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

Reliable estimates of symbiotically fixed N-2 in herbaceous legumes are imp ortant in order to determine their role in maintaining or improving N level s in tropical low-external-input farming systems. We have studied the effec ts of different management systems on the suitability of two non-N-2-fixing reference crops, imperata [lmperata cylindrica (L.) Rauescel] and maize (Z ea mays L.), for estimating N-2 fixation in mucuna [Mucuna pruriens (L.) DC var. utilis (Wright) Bruck] and lablab [Lablab purpureus (L.) Sweet] in th e field. The total-N-difference (TND) method of estimating N-2 fixation was compared to the N-15-isotope-dilution (ID) technique. The two methods did not differ with respect to estimates of N-2 fixation under in situ mulch (I M) systems. In contrast, under live-mulch (LM) systems the TND method under estimated N-2 fixed in mucuna by 29% and in lablab by 40% compared to estim ates made with the ID method. Irrespective of the treatment, estimates of N derived from fixation in both herbaceous legumes were not influenced by ei ther of the reference plants. Using the ID technique, the proportion of N-2 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 un inoculated, cover-crop systems. In view of the nature of cover-crop systems in the derived savanna of West and Central Africa, where imperata is usual ly present as a weed or maize is grown in IM or LM systems, imperata or mai ze could be used to estimate N-2 fixation and N contributions of the legume s to soil fertility and subsequent crop improvements.