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 a nd ultrafiltered (molecular weight cut off < 1000 dalton) to obtain th e dissolved organic carbon (DOC). The DOC was used to test the hypothe sis that burning decreases the soil respiration rate due to a chemical ly modified ''pyromorphic'' humus. Infrared scanning of the two differ ent DOC fractions showed that the unburned DOC was more acidic than th e burned DOC and there were organic carboxylic acids missing from the DOC of the burned humus. When the extracts were applied to unburned hu mus the DOC from the burned humus changed the microbial community stru cture as measured by phospholipid fatty acid analysis (PLFA) and decre ased the respiration rate by 26% as compared to the DOC from the unbur ned humus. The DOC from the burned humus also decreased the light emis sion of the photobacterium Vibrio fischeri within a short time. This i ndicated 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 hydroph ilic acids, bases, and neutrals. When the fractions were tested (at co mparable concentrations of carbon between respective burned and unburn ed fractions) for the inhibition on the CO2-evolution the effect was i n the hydrophilic fraction specifically in the hydrophilic bases it co ntained. (C) 1998 Elsevier Science Ltd. Ail rights reserved.