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Los suelos loéssicos agrícolas de Argentina muestran descensos de los valores de pH, aparentemente asociados con la fertilización nitrogenada. A fin de confirmar la magnitud de esos descensos, el origen de estas disminuciones y sus posibles efectos sobre otras propiedades edáficas, se analizaron diversas propiedades químicas y mineralógicas de Haplustoles, Hapludoles y Argiudoles, fertilizados (F) y no fertilizados (NF) con urea, que evolucionan sobre sedimentos loéssicos. Acerca del primer interrogante, referido a cuál es el estado actual de los valores de pH de suelos F y NF de distintos ambientes edafoclimáticos, los resultados indicaron que el cociente “precipitación media anual : temperatura media anual” (PMA TMA-1) explicó entre un 50 y un 72 % de la variabilidad de los valores de pH entre sitios. Esto confirmó que el clima es el principal factor responsable del pH de los suelos analizados: en ambientes más fríos y húmedos existen menores valores de pH. El cociente PMA TMA-1, las dosis y el número de años de fertilización nitrogenada explicaron un 35 % de las disminuciones de pH actual (pHw) y un 47 % de las de pH potencial (pHKCl) entre tratamientos de fertilización. Las diferencias entre pHw y pHKCl fueron mayores a 1 en todos los suelos analizados, lo que indica cierto grado de acidificación. Los tratamientos fertilizados presentaron valores de pH menores que los no fertilizados, alcanzando diferencias de 0,17 puntos para pHw y 0,09 para pHKCl. Sin embargo, los Argiudoles y algunos Hapludoles fueron los únicos en los cuales la fertilización con urea generó diferencias significativas de ambos valores de pH. También se estudió la influencia de las disminuciones de pH sobre algunas propiedades químicas y mineralógicas de los suelos. Los resultados indicaron que no existieron cambios de la capacidad de intercambio catiónico (CIC), el contenido de bases intercambiables y el porcentaje de saturación con bases (V) entre los tratamientos de fertilización. Tampoco existieron alteraciones en la cristalinidad de los filosilicatos acumulados en la fracción arcilla. En varios suelos, los contenidos de óxidos amorfos de Al, Mn y Fe fueron mayores en los tratamientos F que en los NF, mientras que sus formas cristalinas presentaron tendencias inversas. Esto indicó un cierto grado de transformación de óxidos cristalinos en amorfos como consecuencia de la fertilización. Las sustancias alteradas por efecto de la disminución del pH no fueron adecuadamente identificadas en la presente tesis. Con el objetivo de responder a la pregunta referida a cómo serían las tendencias futuras de los suelos si estos fueran sometidos a las tasas de fertilización más altas utilizadas en la actualidad, se realizó un estudio in vitro. Para ello se simuló la adición de cantidades de H+ equivalentes a las aportadas por una dosis constante de urea de 180 kg urea ha-1 año-1 durante 1, 10, 30 y 50 años. Los resultados indicaron que la adición de H+, en general, no modificó significativamente la CIC ni los contenidos de óxidos amorfos y cristalinos de Al, Mn y Fe. Sin embargo, sí generó alteraciones en los filosilicatos en los tratamientos más acidificados de los Hapludoles. Este efecto no fue tan pronunciado en los otros suelos, principalmente en los Argiudoles, en los cuales la presencia de sustancias buffer como la materia orgánica, carbonatos y fracciones finas (incluyendo el limo), disminuyó este efecto. La capacidad buffer de los suelos fue explicada en un 78 % por las reacciones de intercambio catiónico y de disolución de minerales acumulados en las arcillas y limos. El pH que los suelos tendrían en el futuro si la fertilización con urea continúa, a las dosis y frecuencias usadas en la actualidad, fue explicado en un 75 % por la capacidad de intercambio catiónico y los contenidos de materia orgánica, carbonato, arcila y limo en conjunto y arcilla y limo separadamente. Los valores de pH de los suelos fueron predichos en un 75 % cuando el modelo se validó con los datos de pH obtenidos de las simulaciones y, en un 57 %, cuando el modelo se validó con los datos de pH medidos en los ensayos de siembra directa de larga duración. Se demostró que la presencia de diferentes sustancias buffer estuvo íntimamente asociada con los procesos pedogenéticos particulares que ocurrieron en cada suelo. En suelos de ambientes húmedos (Argiudoles y algunos Hapludoles) las illitas litogénicas acumuladas en fracciones gruesas se habrían transformado en illitas de menor tamaño, acumuladas actualmente en arcillas y también limos. En suelos de ambientes más secos (Haplustoles y algunos Hapludoles), las illitas no fueron alteradas por la pedogénesis pero sí el vidrio volcánico, abundante en los materiales parentales, el que se habría transformado en esmectitas poco cristalizadas, acumuladas en las arcillas pero también el limo fino. Se concluyó que si la fertilización con urea continuara a las tasas ensayadas en nuestro estudio in vitro, los Haplustoles serían los suelos menos afectados por acidificación. Esto es debido a sus contenidos elevados de carbonato, materia orgánica que actúan como sustancias buffer. A pesar de que la proporción de fracciones minerales finas (arcilla y limo) en los Haplustoles no es demasiado elevada, su mineralogía esmectítica les confiere una cierta capacidad buffer a estos suelos. Los Argiudoles son los suelos que cuentan con mejores mecanismos de neutralización de H+, dados por sus elevados contenidos de materia orgánica y de arcillas y limos. Los Hapludoles serían los suelos con el mayor riesgo de acidificación debido a que presentan bajos contenidos de materia orgánica, carbonatos y de fracciones minerales finas. Palabras clave: acidificación de suelos; urea; vidrio volcánico; mineralogía del suelo; capacidad buffer. / Agricultural loessic soils of Argentina show pH decreases apparently linked with N-fertilization. Because of that some chemical- and mineralogical properties of urea-fertilized (F) and non fertilized (NF) Haplustols, Hapludols and Argiudols which evolves from loessic sediments, were analyzed. In relation to the first query, related with the status of pH values of F and NF soils, results indicated that between 50 to 72 % of pH values variability were explained by the ratio “mean annual precipitation:mean annual temperature” (PMA TMA-1). This confirmed that climate is the main driving factor of soil pH: soils of moister and cooler sites show lower pH. The quotient PMA TMA-1, doses and years of fertilization explained, respectively, 35 and 47 % of pHW and pHKCl decreases between fertilization treatments. The difference between the actual- (pHW) and the potential pH (pHKCl) was higher than 1 in all the studied soils, indicating some degree of acidification. pH of fertilized soils were lower than those of non-fertilized soils, being these differences 0.17 and 0.09 for pHw and pHKCl, respectively. However, the Argiudols and some Hapludols were the only studied soils in which urea-fertilization caused significant differences in both pH values. The influence of pH decreases on some chemical- and mineralogical properties were also studied. Results indicated that the cation exchange capacity (CEC), the content of exchangeable bases and the percent of base saturation were not different between fertilization treatments. Neither were detected alterations in the crystallinity of the phylosilicates of the clay fraction. Some fertilized soils showed higher contents of the amorphous Al, Mn and Fe oxides than in the non fertilized pairs, while the crystalline forms showed opposite behaviours. This indicates certain degree of transformation of crystalline oxides into amorphous forms in association with fertilization. Substances altered by the pH decreases were not adequately indentified in this study. In order to answer the question of how will be the future trends of soils if they will be submitted to the highest urea-fertilization rates, an in vitro study was developed. This was based on the addition of equivalent amounts of H+ to those produced by a constant application of urea of 180 kg ha-1 year-1 during 1, 10, 30 and 50 years. Results indicated that mostly H+ additions did not modify CEC and contents of amorphous- and crystalline Al, Mn and Fe oxides. However, the most acidified treatments of the Hapludols showed decreases in the crystallinity of the phylosilicates. This effect was not so pronounced in the other soils, mainly the Argiudols, in which the presence of buffer substances like organic matter-, and fine textural fractions- (including silts), decreased such effect. Soil buffer capacity of all soils was explained in a 78 % by the exchangeand dissolution reactions of minerals accumulated in clays and silts. Soil pH in the future, if nitrogen fertilization continues, was explained in a 75 % by the cation exchange capacity and the contents of organic matter, free lime, clay and silt and clay and silt separately. Soil pH values were predicted in a 75 % when the multiple regression model were validated by data was obteined from the simulations in vitro and, in a 57 %, when the model were validated with data from the long story no till plots. It was demonstrated that the presense of different buffer substances was closely linked with particular pedogenetic processes occured in each soil. In soils of humid environments (Argiudols and some Hapludols) lithogenic illites, accumulated in coarse fractions, were transformed into smaller sized illites, accumulating currently in clay but also in silt fractions. In soils from dryer environments (Haplustols and some Hapludols), illites were not altered by pedogenesis but they were the volcanic glasses, abundant in the parent material, which apparently were transformed into less crystallized smectites, accumulated mainly in clays but also in fine silts. It was concluded that under urea-fertilization rates similar to those used in our in vitro study, Haplustols will be the less affected soils by acidification. This is because they content free lime- and organic matter that act as buffer substances. Though contents of fine mineral fractions (clay and silt) are not so high, their smectitic nature conferes also a relative high buffer capacity to these soils. Argiudols are the soils with better neutralizing mechanisms, due to their high organic matter-, clay- and silt contents. Hapludols are the soils with the highest risk to acidification because they contain low organic matter, free lime and fine textural fractions. Keywords: soil acidification; urea; volcanic glass; soil mineralogy; buffer capacity.

Synthetic nitrogen (N) fertilizers play a key role in human nutrition and crop production. The most widely used N source globally is urea; however, N loss via ammonia volatilization can be great in agricultural systems where urea is surface-applied. The objectives of the experiments reported in this dissertation were: 1) evaluate the performance of a new laboratory ammonia volatilization measurement system for measuring ammonia volatilization from coated granular urea; 2) determine if urease can be extracted from corn and soybean residues; 3) determine if differences in urease activity are present in corn and soybean residues; and 4) evaluate N content and yield of corn treated with surface-applied coated urea fertilizers. The laboratory ammonia volatilization system had a system recovery efficiency (SRE) of 97% of the applied N and the lowest variation in mg N captured in the acid traps when the air flow rate was 1.00 L min⁻¹, at 26°C, and an acid trap volume of 100 ml 0.02M phosphoric acid. Ammonia volatilization was greatest from 12-24 h after N application with a total of 17% of the applied N being lost during that period. The urease inhibitor N-(n-butyl) thiophosphoic triamide (NBPT) was the most effective ammonia volatilization control treatment and reduced ammonia losses 30-40% compared to urea in the laboratory trials. Urease was extracted from soybean residue and retained activity during extraction; however, urease from corn residue could not be identified in extracts. The agronomic field trials indicated that NBPT increased N concentration in corn ear leaves; however the effect on corn grain yield was masked by environmental conditions. The data from this study suggests that ammonia volatilization from granular urea can be effectively controlled using NBPT, and corn tissue N content in the field indicates that NBPT allows for more N to be utilized by the plant. The urease extraction showed that there may be differences in urease activity in different crop residues. Further research is needed to determine if varying levels of volatilization control are needed for urea applied to different crop residues in no-till systems. / Ph. D.

The response of loblolly pine (Pinus taeda L.) to fertilization and weed control at stand establishment, using various formulations of conventional and controlled release N fertilizers was evaluated in a greenhouse study and at two field trials in the Virginia Piedmont, in 2002 and 2003. The greenhouse study evaluated five fertilizer treatments; 1) check with no fertilizer; 2) granular ammonium nitrate (10-10-10 + micro-nutrients) applied to the soil surface; 3) granular methylene urea (40-0-0) applied to the soil surface; 4) methylene urea (20-10-5) in tablet form applied in the planting hole; and 5) isobutylidenediurea (IBDU) (9-9-4) in tablet form applied in the planting hole. Equal amounts of N and P were applied. Fertilization significantly increased seedling root collar diameter and volume at the end of the first growing season in the greenhouse study. Differences in diameter and volume were still significantly different late in the second growing season, however at the last measurement the differences were no longer significant. An analysis of transformed growth curves for 2003 indicated that the ammonium nitrate treated seedlings had a significantly steeper slope than all other treatments. In field trials, at an old-field site and a cut-over site, the same fertilizer products were tested, except granular diammonium phosphate (18-46-0) was used substituted for the ammonium nitrate. Higher rates of N and P were used in the field trials. Complete weed control increased seedling volume by over 700 % after two growing seasons at the reforested old-field site, however fertilizer effects were not significant. At the reforested cut-over site an interaction between weed control and fertilizer treatments was observed. The MU and DAP granular, and the IBDU tablet treatments each had significantly greater seedling volume than the check and the other tablet controlled release fertilizer. IBDU tablets appear to have high fertilizer efficiency, due to the slow release nature and are safe to place in close proximity to the seedling root system. / Master of Science

Nitrogen deficiency is characteristic of many mid-rotation loblolly pine (Pinus taeda L.) plantations in the Piedmont region of the southeastern USA. Fertilization with urea is the most common method used to correct this deficiency. Previous studies show that urea fertilization produces a rapid pulse of available nitrogen (N) with only a portion being utilized by plantation trees. Controlled release fertilizers release available N more slowly over a longer period of time and therefore may result in greater uptake efficiency. The objective of this study was to compare Nitroform®, a urea-formaldehyde controlled release N fertilizer versus urea and a control by measuring the effects of the two fertilizer treatments on N availability and loss as: total KCl extractable-N, total ion exchange membrane-N (IEM-N), N mineralization, and N volatilization, in a mid-rotation loblolly pine plantation in the Piedmont of Virginia. In addition, mid-summer and mid-winter fertilizations were compared to assess fertilizer uptake as a function of season. After the summer fertilization, Nitroform® significantly increased total KCl-extractable N, IEM-N, and N mineralization for two to three months over urea and the control. Three hundred times more N volatilized from urea than from controlled release Nitroform®. Interestingly, seven months after the summer application, the controlled release Nitroform® showed marked immobilization for three months while urea demonstrated greater N mineralization. After the winter application, fertilization with urea demonstrated greater soil inorganic N concentrations for two to three months over Nitroform®, very little N was immobilized, and volatilization was only 10 times that of Nitroform®. After summer and winter fertilizations, both fertilizer treatments significantly increased soil inorganic N concentrations and N volatilization over controls, however did not significantly increase N mineralization over controls when average response was tested over the entire sampling period. In addition to the fertilizer effects measured, a thinning only treatment was also incorporated into this study with soil N-availability indices compared to a control with no thinning or fertilization. The results from the thinning only treatment demonstrated no significant increases over the control in total KCl extractable-N, IEM-N, N-mineralization, or N volatilization when average responses were tested over the entire sampling period. / Master of Science