Microbial processing of humic acids extracted from soils of a long-term field trial

Abstract

Humic acids (HA) extracted with alkali from non-fertilized and fertilized (NPK + organic manure) soils of field trials established 45 years ago were exposed to activities of a mixed population of indigenous soil microorganisms in the laboratory. The individual cultures were kept on a rotary shaker for 21 days (aerobic conditions) or left standing for 12 months (semi-aerobic conditions) in order to determine the utilization of HA as nutrients, and possible HA structural transformations. In a full-strength nutrient broth HA positively affected the yields of microbial biomass, that, however, was strongly diminished if HA served as the only sources of carbon and nitrogen. In contrary, the utilization of HA was up to 47.3% under semi-aerobic conditions in cultures deficient in easy utilizable sources of carbon (glucose). No substantial differences were observed in the microbial processing of HA from non-fertilized or fertilized soils. Usually, microbial activities resulted in elemental and structural transformations of HA. Aliphatic structural units were preferably utilized as determined by FT-IR spectroscopy, while aromatic structures became rather dominating. Conclusively, independent of the type of microbial processing, HA from soils of a long-term field trial showed rather low resistance to microbial activities, and perhaps they should not account for a resistant fraction of the soil organic matter.

Keywords

• soil organic matter
• humic acids
• microbial utilization
• microbial transformation