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Aquatic humic substances (HS) from a bog lake water, a riverwater, and a groundwater were isolated after enrichment on XAD 8 columns and added to a Czapek-Dox nutrient broth which was used either in full strength or without glucose and/or NaNO3. The individual flasks were inoculated with natural microbial populations of corresponding water samples or with a Pseudomonas fluorescens strain isolated from groundwater. The presence of HS resulted in an increase of bacterial numbers in nearly all cultures incubated for 3 weeks at 25 degrees C on a shaker. HS reisolated from cultures without glucose or NaNO3 showed no or only minor quantitative differences as compared to those from sterile controls. In full strength nutrient broth up to 27% of HS were utilized. Data obtained by spectroscopic methods (UV/vis/FTIR) and elemental analysis indicated a decrease in particle size and a loss in aromaticity and aliphatic carbon in HS reisolated from the microbial cultures. Simultaneously an increase in the N content of HS was observed, which probably originated from some constituents of microbial biomass such as proteins and amino sugars. The NMR data also documented that significant transformations of HS occurred in the individual microbial cultures. After incubation, increased amounts of aromatic acids were detected in some liquid media and residual HS by GC/MS or capillary electrophoresis. 1H NMR spectroscopy was less effective in indicating structural differences in the HS than 13C NMR but revealed considerable detail of the microbial degradation of riverine HS, when limited sample was available. The newly developed NMR increment analysis provided substantial detail of aromatic structures in a microbially altered HS. The microbial degradation of HS strongly depended on the composition of the HS, the species selection of the microorganisms, and to a lesser extent on the culture conditions. For any series of identical inoculum and HS, full broth media initiated the most extensive alteration of HS.

A soil sample from the Ap horizon of an arable brown soil was fractionated by wet sieving, and seven size fractions of organic and mineral soil particles were separated. The organic fractions formed only 2.2% of the soil dry mass, but contained 41.5 and 29.12% of the total soil content of carbon and nitrogen, respectively, and thus represented an important reservoir of readily utilizable nutrients. Organic particles also accumulated most of the soil enzyme activities, determined asβ-glucosidase,β-acetylglucosaminidase, and proteinase activity. The highest counts of bacteria, actinomycetes, and fungi per gram of the soil fractions were obtained with the organic particles, but for the most part microorganisms accumulated in the silt-clay fraction. All soil fractions except the coarsest organic particles contained higher counts of oligotrophic bacteria than copiotrophic ones. Microbial counts, ATP contents, and enzyme activities decreased significantly with decrease in size of the organic soil particles, and increased with decrease in size of the mineral soil particles. Thus, the coarse organic particles >5 mm and the silt-clay fraction <0.05 mm represent the sites with the highest concentrations of microorganisms, ATP contents, and enzyme activities in the arable brown soil under test.