In a 2-year field study conducted on a high fertilized Gleyic Luvisol in Stuttgart-Hohenheim significant differences among 10 maize cultivars were observed in soil nitrate depletion. The different capability of the cultivars to utilize nitrate particularly from the subsoil was positively correlated with (a) shoot N uptake at maturity, and (b) root length density (Lv) in the subsoil layers at silking. "Critical root length densities" for nitrate uptake were estimated by (a) calculating uptake rates per unit root length (U), (b) subsequent calculation of needed nitrate concentration in soil solution (Cl) to sustain calculated U according to the Baldwin formula, and (c) reducing measured Lv and proportionate increase of U until needed concentration equaled measured concentration. Uptake rate generally increased with soil depth. "Critical root length densities" for cultivar Brummi (high measured root length densities and soil nitrate depletion) at 60-90 cm depth ranged from 7 % (generative growth) to 28 % (vegetative growth) of measured Lv Measured root length density of each other cultivar was higher than "critical root length density" for Brummi indicating that the root system of each cultivar examined would have been able to ensure N uptake of Brummi. Positive relationships between root length density and nitrate utilization as indicated by correlation analysis therefore could not be explained by model calculations. This might be due to simplifying assumptions made in the model, which are in contrast to non-ideal uptake conditions in the field, namely irregular distribution of roots and nitrate in the soil, limited root/soil contact, and differences between root zones in uptake activity. It is concluded from the field experiment that growing of cultivars selected for high N uptake-capacity of the shoots combined with "high" root length densities in the subsoil may improve the utilization of a high soil nitrate supply.

In a 2-year field experiment conducted on a Gleyic Luvisol in Stuttgart-Hohenheim one experimental and nine commercial maize cultivars were compared for their ability to utilize soil nitrate and to reduce related losses of nitrate through leaching. Soil nitrate was monitored periodically in CaCI 2 extracts and in suction cup water. Nitrate concentrations in suction water were generally higher than in CaC12 extracts. Both methods revealed that all cultivars examined were able to extract nitrate down to a soil depth of at least 120cm (1988 season) or 150cm (1987 season). Significant differences among the cultivars existed in nitrate depletion particularly in the subsoil. At harvest, residual nitrate in the upper 150cm of the profile ranged from 73-110kgNha 1 in 1987 and from 59-119kgNha -1 in 1988. Residual nitrate was closely correlated with nitrate losses by leaching because water infiltration at 120cm soil depth started 4 weeks after harvest (1987) or immediately after harvest (1988) and continued until early summer of the following year. The calculated amount of nitrate lost by leaching was strongly influenced by the method of calculation. During the winter of 1987/88 nitrate leaching ranged from 57-84 kg N ha -~ (suction cups) and 40-55 kg N ha -1 (CaCl 2 extracts), respectively. The corresponding values for the winter of 1988/89 were 47-79 and 20-39 kg N ha -~, respectively.

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