Microbial degradation and transformation of humic acids from permanent meadow and forest soils

Humic acids (HAs) extracted from two soils under permanent meadow, and from a forest soil, were added as either a supplemental source of nutrients, or as the sole sources of carbon or nitrogen to cultures of microbial communities indigenous to the same soil, and incubated under semi-aerobic conditions for 12 months. Depending on nutrient conditions in the individual cultures and the origin of the HA, between 9% and 63% of the added HA was degraded, i.e. microbially utilized. HA from soils under permanent meadow were more resistant to microbial degradation than those from forest soil. The formation of microbial biomass was enhanced up to four times in the full-strength nutrient media supplemented with HA, but was strongly inhibited if HA served as the sole C-source. After 12 months, HA preparations re-isolated from the microbial cultures exhibited different elemental and structural alterations. These included an increase in carbon content and C:N ratio, and in infra-red absorption typical for aromatic structural units. Basically, HA from all three soils under tests appeared susceptible to microbial degradation and transformation.