The study consisted of two experiments conducted over two growing seasons. Urea ammonium nitrate was used as a source of N at rates of 50, 75, 100 and 150% of levels estimated to be ideal for maximum yield of cotton (Gossvpium hirsutum L.). The nitrogen fertilizer was applied through a drip irrigation system. The yield of seed cotton, flowering pattern, boll set, plant N uptake, and dry matter production were studied in relation to four N fertilizer rates and two plant populations in the 1984 study. Yield of seed cotton, plant N uptake and dry matter production were studied in relation to four N rates, three seeding rates, and three cotton cultivars in the 1985 study. Petiole nitrate patterns were studied both seasons. The effect of N applications on seed cotton yield was dependent upon the initial soil N and the yield possibility. In this study the lower rate of N appeared to be sufficient for the yields obtained. Thinning resulted in reduction of the total number of flowers and significantly decreased yield, but percent boll set was not affected. Nitrogen additions significantly increased plant N uptake and dry matter production as well as petiole NO₃-N levels during the growing season. The N need of cotton under drip irrigation was determined throughout the growing season by using petiole analysis. The levels of petiole NO₃-N for N sufficiency and deficiency which are accepted under furrow irrigation cotton were shown to be applicable for drip irrigated cotton. Yield of DPL-775 and DPL-90 cotton cultivars was significantly higher than that for DPL-41 cotton cultivar in 1985.

An incubation study was conducted to evaluate the effects of added urea, NH$ sb4$Cl, and KCl on fixation of NH$ sb4 sp{+}$ and K$ sp{+}$. A field study was carried out to observe the effects of rates and methods of added urea and KCl on the forms and distribution of N and K in soils, and on yields and nutrient uptake of silage corn. / Generally, the fixation of NH$ sb4 sp{+}$ was increased by added N but reduced by added K and vice versa. One exception was that added K increased NH$ sb4 sp{+}$ fixation when NH$ sb4$Cl was the N source. In comparison with NH$ sb4$Cl, urea resulted in greater K$ sp{+}$ fixation but less NH$ sb4 sp{+}$ fixation. / Band placement of urea and KCl showed apparent accumulation of NH$ sb4$-N and NO$ sb3$-N, and exchangeable K$ sp{+}$ in the fertilizer bands, particularly early in the growing season. On the other hand, band placement resulted in lower levels of available N and K in soil between the bands in comparison with broadcast. / Yields of silage corn and nutrient uptake values were increased by N and K fertilizers, except for one soil in the first year. Band placement of urea was superior in the case of frequent and light rainfall but inferior with dry surface soil conditions followed by a relatively large rainfall 3 or 4 days after application. In the latter condition, banding K resulted in less yields on one soil, and banding the two fertilizers together further reduced N utilization on another soil. / Additions of K suppressed Ca and Mg uptake but did not influence N uptake, whereas K uptake was usually enhanced by added N.

One must consider the effects of both soil N and fertilizer N, if rates of N-fertilizer application are to be optimized and NO$ sb3 sp-$ leaching into groundwater be controlled. Objectives were (1) to determine soil $ rm NO sb3 sp-$-N and NH$ sb4 sp+$-N, soil total N, and N fertilizer levels and corn and wheat yields; and, (2) to determine soil sampling times and depths for N analyses that correlated with yields and fertilizer N response. Soil samples taken at seeding and postseeding were analyzed for NH$ sb4 sp+$-N and NO$ sb3 sp-$-N, and for total N in 29 wheat sites and 44 corn sites fertilized with four rates of N. N-Trak (quick test) and laboratory methods were employed to measure soil NO$ sb3 sp-$-N. A significant correlation was found between N-Trak and laboratory methods (R$ sp2$ = 0.61$ sp{**}$ for corn and 0.89$ sp{**}$ for wheat). Compared to the laboratory method, N-Trak overestimated soil NO$ sb3 sp-$-N. Soil NO$ sb3 sp-$-N levels increased upon drying. In some cases soils showed an increase in soil NO$ sb3 sp-$-N from seeding to postseeding. Wheat yields and wheat grain N uptake were better correlated with soil NO$ sb3 sp-$-N at seeding than at postseeding. For corn, yields and grain uptake showed a higher correlation with soil NO$ sb3 sp-$-N at postseeding. Corn regression models predicted 180 to 240 kg ha$ sp{-1}$ of N fertilizer to obtain optimum economic yields and 0 to 40 kg ha$ sp{-1}$ N for wheat. Soil total N was not correlated with corn yields as highly as soil NO$ sb3 sp-$-N. Soil NO$ sb3 sp-$-N can be used as an indicator of fertilizer N requirements for corn and wheat in Quebec.

Fertilized croplands unintentionally export large amounts of reactive nitrogen (N), which degrades water and air quality and contributes to climate change. In this dissertation, I focus on how these reactive N losses are likely to change in the near future as agriculture intensifies in the tropics, and ecological intensification strategies to mitigate N losses are more widely adopted. I use a combination of empirical field measurements in Mato Grosso, Brazil and Skåne, Sweden, literature review, and statistical models to quantify trends. In chapter one, I quantified emissions of nitric oxide (N₂O) and nitrous oxide (N₂O) in forest, single cropped soybean, and N-fertilized double-cropped soybean-maize at three nitrogen fertilizer levels within the largest area of recent cropland expansion on earth, in the Amazon and Cerrado biomes in Mato Grosso, Brazil. I found that NO emissions do not increase when forests are converted to croplands under current fertilization levels, and that NO will respond more strongly than N₂O fluxes to increases in fertilizer applications. In chapter two, I investigated anion exchange capacity and soil nitrate (NO₃¯) pools in deep soils in Mato Grosso, Brazil in the southern Amazon. I found that soil NO₃¯ pools in the top 8 m increased from 143 kg N ha¯¹ in forest to 1,052 and 1,161 kg N ha¯¹ in soybean and soybean-maize croplands. This NO₃¯ accumulation in croplands compared with forest soils matched the estimated amount of surplus N from the croplands, and could be explained by the soil’s positive charge through its anion exchange capacity. In chapter three, I conducted a meta-analysis of the effects of fertilization amount on of NO₃¯ leaching, N₂O emissions, NO emissions, and ammonia (NH₃) volatilization, totaling over 1,000 observations. I found that the relationship between N inputs and losses differed little between temperate and tropical croplands, although total NO losses were higher in the tropics. Among the potential drivers I studied, the N input rate controlled all N losses, but soil texture and water inputs also controlled NO₃¯ leaching losses. In chapter four, I explored the differences in NO₃¯ leaching, fertilizer N use efficiency, and soil N cycling in perennial wheat, which is being domesticated as a more sustainable alternative to annual crops, and annual wheat at a long-term experimental site in Skåne, Sweden. I found that NO₃ leaching was more than two orders of magnitude lower in perennial wheat, overall ecosystem recovery of fertilizer was quite high and not significantly different between perennial and annual wheat after the first growing season, and that measures of soil N cycling were largely the same between both crops. Together, these chapters highlight that reactive N losses will remain a critical global challenge in the coming decades, but that there are also key opportunities to reduce N losses by increasing the use of perennial crops and focusing tropical agricultural intensification on Oxisol soils which buffer against NO₃¯ leaching.

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Use of legumes as an alternative to nitrogen (N) fertilization in pasture management improves forage quality and animal performance and has been suggested to reduce the potential for environmental pollution. "Kentucky 3l" tall fescue fertilized with 160 kg N ha-1 yr-1 (split application) was compared to tall fescue grown with alfalfa or red clover in a 5-yr pasture experiment on a mixed Typic Hapludult. During yr 6, effects of N fertilization or the legume on soil N, forage N concentration, yield, botanical composition, N intake by esophageally fistulated steers grazing the pastures and N utilization by wethers fed the harvested forages were investigated. Soil ammonium was higher (P ≤ .01) in the A and B horizons in the tall fescue-red clover pastures compared to the other treatments and nitrate was lower (P ≤ .05) in the A horizon, but concentrations differed (P ≤ .01) by date. Nitrate in the A horizon averaged 2.65, 1.38 and 2.21 ppm for tall fescue-N, tall fescue-red clover and tall fescue-alfalfa, respectively. In the B horizon, average soil NO3 was .43, .23 and .53 ppm for tall fescueâ N, tall fescue-red clover and tall fescue-alfalfa, respectively. Tall fescue-alfalfa pastures were higher (P ≤ .01) in percentage legume than tall fescue-red clover, overall, but differed by date (P ≤ .01). Alfalfa was generally higher (P ≤ .05) in N concentration than red clover. Total kg N accumulated ha⁻¹ in above-ground herbage was higher (P≤ .05) for the grass-legume mixtures than N-fertilized tall fescue. Esophageally fistulated steers grazing stockpiled tall fescue-alfalfa selected forage higher (P ≤ .05) in N concentration than steers grazing the other pastures. Stockpiled tall fescue-alfalfa fed to wethers in a metabolism trial was higher (P ≤ .01) in N concentration, dry matter digestibility (DMD), apparent N absorption, and N retention than the other treatments. All treatments differed, with wethers fed tall fescue-red clover having the lowest DMD, apparent N absorption and N retention. Wethers fed tall fescue-alfalfa and tall fescue-red clover had higher blood urea N then those fed tall fescue-N. Results of this research demonstrate that soil NO₃ concentrations were low for all three forage treatments and would not contribute to ground water contamination. Legumes supplied adequate N to achieve yields similar to tall fescue fertilized with N and increased N production ha⁻¹ in the above ground biomass. Digestibility and utilization of the N in stockpiled tall fescue were improved by inclusion of alfalfa but not red clover. / Master of Science

In Mediterranean-type climates, topsoil frequently dries out during spring. Problems associated with reduced nutrient (P, K) availability in dry topsoil may be overcome by placing fertilisers deeper in the soil, where the soil is more likely to remain moist for longer periods as opposed to conventional fertiliser placement. Deep-P placement has resulted in significant yield improvements for lupin crops in Mediterranean environments because lupin crops generally require soil P supply during spring (throughout the flowering stage); in contrast, wheat yields have seldom improved with deep P placement, presumably because plants have accumulated sufficient P prior to spring (grain filling stage) for maximum grain yields. The P and K accumulation patterns of canola had not been investigated, and therefore any potential yield benefits of deep placed fertilisers were unknown. This study aimed to define the P and K demands of canola throughout the growing season, and assess the viability of deep placement of fertiliser in matching soil P and K supply to crop demand. The study further investigated the impact of deep placement of P fertiliser on root growth and distribution throughout the soil profile. Initial glasshouse studies compared the P and K accumulation patterns of several canola cultivars with wheat, and found that the P and K demand of canola continued until later into the season than wheat, but there was little difference in the P and K accumulation patterns of the various canola cultivars. Further experiments in sand culture determined that regardless of the level of K supply, canola plants had accumulated sufficient K for maximum seed yields by early flowering. Under high P supply, canola plants had accumulated enough P for maximum seed yields by early flowering, but when P supply during vegetative growth was just adequate, plants required a continual P supply until mid silique-filling to attain maximum yields. Because plants had accumulated sufficient K for maximum seed yields by early flowering (therefore topsoil drying in spring was unlikely to affect yields), further field experiments examined only deep placement of P fertiliser to improve P uptake and yields.

Hybrid poplars are fast-growing trees, well-suited for the production of bioenergy and wood products. They are often planted on marginal agricultural lands where soil fertility is low. The application of nitrogen (N) fertilizers is expected to replenish soil N reserves and support hybrid poplar productivity throughout the short-rotation growth period. The objective of this thesis was to assess the effect of two organic N fertilizers, papermill biosolids and pig slurry, on tree growth, nutrient uptake and soil fertility in a Populus trichocarpa x Populus deltoides hybrid poplar plantation. I also evaluated how these organic amendments influenced N transformation rates and denitrification losses. Hybrid poplar growth was greater when trees were fertilized with mixed biosolids: pig slurry fertilizer treatments compared to biosolids or pig slurry alone. Assessment of foliar nutrition with the critical value approach (CVA) and compositional nutrient diagnosis (CND) methods revealed that foliar N and P concentrations were limiting to tree growth. Vector analysis (VA) reflected growth responses in fertilized trees, relative to an unfertilized control, but may not be useful as a diagnostic tool due to the indeterminate growth habit of hybrid poplars. Nevertheless, surface application of papermill biosolids and pig slurry had the potential to improve soil fertility by increasing soil pH and extractable nutrients in the hybrid poplar plantations. Nutrient use efficiency (NUE) was greater in plots receiving the mixed biosolids:pig slurry treatment than biosolids or pig slurry alone. Laboratory incubations and 15N isotope pool technique revealed that pig slurry was a source of readily-mineralizable N one month after fertilizer application, and stimulated the gross nitrification rate and immobilization, while papermill biosolids contributed to a larger mineral N pools at the end of the season. Our results suggest that denitrification will be greater in biosolids-treated soils, which have a higher extractable Ca concentration and soil pH, as well as more NO3-N and available C for denitrifiers. Further investigation under field conditions would help us to evaluate the N transformations and losses mediated by microbial activities following organic fertilizer application in a hybrid poplar plantation.

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