Nitrogenase activity by diazotrophs grown on a range of agricultural plant residues

Abstract Six diazotrophs, each representative of a different genus, were grown on a range of plant residues in pure culture and also in co-culture with a cellulolytic bacterium, Cellulomonas sp. CS1-17. The residues were analysed for total nitrogen, and hot and cold water-extractable carbohydrate. In the absence of Cellulomonas sp., nitrogenase activity by all diazotrophs was low on most residues. Cereal straws (wheat, oat and rice), rice hulls and palmnut residues yielded the highest nitrogenase activities with Azospirillum sp., Pseudomonas sp., Azomonas agilis and Beijerinckia indica being the more active diazotrophs. When grown in co-culture with Cellulomonas sp., all diazotrophs showed substantial nitrogenase activity on all straws and. with the exception of Azotobacter chroococcum , on palmnut residues. The residues which were not readily utilized to support nitrogenase activity were sawdust, oil-extracted melaleuca leaves and sugar cane trash. Straw from five varieties of wheat, one variety grown under differing cultural conditions to yield straw with a range of N contents, showed significant variation in the nitrogenase activity recorded between both the wheat variety and the diazotroph strain. There was no significant correlation with nitrogenase activity and the N content of any of these residues, but for Pseudomonas sp., Derxia gummosa and B. indica there was a positive correlation between nitrogenase activity and the cold water-extractable carbohydrate component of the residue. Examination of the relationship between respiration and nitrogenase activity indicated that a respiratory threshold was essential for significant rates of nitrogenase activity, and that above this threshold, the respiratory efficiency of nitrogenase activity improved. However, some residue-diazotroph combinations achieved high rates of respiration without achieving significant rates of nitrogenase activity. Even with acid-tolerant strains such as B. indica , nitrogenase activity in co-culture with Cellulomonas was promoted at pH 6.5. and higher, relative to pH 5.5, but this may reflect a pH effect on the cellulolytic activity of the Cellulomonas .