CHARACTERIZATION OF DENITRIFICATION AND LEACHING LOSSES OF FERTILIZER NITROGEN FROM SOIL (EVOLUTION, ISOTOPE, NITROUS OXIDE, DINITROGEN)

Quantification of leaching and denitrification losses of fertilizer N from soil requires that appropriate methods be developed for their measurement. The objectives of this work were to: (1) evaluate potential errors in measuring denitrification by use of ('15)N-tracer techniques recently developed for determination of N(,2) and N(,2)O evolved from soil; (2) evaluate three methods of estimating fertilizer N loss under field conditions (grain yield reductions, an ('15)N difference method, and a direct measurement of denitrification using the procedures in 1); and (3) evaluate the extent of N loss through leaching and denitrification in a laboratory study using semi-disturbed soil columns.;The ('15)N difference study was conducted to determine the amount of fertilizer N loss that would occur from ambient rainfall (0-cm addition) and ambient rainfall plus 7.6 cm of additional water (7.6-cm addition). Of the 168 kg N/ha applied, 141 and 104 kg N/ha were accounted for at harvest for the 0 and 7.6-cm water additions, respectively. Gas analyses accounted for losses of 24 and 44 kg N/ha, respectively, with very little leaching occurring for either treatment. Essentially all of the N loss occurred by the time of the 11-leaf growth stage. The yield study indicated that average corn (Zea mays L.) yield reductions across N rates of 0-224 kg N/ha were great enough to justify application of supplemental N. Using yield as a measure of N loss provided larger estimates than either the ('15)N difference method or gas analyses.;The soil column study included water and fertilizer treatments similar to those used in the field study. Nitrogen evolution amounted to only 1 kg N/ha for the highest water treatment. A separate investigation indicated C availability was apparently limiting denitrification despite the addition of plant residue. The reason for the discrepancy between large N evolution rates under field conditions and small losses under laboratory conditions was not evident.;Computer simulation studies showed that when nitrate undergoing denitrification is not isotopically uniform, N evolution rates will be underestimated. The degree of error increases with an increase in the number of NO(,3)('-) pools present where their isotopic diversity increases (range or standard deviation of the ('15)N enrichments). The error will generally not exceed 25% where the ('15)N enrichments are low (<40 atom %). Where the enrichments remain high (>40 atom %), the error generally will not exceed 10%.