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Nitrogen (N) fertilizer application constitutes a major portion of farmers' cost of production since N is the most applied nutrient in U.S. cotton production. Despite this, N uptake and use efficiency (NUE) in cotton remains below 50%, which presents challenges of environmental quality. Studies were conducted across 4 states in the US Cotton Belt with the overall objective of evaluating strategies to reduce loss of N to the environment, increase N uptake and NUE. The first study had two objectives: 1) compare NH3 volatilization from surface versus subsurface application/placement of granular (urea) and fluid N source (urea ammonium nitrate; UAN32); and 2) compare NH3 volatilization from urea and UAN treated with enhanced-efficiency fertilizer products. For this study, four A horizon soils of different types were collected from four sites in Virginia (VA), Georgia (GA), Tennessee (TN), and Texas (TX). The EEF products were N-(n-butyl) thiophosphoric triamide (NBPT), nitrapyrin, and ESN. In the first set of experiments (N placement experiments), it was found that across soil types, subsurface placement of granular N source reduced NH3 volatilization by 58 – 81% and subsurface placement of UAN reduced NH3 volatilization by 56 – 98%. In the second set of experiments (EEF experiments), it was found that NBPT reduced NH3 volatilization by 5 – 77% across soil types, and the highest reduction in losses by NBPT was observed on sandier and low CEC soils. Treating urea with both nitrapyrin and NBPT was more effective at reducing NH3 volatilization compared to treating urea with nitrapyrin alone. Based on our findings, subsurface application of granular and fluid N sources is recommended as strategy to reduce NH3 volatilization. Where subsurface placement is not possible, EEF products should be considered. The objectives of the second study were: 1) determine the effects of small grain and legume cover crops on N cycling; 2) evaluate the effects of cover crops and N fertilization on N uptake; and 3) evaluate the effects of cover crops on lint yield. Cover crops were winter fallow (winter weeds), small grain [cereal rye (Secale cereale)], legume mix [(50% crimson clover (Trifolium incarnatum): 50% hairy vetch (Vicia villosa)], and legume mix + rye [(67% legume mix:33% hairy vetch)]. Fertilizer N application rates were 0, 45, 90, and 135 kg ha-1. Soil inorganic N in the top 30 cm depths of the legume mix and legume mix + rye plots was consistently higher than in the rye lone or fallow plots. Cotton lint yield following legume mix with 45 kg ha-1 fertilizer N application was comparable to following fallow plots with 135 kg N ha-1. Thus, fertilizer N rate could considerably be reduced when cotton follows legume cover crops. The objectives of the third study were: 1) evaluate urea and UAN placement (broadcast, dribbling, and injection) on lint yield and fiber quality of three cotton maturity groups (early-, mid-, and full-maturity); (2) assess N use and agronomic efficiencies as influenced by N source, rate, and placement; (3) evaluate the impact of N source and placement on fiber quality. A study including 9 site-years studies was conducted in VA, GA, and TX. It was found that placement had no effect on yield in VA, had effect in all 3 years in TX, and had effect in 1 year in GA. Yield responded to N application in 8 out of 9 site-years in this study. Nitrogen use efficiency was highest among the early- and mid-season varieties. Overall, N rate and variety, rather than application/placement strategy, had the most pronounced effects on lint yield. Doctor of Philosophy Nitrogen (N) is usually the major limiting nutrient in cotton production and represents a significant cost of production. On average, the current proportion of applied N recovered in the aboveground crop biomass, (i.e., nitrogen use efficiency, or NUE) ranges from 33 – 50%, meaning that up to two-thirds of applied N is not recovered by crops. This unrecovered N not only represents economic loss to growers, but acts as a potential pollutant in the environment. There is a need for practices which increase N uptake in cotton production, agronomic efficiency, and environmental sustainability. Previous studies conducted outside the U.S. Cotton Belt reported that NUE is influenced by N source and rate of application. Data on NUE of contemporary cotton varieties utilized in the humid and semi-arid regions of the U.S. Cotton Belt would prove useful in efficient N management in the region. First study evaluated gaseous N loss from fertilizer application. It was found that subsurface placement of granular urea reduced NH3 loss as much as 58 – 81% compared to surface broadcast granular urea and subsurface placement of fluid N source reduced NH3 loss by 56 – 98%. In a second study, N rate and method of application/placement were evaluated. Here, it was found that N rate and cotton variety, rather than application/placement strategy had a more pronounced effect on cotton yield. Lastly, the potential of cover crops as alternate N source in cotton production was investigated. It was found that cotton yield following legume mix and fertilized with 45 kg of N per hectare was comparable with cotton yield following no cover crop and fertilized with 135 kg of N per hectare. The results of these studies indicate that subsurface placement of granular and fluid N sources can reduce NH3 loss. In addition, all other things being equal, choosing the right cotton variety as well as applying the right N rate are critical for yield. Furthermore, by growing cotton after legume cover crops, N fertilizer application rates can be significantly reduced.

Surface application of broiler litter to grasslands can increase concentrations of ammonium (NH₄+-N) and dissolved reactive phosphorus (DRP) in surface runoff. It is not known, however, for how long after broiler litter applications that NH₄+-N and DRP concentrations remain elevated. Five 0.75-ha, fescue-bermudagrass paddlocks received four broiler litter applications in 1995 and 1996, and only inorganic fertilizer N in 1997 and 1998. Runoff from each paddock was measured, sampled, and analyzed for NH₄+-N and DRP. Flow-weighted NH₄+-N and DRP concentrations increased from background values of 0.5 and 0.4 mg L-1, respectively, to values as high as 50.7 mg NH₄+-N L⁻¹ and 18.8 mg DRP L-1 in a runoff event that occurred immediately after the third litter application. Concentrations remained high while broiler litter was being applied but decreased steadily after the last application, reaching values near 1 mg L⁻¹ (for NH₄+-N and DRP) by 19 months after the final application. Among the factors that affected the average concentration of NH₄+-N and DRP in cumulative runoff after a litter application were cumulative runoff, rates of total N and NH₄+-N applied, and cumulative total litter N, total litter P, and water-soluble litter P applied during the four years of the study. Soil test P also affected DRP concentrations, but its effect depended on when the paddocks last received broiler litter. There is a need for tools to identify situations in which the application of broiler litter may enrich surface runoff with P. One such tool is the simulation model Erosion Productivity Impact Calculator (EPIC). EPIC's ability to simulate runoff volume and losses of dissolved reactive P (DRP) was evaluated. Data from the five 0.75-ha, tall fescue-bermudagrass plots that were fertilized with broiler litter during two years, and received only inorganic fertilizer N for the two subsequent years, were compared with EPIC estimates. EPIC simulations of runoff volume in individual events did not show bias in three of the plots but underestimated runoff in one plot and overestimated runoff in another. On an annual basis, the runoff volumes simulated by EPIC were similar to the observed values. A modified version of EPIC yielded better estimates of event DRP losses than the original EPIC and generated estimates of annual DRP loss that were similar to observed values. These results suggest that the modified EPIC may be useful for identifying situations where there is a high risk of large annual P losses from grasslands fertilized with broiler litter. Ph. D.