International Institute of Tropical Agriculture, Humid Forest Eco-regional Center, Cameroon
The cover crops Mucuna pruriens var. utilis and Pueraria phaseoloides are gaining importance in annual sole maize cropping and horticultural systems in the humid forest zone of West and Central Africa. Their contribution to the N balance of a sole maize relay cropping system was estimated. Over four consecutive years (1998 to 2001), amounts of N in 8-9 months old aboveground M. pruriens, P. phaseoloides and natural fallow biomass and litter were determined. In all years fallow biomass was either burned or retained as mulch and a sole maize crop was grown. Maize aboveground N-accumulation and N-export with grain was determined. The amount of N2-fixed was estimated with the N difference method. In 1998, 1999, and 2000, nitrogen content in biomass of M. pruriens and P. phaseoloides fallow was higher than in natural fallow. In 2000 and 2001 fallow biomass in previously burned plots contained less N than when biomass had been retained. There was no fallow-type x biomass management interaction. Maize N uptake was higher in the biomass burned treatments in 1998, yet thereafter higher in the biomass retained treatments. Fallow type had only in 1999 an effect with highest maize N uptake in the P. phaseoloides system, followed by the M. pruriens system and the natural regrowth. The estimated amounts of N2-fixed were higher in the P. phaseoloides system; biomass management had no effect. Nitrogen export with maize grain was highest in the P. phaseoloides system, followed by the M. pruriens system; biomass management had no effect across years. Nitrogen uptake into the aboveground maize biomass and N export with grains was balanced by the amount of N in fallow biomass and litter in all systems where biomass was retained. The amount of N2-fixed did not balance the amount taken up by the maize. In the M. pruriens system the N exported with grains was not balanced by N2-fixation. Only the P. phaseoloides system had a positive balance of N export in grain versus the amount of N2-fixed.
Keywords: Cameroon, cover crop, maize, Mucuna pruriens, N balance, Pueraria phaseoloides, symbiotic N fixation, Ultisol