Diffusion and Mass Flow of Nitrate-Nitrogen into Corn Roots Grown Under Field Conditions

Nitrate-nitrogen, the most important source of N of non-leguminous plants, is soluble in soil water and is transported to plant roots by both mass flow and diffusion. It is, therefore, important to evaluate the relative importance of each of these two mechanisms of transport of NO₃-N to plant roots and the environmental conditions under which each is the dominant mechanism of transport. The objective of this paper was to present measurements of plant and soil parameters necessary for estimating accumulation of N grown under field conditions with the use of a theoretical model and to compare estimates of accumulation of N in the corn plant with measured accumulation in the plant. Plant and soil parameters were measured 37, 49, 76 and 97 days after planting on corn grown under field conditions for use in the theoretical model to predict uptake of N by the roots. The plant and soil parameters measured were volumetric soil water content, concentration of NO₃-N in soil solution, porous diffusion coefficient of NO₃-N in soil, average macroscopic velocity of soil water at plant root surface, radius of root, transpiration rate, and plant root length. In the derivation of the theoretical model it was assumed that the flux of NO₃-N into the root was proportional to the concentration of NO₃-N in the soil solution surrounding the root; the constant of proportionality, k, relating these two quantities has been referred to as the “absorbing power of the root.” A linear relationship between flux and concentration was found to exist up to a concentration as high as 160 μg NO₃-N/cm³ of soil solution for corn grown under field conditions. The proportionality constant, k, was found to decrease with time and/or age of the plant; k values of 0.51, 0.35 and 0.26 cm²/sec were found for three harvest dates corresponding to 34 to 49, 49 to 76, and 76 to 97 days after planting, respectively. The errors of the predicted uptake of N as compared with measured accumulation were as large as 50% for the individual harvest periods. However, the error was greatly reduced when the predicted values and measured values were summed over the three harvest periods. In general, the predicted uptake values were smaller than the measured values when the measured values of accumulation were less than 1,200 mg of N/plant; the reverse was true when the measured values of accumulation were greater than 1,200 mg of N.