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Evaluation of symbiotic dinitrogen inputs of herbaceous legumes into tropical cover crop systems
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Reliable estimates of symbiotically fixed N2 in herbaceous legumes are important in order to determine their role in maintaining or improving N levels in tropical low-external-input farming systems. We have studied the effects of different management systems on the suitability of two non-N2-fixing reference crops, imperata [Imperata cylindrica (L.) Rauescel] and maize (Zea mays L.), for estimating N2 fixation in mucuna [Mucuna pruriens (L.) DC var. utilis (Wright) Bruck] and lablab [Lablab purpureus (L.) Sweet] in the field. The total-N-difference (TND) method of estimating N2 fixation was compared to the 15N-isotope-dilution (ID) technique. The two methods did not differ with respect to estimates of N2 fixation under in situ mulch (IM) systems. In contrast, under live-mulch (LM) systems the TND method underestimated N2 fixed in mucuna by 29% and in lablab by 40% compared to estimates made with the ID method. Irrespective of the treatment, estimates of N derived from fixation in both herbaceous legumes were not influenced by either of the reference plants. Using the ID technique, the proportion of N2 derived from fixation in mucuna and lablab at 12 weeks varied from 52% to 90% depending on whether the treatments were N fertilized, inoculated or uninoculated, cover-crop systems. In view of the nature of cover-crop systems in the derived savanna of West and Central Africa, where imperata is usually present as a weed or maize is grown in IM or LM systems, imperata or maize could be used to estimate N2 fixation and N contributions of the legumes to soil fertility and subsequent crop improvements.