Aiken, R. M. 1992. Functional relations of root distributions with the flux and uptake of water and nitrate. Dissertation , Michigan State University, East Lansing, Michigan, USA.
Nitrite leaching reflects poor nutrient retention, poses a hazard to public health and a challenge to solute transport theory. Field observations and numerical simulation of soil-plant interactions are integrated to identify sensitivity of simulated nitrate leaching to errors in predicted root function. Seasonal changes in maize root distributions, canopy development, and gradients in soil water content and carbon dioxide partial pressures were quantified in 1990 and 1991 during water deficits in a field lysimeter under an irrigated, rain shelter. Seasonal changes in these parameters and in soil mineral and leached N were determined during 1991 in four field lysimeters subjected to conventional or no-till crop culture. Horizontal and vertical gradients in root intersections with 0.05 (I.D.) x 1.4 m polybutyrate tubes corresponded with transient deficits in plant water supply, and subsequent root proliferation during mid-vegetative growth stages. A horizontal complement to the vertical rooting front is characterized by exponential distributions of inter-root distances under a row crop. Geostatistical measures of clustered root distributions indicate spatial correlation up to 0.45 m at anthesis. Failure to consider depth-dependent gradients in root xylem potential most likely accounts for systematic bias in soil water depletion predicted by a simplified solution to a cylindrical model of root water uptake., Soil plus root respiration is related to vertical root distributions, but vertical gradients in C02 and C02 flux fail to satisfy conditions for the steady state assumption. Increased N03-N retention in conventional till soil, relative to no-till soil indicates solute partitioning among mobile and immobile regions of soil water may be modified by historic tillage effects. Deviations in N03-N concentrations of leachate from seasonal trends coincide with extreme high or low drainage flux conditions, invalidating assumptions of homogeneous pore velocities and solute concentrations. Simulated N03-N leaching rates are sensitive to errors in predicted infiltration, canopy dimensions and drainage below the root zone, but are insensitive to reductions in maximum root length density. Managing soil-plant interactions for optimal productivity and solute retention requires accurate simulation of system behaviour when regulation shifts from atmospheric boundary conditions to soil system transport and transformation processes.
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