The development and suppressive activity of soil microbial communities under compost amendment

Soil is a highly complex system in which bacteria play a dominant role. Soil microorganism biodiversity is extremely important for sustaining the nutrient cycles of soil ecosystems. The diversity of soil bacterial and fungal communities can be influenced by agricultural management. The use of amendments, such as the incorporation of compost into the soil, strongly affects soil microbial communities directly or indirectly. The use of compost can be an important tool to control soil-borne pathogens. Certain groups of microorganisms (bacteria and fungi) present in compost produce metabolites, such as siderophores and antibiotics, with specific suppressive activity against soil-borne pathogens: among these compost bacteria, species of Pseudomonas and Bacillus are very important. In this study, we investigated the effects of increasing doses of compost on the density of several cultivable soil microbial groups in an agricultural system of Southern Italy. The number of total bacteria, total fungi, siderophore-producing bacteria (principally Pseudomonas spp.) and spore-forming bacteria (principally Bacillus spp.) was evaluated over a period of five months. Moreover, we analyzed the ability of spore-forming bacteria isolated from soil and compost to inhibit, in vitro, five soil-borne phytopathogenic fungi (Rhizoctonia solani, Fusarium oxysporum, Sclerotinia minor, Fusarium solani, Pyrenochaeta lycopersici). The number of total bacteria and siderophore-producing bacteria was greater in the soil treated with compost than in soils treated with mineral fertilization or not manured, with an increase related to the dose of compost applied. Moreover, the compost used in this study produced a substantial increase in the number of spore-forming bacteria in the soil. Approximately 80% of these bacteria were able to inhibit the soil-borne phytopathogenic fungi Rhizoctonia solani, Fusarium oxysporum, Sclerotinia minor, Fusarium solani and Pyrenochaeta lycopersici. These results show that compost from the organic fraction of municipal solid wastes furnishes a high number of spore-forming bacteria exhibiting antibiotic activity against phytopathogenic fungi. The application of compost can change the composition of the soil microbial community, modifying the relationships among microorganisms, both competitive and/or antagonistic, producing a decrease in the activity of plant pathogens. These results were in agreement with the decrease in the symptoms of disease on tomato plants cultivated in the soils treated in this study.