Graduate Thesis Or Dissertation


Investigating the Potential of Slow Release Nitrogen Fertilizer to Increase Agricultural Nitrogen Management Efficiency for Tall Fescue in the Southern Willamette Valley Public Deposited

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  • Nitrogen (N) fertilizer application can promote nitrate leaching below the rooting zone and into groundwater drinking water sources in many agricultural settings. The proper management of N application can minimize non-point source nitrate pollution to groundwater. This study examines the potential for enhanced efficiency fertilizers, specifically urea treated with a slow release coating, to improve agricultural N management in tall fescue fields in the southern Willamette Valley of Oregon. Slow release and conventional urea applications were compared using four replicates of two, one-acre plots of tall fescue grass. Four lysimeters were installed in each plot at a depth of one meter to allow collection of soil solution. Lysimeter sampling occurred biweekly from March 13, 2018 to March 12, 2019. Lysimeter soil solution ammonia and nitrate concentrations were determined and used in conjunction with a farm specific water budget to estimate N leaching below the rooting zone. The water budget was created using physical characteristics of the soil, local models of evapotranspiration and precipitation, and soil moisture measurements at 10cm. Crop yield and aboveground biomass N content were used to assess the treatment’s effect on grass seed production and crop N use efficiency (NUE). The average yield of conventional urea plots and slow release urea plots was similar in terms of total harvested aboveground biomass at 10,100 and 10,600 kg ha-1, seed mass at 2,560 and 2,530 kg/ha, and hay mass at 7,420 and 7,480 kg ha-1 respectively. Mean NUE for conventional urea plots was not significantly higher than slow release urea plots at 50.7% compared to 46.6%. Total dissolved inorganic nitrogen (DIN) leaching below the rooting zone was 45.3 kg N ha-1 yr-1 (23.6%) on average for conventional urea plots compared to 58.6 kg N ha-1 yr-1 (28.7%) for slow release urea plots. While the slow release urea did not affect crop yield or NUE when compared with conventional urea, there was also no significant difference in DIN losses below the rooting zone. Slow release fertilizers did not provide any environmental benefit for reducing nitrate leaching in tall fescue fields. The majority of the N leaching occurred during the winter, suggesting that further efforts to reduce N leaching should focus on maximizing crop N uptake and reducing soil available N pools prior to fall and winter rains.
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