In the Zapotitan Valley near San Andres, El Salvador, Central America, an experiment was conducted to determine the availability of residual soil N to corn grown during the rainy season. This was an extension of an experiment conducted during the preceding dry season. The variables of the dry season experiment were irrigation method, crop, and rate of fertilizer N application. Soil N03-N and NH4-N were determined by soil sample analysis to a soil depth of 120 em by 30 em depth increments. The samples were taken at the end of the dry season experiment and again at harvest time of the wet season experiment. Yield of corn grown during the rainy season was measured. The results indicate the following: (1) soil N03-N alone was an efficient indicator of residual soil N; (2) there was a linear increase of soil N03-N with N applied four months previously at the beginning of the dry season crops; (3) soil sampled to the 30 cm depth was sufficient to estimate availability of the residual N; (4) corn yields increased linearly with the increase of soil N03-N; (5) the measurement of residual soil N03-N can be used as a soil text index in connection with N prediction equations for estimating fertilizer N requirements. The measurements of soil N03-N can, therefore, increase the efficiency of fertilizer use in a wet-dry tropical climate.

AN ABSTRACT OF THESIS Jarrod Armstrong, for the Master of Science degree in Forestry, presented on December 10, 2014, at Southern Illinois University Carbondale. Title: Nitrogen Availability and Transport Following Drought in Three Agricultural Watersheds in Central Illinois Major Professor: Dr. Karl Williard The use of inorganic nitrogen (N) fertilizers has become an essential part of modern agriculture and has helped increase yields to keep pace with an ever growing population. N is the most dynamic nutrient in nature, and biological activity can transform it into several mobile forms. Nitrate (NO3-N) is the most mobile form of N and is highly susceptible to transport to ground and surface waters. The purpose of this study was to assess N dynamics in three agricultural watersheds during and following a significant drought in 2012. Specifically, the study focuses on the mobilization and transport of residual N. The research was conducted on a ninety‐seven hectare agriculture field in Macon, County Illinois. The study site (BRKA) was divided into three watersheds, with four plots per watershed, and two topographic positions per plot. Volumetric water content (VWC) was measured continuously in each of the two topographic positions. In each watershed, stream stage collected over storm hydrographs using automated water samplers was compared to volumetric water content and NO3-N concentrations over the hydrograph. Four 6.1m groundwater monitoring wells and eight vacuum lysimeters in each watershed were monitored to determine the fate and transport of N to soil water and groundwater. Soil sampling at the 15cm depth was completed on a 0.4 hectare grid over the entire field during the fall of each year of the study. Soil and groundwater samples were analyzed monthly to compare NO3-N concentrations across topographic positions. NO3-N concentrations were highest in soil water, followed by groundwater, and lastly surface runoff. Studies in Illinois and Iowa both confirmed large amounts of residual N in the soil after the growing season in the fall of 2012 (Sawyer 2013, and Nafziger 2013). Residual N was apparent at BRKA in elevated NO3-N concentrations in soil water and groundwater after the 2012 growing season. Runoff events in April 2013 also showed increased NO3-N transport. However, due to precipitation events in the late fall and winter the residual N was flushed from the soil profile rendering it unavailable for the 2013 growing season. The soil NO3-N deficit after the 2012 drought was likely the result of decreased N fixation, N mineralization, nitrification, and leaching of any residual NO3-N. Bottomland positions consistently displayed higher soil water and groundwater NO3-N concentrations compared to uplands. However, due to a lack of plant uptake during the 2012 drought this trend was reversed and caused upland positions to exhibit higher NO3-N concentrations compared to bottomlands. This study demonstrated that even during a soybean year when no N fertilizer was applied significant drought can effectively alter the normal N dynamics at the field scale. Furthermore, this change in dynamics can lead to elevated NO3-N concentrations in soil water and ground water. These findings also suggest that precipitation events following periods of drought, like those observed after the 2012 growing season, can flush excess nutrients from the rooting zone further depleting the NO3-N pool and posing a risk to water quality. Data from a June 15, 2011 storm showed that on the falling limb of the hydrograph subsurface flow flushed soil water from the top of the slope to the bottom of the slope. This is indicative of a variable source area controlled watershed where the near stream zones undergo prolonged saturation from the subsurface drainage of the upland areas. Additionally, the early peak of NO3-N during an April 18, 2013 surface runoff event could be attributed to increased mineralization and nitrification following a rewetting of the soil profile after the 2012 drought. Lastly, topography was shown to have a strong influence on soil NO3-N concentrations across the field. This finding suggests that fertilizer applications based on topography and hydrology could help to mitigate the loss of excess NO3-N from agricultural watersheds. Furthermore, fertilizer applications should be adjusted for drought conditions that extend into the following growing season to account for residual N in the soil.