Contributions of rhizobia to soil nitrogen fertility

Agricultural systems are dependent on the input of nitrogen (N) to offset losses sustained through produce removal and through processes such as leaching, denitrification and the volatilisation of ammonia. A major source of N in agricultural systems is provided through the symbiosis between rhizobia (root nodule bacteria) and legumes. Rhizobia enter legume root systems through root hair infection or through cracks in the epidermis and induce cortical cells to divide and form nodules. Within the root nodules, rhizobia transform N between its abundant form as atmospheric gas which is metabolically unavailable to plants and metabolically available combined N. This process is known as biological N fixation.