Long-term effects of organic farming on fungal and bacterial residues in relation to microbial energy metabolism

Samples from the bio-dynamic, bio-organic, and conventional trial, Therwil, Switzerland, were analyzed with the aim of determining the effects of organic land use management on the energy metabolism of the soil microbial biomass and on the fraction of microbial residues. The contents of adenylates, adenosine triphosphate (ATP), glucosamine, muramic acid, and galactosamine were significantly largest in the biodynamic organic farming (BYODIN) treatment and significantly lowest in the conventional farming treatment with inorganic fertilization (CONMIN). In contrast, the ergosterol-to-ATP ratio and fungal C-to-bacterial C ratios were significantly lowest in the BYODIN treatment and significantly largest in the CONMIN treatment. No clear treatment effects were observed for the ergosterol content and the adenylate energy charge (AEC), the ATP-to-microbial biomass C ratio and the ergosterol-to-fungal C ratio. Ergosterol, an indicator for saprotrophic fungal biomass, and fungal residues were significantly correlated. The microbial biomass carbon-to-nitrogen ratio showed a negative relationship with the AEC and strong positive relationships with the ratios ergosterol-to-microbial biomass C, ergosterol-to-ATP and fungal C-to-bacterial C. In conclusion, the long-term application of farmyard manure in combination with organic farming practices led to an increased accumulation of bacterial residues.