Enhancing biological nitrogen fixation and yield of soybean and common bean in smallholder farming systems of Rwanda

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Legumes play a key role in soil fertility replenishment, yet the yields achieved are far below their potential due to poor management practices. The main objective of this thesis was to increase soybean and common bean productivity in the smallholder farming systems of Rwanda. Inputs of manure at different rates, mineral fertilizers and rhizobial inoculants were used.
Field trials evaluating the response of common bean and soybean to inoculation, P and manure (0,5 and 10 t ha-1) in three agro-ecological zones of Rwanda showed greater grain yield, biomass and stover yields when inputs were  combined. The %Ndfa, amount of N2-fixed, N and P uptake were larger in treated plots compared to control plots. Large variability in the data following inputs application was observed, but there was no clear relationship between the response to inputs and soil parameters. Inputs applied to the legumes lead to substantial increase in the yield of a subsequent maize crop. However, maize grown after soybean failed to yield in Bugesera due to the long maturity of the soybean variety used which resulted in late planting of the maize.
The role of manure on the survival of rhizobia in the soil was explored. The population of rhizobia in the soil was higher in plots that had received manure two seasons earlier compared with plots that had been inoculated or plots that had received P fertilizer only. The number of rhizobia in manured plots was still higher eighteen months from the first sampling. In the dry season rhizobial numbers decreased and increased again soon after during the rainy season.
The Northern Province of Rwanda is the best region for climbing bean. However, yields achieved are very poor. Trials evaluating the response of climbing bean to manure (0, 2 and 5 t ha-1) and mineral fertilizers (N, P, K and their combination) were established in Kinoni and Muko villages with seven fields in each village. Results showed consistent yield increase when inputs were used together. Greater yields were achieved when manure was combined with NPK. In all cases larger responses were observed with the higher rate of manure. Similarly, inputs application increased the amount of N2-fixed, N and P uptake.
Determination of limiting nutrients to climbing bean was performed using the Compositional Nutrient Diagnosis (CND) and the Diagnosis and Recommendation Integrated System (DRIS). The two approaches were useful in identifying nutrient limitations to climbing bean in Northern Rwanda. We observed deficiencies of Zn, N, K and P in Kinoni, and Zn, Mg, Ca, P and N in the Muko site.

Edouard Rurangwa
Wageningen University