At each harvest, each single plant was excavated with a soil volume of 50 cm × 33 cm and a depth of 40 cm. For dry weight and N determination, the plant was separated into the shoot and roots. The whole root system was obtained from the soil and washed free of soil. All samples were killed at 105 °C for 30 min, dried at 70 °C until constant weight, weighed (dry weight) and ground into powder. Appropriate amounts of the ground plant material were used to determine the total N content using a modified Kjeldahl digestion method (Nelson and Somers, 1973).