Quantitative microbial indices in biogas and raw cattle slurries

Biogas slurry, the secondary product of the anaerobic digestion process, is increasingly being used as fertilizer. Information is available on its chemical and physical properties and their effects on plant growth. However, there is a demand to characterize the microbial quality of slurries, which may control further mineralization processes after application to soil. In this study, biogas and raw slurries obtained from six farms were analyzed for their ergosterol and amino sugar concentrations as indices for microbial biomass. A reliable, precise method for determining ergosterol in slurries is presented. Biogas slurries contained significantly less ergosterol (?34%), muramic acid (MurN; ?42%), galactosamine (GalN; ?32%), and fungal glucosamine (GlcN; ?40%) than raw slurries. The mean fungal GlcN to ergosterol ratio (50) and also the mean fungal carbon (C) to bacterial C ratio (0.29) did not significantly differ between the slurry types. The mean microbial C concentration in the biogas slurries was significantly lower than in the raw slurries. Consequently, the contribution of microbial C to slurry organic C was 3.6% in the biogas slurries and 5.7% in the raw slurries. Microbial C revealed significant nonlinear relationships with the fiber and ash concentration, pH, as well as the C/N ratio of the slurries.