INOCULATION OF WHEAT STRAW TO ENHANCE LIGNOCELLULOSE BREAKDOWN AND ASSOCIATED NITROGENASE ACTIVITY

A range of lignocellulolytic fungi isolated from decaying straw was examined for ability to increase the rate of straw breakdown (as measured by CO2 production), to cross-feed two selected diazotrophs and hence to increase the rate of nitrogen fixation (measured as C2H2 reduction) under both axenic conditions and in native soil. Cyathus stercoreus was the only isolate obtained from decaying straw which was capable of degrading lignocellulose. Three out of 20 fungi which were isolated on their ability to degrade cellulose, showed varying degrees of ability to cross-feed the two diazotrophs. With straw as the sole source of carbon and nitrogen, co-inoculation of Beijerinckia indica B15 with Cyathus stercoreus stimulated nitrogenase activity in both axenic culture (1.7-12.22-fold increase according to the cellulolytic co-inoculant) and a native soil (pH 6.0; 2.2-fold increase). Azospirillum sp. DN64 in axenic culture was not stimulated when co-inoculated with Cyathus stercoreus alone but did respond to co-inoculation with a mixed group of cellulolytic fungi (22-fold increase in nitrogenase activity). The proportion of carbon released from the decaying straw as CO2 was as high as 33% over an 8-week period when Cyathus stercoreus was present, but was only 16% in the absence of C. stercoreus, when only the cellulolytic fungi were present. The results indicate that, with the selection of suitable lignocellulolytic, cellulolytic and N2-fixing microorganisms, it may be possible to improve the rate of straw breakdown and to fix enough atmospheric N2 to supply the requirements of the cellulolytic organisms and to contribute positively to the N2 balance of the following crop.