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.

Dinitrogen fixation of microbe-plant associations as affected by nitrate and ammonium supply.

Abstract The dinitrogen fixation activity of Azospirillum sp., and Pantoea agglomerans strains was determined by (15)N(2) incorporation after incubation with (15)N(2) labeled air or/and by acetylene reduction. These bacterial strains were able to fix N(2) both in pure culture and in association with wheat plants in hydroponics. Nitrogenase activity of Azospirillum sp., in pure culture was more rapidly inhibited by the addition of NH(4) (+) than NO(3) (-). The N(2) fixation of P. agglomerans decreased only by NH(4) (+) -addition, but was stimulated by NO(3) (-). Nitrogen fixation in association with wheat plants remained unaffected by both N compounds. However, nitrogen derived from the atmosphere (N(dfa)) contributed only very little to the overall nitrogen nutrition of the plants.

Nitrogen fixation and nitrogen fertilization of soybeans.

Abstract In pot experiments with (15)N labelled soil and mineral (15)N, the influence of Bradyrhizobium (Rhizobium japonicum) inoculation and N fertilization on the symbiotic N(2) fixation and yield of soybeans [Glycine max (L.) Merill., cv. 'Fiskeby V'] was investigated. Symbiotic N(2) fixation only occured after inoculation with Bradyrhizobium. Considerable differences in efficiency of the bacterial preparations were observed. Shortly after flowering, the symbiotic nitrogen fixation was finished and, subsequently, soybeans took up considerable N amounts from the soil. N fertilization at seeding suppressed N(2) fixation of soybeans. In this case, the dry matter and nitrogen yield increased, because the loss of fixed nitrogen was overcompensated by the mineral N uptake. During flowering of soybeans, the N(2) fixation was not affected by N supply, because this process was already terminated. The mineral N was additionally available to the plants and led to increased N amounts in plants. It was absorbed to a considerable degree by soybeans. The mineral N was translocated (partly, after intermediate storage in the vegetative organs) into the seeds thus increasing their yields.

[Relation between mineral fertilization and supply of potassium, calcium, magnesium and phosphorus to the population under conditions of intensive plant production in East Germany]. / Beziehungen zwischen Mineraldüngung und Versorgung des Menschen mit Kalium, Calcium, Magnesium und Phosphor unter den Bedingungen der intensiven Pflanzenproduktion der DDR.

In the past 25 years, mineral fertilization increased considerably in the GDR in the course of the intensification of plant production. This happened on the basis of computer-assisted advice on mineral fertilizing and resulted in the improvement of the mineral contents of the soil and also in a considerable increase in crop yields, the mineral contents typical of the respective plants remaining mostly unchanged or being in part increased. During the same period, the proportion of vegetable foods in the mineral supply to man decreased due to changes in food habits. Nevertheless, most of the P, K and Mg consumed in the nutrition still originates from vegetable products; only in case of Ca, milk is the main course. In the course of this development, the amounts of P, K and Ca consumed in the human nutrition increased, whereas the amount of Mg remained almost unchanged. There is some evidence of the (at last potential) danger of an insufficient supply of Ca and Mg. As the intensive plant production in the GDR furnishes high-mineral crops in sufficient quantity, a better supply of these minerals might be realized by reducing the losses due to processing (extraction rate of cereals, preservation) and by changing the food habits (more vegetable foods, greens).

[Effectiveness of symbiotic n2-fixation in leguminous plants, as affected by inoculation with rhizobia, by substrate, n-fertilizing, and 14c-sucrose application (author's transl)]. / Wirksamkeit der symbiontischen N2-Fixierung der Körnerleguminosen in Abhängigkeit von Rhizobienimpfung, Substrat, N-Düngung und 14C-Saccharoselieferung.

Cultivation experiments (Mitscherlich-vessels, quartz sand, 15N-labelled soil, 15N-fertilizer) showed, that various strains of Rhizobium lupini (white and yellow lupines) and of Rhizobium leguminosarum (field beans and peas) induced a different N2-fixation of the inoculated plants, the most effective Rhizobium strains being 367a, Cz, T3, 271 (Rh. lupini), and Azotogen (Rh, leguminosarum). Yellow lupines and field bean plants were supplied with N2 from the air considerably better than white lupines and peas after inoculation with the most effective Rhizobium strains. Application of mineral N to the white lupines and peas not only substituted the inhibited N2-fixation, but increased N amounts in the plants. White lupines fixed more N2 under soil conditions than in quartz sand. An experiment with steam-sterilized and 15-labelled soil as a comparative substrate showed, that this finding was mainly caused by an additional Rhizobium infection from the soil. Contrary to field beans and yellow lupines which fix N2 up to ripeness, white lupines and peas finished N2-fixation in the time of flowering. Mineral-N applied at that time was an additional source of N for last-named plants and they utilized it for production of higher protein yields. Continual spraying of white lupine plants with 14C-labelled sucrose solution after the time of flowering caused continuance of N2-fixation up to the stage of ripeness. It is assumed that the cause of this effect was the competition of growing seeds and nodules for the photosynthates. The supply of nodules was inadequate without external sucrose application. Mineral N inhibited the sucrose-induced N2-fixation of white lupine nodules and their consumption of photosynthates. Consequently, the applied 14C was transported into seeds to a larger extent. The investigations allow the following conclusion: Effective N2-fixation requires nodules being a powerful sink for assimilates on the basis of a highly efficient photosynthetic system of the host plant.