Effect of bradyrhizobial inoculation and n fertilization on promiscuous soybean and subsequent maize yield grown in degraded “terre de barre” in Benin

Poor adoption of green manure and agro forestry systems for sustainable intensification of agricultural production in the moist savannah of West-Africa, as well as the low contribution of traditional grain legumes such as groundnuts, cowpea and common bean have prompted the search for alternative socio-economic solutions for smallholders. One of them is the development of N2 fixation by promiscuous soybean to increase food production and to improve soil fertility, in particular on degraded lands. Fifteen farmers’ fields were selected for soybean trials in 2001 and 2002, and again in 2002 and 2003 for the maize trials. Each farmer’s field represented one replication. The 15N isotope dilution method was used to assess symbiotic N2 fixation of the International Institute for Tropical Agriculture (IITA) promiscuous soybean variety TGX 1448 2E and its response to inoculation and fertilization. In both 2001 and 2002, the application of 20 kg N did not affect nodulation, biomass production and N accumulation of soybean uninoculated in 2001, or inoculated in 2002. However, inoculation produced the highest soybean nodule number and nodule weight in 2002. The highest values of biomass production and N accumulation were found with soybean that received poultry manure in 2001 and 100 kg N ha-1 as urea in 2002. The highest biomass and N accumulation in 2001 was 1600 kg DM ha-1 and 41 kg N ha-1 with soybean amended with poultry manure. Shoot N content in 2002 averaged only 25 kg N ha- 1, while the average N accumulation in soybean seed was 64 kg N ha-1. The best percentage of N derived from atmosphere (Ndfa) (54 %), amounting to only 13 kg N ha-1, was obtained with soybean fertilized with 20 kg N ha-1 and inoculated. These values are very low indicating that soybean cultivated in the study area was far from satisfying its requirements through N fixation. The N balance calculated on the basis of the amount of N fixed removed in the grain is negative (-48 kg ha-1). Significant increase occurred only when plots were previously grown to soybean and fertilized with poultry manure being 5124 and 3211 kg DM ha-1 compared to the previous soybean plot without N, for maize shoot and grain, respectively. Total N yield in shoots was significantly higher in plots previously cropped to soybean than to maize. The contribution of soybean to maize yield was not significantly different from the contribution of maize-to-maize in 2003. Soybean growing in West-Africa is a promising technology that has multiple benefits.Keys words : Benin Republic ; promiscuous soybean ; symbiotic N fixation ; 15N isotope dilution; N accumulation ; N Balance ; % Ndfa