In-stream nitrogen processing and dilution in an agricultural stream network

The interaction of agricultural fertilizer application and a drought-to-flood transition in 2012-2013 set up conditions for a watershed-scale experiment on 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 stored in the soil column through the winter. The wet conditions in the Midwest in spring of 2013 mobilized the unused fertilizer from the landscape into the stream network. A six-week period of extremely high in-stream nutrient concentrations followed. This study analyzes results of three synoptic sampling campaigns in the Iowa-Cedar River Basin in 2013 to quantify patterns in nitrogen dynamics. I estimate nutrient spiraling metrics by interpreting the fertilizer mobilization from the landscape to the stream as a constant rate nutrient addition at the scale of the entire watershed—a vital scale on which to understand these processes, but for which costs and logistics of standard experiments are prohibitive. Results of this study compare patterns in dilution and uptake across spatial and temporal scales, and bound feasible explanations for hydrologic and reactive function in each reach of the network.