| Abstract: | The underlying question of these investigations asked, how and to which extent rape plants react with transpiration and soil water uptake to different degrees of nitrogen fertilization. Therefore repeated campaigns with concurrent measurements of plant surfaces (leaves, stems, pods), diurnal courses of leaf transpiration and root length density of rape plants growing on heavily (240 kg ha—1), moderately, (120 kg ha—1), and nil N‐fertilized plots of an experimental field in northern Germany were performed during two growing seasons. Additionally, matric potentials at different soil depths were measured. In the first year (1994) investigations were concentrated primarily on shoot area development and transpiration, whereas in the subsequent year (1995) root measurements were mainly undertaken. Also, the influence of soil management (ploughing, conservation tillage) was taken into consideration. The plots where the shoot measurements were carried out were ploughed in 1994 and rotovated in 1995. Matric potentials were measured in both years in ploughed soil and, for comparison, also in soils with conservation tillage. Shoot area index, as measure of the transpiratory capacity of the canopy, increased on ploughed soil and reached a maximum before flowering. Thereafter it decreased until harvest when the relative amount of green stems and pods was increasing. Then, the measured transpiration rate per pod surface area was equal to, or higher than, the transpiration rate per leaf surface area. Plant surface area was smaller in plots with conservation tillage and decreased generally with decreasing N‐fertilization. Increasing plant surface area was joined by an increasing density of plant canopy. Light interception was thus highest in the plots receiving 240 kg N ha—1. Although the shading effect may cause a reduction of transpiration per plant, the total plant mass per area generally resulted in a greater water loss from these plots. Roots reached at least 110 cm depth. Root length density was significantly higher in the upper 10—30 cm of soil than at greater depths. Root mass was smaller in soil with conservation tillage than in ploughed soil. Oscillations of soil matric potentials in the diurnal and long‐term periods were highest in the upper 10 cm of soil. Here, they corresponded well with the cumulative diurnal transpiratory water loss. It is concluded that the soil water dynamics depends largely on the distribution of plant roots. As a result, rape plants did not change their specific transpiration capacity as a response to increased nitrogen fertilization. However, the transpiring plant surface and root length density increased the turnover rate of water by a higher plant density per plot. This effect was more pronounced in ploughed than in rotovated plots. |
