Diurnal and Seasonal Variation in Groundwater Nitrate-N Concentration in a Riparian Buffer Zone

The most notable observations in this study are the 24-hour time series vegetation uptake and multi-process increase signals. Time of maximum and minimum NO3- -N concentration data indicate vegetation uptake generates the sinusoidal trend. Time of maximum and minimum NO 3- -N concentration occurs when it would be expected for a photosynthetically controlled process. Maximum NO3- -N concentration occurs during the dark period, while minimum NO3- -N concentration occurs at the end of the photoperiod. Vegetation uptake is observed under conditions where vegetation growth is possible and sufficiently isolated from competing processes by environmental conditions. The alternate explanation for the sinusoidal trend, denitrification, is not supported by these data. If denitrification were the process responsible for the time of maximum and time of minimum NO3- -N concentration, then the denitrification rate would need to change and would have to be driven by water temperature changes. No relationship exists between 24-hour water temperature difference and NO3–N concentration difference. Additionally, these data indicate multiple processes are responsible for the increase trend. Anoxic conditions in the saturated zone coupled with water column height drops from ET provide ideal conditions for conversion of accumulated NH4+ to NO3- in the unsaturated zone. NO3- produced by nitrification in the unsaturated zone was then transported to the saturated zone by unsaturated flow. NO3- transport from the upgradiant agricultural land use are best accounts for increases during the winter.As an area that is intensively farmed, Illinois fields receive a large amount of NO3- fertilizer yearly. Tile drains make many forms of NO3- management ineffective, exporting NO3- through soil into streams, bypassing riparian zones and making best management practices such as riparian buffers less effective. The results of this study reinforce the importance of vegetation NO3- uptake. The results presented here provide evidence for a measurable contribution of NO3- removal by vegetation. Seasons that have a significantly different μ daily [NO3–N] do not have a significantly different μdifference [NO3–N]. This means the amount of NO3- available does not influence the magnitude of change over 24-hours. If implemented, conservation practices that intercept and increase residence times of water will reduce NO3- loads. Future research could focus on quantification of total nitrogen captured by vegetation, NO3- flux in the unsaturated zone, and stable isotope methods for NO3- source and fate. This study focused on NO3- -N in the saturated zone, but other forms of nitrogen such as ammonium and nitrite in the unsaturated zone may be of interest.

  • PI: Eric Wade Peterson
  • PI Institution: Illinois State University
  • March 1, 2017 – February 28, 2019