BIDIRECTIONAL TRANSFER OF NITROGEN BETWEEN ALFALFA AND BROMEGRASS - SHORT AND LONG-TERM EVIDENCE

Transfer of N from legumes to associated non-legumes has been demonstr ated under a wide range of conditions. Because legumes are able to der ive their N requirements from N-2 fixation, legumes can serve, through the transfer of N, as a source of N for accompanying non-legumes. Stu dies, therefore, are often limited to the transfer of N from the legum e to the non-legume. However, legumes preferentially rely on available soil N as their source of N. To determine whether N can be transferre d from a non-legume to a legume, two greenhouse experiments were condu cted. In the short-term N-transfer experiment, a portion of the foliag e of meadow bromegrass (Bromus riparius Rhem.) or alfalfa (Medicago sa tiva L.) was immersed in a highly labelled N-15-solution and following a 64 h incubation, the roots and leaves of the associated alfalfa and bromegrass were analyzed for N-15. In the long-term N transfer experi ment, alfalfa and bromegrass were grown in an N-15-labelled nutrient s olution and transplanted in pots with unlabelled bromegrass and alfalf a plants. Plants were harvested at 50 and 79 d after transplanting and analyzed for N-15 content. Whether alfalfa or bromegrass were the don or plants in the short-term experiment, roots and leaves of all neighb ouring alfalfa and bromegrass plants were enriched with N-15. Similarl y, when alfalfa or bromegrass was labelled in the long-term experiment , the roots and shoots of neighbouring alfalfa and bromegrass plants b ecame enriched with N-15. These two studies conclusively show that wit hin a short period of time, N is transferred from both the N-2-fixing legume to the associated non-legume and also from the non-legume to th e N-2-fixing legume. The occurrence of a bi-directional N transfer bet ween N-2-fixing and non-N-2-fixing plants should be taken into conside ration when the intensity of N cycling and the directional flow of N i n pastures and natural ecosystems are investigated.