Transfer of symbiotically fixed nitrogen in an alfalfa-grass mixture studied through isotope dilution in a pot experiment.

The nitrogen transfer between alfalfa and ryegrass was studied through isotope dilution at three different levels of N fertilization (20 mg N/pot, 200 mg N/pot, 400 mg N/pot) in a pot experiment using quartz sand as a substrate. An isogenic, nodulating, but non nitrogen fixing alfalfa line was used as a reference crop. Fixed N was transferred to the grass in the 20 mg N treatment and contributed markedly to the N nutrition of the grass (about 50% of the N in the plants). No transfer of fixed N could be detected in the higher fertilized treatments, although nitrogen fixation was only slightly inhibited by the presence of the fertilized mineral N. It is concluded that N transfer is strongly influenced by the N concentration in the substrate and transferred N contributes only slightly to the productivity of the legume/grass mixture under the given experimental conditions.

The Effect of an 15NH415NO3 Fertilization at Flowering on Growth and Nitrogen Fixation of White and Blue Lupins.

It is well established that legume species differ in intensity and duratio of nitrogen fixation during pod-filling. This was studied on new lines of white and blue lupins by comparing a solely nitrogen fixing and a treatment supplied with 15N-labelled fertizlier at flowering in pot experiments. Nitrogen application at flowering markedly incresed seed and total dry matter and seed and total N yield in white lumpins but remained without effect in blue lupins. simply replacing nitrogen fixation. Nitrogen fixation virtually ceased with the onset of pod-filling in white lupins, causing a massive N-remobilization from vegetative plant parts. In contrast, the blue lupins fixed about 70% of total nitrogen fixation after flowering and displayed growth and net-N-assimilation of the vegetative plant parts. In both species the stems and pods especially supported seed N-filling, indicated by a low% N and a high C/N ratio at ripeness. It is concluded that pod-filling proved to be a critical stage for the seed yield in grain legumes. Exploiting differences in nitrogen fixation in this growth stage, e.g., by breeding, needs more precise knowledge about the regulation mechanisms and source-sink relations.