Go. Tomm et al., BIDIRECTIONAL TRANSFER OF NITROGEN BETWEEN ALFALFA AND BROMEGRASS - SHORT AND LONG-TERM EVIDENCE, Plant and soil, 164(1), 1994, pp. 77-86
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.