Time‐course of N 2 ‐fixation ( 15 N) in the field by clover growing alone or in mixture with ryegrass to improve pasture productivity, and inoculated with vesicular—arbuscular mycorrhizal fungi

SUMMARY The isotope 15 N was used to estimate N 2 ‐fixation by white clover, and N‐transfer from clover to perennial ryegrass. Pure stands of each of these pasture plants and a mixed sward of both were established in a field soil in which it had been demonstrated that vesicular‐arbuscular mycorrhizal inoculation was necessary for suitable development of the clover. All plants, whether clover or ryegrass, received the same amount of a selected mycorrhizal inoculum. The 15 N/ 14 N ratio of the grass in pure stands was used to evaluate N 2 ,‐fixation by clover growing alone, and that of the grass in the mixture to estimate that by the companion clover. Tentative evidence of N‐transfer was obtained by comparing the 15 N/ 11 N ratio in the grass growing alone with that when mixed with the legume. In spite of a decline with time in 15 N enrichment, the N 2 ‐fixation rates were about the same throughout the growing season and for the two cropping systems. The total N, P and dry matter yields in the grass/clover mixture were greater than in monocultures. A lower 15 N enrichment of the grass growing mixed compared to that growing alone, suggesting N‐transfer, was only obtained at the second harvest. The mycorrhizal colonization of the grass in the mixed grass/closer sward was significantly enhanced, as compared with that of the grass in pure stands. Mycorrhizal hyphae are known to be involved in NH 4 + uptake, translocation and transfer to the host plants, and this may partly explain the improvement of pasture productivity when grass and mycotropic legumes are grown together.