The influence of applied nitrogen on nitrogen fixation and Bradyrhizobium interstrain competition in intercropping systems

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Sole cropping of cereals has been viewed as contributing to a rapid decline in soil fertility. The incorporation of legumes in such farming systems, apart from offering food security, has the potential to improve soil fertility, through dinitrogen fixation. Such benefits are influenced by available soil nitrogen and indigenous rhizobial populations. The study was designed to determine (1) the starter nitrogen requirement of some cowpea and peanut cultivars (ii) Bradyrhizobiuin inter-strain competition and (iii) quantify the amount of dinitrogen, fixed in such cropping systems using the 15N-dilution technique. To achieve the above objectives, greenhouse and field studies were conducted. The pot experiment evaluated the influence of starter nitrogen application (0, 5, 10, and 35 mgkg-1 soil), legumes-cereal cultivar combinations on dinitrogen fixation in a soil of high indigenous rhizobia population. Selected legume-cereal combinations were put on a field trial at higher nitrogen fertilization rate (5, 50 and 300 g ha-1) and also evaluated dinitrogen fixation. Both pot and field experiments were split-split plot designs with four replicates. In the starter nitrogen experiment, there was response to inoculation for measured parameters for both legumes and component cereals. The starter nitrogen application of 10 and 35 mg N kg-1 was found to be suitable for both cowpea and peanut. Mixed cropped legume components fixed significantly more dinitrogen than the sole crops. Aburotia and Local-29 were found to be suitable reference crops for Soronko an Sinkarzej respectively. In the field, the rate of 50kgNha-1 enhanced growth parameters compared with 5 and 300kgNha-1. 300 kg ha-1 of nitrogen application suppressed nodule formation and development and consequently dinitrogen fixation. Nodule occupancy was highest with TAL 209 (61.27%) compared with TAL 658 and TAL 169 which occupied 48.47% and 30.86% respectively of nodules. With peanut strains TAL 169 was a better competitor occupying 56.15% of nodules followed by TAL 1371 (45.10%) and TAL 1000 (31.30%) was the least. The results indicate that (i) •a good selection of compatible cultivars of legumes and cereals for use in field experiments is a pre-requisite for increased yield and dinitrogen fixation in intercropping systems (ii) Response to inoculation can be achieved provided the strains are effective and more competitive than the native rhizobia populations and may not be influenced by these populations. (iii) Though nitrogen is required for increased yield production, nitrogen application as high as 300 gNha-1 inhibited nodulation and dinitrogen fixation and (iv) Interspecific competition among intercropped components is much more antagonistic to the legume than the cereal.
A thesis submitted to the Board of Postgraduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirement for the award of the Degree of Master of Philosophy, 1992