Manure and soil properties affect survival and persistence of soyabean nodulating rhizobia in smallholder soils of Zimbabwe

Persistence of the soyabean rhizobial inoculant strain MAR 1491 was assessed in 52 soils from Guruve, Hurungwe and Goromonzi districts of Zimbabwe, which had been inoculated 1¿4 or 6 years previously. Most probable number estimates of rhizobia in the soils showed that population sizes decreased with increasing time since the last inoculation. Rhizobial populations of up to 102 cells g¿1 soil were found in Guruve soils inoculated 3 years before, while persistence in Hurungwe and Goromonzi soils was significant for soils inoculated 2 years before. The greater rhizobial persistence in Guruve soils was attributed to their higher clay (>20%) and organic C (>1%) compared with the sandier, relatively less fertile Hurungwe and Goromonzi soils. Farmers with favourable soils such as those in Guruve can grow soyabean for at least 3 years without the need for repeat inoculation. Manure application led to increased indigenous rhizobial numbers at two smallholder sites and larger numbers of rhizobia in inoculated plots in sandy soils from Goromonzi. Inoculation and manure addition increased rhizobial numbers and soyabean yields in field experiments; soyabean yields varied from 0.5 t ha¿1 in uninoculated, unmanured plots to 1.1 t ha¿1 in inoculated plots receiving 10 t manure ha¿1. Increases in numbers closely followed the progress of the wet season. The highest rhizobial population of 105 cells g¿1 soil was obtained in plots that had received 10 t ha¿1 manure when a high soil moisture (15%, w/w) was recorded during the cropping season, while the lowest population of 101 cells g¿1 soil was recorded in treatments during the dry season when soil moisture was low