Initial studies on the legume: rhizobium symbiosis

Twenty strains of cowpea Rhizobium have been screened to date for their ability to form nodules and to fix nitrogen using the Leonard jar assembly. The strains were tested on Ife Brown, TVu 1190, and TVx 222-01B. From among the twenty, four promising strains were selected. The nodulation and nitro-gen fixing ability of these strains plus Nitragin commercial inoculant on the three cowpea varieties were tested in two soils in the greenhouse. Many of the selected strains caused an increase in the count of nodules on Ife Brown in both Apomu and Egbeda soils. Inoculation with the selected strains improved nodulation on TVx 22-01B in Apomu but not in Egbeda soil. Innoculation of cowpea Rhizobium did not increase the numbers of nodules formed on TVu 1190 in any of the soils. In an experiment performed in the greenhouse in which inoculated seeds were either pelleted or not pelleted, it was shown that nodulation was not affected by the pelleting material used. However, the uninoculated controls were practically not nodulated. Counts of the rhizosphere population of R. japonicum revealed that greater numbers of the bacteria persisted in the soil planted with pe11eted seeds than where only inoculation was practised. The best pelleting material was rock phosphate followed by CaSi03

Biological nitrogen fixation in trees in agroecosystems: twenty years of biological nitrogen fixation research in Africa.

With the low input of fertilizers into African farming systems, it is necessary to harness biological nitrogen fixation to its fullest extent. The greatest potential, at least in the short term, lies in nitrogen fixation in grain and pasture legumes and in nitrogen-fixing trees. To maximise nitrogen fixation in these plants and under various cropping systems requires intensified research. Inoculation techniques and inoculum production in Africa are not yet advanced, and this is unlikely to change until unequivocal proof for their need has been established. The present indication is that several indigenous grain legumes do not respond to inoculation. This has been interpreted to mean that, for locally cultivated grain legumes, highly competitive and effective Rhizobium strains abound in the soils of Africa. More research is needed to prove these claims. Studies on the ecology ofRhhobium need to be intensified, with some mechanistic studies to understand the possible factors underlying competition between Rhizobium strains. For most nitrogen-fixing trees, although the microbial component (Rhizobium or Frankia) has been little studied in Africa, there is a strong indication that rhizobial or Frankia inoculation will be most beneficial

Field measurements of nitrogen fixation in leguminous trees used in agroforestry systems: influence of 15Nlabelling approaches and reference trees

Appropriate 15N-labeling methods are crucial for estimating N2-fixation in trees used in agroforestry systems. A 4-year field experiment was conducted on an Alfisol in Southwestern Nigeria to compare the estimates of Na fixed in Leucaena leucocephala, using two non-N2-fixing leguminous trees, Senna siamea and S. spectabiIis, as reference plants and three different methods of introducing 15N into soil. The atom % 15N uptake pattern (as reflected in the leaves) was identical in both N2- and non-N2-fixing tree species irrespective of the 15N-application method. There was a significant decline in atom % 15N excess in the leaves of L. leucocephala (from 0.266 to 0.039), S. siamea (0.625 to 0.121), and S. spectabilis (from 0.683 to 0.118) from the first sampling 12 months after planting and the second sampling 18 months after sampling. From the second harvest in 1991 until the end of the experiment (fifth) harvest in 1993, however, the atom 15N % excess decline in leaves of the three species was less pronounced and depended on the method of 15N application. In these plants to which the tracer was applied once at planting, the 15N decline was steady between the second and the last prunings. In the split- application treatment, the atom 15N % excess increased slightly at the third pruning and decreased during the subsequent two prunings. The reference tree and the method of 15N application influenced the estimated proportion of N derived from atmospheric N2 by L. leucocephala, calculated as 73 and 64%, corresponding to 119 and 98 kg N ha-1 of N2 fixed per 6 months, when S. spectabilis and S. siamea were used as reference trees, respectively. The approach by which 15N- labeled fertilizer was applied to the soil in three splits gave slightly higher estimates of N derived from the atmosphere but this was of little agronomic significance because total N2 fixed was similar for all methods

Assessment of genetic variability for N2 fixation between and with provenance of Leucaena leucocephala and Acacia albida estimated by 15N labelling techniques

Nitrogen fixed in 13 provenances of Acacia albida and 11 isolines of Leucaena leucocephala inoculated with effective Rhizobium strains was measured by 15N techniques and the total N difference method. In the test soil, on the average, L. leucocephala derived about 65% of its total N from atmospheric N2 fixation compared to about 20% by A. albida. Significant differences in the percentage of N derived from atmospheric N2 (% Ndfa) occurred, between provenances or isolines within species. The % Ndfa ranged from 37 to 74% within L. leucocephala and from 6 to 37 within A. albida ; (equivalent to 20-50 mg N plant-1 and 4-37 mg N plant-1 for the two species over three months, respectively) and was correlated with the nodule mass (r = 0.91). The time course of N2 fixation of three selected provenances (low, intermediate and good fixers) was followed at 12 weekly intervals over a 36 week period. The % Ndfa of all provenances and isolines increased with time; and except for one of the L. leucocephala provenances, % Ndfa was similar within species at the 36 weeks harvest. There was a significant correlation between % Ndfa and the amount of N2 fixed (r = 0.96). Significant interactions occurred between provenances and N treatments and often growth of uninoculated but N fertilized plants was less variable than for inoculated unfertilized plants

Genetic variability in symbiotic nitrogen fixation within and between provenances of two Casuarina species using the 15Nlabelling methods

Differences in the nitrogen-fixing abilities of provenances of Casuarina equisetifolia and C. cunninghamiana were assessed in pot experiments. Three methods, the 15N isotope dilution, the A value and total N difference were used to measure N2 fixed. There was a good agreement between the A value and isotope dilution methods for measuring N2 fixed. The total N difference method gave unreliable values, with large coefficients of variation. There were significant differences in the proportions and amounts of N2 fixed in the two Casuarina species with C. equisetifolia deriving on the average 63% or 45 mg N plant−1 from atmospheric N2 fixation, compared to 43% or 22 mg N plant−1 by C. cunninghamiana. Nitrogen fixation also varied substantially within provenances of each species with the percentage of N derived from atmospheric N2fixation (% Ndfa) ranging from 14 to 76% for the C. cunninghamianaprovenances and from 25 to 75% within C. equisetifolia (equivalent to 2–25 mg N plant−1 and 4–29 mg N plant−1 for the two species, respectively). Growth of C. equisetifolia and C. cunninghamianaincreased with either inoculation with Frankia or N fertilizer addition, but marked differences developed between these N treatments with time. Growth of inoculated plants was more variable (CV = 38%) than that of plants dependent on soil N of fertilizer N. This variation in the growth of the inoculated plants was thus due to the large differences in the N2-fixing abilities than to intrinsic growth differences

Intraspecific variation in growth and P accumulation of Leucaena leucocephala and Gliricidia sepium as influenced by soil phosphate status

Twenty-three provenances of Gliricidia sepium and eleven isolines of Leucaena leucocephala were examined at a low and at high phosphate levels (20 and 80 mg P kg-1 soil) for growth, phosphate (P) uptake and use efficiency. Large differences in growth at the low P level, and in growth response to the higher P rate occurred among L. leucocephala isolines and G. sepium provenances. Shoot dry weight atlow P varied from 1.30 to 3.01 g plant-1 for L. leucocephala and from 1.44 to 3.07 g plant-1 for G Sepium. Leucaena isolines had only half the root weight of G. sepium provenances yet produced approximately 90% of the shoot weight of the corresponding G. sepium treatments, i.e. more than 2-fold difference in root/shoot ratios. Total P in shoots of G. sepium was some 85% greater than of the respective L. leucocephala isolines in corresponding treatments. Physiological phosphate use efficiency (g shoot/mg P in shoots) (PPUE) was not a simple reciprocal relation, being markedly lower at higher shoot % P and content. However, for the same shoot P both species produced the same shoot weight. Nevertheless, there were apparent genotypic differences within species in the root development, shoot P and PPUE In another study, the numbers of rhizobia in the rhizosphere of L. leucocephala nodulation, N2 fixation at five different levels of P were determined. The numbers of rhizobia in the rhizosphere of inoculated L. leucocephala during the first two weeks were lower when P was added but later became similar to those without added P. Nodules formed earlier than inoculated plants fertilized with P and in greater numbers (4- to 5-fold) and dry weights than in those without P. However, the percentage of N2 derived from fixation did not change with increasing levels of P application. These results suggest that the observed P effect did not operate via stimulated growth of rhizobia in the rhizosphere, nor through increased N2 fixation rate. The major effect appeared to be due to effects via plant growt