Towards indicators of soil biological quality: use of microbial characteristics

The aim of this study is to establish correlations between different descriptors and pathological potential of soils towards plants, in order to identify indicators of soil biological quality. Three cultural practices were applied in an experimental field before carrot culture (conventional, organic amendment with cattle manure and biofumigation with fodder radish). Twenty-one soil samples were collected in each plot at 3 key points during the cropping sequence (before treatment, before and after carrot culture). Physicochemical (carbon, nitrogen and organic matter contents, C/N ratio, pH, EC, ions and trace elements), biological (densities of cultivable bacteria and fungi, microbial biomass and basal respiration, densities of plant-parasitic nematode populations), microbial diversity (bacterial and fungal community structure by T-RFLP) and plant-pathological (receptivity to Rhizoctonia solani damping-off) properties were measured and correspondences were analysed by Principal Component Analysis. Before application of the cultural practices, soil characteristics were very similar in the 3 plots. Biofumigation resulted in a significant increase of the microbial densities, biomass and basal respiration. Both organic amendment and biofumigation modified the microbial community structures. Intra and inter-class PCA enabled to distinguish any effect between treatments and dates. The soil biological characteristics showed resilience to the situation that occurred before cultural practices. Relations between physicochemical, biological and molecular data sets, provided by co-inertia analysis, will be discussed and dealt with the receptivity of the soil samples to R. solani damping-off. This strategy will enable us to propose a bunch of soil biological quality indicators