treating the fertilizer pulse as a constant rate nutrient addition across the watershed—a scale on which these processes are increasingly modeled numerically, but on which standard nutrient addition experiments are simply not feasible. Results of this study compare patterns in dilution and uptake across spatial and temporal scales, and bound feasible explanations for each reach of the network.