Selection of rhizobia for Trifolium repens in flooded soil conditions

The Brazilian state of Rio Grande do Sul has approximately 5.4 million hectares of lowland. The introduction of winter forage legumes, such as white clover, can increase the productivity of these lowland areas through biological nitrogen fixation (BNF). However, this benefit can only occur if the legume is associated with rhizobia that can efficiently fix nitrogen under available soil conditions. The objective of this study was to select rhizobia of Rio Grande do Sul soils, which are efficient in BNF when in symbiosis with white clover, and evaluate the tolerance of this symbiosis under flooded soil conditions. The experiment was conducted in a greenhouse, in soil-filled pots, where 14 isolates were evaluated for nitrogen fixation ability under flooded soil conditions. Each treatment was tested under two soil moisture conditions (flooded and field simulated). Flooding significantly reduced the number and nodule weight as well as the production of dry matter and nitrogen accumulation in the shoots of plants inoculated with rhizobia. The effect of soil flooding on BNF varied according the isolate evaluated. The strains CVII, P3 4, T4 and VP16 exhibited high nitrogen fixation efficiency under flooding conditions, surpassing the nitrogen-added control treatment, on nitrogen accumulation in shoots.