Competition between Medicago truncatula and wheat for N-15 labeled soil nitrogen and influence of phosphorus

To determine competition for nitrogen uptake, a greenhouse experiment was conducted using annual Medicago truncatula cf. Jemalong (medic) and Triticum turgidum spp. durum cv. Karim (wheat) grown alone or in mixture, over five successive crop cycles. Pots were supplied with P equivalent to 0, 50, 100 and 150 kg P2O5 ha(-1) at the beginning of each crop cycle. The soil was labeled by growing wheat plants on soil fertilized with N enriched with 10.43 atom % N-15 excess and by incorporating the labeled wheat as organic matter (2.08 atom % N-15 excess) back into the soil. This method allowed estimation of N, fixed by medic and N transfer to wheat. Compared to the sole medic crop, dry herbage yield and total N of medic grown in mixture, were reduced. Medic plants alone (unmixed) assimilated 80% more soil N that wheat alone at the higher rate of P. When in mixture, wheat plants took up more soil N than medic plants. The percentage N derived from synbiosis (%Ndfa), by the sole medic, averaged 60%. In mixture, %Ndfa of the medic was 80%. The total N difference method gave higher values for %Ndfa (6-22%) than the isotope dilution technique, depending on the P rate. This was because P rates affected the N absorption of medic and wheat differently. Dry herbage yield and total N of medic and wheat grown alone and in mixture were improved by P supply. Competition between medic and wheat was affected by P fertilization, favoring wheat which became more competitive for uptake of soil N at the higher P rates. When no P was added to the soil, mixed medic took up 60% of the soil N absorbed by sole medic; at the higher P rate, this was reduced to 34%. Transfer of N fixed by medic to wheat was detected only after the second crop cycle.