Field experiments to assess the Nitrogen (N) contribution of legumes to a sorghum based cropping system in the southern Guinea savanna of Nigeria were carried out during the 2004 and 2005 cropping seasons. The studies were conducted at the experimental station of the National Cereals Research Institute, Badeggi, Nigeria (09° 451 N, 06° 71 E). The experimental design was a split plot randomised complete block replicated three times. Species of herbaceous legumes (Canavalia ensiformis, Mucuna cochinchinesis), and grain legumes (Cowpea - [Vigna unguiculata] varieties L25 and IR48) as well as a fallow control were assigned to the main plots. N levels (0, 20, 40 and 60 kg/ha) were assigned to the sub plots. Plant residues of M. cochinchinesis and V. unguiculata -IR48 had the highest percent of N content (1.55 and 1.54% respectively), while C. ensiformis had the lowest (0.93%). Only the two cowpea varieties had C:N ratio of less than 30:1. The lignin and polyphenols content of all the legumes species were less than the critical levels of 15 and 4% respectively. The soil textural classification is of sandy loam with low initial N and cation exchange capacity. Application of N fertilizer to soils incorporated with legumes resulted in significantly higher mineral N (NO3 and NH4-N) accumulation than those without fertilizer N in both soil and sorghum N. Generally, higher mineral N was recorded in legume residue incorporated treatments than the control. Application of up to 40 kg N/ha to sorghum grown after incorporation of legumes except canavalia resulted in significantly higher yield of sorghum than those after fallow control.
Key words: Nitrogen, Sorghum, canavalia, mucuna, cowpea.
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