THE EFFECT OF FERTILIZER PLACEMENT ON NITROGEN UPTAKE AND YIELD OF WHEAT AND MAIZE IN CHINESE LOESS SOILS

Field trials were carried out to study the fate of N-15-labelled urea applied to summer maize and winter wheat in loess soils in Shaanxi Pro vince, north-west China. In the maize experiment, nitrogen was applied at rates of 0 or 210 kg N ha(-1), either as a surface application, mi xed uniformly with the top 0.15 m of soil, or placed in holes 0.1 m de ep adjacent to each plant and then covered with soil. In the wheat exp eriment, nitrogen was applied at rates of 0, 75 or 150 kg N ha(-1), ei ther to the surface, or incorporated by mixing with the top 0.15 m, or placed in a band at 0.15 m depth. Measurements were made of crop N up take, residual fertilizer N and soil mineral N. The total above-ground dry matter yield of maize varied between 7.6 and 11.9 t ha(-1). The c rop recovery of fertilizer N following point placement was 25% of that applied, which was higher than that from the surface application (18% ) or incorporation by mixing (18%). The total grain yield of wheat var ied between 4.3 and 4.7 t ha(-1). In the surface applications, the rec overy of fertilizer-derived nitrogen (25%) was considerably lower than that from the mixing treatments and banded placements (33 and 36%). T he fertilizer N application rate had a significant effect on grain and total dry matter yield, as well as on total N uptake and grain N cont ents. The main mechanism for loss of N appeared to be by ammonia volat ilization, rather than leaching. High mineral N concentrations remaine d in the soil at harvest, following both crops, demonstrating a potent ial for significant reductions in N application rates without associat ed loss in yield.