Characterization of dissolved organic carbon from burned humus and its effects on microbial activity and community structure

Samples of burned and unburned humus layer were extracted with water and ultrafiltered (molecular weight cut off < 1000 dalton) to obtain the dissolved organic carbon (DOC). The DOC was used to test the hypothesis that burning decreases the soil respiration rate due to a chemically modified "pyromorphic" humus. Infrared scanning of the two different DOC fractions showed that the unburned DOC was more acidic than the burned DOC and there were organic carboxylic acids missing from the DOC of the burned humus. When the extracts were applied to unburned humus the DOC from the burned humus changed the microbial community structure as measured by phospholipid fatty acid analysis (PLFA) and decreased the respiration rate by 26% as compared to the DOC from the unburned humus. The DOC from the burned humus also decreased the light emission of the photobacterium Vibrio fischeri within a short time. This indicated toxicity within the DOC of the burned humus. The DOC samples were further fractionated according to chemical characteristics using chromatographic techniques into six fractions: hydrophobic and hydrophilic acids, bases, and neutrals. When the fractions were tested (at comparable concentrations of carbon between respective burned and unburned fractions) for the inhibition on the CO2-evolution the effect was in the hydrophilic fraction specifically in the hydrophilic bases it contained. (C) 1998 Elsevier Science Ltd. Ail rights reserved.