Monocropping and Intercropping of Maize with Six Food Legumes at Malkerns in Eswatini: Their Effects on Plant Growth, Grain Yield and N2 Fixation, Measured using the 15N Natural Abundance and Ureide Techniques

Intercropping of legumes and cereals has many benefits to both plant partners. In this study, the effect of legume-maize intercropping on plant growth, grain yield and N₂ fixation of six legumes was assessed using the ¹⁵N natural abundance and ureide techniques. For this, a field experiment involving six legume species and two cropping systems was established at the Malkerns Research Station, Eswatini during the 2017/2018 cropping season. Based on the ¹⁵N isotopic and ureide analysis, the six test legumes respectively obtained 39.06 – 70.19% and 16.46 – 55.79% of their N nutrition from symbiosis. The amounts of N-fixed ranged from 12.66 to 66.57 kg ha⁻¹. In general, high amount of N-fixed by legumes correlated strongly with greater shoot dry matter accumulation (r = 0.7981; p < 0.001) and high grain yield (r = 0.5905; p < 0.001), indicating the importance of N₂ fixation in plant growth and reproduction. Legumes grown under monocropping recorded higher plant growth, symbiotic performance and grain yield when compared to those grown in mixed culture with maize. However, shoot %Ndfa was much higher under intercropping than sole cropping due to competition by cereal and legume for soil N. Components of maize yield were similar for the two cropping systems. The %N derived from fixation and %relative ureide-N abundance were significantly correlated (r = 0.4005; p < 0.001), indicating that the ¹⁵N natural abundance technique and the ureide method were complementary in measuring N₂ fixation in the test legumes. These results have provided some insights on the impact of cropping system on plant growth, symbiotic performance and grain yield of six selected legumes.