EFFECT OF WATER-STRESS ON COMPETITION BETWEEN MEDICAGO-TRUNCATULA ANDWHEAT IN N-15 LABELED SOIL

Crop mixtures that include legumes increase yield and restore soil fer tility. These mixtures are limited by drought, which is frequent in Mo rocco due to lack of and poor distribution of rain. However, annual Me dicago species are very important to forage production, and they have a tolerance to water stress. The effect of water stress on N-2 fixatio n and N transfer by Medicago truncatula cv. Jemalong (medic), when mix ed with wheat (Triticum durum cv. Karim), was investigated in three gr eenhouse experiments. Annual medic and wheat were seeded in 20-L pots in pure and mixed stands. Three watering treatments were used: -0.9, - 1.5, and -2.5 MPa in the first experiment; and -0.9, -2.5, and -3.5 MP a in the second and third experiments. Competition between medic and w heat was affected by hydric treatments; the medic was more competitive for soil water than the wheat, especially under severe water stress. Shoot dry weight of wheat grown with medic was higher than that of whe at grown in pure stands. Medic alone took up more soil N than wheat al one, while in the mixture, medic took up less soil N than wheat, indic ating that the mixed medic was not competing for soil mineral N. Using the N-15 dilution technique, it was found that medic N-15 concentrati on was lower than that of the wheat in all treatments, suggesting acti ve N-2 fixation. Apparently, the medic obtained from 20% to 73% of its N from the atmosphere, depending on the water regime. The atoms-perce nt excess N-15 in medic decreased significantly in the mixture, showin g higher N-2 fixation (N-15 dilution method) than in pure medic. Mixed medic derived 82% of its N from fixation at severe water stress (T-2) and 92% at the control (T-0), showing that N-2 fixation by medic was less affected by water stress in the mixed stand than in the pure stan d. In the first experiment, N transfer was detected only in the contro l treatment, but in the second and third experiments, transfer of N wa s significant (p = 0.05).