The Potential of Nitrogen-Fixing Bacteria in the Sustainability of Agro-Forestry Ecosystems

Microorganisms are critical for the maintenance of soil functions in natural and managed agro-forestry ecosystems. This explains the importance of evaluating a particular group of microorganisms, such as legume root nodule bacteria. Legumes and their root nodule bacteria are considered powerful management tools for improving pasture yield in the montado ecosystem, which is an agro-forestry system associated with the exploitation of cork and holm oaks. In Portugal, the widespread mortality registered in cork and holm oaks in recent decades has been attributed to infections by Phytophthora cinnamomi. In addition to nitrogen fixation, evaluated by the size and symbiotic effectiveness of the rhizobial population, other important functions were also investigated, such as mineral phosphate solubilization and cellulase activity, as well as the antagonistic activity against P. cinnamomi. This work showed the important role that root nodule bacteria can play in the sustainability and recovery of these ecosystems by promoting biological nitrogen fixation, especially in low fertility soils, through the establishment of pastures with legumes using pre-selected and characterized rhizobia as biofertilizers. The Phytophthora antagonistic activity from some of these bacteria, as well as their ability to degrade cellulose, an important component of Phytophthora cell walls, indicate that they may be used as potential biocontrol agents against this disease.