Microbial utilization and transformation of humic substances extracted from soils of long-term field experiments

Humic acids (HA)  extracted with alkali from unfertilized and fertilized (NPK + organic manure) plots of  long-term, i.e. 45 years, field experiments were added as a supplemental source of nutrients or as the sole sources of carbon and nitrogen to semi-aerobic cultures of complex microbial communities indigenous to the same individual soils. Depending on the experimental conditions, between 2.7% and 47.3% of the added HA preparations were utilized in 12 months. In full strength nutrient media these numbers seemed affected by some novel microbially produced HA-like substances. An average utilization rate was 33% for the HA from an unfertilized   Orthic-Luvisol plot, whereas HA preparations extracted from fertilized plots of the Orthic-Luvisol or  a Dystric-Cambisol soil site were utilized to 27% and 15% respectively. The HA preparations re-isolated after 12 months from microbial cultures exhibited differences in elemental and structural characteristics, corresponding mainly with the nutrient status of the individual  cultures. Those from nitrogen-deficient cultures, e.g., were partly depleted in N. The Fourier-transformed infrared (FTIR) spectra of  the re-isolated HA preparations indicated loss in aliphatic structures. Aromatic structures were strongly associated with  mineral moieties and  remained rather  unaffected. The results indicate in general that an increase in soil organic matter contents as frequently observed in long-term fertilized soils could be attributed to an enhanced resistance of humic acid fractions to microbial degradation.