Evolution of humic substances from unripe compost during incubation with lignolytic or cellulolytic microorganisms and effects on the lettuce growth promotion mediated byAzotobacter chroococcum

Some recalcitrant organic wastes, which contain a large proportion of lignin or cellulose, are not changed much by composting, and thus the effectiveness of the compost as fertilizer is usua.lly low. In this study, incubation of unripe compost with ligno-cellulolytic microorganisms —Trichoderma viride orBacillus spp. — was investigated to increase the degree of humification of the organic matter present, and improve ils quality as a soil amendment. High-performance liquid chromatography (HPLC) analyses together with humification indices and electrofocusing patterns were used to monitor the evolution of the humic substances during the incubation process. Plant growth effects exerted by Azotobacter chroococcum on lettuce plants growing on the previously incubated compost were affected by the length of incubation and by changes in the composition of humic substances. Higher organic matter content and better humification seem to be important factors for predictingA. chroococcum hebaviour in the rhizosphere.