Comparative analyses of the nitrogen fixing potential of different legume species grown under different agronomic practices

Abstract

Biological nitrogen fixation (BNF) is an ecologically and economically beneficial process that can reduce the use of costly conventional inorganic nitrogen (N) fertilisers. This study was therefore conducted to assess the N fixing potential of four different legumes grown under different agronomic practices such as inoculation and fertilisation under both glass house and field conditions. One rainfed field trial and one glasshouse experiment were conducted during 2013/14 summer cropping season at the Agricultural Research Council- Grain Crops Institute (ARC-GCI), Potchefstroom. Treatments consisted of four legume species namely, cowpea, dry bean, groundnut and soya bean. These legumes were fertilised, inoculated, and fertilised + inoculated. Control plots with neither fertiliser nor inoculation were also included as standard checks. The different legumes were fertilised at optimum recommended rates for P and K, while N was only applied to specific treatments to determine the minimum accretion of whether nitrogen would have influence on nodulation and productivity. The legumes were inoculated with the rhizobium inoculant registered for each respective crop. The sources of N, P and K were limestone ammonium nitrate, superphosphate and potassium chloride, respectively. All the treatments were replicated four times and were arranged in a randomised complete block designin factorial arrangement ( 4 legumes x 2 inoculants x 2 fertiliser = 16 treatments) in all experiment. Data collected included total number of nodules, viable and non-viable nodules, ¾N, ¾C, ¾Ndfa, total amount of nitrogen in the plant, total amount of nitrogen fixed and seed yield. The treatment effects showed statistically significant differences (P<0.05) on the number of nodules, percentage of nitrogen from atmosphere and total amount of nitrogen fixed per plant. Significantly higher total number of nodules, viable nodules, percentage of nitrogen from atmosphere and total amount of nitrogen fixed per plant were observed from inoculated plants irrespective of the legume species under both field and glasshouse conditions. Under field conditions, groundnut (48.5) and soya bean (42.9) recorded significantly higher average total number of nodules at full flowering, while dry bean (83 .2) and groundnuts (70.6) produced significantly higher total number of nodules at physiological maturity. The crops studied showed that fertiliser application had a depressive effect on nodulation, but promoted nodulation when the different crops were inoculated comparable to when used in combinations. Nodulation in groundnuts was higher in both samplings, but lower with cowpea. Nonetheless, the response of dry bean and soya bean was infrequent across the sampling. Dry bean showed lower percentage of nitrogen (¾N) from the atmosphere and total amount of nitrogen fixed across the cycle under field and glasshouse conditions. Groundnut showed higher percentage of nitrogen from atmosphere and total amount of nitrogen fixed across the cycle under field and glasshouse conditions. Cowpea had higher percentage of nitrogen from atmosphere and total amount of nitrogen fixed at 100% flowering and 100% pod formation, while for soya bean that was achieved at 100% pod formation and physiological maturity in the field. The results highlight that groundnuts among other legumes has been noted to be more recommendable for use as crop that can fix nitrogen. This crop has been noted that it can fix N throughout its cycle. It has also been proven that it contributes to the improvement of chemical, physical and biological soil caractreisticas.