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African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 5, Num. 1, 1997, pp. 87-92
African Crop Science Journal, 1997, Vol. 5. No. 1, pp. 87-92.

SHORT COMMUNICATION

Influence of molybdenum and cobalt fertilisation on symbiotic nitrogen fixation indicators in an oxisol in Uganda

J.S. Tenywa

Department of Soil Science, Makerere University, P.O. Box 7062, Kampala, Uganda

(Received 13 February, 1996; accepted 5 February, 1997)


Code Number: CS97011
Sizes of Files:
    Text: 16.2K
    Graphics: Tables (gif) - 17K 
ABSTRACT

Molybdenum (Mo) and cobalt (Co) are important micro-elements involved in biological N2-fixation. Uganda has embarked on a campaign to increase the productivity of legume crops through utilisation of rhizobial inoculants, unfortunately, non-responsiveness is frequently encountered. Among the suspected causes are inadequacies of Mo and Co supply in the soil. A glasshouse study was, therefore, conducted with an Oxisol to examine this relationship. Treatments included provision of lime at 0 and 1.0 t ha^-1, Mo at 0, 390 and 780 g ha^-1 and Co at 0, 454 and 907 g ha^-1. Soybean (Glycine max Merrill) cultivar NAM.1 was the test crop. Cobalt application at a rate of 454 g ha^-1 resulted in the greatest nodulation and number of effective nodules per plant, as well as total N accumulation in the shoots. Liming depressed soil solution acidity by 0.2 pH units. A combination of lime (1 t CaCO3 ha^-1) and Co (454 g ha^-1) increased nodulation and the number of effective nodules per plant, but masked cobalt's positive effect on total N accumulation in shoots. Liming alone did not affect the biological N fixation indicators considered in this study.

Key Words: Biological nitrogen fixation, inoculation, liming, soybean

RESUME

Le molybdene (Mo) et le Cobalt (Co) sont des micro-elements importants pour la fixation d'azote biologique. L'Ouganda mene une campagne pour elever la productivite des legumes en utilisant des inoculations Rhizobiales, mais souvent sans succes. Parmi les raisons possible, on a de quantites insuffisantes de Mo et Co dans le sol. Pour examiner cette relation, une etude a ete conduite dans la serre avec un Oxisol. On utilisait des concentrations sub-normales de la chaux, Mo et Co. Le soya (Glycine max Maerril), cultivar NAM.1 etait teste. L'application de 454 g/ha de Co resultait en une nodulation elevee et une augmentation du nombre de nodules effectives par plante, mais aussi dans l'accumulation d'azote totale. L'application de la chaux diminuait l'acidite du sol avec 0.2 unites de pH. Une combinaison de chaux (1 t CaCO3 /ha) et Co (454g/ha) augmente la nodulation et le nombre de nodules effectives par plante, mais masquait l'effet positive de l'accumulation de l'azote totale dans les pousses. L'application uniquement de la chaux n'influencait pas les indicateurs de fixation biologique N dans cette etude.

Mots Cles: la fixation d'azote biologique, inoculation, chauler, le soya

INTRODUCTION

The importance of nitrogen (N) availability in crop production has gained considerable appreciation by Ugandan farmers. This is evidenced by their enthusiastic purchase of rhizobial inoculants currently produced at Makerere University. Higher legume yields have been realised with enhanced biological N2-fixation (BNF), both on-station and in farmers' fields. The long-term success of this technology, however, will depend not only on the continued refinement of the capability of the microsymbiont association to fix N, but also on the suitability of the soil conditions for the association. Appropriate soil reaction, as well as adequate supply of molybdenum (Mo) and cobalt (Co), are necessary for the successful establishement of the legume-Rhizobium association (Bergersen, 1971; Evans and Russell, 1971; Somasegaran and Hoben, 1994). Molybdenum is a vital component of nitrogenase, the enzyme responsible for mediating the actual N2 reduction processes during N2-fixation (Bergersen, 1971; Marschner, 1986; Somasegaran and Hoben, 1994). Cobalt, on the other hand, is the metal component of Vitamin B12 (Mengel and Kirkby, 1982; Somasegaran and Hoben, 1994) which appears to serve as a coenzyme in BNF reactions (Kliewer and Evans, 1963; Marschner, 1986). Furthermore, Co has been associated with the functioning of leghaemoglobin, the substance responsible for controlling oxygen dynamics within the nodule during N2-fixation (Evans and Russell, 1971; Marschner, 1986; Somasegaran and Hoben, 1994).

The supply of Mo largely depends on the inherent presence of the element in the soil as well as the prevailing pH. Optimum Mo availability occurs over a pH range of 5.8-6.8 (Wild, 1988). Most Uganda soils used for legume cultivation have pH values in the range of 5-5.9 (Nkwiine, C., Makerere University, Kampala; pers. commun.). Hence, Mo could be a potentially limiting factor to biological N2-fixation. However, there is a lack of information pertaining to the status of Mo and Co in Ugandan soils as well as their influence on biological N fixation. This study, therefore, was designed to: (i) determine the relationship between soil pH and N2-fixation in a Ugandan Oxisol, (ii) establish the status of Mo and/or Co as limiting factors in symbiotic N2-fixation in the study soil, and (iii) determine the amount of supplementary Mo and/or Co fertilizers needed to enhance symbiotic N2-fixation in the study soil.

MATERIALS AND METHODS

A glasshouse experiment was conducted at Makerere University Agricultural Research Institute, Kabanyolo from March to December, 1993 using soils from a continuously cultivated but unfertilised field. The study soil was an Oxisol (Yost and Eswaran, 1990). Soil samples were collected randomly in the field at a depth of 0-20 cm, and air-dried at room temperature. Treatments included application of lime at two rates (0 or 1.0 t CaCO3 ha^-1); molybdenum at three levels 0, 1.0 and 2.0 kg sodium molybdate (0, 390 or 780 g Mo) ha^-1; and Co at three rates 0, 1.0 and 2.0 kg C Cl2 (0, 454 or 907 g Co ) ha^-1.

The experiment was of a completely randomised design with three replications. Plastic pots of four-litre capacity and lined with a polythene sheet were filled with 4 kg of air-dried soil previously bulked and thoroughly hand-mixed with the treatment additions. Equal amounts of other nutrient elements including P, K, Ca, Mg, and S were supplied to each pot as basal fertilisation (Table 1). Each pot was then sown with eight seeds of soybean (Glycine max L.) cultivar NAM.1 dressed with Bradyrhizobium inoculum (strain TAL 102) obtained from the Department of Soil Science, Makerere University. This is a commercially available product which is used by farmers. Pots were watered with de-ionised water up to field capacity (16%). Subsequently, watering was calibrated by weight as frequently as was necessary to maintain the soil near field capacity. Seven days after germination, the seedlings were selected for uniformity and thinned to two plants per pot.

Base-line soil analysis was done in order to characterise the study soil. Standard laboratory procedures (Page et al., 1982) were used to determine soil pH, exchangeable K^+, Ca^2+, Mg^2+, Bray 1 P and organic carbon content (Table 2). Available Mo and Co were not assessed due to lack of facilities.

The plants were harvested from each pot at 30% bloom, by hand-cutting the stems at 1 cm above the soil surface. The samples were oven-dried at 70 C for 48 hr, weighed, ground and analysed for total nitrogen content using the micro-Kjeldahl procedure. The remaining pot contents were emptied, each on a polythene sheet, and the root nodules sorted out carefully. Nodules were counted; each one was cut cross-sectionally and scored for effectiveness with respect to N2-fixation based on red-pinkish colouration. All nodule samples were oven-dried at 70 C for 48 hr and weighed. Soil samples were collected from each pot, air-dried and analysed for soil pH, exchangeable K, Ca, and Mg, Bray 1 P, and organic matter using the same standard procedures. The experiment was repeated once. Data analysis was performed using MSTAT-C software and means separated by Fisher's LSD test.

RESULTS AND DISCUSSION

Base-line soil analysis results are presented in Table 2. The soil was moderately acidic with a pH value below the optimal N fixation range of 5.6-6.8 (Somasegaran and Hoben, 1994). Soil organic matter was fairly high as expected of many Oxisols (Sanchez, 1981). The concentrations of exchangeable bases, namely K^+, Na^+, Ca^2+, and Mg^2+, were typical of most Oxisols. The concentration of Bray 1 extractable P, however, was rather low for soybean nutrition perhaps reflecting the P precipitation (fixation) characteristics of the soil.

Liming at a rate of 1.0 t CaCO3 ha^-1 effectively depressed hydronium ion concentration (pH) of the soil by 0.2 pH units (Table 3). This pH change was realised even at the low lime rate. Neither Mo, Co nor their combined application with lime influenced the hydronium status of the soil.

Combined application of lime and Co significantly increased the number of nodules produced per soybean plant (Table 4). The effect of Co was evident at both levels of lime, except at the highest rate of application of both materials where a depressive effect occurred. The positive interaction effect of lime and Co (up to 454 g Co ha^-1) on soybean in these results nodulation is difficult to explain (Table 5). Lime, however, was likely to have reduced the toxic effects of Al^3+ and Mn^2+ (common in Uganda soils; Chenery, 1954) to plant growth and nodulation, apart from supplying Ca and increasing the availability of other nutrients (Table 6). The effect of Co, however, requires further investigation since this element is not universally a plant nutrient. At the highest rate of application, Co suppressed nodule formation. Application of Mo, either alone or in combination with lime or Co, did not significantly influence nodule production per plant. None of the treatments, either in combination or in single application, affected nodule or plant shoot weights significantly.

Total N in soybean shoots was significantly increased by Co application. Lime or Mo application, even in combination with Co, did not change the total N status of the plants. On the other hand, liming along with Co application increased the number of effective nodules per plant, suggesting that Co is needed for effective BNF in this soil (Table 5). This is likely to be true in light of the age, origin and weathering history of most Ugandan Oxisols. The soils are largely derived from old metamorphic rocks of Pre-cambrian basement complex origin, which are highly weathered and leached of most structural cations (King and DeSwardt, 1967). Neither liming nor Mo application influenced plant N content. The effect of Co on shoot N content is unlikely to have been due to a synergistic effect on plant available N forms; at least the literature is not suggestive of this relationship. The lack of N content response to Mo application may suggest that Mo is present in sufficient concentration in the soil for BNF. The lack of response to liming might also be a reflection of the Mo adequacy for BNF activities. The effect of lime on the number of effective nodules may have been through improved soil conditions for nitrogen fixation, for instance, better pH, higher availability of P, Ca (Tables 6) and Mg (data not shown).

The relationship between dry nodule and shoot weight was not significant (r = 0.44; P >0.05). This deviated from previous reports that both parameters are positively correlated (Somasegaran and Hoben, 1994). Whether or not this relationship reflects nitrogen fixation potential remains a question for investigation.

In conclusion, it is apparent that biological N2 fixation by soybean is responsive to Co and not Mo application on the study soil. Cobalt at a rate of 454 g ha^-1 resulted in the highest shoot total N accumulation, which could have been a reflection of biological N2-fixation capacity. Liming per se did not influence any of the N2-fixation indices measured. Liming in combination with Co application, but only up to 454 g of Co ha^-1, increased nodulation and the development of effective nodules, but did not increase shoot total N content. Neither of the measured parameters responded to Mo application. This contradicts the general belief that Mo is deficient in most soils non-responsive to rhizobial inoculation. Further research is necessary to compare these results generated under glasshouse conditions with field conditions and for the different soils across the country which are non-responsive to Rhizobial inoculum.

ACKNOWLEDGEMENTS

The United States Agency for International Development and the Manpower for Agricultural Development Project are appreciated for providing funds. Christine Najjuma and Harriet Nambaziira assisted with glasshouse operations and David Bwamiki with data analysis, for which I am grateful. Charles Nkwiine provided valuable technical input in this project.

REFERENCES

Bergersen, N.F.J. 1971. Biochemistry of symbiotic N2-fixation in legumes. Annual Review of Plant Physiology 22:121-140.

Chenery, E.M. 1954. Minor elements in Uganda soils. In: Proceedings of the 2nd Inter-African Soils Conference. pp. 1157-1163. Leopodville.

Evans, H.J. and Russell, S.A. 1971. Physiological chemistry of symbiotic N2-fixation by legumes. In: The Chemistry and Biochemistry of N Fixation. Postgate, J.R. (Ed.), pp. 191-244. Plenum Publishing Co.

King, B.C. and DeSwardt, A.M.J. 1967. Geological Survey of Uganda, Memoir No. XI. Problems of Structure and Correlation in the Precambrian Systems of Central and Western Uganda. Published by authority of the Uganda Government.

Kliewer, M. and Evans, H.J. 1963. Identification of cobamide coenzyme in nodules of symbionts and isolation of the B12 coenzyme from Rhizobium meliloti. Plant Physiololgy 38:55-59.

Marschner, H. 1986. Mineral Nutrition of Higher Plants. Academic Press, U.K. 674 pp.

Mengel, K. and Kirkby, E.A. 1982. Principles of Plant Nutrition (3rd Ed.). International Potash Institute, Bern, Switzerland.

Page, A.L., Miller, R.H. and Keeney, D.R. 1982. Methods of Soil Analysis Part 2: Chemical and Microbial Properties. 2nd Ed. American Society of Agronomy, Monograph No. 9. ASA, Madison Wisconsin, U.S.A.

Sanchez, P. 1981. Properties and Management of Soils in the Tropics. Wiley and Sons, New York. 618 pp.

Somasegaran, P. and Hoben, H.J. 1994. Handbook for Rhizobia: Methods in Legume-Rhizobium Technology. Springer-Verlag, New York. 450 pp.

Wild, A. 1988. Soil Conditions and Plant Growth. 11th Ed. Longman, U.K. 972 pp.

Yost, D. and Eswaran, H. 1990. Major Land Resources of Uganda. World Soil Resources, Soil Conservation Service-USDA. Washington D.C., USA. 220pp.

Copyright 1997 The African Crop Science Society


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