Root Morphology and Nitrogen Uptake of Maize Simultaneously Supplied with Ammonium and Nitrate in a Split-root System

We studied the response of maize (Zea mays L. cv. Anjou 256) to a simultaneous, but separated supply of ammonium and nitrate (localized supply, LS). A split-root system was used to supply half of the roots with ammonium and the other half with nitrate. A homogeneously distributed supply of both nitrogen forms (HS) was the control treatment. Seedlings were grown for 12 d from the two-leaf to the three-leaf stage in hydroponics at three pH levels (4, 5·5 and 7). The total N concentration was 3 mol m-3. The split-root system was established by removing the seminal root system and using only four nodal roots per plant. Total root length and root surface area were recorded automatically with a modified Delta- T area meter. Other morphological root traits (such as main axis length and diameter, number, density, and length of laterals) were recorded manually. Uptake of ammonium and nitrate was measured by the depletion of the nutrient solution. As compared with LS, HS was superior in shoot and root DM, total root length and root surface area, ammonium and nitrate uptake and shoot nitrogen concentration, irrespective of pH level. This indicates that, also under field conditions, mixed ammonium and nitrate fertilization is only beneficial to plant growth if both N forms are evenly distributed in the soil. At both HS and LS, ascending pH increased the ammonium:nitrate uptake ratio. At LS, declining pH induced a considerable shift in the distribution of root DM, root length, and root surface area the nitrate-fed compartment.