TIME-COURSE OF NITROGEN IN SOIL SOLUTION AND NITROGEN UPTAKE IN MAIZE PLANTS AS AFFECTED BY FORM AND APPLICATION TIME OF FERTILIZER NITROGEN

Field experiments with silage maize were conducted in 1987 and 1988 on a loess-derived Luvisol in southwest Germany. Four nitrogen fertilizer treatments were compared: application of preplanting NH4-N (plus a nitrification inhibitor, dicyandiamide as Didin(R)) and preplanting NO3-N, split application of NO3-N (preplanting and side dressed 45 days after planting) and a control without nitrogen fertilizer in 1987 and with 64 kg N ha(-1) as calcium ammonium nitrate in 1988. The total amounts of soil mineral nitrogen (N-min + fertilizer N) were 200 kg N ha(-1) in 1987 and 240 kg N ha(-1) in 1988. Suction cups and tensiometer were installed at five depths and samples were taken in regular intervals. Nitrate concentrations in the suction solution steeply increased at 15 cm and 45 cm soil depth 3-4 weeks after fertilizer application (1987 up to 160 mg N l(-1); 1988 up to 170 mg N l(-1)) and steeply decreased up to 75 cm depth with the onset of intensive N uptake at shooting. Ammonium concentrations in the suction solution were very low (0-0.16 mg N l(-1)). Compared to preplanting NO3-N application, preplanting NH4-N and split NO3-N application decreased nitrate concentrations in the suction solution in spring 1987. In 1988, however, nitrate concentrations in the suction solution of preplanting NH4-N and split NO3-N application plots did not fall below 50 mg N l(-1) at 15 cm depth during the growing season. Nitrate concentrations of split NO3-N application increased again in autumn 1988 and hence doubled the calculated N losses by leaching during the winter months compared to preplanting N applications. At shooting, plants of the preplanting NH4-N treatment had lower nitrate concentrations in leaf sheaths compared to plants of preplanting NO3-N application. Total N uptake of maize between shooting and early grain filling of preplanting NH4-N and split NO3-N application tended to be higher compared to preplanting NO3-N application, reflecting the higher N availability in the soil later in the season. However, final dry matter yields and N uptake were not significantly affected by N form or time of N application. Since N losses by nitrate leaching between N application and onset of N uptake by plants were negligible on the experimental site, preplanting NH4-N application and split NO3-N application showed no agronomic advantages. High amounts of side dressed NO3-N may increase nitrate leaching during the winter months, especially in years with delayed rainfall after application.