Spectroscopic properties of dissolved humic substances - a reflection of land use history in a fen area

The elemental composition and spectroscopic properties of dissolved fulvic acids isolated from different sampling media (topsoil, ground and surface w ater) of a natural fen area (high portion of organic soils) were examined t o reveal the effects of land use history. These effects need to be known if dissolved humic substances are to be a major factor in identifying the imp act of present and future changes in land use. Dissolved fulvic acids (tops oil, groundwater) from highly degraded peatlands (due to a long-term agricu ltural use) exhibit lower C/N ratios, higher absorption in the UV spectra, and higher absorption at 1,620 cm(-1) in the FTIR spectra compared with ful vic acids from relatively intact peatlands. These properties illustrate tha t long-term agricultural use with high inputs results in increased aromatic structures and a further humification of dissolved fulvic acids due to ver y strong peat decomposition compared with relatively intact peatlands. Sync hronous fluorescence spectra also indicate the higher level of aromatic str uctures within fulvic acids isolated from sites with long-term agricultural use (high peat decomposition) compared with a land use history resulting i n a lower peat decomposition. The different sources of fulvic acids in surf ace water (precipitation, runoff, interflow, groundwater) are the main reas on for these effects not being detected in fulvic acids isolated from surfa ce water. Short-term changes in land use characterized by a transition from crop farming to an unimproved grassland were found not to affect the spect roscopic properties of dissolved fulvic acids. A humification index deduced from the synchronous fluorescence spectra is proposed. We have strong evid ence that dissolved humic substances indicate changes in the environmental conditions (both anthropogenic and natural) of wetlands with a high proport ion of organic soils.