Transferencia de nitrógeno de ecosistemas agrícolas recientemente intensificados a acuíferos: efecto de la variación en las precipitaciones
Date
2012
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Abstract
A partir del monitoreo a campo de cultivos, suelo y agua subterránea, modelos de
simulación y un experimento en lisímetros se estudió el intercambio de nitrógeno (N)
inorgánico entre cultivos y el agua subterránea en sistemas agrícolas recientemente
intensificados con suelos ricos en materia orgánica (MO). El monitoreo (1998-2010) en
tres posiciones del paisaje de la Pampa Ondulada permitió captar períodos plurianuales
húmedos y secos (1506-444 mm año-1) disparadores de cambios intensos en las
profundidades freáticas (<1 a >6 m) que afectaron la función del agua subterránea y de
los cultivos como moduladores de los flujos de agua y N. Con niveles freáticos
superficiales, se redujo la capacidad de almacenaje del agua drenada hacia la zona no
saturada, produciéndose un flujo lateral subsuperficial que redistribuyó el N lixiviado
desde posiciones altas hacia los bajos y contribuyó a la recarga del acuífero aguas abajo.
Este flujo, favorecido por fuertes gradientes hidráulicos, y la concentración local de
solutos disparada por el consumo de agua subterránea por los cultivos en la posición de
pie de loma, produjeron picos de concentración de cloruro (>500 mg Clֿ l-1) y nitrato
(>45 mg N-NO3ֿ l-1) en el agua freática superficial. Durante períodos normales a secos
los drenajes de diferentes eventos de lluvia se alojan en la zona no saturada, disipando y
retrasando la respuesta del nivel freático a eventos individuales. El agua de lluvia fluye
verticalmente arrastrando N del suelo hacia una superficie freática profunda (con mayor
desfasaje temporal entre la concentración de nitrato del drenaje y la del agua freática a
medida que el nivel se profundiza) y luego descarga en el arroyo. La aplicación de un
fertilizante enriquecido con 15N a un cultivo de maíz en lisímetros permitió demostrar
que la fertilización representa un aporte insignificante (1%) al flujo de N lixiviado y que
el N derivado de la mineralización de la MO del suelo constituye una fuente importante
de N lixiviable durante períodos con balance hídrico positivo y/o baja demanda de los cultivos. A partir de los resultados obtenidos se propone un modelo conceptual de los
flujos hidrológicos y de N en paisajes ondulados de uso agrícola para situaciones
climáticas contrastantes.
We combined field monitoring of crops, soils and groundwater with the use of simulation models and an experiment in lysimeters, to study inorganic nitrogen (N) exchange between terrestrial ecosystems and groundwater under recently intensified cropping schemes of rich organic matter soils. More than ten years of monitoring (1998- 2010) at three landscape positions of the Rolling Pampas allowed to capture plurianual periods of highly variable precipitation (1506 to 444 mm yr-1) that triggered large changes in water table depths (<1 to >6 m), which altered the role of groundwater and crops as modulators of water and N fluxes. Shallow water tables reduced drainage water storage capacity in the unsaturated zone and promoted sub-horizontal water flow enhancing N transport towards the lower landscape positions and contributing to lowland aquifer recharge. This flow, favored by strong lateral hydraulic gradients, and local concentration of solutes by groundwater consumption by crops in the toe slope position, triggered a striking increase of chloride (>500 mg Clֿ l-1) and nitrate (>45 mg N-NO3ֿ l-1) concentrations in phreatic groundwater. During normal to dry periods, drainage fronts from different rainfall events were mixed within the unsaturated zone, delaying water table level response to individual events. Rain water flowed vertically flushing soil N towards a deep water table (with increasing time lags between nitrate concentration in percolating water and phreatic water as the water table deepened) and was ultimately discharged to the stream. The use of a 15N labelled fertilizer applied to corn in field lysimeters allowed to demonstrate that N fertilization represents a minor contribution (1%) to nitrate leaching and that N derived from soil organic matter mineralization constitutes a key source of leachable N during periods of excess water and/or low crop demand. Our results were used to state a conceptual model that explains water and N fluxes in a smoothly undulating agricultural landscape under contrasting climatic scenarios.
We combined field monitoring of crops, soils and groundwater with the use of simulation models and an experiment in lysimeters, to study inorganic nitrogen (N) exchange between terrestrial ecosystems and groundwater under recently intensified cropping schemes of rich organic matter soils. More than ten years of monitoring (1998- 2010) at three landscape positions of the Rolling Pampas allowed to capture plurianual periods of highly variable precipitation (1506 to 444 mm yr-1) that triggered large changes in water table depths (<1 to >6 m), which altered the role of groundwater and crops as modulators of water and N fluxes. Shallow water tables reduced drainage water storage capacity in the unsaturated zone and promoted sub-horizontal water flow enhancing N transport towards the lower landscape positions and contributing to lowland aquifer recharge. This flow, favored by strong lateral hydraulic gradients, and local concentration of solutes by groundwater consumption by crops in the toe slope position, triggered a striking increase of chloride (>500 mg Clֿ l-1) and nitrate (>45 mg N-NO3ֿ l-1) concentrations in phreatic groundwater. During normal to dry periods, drainage fronts from different rainfall events were mixed within the unsaturated zone, delaying water table level response to individual events. Rain water flowed vertically flushing soil N towards a deep water table (with increasing time lags between nitrate concentration in percolating water and phreatic water as the water table deepened) and was ultimately discharged to the stream. The use of a 15N labelled fertilizer applied to corn in field lysimeters allowed to demonstrate that N fertilization represents a minor contribution (1%) to nitrate leaching and that N derived from soil organic matter mineralization constitutes a key source of leachable N during periods of excess water and/or low crop demand. Our results were used to state a conceptual model that explains water and N fluxes in a smoothly undulating agricultural landscape under contrasting climatic scenarios.
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Fecha de defensa de la tesis: 29 de agosto de 2012
Fecha de defensa de la tesis: 29 de agosto de 2012
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Keywords
HIDROLOGIA SUBTERRANEA, LIXIVIACION DE NITRATO, AGRICULTURA, FERTILIZACION, PRECIPITACION, FLUCTUACION DEL NIVEL FREATICO, MATERIALES ORGANICOS, GROUNDWATER HYDROLOGY, NITRATE LEACHING, AGRICULTURE, FERTILIZATION, RAINFALL, WATER TABLE LEVEL FLUCTUATION, SOIL ORGANIC MATTER
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