Characterizing the effect of surface topography on Nitrogen transport through simulation modeling

Jessica Fry, Moslem Ladoni, Juan-David Munoz-Robayo, Alexandra Kravchenko, and Andrey Guber
Department of Plant, Soil, and Microbial Sciences. Michigan State University

Presented at the All Scientist Meeting (2015-04-15 to 2015-04-16 )

The movement of nitrogen in the soil is known to be influenced by the surface topography of a field. In other words, nitrate fluxes varies with the landscape. Within the same field, there are differences in flows between areas that can be defined as summit, slope, or depression. These differences are due to variations in soil properties, such as soil texture, structure, thickness of soil layers and their elevation. These variations in nitrate flows are relevant in the context of nutrient management, minimizing fertilizer runoff, and water conservation. The objectives of our study were (1) to study how the field dynamics of nitrate are influenced by topography during the growing season. For example, how does nitrate move through the soil on a slope? How does it move in a depression area? And (2) to gain insight into how topography influences these flows using computer simulations of water flow and nitrate transport. To meet the first objective, we took soil samples from the three topographical positions (summit, slope, and depression) once a month during the growing season and analyzed them for nitrate content. We also measured soil texture (the percentages of sand, silt, and clay) at these same locations. We then used the computer model to simulate water flow and nitrate flow in the soil. The results showed consistently higher concentrations of nitrate in depression areas compared to slopes and summits. The soil texture analysis showed that the top 12” (30 cm) soil layer in slopes and summits was mainly a sandy loam (more coarse particles). The soil texture in depressions was different, it was mainly silt loam (more finer particles). This difference has several effects. In depressions, the soil had a larger water holding capacity and smaller water conductivity, that resulted in smaller downward water and N fluxes compared to summit and slope areas. There was also more ammonium adsorption and a higher nitrate concentration in the top 12” (30 cm) soil layer in depression plots compared to summit and slope. Based on these results, we conclude that management practices for sustainable crop production need to focus on reduction of nitrogen losses through infiltration in summit and slope areas.