Increased N fertilizer efficiency in crop production is essential for agronomic, economic, and environmental improvement. In order to increase efficiency, a basic knowledge of fertilizer-soil-crop relationships and components is required. Nitrogen components in the soil-corn system were determined on two soils (Chicot sandy clay loam, Grey Brown Luvisol; Ste. Rosalie clay, Humic Gleysol). Fall soil NO$ sb3 sp-$-N levels increased linearly with increasing N rates above the 170 kg ha$ sp{-1}$ N rate. Changes in soil NO$ sb3 sp-$-N over winter were a function of both fall soil NO$ sb3 sp-$-N levels and winter precipitation. Denitrification rates during the non-growing season ranged from 7 to 24 kg N ha$ sp{-1}$, mainly dependent on N fertilizer rates the previous growing season on the Ste. Rosalie soil. Denitrification losses were a small portion of NO$ sb3 sp-$ disappearance over the non-growing season. Almost all fertilizer N at 170 kg N ha$ sp{-1}$ was recovered as crop N, clay fixed NH$ sb4 sp+$ and organic immobilized N at the end of the growing season, where at 400 kg ha$ sp{-1}$ N fall mineral N and unaccounted for N were a major component of the N fractions. High rates (400 kg N ha$ sp{-1}$) compared to normal rates (170 kg N ha$ sp{-1}$) resulted in some increase in yield, greater microbial activity and greater soil organic N, and a significant loss of fertilizer N by denitrification or leaching.

Note: / The soil-solution partitioning of dissolved organic carbon (DOC) withinmineral soil horizons is primarily controlled by processes of adsorption and desorption. These abiotic processes largely occur within a short equilibration time of seconds to minutes, which generally occur faster than microbial processes. To characterise the adsorption of DOC to mineral soils, I used the Langmuir adsorption isotherm, which holds several advantages to the commonly used linear initial mass (IM) isotherm. One advantage to using the Langmuir isotherm is anestimation of the maximum DOC adsorption capacity (Qmax). The Qmax estimates the number of remaining DOC binding sites available on the mineral soil particle surfaces. I modified the traditional Langmuir isotherm in order to estimate the DOC desorption potential of native soil organic matter (SOC).[...] / Le partitionnement entre les solutions de sols du carbone organiquedissous (COD) dans les horizons des sols minéraux est essentiellement contrôle par les processus d'adsorption et de désorption. Ces processus abiotiques se déroulent normalement dans un bref temps d'équilibration variant de quelques secondes a quelques minutes, ce qui est en général plus rapide que les processus microbiens. Pour caractériser Fadsorption de COD aux sols minéraux, nous avons utilise l'isotherme d'adsorption de Langmuir. Cette isotherme présente plusieurs avantages par rapport a Fisotherme de masses initiales (IM) linéaires couramment utilisée, en particulier F estimation de la capacité d'adsorption maximale du COD (Qmax). Le Qmax estime le nombre de sites de liaison de COD restants a la surface du sol minéral. Nous avons aussi modifie Fisotherme de Langmuir traditionnelle afin d'évaluer le potentiel de désorption de COD de la matière organique du sol indigène (MOS).[...]

The soil-solution partitioning of dissolved organic carbon (DOC) within mineral soil horizons is primarily controlled by processes of adsorption and desorption. These abiotic processes largely occur within a short equilibration time of seconds to minutes, which generally occur faster than microbial processes. To characterise the adsorption of DOC to mineral soils, I used the Langmuir adsorption isotherm, which holds several advantages to the commonly used linear initial mass (IM) isotherm. One advantage to using the Langmuir isotherm is an estimation of the maximum DOC adsorption capacity (Qmax). The Qmax estimates the number of remaining DOC binding sites available on the mineral soil particle surfaces. I modified the traditional Langmuir isotherm in order to estimate the DOC desorption potential of native soil organic matter (SOC). / Sorption characteristics were derived for a broad range of52 mineral soils collected from 17 soil profiles spanning across Canada from British Columbia to Quebec. Mineral horizons with the greatest Qmax included the Fe-enriched B horizons of acidic Podzols and Volcanic soils, followed by B horizons not enriched in Fe, followed by A and C horizons. Podzol B horizons were distinct from all other horizons due to significantly higher desorption potential. Soil properties predicting the adsorption characteristics of DOC also predicted the adsorption characteristics of dissolved organic nitrogen (DON). Adsorption of DOC and DON was tightly coupled (R 2 = 0.86), however the ratio of DOC:DON in the final equilibrium solution lowered for 48 out of 52 minerals horizons. These results suggest that DON may be slightly more mobile than DOC. / A short-term (32 day) incubation was perform to establish the fate of indigenous soil C, relative to newly adsorbed soil C to four mineral soils with different adsorption characteristics. Soil columns were leached periodically and sampled for DOC and CO2 production. Two Fe-enriched mineral horizons with high adsorption capacity released low amounts of old SOC, yet released almost all of the newly adsorbed SOC. In contrast, two B horizons without Fe-enrichment released greater amounts of old SOC, and retained a greater fraction of the newly adsorbed SOC than the Fe-enriched horizons. These results identify a contrast between the fate of indigenous and newly adsorbed SOC on mineral soils with differing Qmax. / The final component of this study examined changes to the molecular structure of DOC after equilibration with mineral soils. Multiple techniques were used to assess changes in the molecular composition of DOC, including the analysis of aromatic content by specific UV absorbance (SUVA) and fluorescence spectroscopy, analysis of molecular weight distribution (MWD) with high performance size exclusion chromatography (HPSEC) and functional group analysis with Fourier transform infra-red spectroscopy (FTIR). The solution phase DOC generally showed a reduced aromatic content, along with the removal of organic compounds with carboxyl groups. The MWD of DOC was reduced after equilibration to mineral soils, and the reduction in average molecular weight was related to the Qmax of mineral soils. / The various components of this thesis have contributed to the overall understanding of controls on the adsorption of DOC and DON species to mineral soils of the Canadian temperate and boreal forest.

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Earthworms have an important role in the decomposition of organic matter, mineralization of nutrients and physical mixing of soils. Despite a large number of laboratory and greenhouse-level studies investigating how earthworms modify soil properties and promote soil fertility, we lack reliable methods to scale-up and quantify earthworm contributions to nutrient cycling at the agroecosystem level. The objective of this thesis is to determine the influence of earthworm communities on nitrogen (N) transformations in soils and to quantify their contribution to nitrogen flux through soils for soybean and maize cropping systems of Quebec. Laboratory growth rates were used to predict how earthworm growth responded to seasonal fluctuations in soil temperature and moisture. The relationships between earthworm populations, soil-N pools and annual crop production were evaluated in a field experiment. When favourable conditions occurred in 2004 (temperatures <20°C, and rainfall at least once a week), a positive relationship was found between earthworm numbers and the plant available-N, including soil mineral-N, microbial biomass-N and total-N removed in soybean grain. In 2005, soil conditions were unfavourable (temperatures > 20°C and little or no rainfall) to earthworm survival and growth, and no relationship was found between earthworm populations, soil N pools and corn production. These data permitted me to make assumptions about earthworm activity and life histories under field conditions, which were used to estimate N flux through earthworm communities with two models. The models were tested for their sensitivity to varying parameter values within the range reported in the scientific literature. During a crop growing period with favourable climate conditions, a large earthworm population (100 g fresh weight biomass m-2 or greater) is predicted to cycle as much as 120 kg N ha-1. Model predictions were very sensitive to input parameters and did not correspond to the partial N budget calculated at the site. Accurate predictions of N mineralization by earthworms require more species- and site-specific parameter values. Further investigation using stable 15N isotopes as tracers would help us to follow the N transformations and evaluate the N flux mediated by earthworms at the field scale.

Urea and triple superphosphate (TSP) are important N and P fertilizers, respectively, but they suffer from inefficiency because of NH$ sb3$ volatilization and phytotoxicity and P fixation. Combinations of urea and TSP or KCl might improve N and P use efficiency primarily through pH changes. Mixtures of urea, TSP and KCl, either compacted or blended, were studied as to NH$ sb3$ volatilization, denitrification and P transformation in soils, and N and P use efficiency in crops. Soils studied included a Ste. Rosalie clay (Humic Gleysol), an Ormstown silty clay loam (Humic Gleysol), a Chicot sandy clay loam (Grey Browm Luvisol), a St. Bernard loam (Melanic Brunisol) and an Upland loamy sand (Humo Ferric Podzol). Triple superphosphate or KCl reduced urease activity and delayed urea hydrolysis primarily through acidification. Ammonia volatilization decreased when urea was surface-applied with TSP or KCl. The depressive effect of TSP on NH$ sb3$ volatilization was reduced and the KCl effect was increased in the soil with high exchangeable acidity. Phytotoxicity from banded urea, NO$ sb2 sp-$ accumulation and denitrification declined with added TSP. Urea hydrolysis caused a rise in pH and dissolution of organic matter in soils, and as a result, Mehlich (3) and water extractable P increased when urea were banded with TSP. Corn yield and N and P use efficiency were improved with application of mixtures of TSP and urea as compared with urea or TSP alone. Compacted mixtures of urea and TSP or KCl were superior to blended mixtures, possible because of the intimate association of fertilizer dissolution products. Compacted mixtures of urea and TSP or KCl hold promise for improved agricultural productions.

Five species, Acacia karroo, A. robusta, A. nilotica, A. sieberana and A. tortilis, were inoculated with Rhizobium and grown in potted sand in a temperature controlled greenhouse. After six months, results showed a higher percentage plant nitrogen for all five species when inoculated plants were compared to uninoculated controls. Inoculated treatments of A. karroo and A. sieberana had the greatest growth in shoot length and biomass. Acacia robusta showed the highest nitrogenase activity when nodules were tested using acetylene reduction methods. Inoculants of A. tortilis showed the poorest growth for all parameters measured. A. karroo and A. nilotica were studied at a field site at Ashburton, 15km east of Pietermaritzburg. Acacia karroo and A. nilotica had similar mean percentage leaf nitrogen, but A. karroo had a significantly higher mean percentage stern nitrogen than A. nilotica. Rainfall, canopy throughfall and stemflow from A. karroo and A. nilotica were collected in late spring and examined for inorganic nitrogen content. Acacia nilotica yielded the highest nitrate levels in both throughfall and stemflow samples. Acacia karroo produced lower nitrate concentrations in samples of both throughfall and stemflow, than was found in rainfall. Both A. nilotica and A. karroo exhibited higher concentrations of ammonium in samples of throughfall and stemflow as compared to levels. Soil analyses yielded highest levels of organic nitrogen at the surface (0 - 5 cm) but this decreased significantly at 20 cm deep. Surface organic nitrogen was highest under A. karroo canopies and lowest in open grassland. At 20 cm, there was little difference in organic nitrogen content between soils sampled from open patches and those under canopies of A. nilotica or A. karroo. Nitrate showed little variation with species, but highest levels were found in the top five centimetres and levels were higher under grasslands than under canopies. Ammonium showed no significant differences between different depths but was higher in open grassland sites than under canopies. No pattern could be found to relate tree size to soil organic nitrogen content. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1995.

The focus of this mini-dissertation is the use of riparian buffer zones to manage nitrate pollution of water resources. Riparian buffer zones are vegetated areas adjacent to streams, lakes and rivers, that are managed to enhance and protect aquatic resources from the adverse impacts of agricultural practices. These zones are recognised globally for their function in water quality amelioration. Despite the growing literature, there is little consensus on how to design, assess and manage these riparian buffer zones specifically for nitrate attenuation. For the purpose of this mini-dissertation, a literature review of world-wide research into the nitrate attenuation efficiencies of riparian buffer zones was undertaken. A database was created using the key information from this literature. Two key processes responsible for immobilising and/or removing nitrate from surface and subsurface flows are generally recognised in the available literature, namely: vegetative uptake and the process of denitrification. A comparison of the available riparian studies indicated that there are similar characteristics in riparian buffer zones that may be responsible for enhancing these key mechanisms. Studies where there was shallow lateral subsurface or uniform surface water delivery pathways, vegetation of close structure and composition, high organic matter in the soils and fluctuating soil surface saturation rates showed the most significant nitrate attenuation efficiencies. The mini-dissertation proposes that these similarities can be used to both assess a riparian landscape for its potential to attenuate nitrate, and to size a riparian buffer zone specifically to meet this function. A set of proposed guidelines based on the findings of the dissertation attempt to illustrate how riparian pollution control recommendations can be achieved. These guidelines are an example of how to assist a farmer or similar landowner in achieving good nitrate removal efficiencies from a riparian buffer zone. The guidelines work through three steps, which help to establish and prioritise management zones, assess each zone's potential for nitrate attenuation, and determine adequate riparian buffer widths for each management zone. A case study was used to illustrate the practical application of the guidelines. Full testing of these guidelines was not within the scope of this mini-dissertation, however the guidelines are an indication of how information regarding riparian function can be applied to a system to determine effective management of water resources. / Thesis (M.Env.Dev.)-University of Natal, Pietermaritzburg, 2002

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.