Impact of organic amendments on the dynamics of soil microbial biomass and bacterial communities in cultivated land

In the course of a 1 year study on agricultural plots (mustard-beet) the impact of organic materials and soil management on soil microbial communities was assessed after amendment with three types of organic materials (sewage sludge, turkey manure and compost made of turkey manure and ligneous waste) and a mineral fertilizer. Transient impacts on potential metabolic activity, on genetic structure and on quantities of soil microorganisms, measured with three different methods (Cmic, total DNA content and CFU enumeration), were observed. The extent of these impacts depended on the composition of the amendment. The proportions of mineralisable carbon and lignin in organic materials seemed to influence the behaviour of the bacterial communities during the first 3 months after amendment. After a period of 6 months, no effect specific to the type of organic materials was detected. The dynamics of microbial biomass was assessed by both microbial carbon and by total DNA, but no correlation was found between the results; DNA seemed to be more sensitive to time-related variations in microbial biomass. Finally, although the bacterial functional and genetic structures were strongly modified between 3 and 6 months, this modification was not related to the type of amendment. This result suggests that organic amendments have less effect than seasonal variations or others anthropic factors such as the mechanical management of the soil.