Aerobic short-term microbial utilization and degradation of humic acids extracted from soils of long-term field experiments

A humic acid (HA) fraction of the soil organic matter (SOM) was extracted with alkali from soil samples originated in non-fertilized and fertilized (NPK + organic manure) plots of long-term (45 years) field experiments. The HA preparations served as supplemental sources of nutrients or as sole source of either C or N for soil micro-organisms indigenous to the same soils. Under aerobic conditions (shake cultures) between 15% and 45% of HA were degraded in 21 days. The degradation was minimum if HAs were added supplementary, although the biomass formation was strongly enhanced. Preparations of HA from long-term fertilized soils appeared somewhat less susceptible to microbial degradation but they were capable of supporting microbial growth. Under copious nutritional conditions some novel HA-like substances were formed. The HA preparations re-isolated from individual cultures exhibited differences in elemental and structural characteristics. The FTIR spectra indicated an increasing proportion of aromatic structures that appeared as associated with mineral moieties. Conclusively, HAs from long-term fertilized and manured soils could be considered as more resistant to microbial activities than those from control soil, but under limited nutrient conditions their aliphatic constituents appear utilizable by micro-organisms.