Biological nitrogen fixation in legume - cereal intercropping systems

In West Africa legumes are important food crops grown mainly in association with maize, sorghum and millet. Although legume/cereal association is a common practice, nitrogen contribution by legumes in such cropping systems and in rotation with cereals has not been given much attention and there have been relatively few studies on the biological consequences of this association. In order to address these issues four sets of experiments were conducted for two seasons (1992/1993) at Ibadan, Zaria and Kano situated in the Forest, Guinea savanna and Sudan savanna zones respectively. The first experiment consisted of two soybean cultivars intercropped with maize; the second experiment consisted of two cowpea cultivars intercropped with maize; the third experiment, consisted of two soybean cultivars intercropped with sorghum; and the fourth experiment, consisted of three cultivars of cowpea intercropped with millet. In the first season the legumes in all the experiments were planted either as sole crops or intercropped with cereal (either on the same-row or on alternate-rows), with fertilizer nitrogen at three levels (0, 20 or 100 kg h-1). Soybeans were either uninoculated or inoculated with a mixture of Bradyrhizobium japonicum strains TAL 102, TAL 377 and TAL 379. Sole crops of the cereals were included as checks. In the second season, the fields were cropped to the respective cereals as test crops. The design used was randomized complete block with factorial treatments in three replicates. Nodulation was assessed at flowering. At physiological maturity the total dry matter and nitrogen yields were determined. Results of experiment 1 indicate that inoculation increased nodulation, grain yield, N2 fixed and %Ndfa of soybean, the response was higher in Bossier than in TGX1485-1D. Intercropping decreased nodulation, grain yield, total N uptake and N2 fixed by soybeans. However, the extent of decrease depended on the soybean cultivar. The two methods of estimating nitrogen fixation (i.e. the Total Nitrogen Difference and the Isotope Dilution), gave similar rankings of treatments in this experiment although the methods gave different estimates of N derived from nitrogen fixation. Intercropped soybean contributed more to the succeeding maize yield than sole soybean, the highest maize yield obtained (4125 kg ha-1 was from the treatment BPN1 (Same-row with 20 kg N ha-1 fertilizer). In experiment 2, cowpea cultivar 1T86D-719 produced higher nodule weights, grain yield, total N uptake, N2 fixed, and %Ndfa than Suvita-2. Intercropping with maize decreased nodulation, total dry matter, total N uptake, and N2 fixed by cowpea compared to sole crop. However, the extent of decrease was dependent on the cowpea cultivar. For example, nodule weight of IT86D-719 was decreased by 35% (same-row) and 46% (alternate-row); whilst that of Suvita-2 was reduced by 26% (same-row) and 5% (alternate-row). Grain yield of maize was increased by 58% when grown after 1T86D-7 19 and 15% after Suvita-2 compared with when grown after maize. In Experiment 3 results showed interactive effects of cultivar x inoculation, cultivar x nitrogen and cultivar x cropping system. Soybean cultivar TGX148 5-1D out yielded Bossier in all the parameters measured under all experimental conditions. Inoculation increased nodule weight of soybean cultivar Bossier by 77% and that of TGX1485-ID by 11%. Highest nodule weights, total dry matter yield, total N uptake, N2 fixed and %Ndfa by TGX1485-ID were obtained with 20 kg N ha-1 whereas those of Bossier were obtained with 100 kg N ha-1. Intercropping with TGX1485-ID on the same-row increased sorghum yield by 10 and 26% at the 0 and 20 kg N ha-1 levels respectively, but decreased the yield at 100 kg N ha-1. However, sorghum yields were reduced to 68, 71 and 78% that of sole sorghum at 0, 20, and 100 kg N ha-1 respectively, when grown with Bossier. Grain yields of sorghum increased by 14% when sorghum was grown after Bossier and 38% after TGX 1485-ID compared with when grown after sorghum. Results obtained in Experiment 4 show that cowpea cultivar IT89KD-245 out yielded Kanannado and IT89KD-2246 in almost all parameters measured. There were interactions of cultivar x nitrogen and cultivar x cropping system with regards to the parameters measured. For instance, intercropping increased the nodule weight of IT89KD-245 by 5% but decreased those of Kanannado (22%) and IT89KD-2246 (2 8%). Grain yield of millet was increased by 4% when intercropped on the same-row with IT89KD-245 but was decreased by up to 29% and 26% when intercropped with Kananriado and IT89KD-2246, respectively. IT89KD-245 fixed consistently the highest amount of N2 with an average of 47 kg N ha-1 which was 15% and 147% more than the amounts fixed by Kariannado and IT89KD-2246, respectively. The yield of millet on previous cowpea plots ranged from 1200 to 2440 kg h&’ while previous millet plots yielded 1440 kg h-1. The results of these experiments showed that the productivity of cereal and legume intercropping is dependent upon a number of factors, whose interactions should be considered in research work. Different genotypes of the same legume have been shown to have different capacities to nodulate and fix N2 under intercropping. For instance, large differences in nodulation in the field have been shown between genotypes of cowpea and soybean intercropped with maize, sorghum or millet. Given the lack of inputs commonly available to small holders in the tropics, particularly in Africa, legumes which fix N2 and grow well without the need for inoculation such as the promiscuous soybean cultivar TGX 1485-ID, are the best solution for the immediate future. Planting arrangement of legume on either same- row or on alternate-rows with cereal affected the growth and yield performance of both legume and cereal in the mixture. TGX1485-1D intercropped on same-row with sorghum showed more vigour and gave higher yields than on alternate-rows with sorghum, whereas the reverse occurred in the case of Bossier. Total N accumulation, which is a good indication of the total amount of N2 fixed, at least in soils with a poor capacity to supply combined N is probably the best criterion for selection programmes in these ecologies. More research work needs to be directed towards nitrogen and other elements partitioning in intercropping situation. Serious attempts should be made to enhance N2 fixation in grain legumes. There is the need for agronomists and plant breeders to work in close collaboration with microbiologists in the development of better legume /rhizobial combinations.

Source: KNUST