Growth and n 2 fixation by field grown Medicago littoralis in response to added nitrate and competition from Lolium multiflorum

Abstract

Medicago littoralis (medic) plants were grown with varying proportions of Lolium multiflorum (ryegrass), in field microplots, containing 15N labelled soils. Nitrate was applied to the microplots at either of two depths (5 or 20 cm) and at two times (germination and flowering of the medic). Plants were regularly clipped and the harvested plant materials analyzed to estimate N 2 fixation by the 15N dilution technique. The labelling method used ensured that the added 15N was incorporated into the soil organic fraction, producing a stabilized enrichment of the mineral N released, thus eliminating errors due to uptake missmatches between legume and grass. Nitrate, applied at germination, at 5 or 20 cm, significantly reduced the proportion of N fixed by medic grown with ryegrass by similar amounts. The proportions of N fixed by medic plants generally decreased with increasing proportions of medic, and when nitrate was added at flowering. However, when the nitrate was applied at 20 cm, this effect was not evident—probably due to strong competition from ryegrass roots removing most of the nitrate applied at that depth. Surface-applied nitrate always decreased amounts of N fixed, and also decreased amounts of N fixed g − plant material (from 29 for the nil nitrate to 13 and 20 mg g −1 for nitrate at germination and at flowering, respectively). Increasing proportions of ryegrass in the mixtures also decreased amounts of N fixed. Despite defoliation and water stress, there was no evidence of transfer of fixed N from medic to ryegrass in any treatment.