Submitted to: ASAE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: 12/1/1999
Publication Date: N/A
Citation: Interpretive Summary: Experimental wheat fields at Oklahoma State University are used to test for the existence of a Soil-Plant Buffer (SPB) in the root zone which inhibits nitrate leaching to the groundwater. Such a SPB is believed to provide a "safety zone" that has the capability to regulate or remove significant amounts of fertilizer nitrogen applied over that required to achieve maximum crop yield. To establish the existence of a SPB, it is necessary to estimate the rate of water flow in the soil. This investigation evaluates two methods to estimate water flow rate the soil for conditions encountered at the experimental wheat fields at Oklahoma State University. The first method is based on traditional infiltration theory. Unrealistic small water flow values were obtained by this approach. This was attributed to the fact that infiltration theories are mostly geared towards sandy- and silt-loam soils, and cannot be readily extrapolated to soils with high clay contents such as those encountered at the experimental sites. The second approach for estimating water flow is based on measuring the concentration in the soil of a tracer chemical that was applied at a known rate. The known application rate of the tracer chemical and the resulting concentration in the soil are used to calculate the water flow rate in the soil. Water flow values ranging from about 1 to 4 centimeters per year were produced by this approach. These values are in line with expectations for the soils at the experimental fields. The values are used in a follow-up study to test the existence of a SPB on four winter wheat fields.
Technical Abstract: Nitrate loading through the vadose zone beneath Oklahoma wheat fields has been measured to test the presence of a Soil Plant Buffer (SPB) which inhibits nitrate leaching to the ground water. The experiments have been completed on soil cores obtained from long term experimental plots locate near Stillwater, OK, which have been uniformly fertilized for 29 years. The nitrate mass flux can be calculated as the product of the soil-water Darcy flux and the nitrate concentration. Two indirect procedures have been used to estimate the water flux: the Darcy Flux Method (DFM) and the Chloride Mass Balance (CMB). Both techniques required core sampling of water content, and nitrate-nitrogen. In addition the DFM requires bulk density and particle size to estimate hydraulic properties, which are used to calculate the Darcy flux. In contrast, the CMB requires soil chloride concentrations and chloride mass input to solve for the Darcy flux by simple mass balance. The CMB is considered the superior method in this case due to well-defined chloride inputs. The DFM yielded water fluxes an order of magnitude less than the CMB and well below physically reasonable levels. Results of this study support the SPB theory that fertilizer application up to the amount needed for maximum yield of winter wheat does not increase nitrate leaching to the ground water.