Evaluation of symbiotic dinitrogen inputs of herbaceous legumes into tropical cover-crop systems

Abstract

 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.