Options to enhance biological nitrogen fixation by soyabean and common bean in smallholder farming systems of Rwanda

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Legumes are inoculated with expectation that it will increase nitrogen fixation and grain yield. However, this is not always the case. Among the possible factors affecting the response of legumes to inoculation are competition between indigenous soil strains with introduced ones, sufficient population of soil rhizobia and environmental stresses.

The survival of rhizobia population in soil in the other hand is affected by soil properties and management activities. To understand how management and soil properties affect the population of soil rhizobia, hence the nitrogen fixation and grain yield, 18 field trials of both soyabean and bean were established in three different agroecological zones of Rwanda during the agricultural season 2014 A and will run for three consecutive seasons, with the second season (2014B) a rotational trial with maize to evaluate how different manure rates and soil characteristics affect the survival of rhizobia in soil and the need to re-inoculate in previously inoculated plots.

The results so far showed variability in rhizobia population following soil origin. Both soyabean and bean nodulated well in some fields but not in others. The grain yield in all sites increased with increasing manure rates. Grain yield of uninoculated plots also increased with increasing manure rates. The pictures below show well nodulated soyabean in one site (Plate 1) and two plots of soyabean showing differences: one with manure addition (Plate 2) and the other one without (Plate 3).

Plate 1: Well nodulated soyabean in field

Plate 2: +P+10t manure, not inoculated

Plate 3: no P, no manure, not inoculated

In the last week of March, 2014, 14 field trials have been established in Northern Province of Rwanda. This area is one of the major climbing bean growing areas in Rwanda. Fields were selected based on soil fertility differences. These trials aimed at exploring the possible mechanisms behind the response of climbing bean to manure application even at lower rates. Different farm yard manure rates were used with or without N, P, K or NPK application. The pictures below show farmers weighing manure to be used and a field on preparation for planting. Farmers were curious as weighing prior to application is not a common practice.

Plate 4: Farmers weighing farm yard manure


Plate 5: Field preparation prior to planting

Edouard Rurangwa, PhD student at Wageningen University