FATE OF FERTILIZER NITROGEN APPLIED TO WINTER WHEAT AS Na15NO3 AND (15NH4)2SO4 STUDIED IN MICROPLOTS THROUGH A FOUR-COURSE ROTATION

Abstract

Two splitting schedules of a 100 kg/ha nitrogen dressing applied to winter wheat as Na15NO3 and (15NH4)2SO4 have been compared in a microplot study through a four-course rotation (winter wheat, oats, fodder maize, fodder maize) for their influence on soil and fertilizer N. The soil was a well-drained silt-loam containing 0.25 percent CaCO3 by weight. The treatments investigated are referred to as 3-split (NO3), 2-split (NO3), 3-split (NH4), and 2-split (NH4). The percentage uptake of the fertilizer N by the winter wheat is markedly related to the splitting schedule and, to a lesser extent, to the kind of carrier. The 3-split (NH4) treatment leads to a significant 25 percent increase in soil N uptake as compared with the O-N reference microplots; the priming effect calculated for this dressing amounts roughly to 12 kg N/ha. The evolution of the residual fertilizer N in the 0–70 cm soil profile shows higher half-life values for the 2-split than for the 3-split applications, namely 10.0 versus 5.1 years and 13.4 versus 10.9 years for the NO4 and the NH4 carriers, respectively. A tentative evaluation of the denitrification and of the NH3-N volatilization losses related to each split, and of the residual fertilizer N left beyond the 70-cm depth has been made. This approach is based on the evolution of the soil conditions (actual water deficit, drainage, temperature) during the first cropping season and on the regression curves of the residual first year fertilizer N during the last three years of the crop rotation. The NH3-N volatilization loss represents 5 percent of the surface-applied ammonium sulfate. Residual fertilizer N beyond 70 cm does not exceed 2 percent of the applied N dressing. The percentage of N in the crop derived from fertilizer (NDFF) is related to the splitting schedule. Significantly higher NDFF values have been found for the 3-split dressings. Higher values have been calculated for the grain than for the straw. This is true for the fertilizer N taken up by the wheat crop and for the residual N taken up by the oat the second year of the crop rotation.