Effects of liming and legume/cereal cropping on populations of indigenous rhizobia in an acid Brazilian Oxisol

Given the acid soil conditions in many regions of common bean production in the tropics and the deleterious effects of soil acidity on rhizobia, studies to assess survival of Phaseolus-nodulating rhizobial populations in acidic soils are important to ensure establishment of effective N 2-fixing symbioses. The abundance of indigenous Phaseolus-nodulating rhizobia in a Brazilian acidic soil varied with the rate of lime applied more than 6 yr prior to sampling, with the season of sampling and with the previous cropping history and showed a significant interaction between these factors. The abundance of rhizobia was enhanced by both soil liming and by presence of common bean with populations ranging between less than 10 cells g −1 soil and more than 10 5 cells g −1 soil. In experiments cropped with upland rice, maize and a soyabean/wheat rotation differences between the sizes of rhizobial populations were highly significant where small amounts of lime had been added. Rhizobial populations were smallest in soils continuously cropped with rice compared with those cropped with maize and the soyabean/wheat rotation only when effective Al saturation was high (>20%). Conversely, the numbers of Leucaena-nodulating rhizobia in the soil samples from a field cropped with common bean were small and were not related to lime application. After incubation for 25 months rhizobia in an acidic soil sampled from the common bean field remained at around 90–94% of the initial population in limed soils containing 4.0, 7.0 and 11.0% effective Al saturation, whilst in soils with 36 and 27% effective Al saturation the abundance declined to 3–7% of that initially observed. An effective Al saturation of >20% is critical for survival of Phaseolus-nodulating rhizobia only in the absence of the host plant.