Tillage method and crop rotation effects on the population sizes and diversity of bradyrhizobia nodulating soybean

This study was conducted in an area of Brazil cultivated with soybean since the early 1960's but which for the last 17 yr was under different tillage (no-tillage, NT; conventional tillage, CT) and crop rotation (soybean, S/wh eat, W/maize, M; S/W; M/W) systems. The area had not received any inoculant for the last 15 yr and our objective was to investigate the effects of til lage and cropping systems on the bradyrhizobia population. The NT system an d crop rotations with soybean resulted in high populations of bradyrhizobia , but even in the treatment where soybean had not been cultivated for 17 yr (M/W) the number of viable cells in the soil was high. A total of 142 brad yrhizobia isolated from the different treatments were characterized based o n colony morphology, serological reaction, DNA analysis by RAPD, protein an d Nod factors profiles. The analyses resulted in grouping of the isolates i nto 16 DNA, five protein and three Nod factors profiles. A high proportion (37.5%) of the isolates did not react with any known serogroup. Both NT and crop rotations with soybean resulted in a higher bradyrhizobia diversity, with the lowest number of genomic patterns occurring in the CT with M/W rot ation. However, there was no relationship between the treatment combination s and genetic relatedness. The evaluation of symbiotic performance under gr eenhouse conditions showed that the isolates with higher rates of Nz fixati on were also isolated from NT with S/W or S/W/M crop rotations. Consequentl y, the use of agronomic practices such as NT and crop rotation with legumes will not only contribute to agricultural sustainability, but also help to maintain bradyrhizobia population and diversity. (C) 2000 Elsevier Science Ltd. All rights reserved.