FLUORESCENCE SPECTROSCOPY OF AQUEOUS PINE LITTER EXTRACTS - EFFECTS OF HUMIFICATION AND ALUMINUM COMPLEXATION

Fluorescence excitation, emission, and synchronous-scan spectra were obtained for aqueous extracts of needle and litter layer (0-horizon) samples from ponderosa pine collected at a plantation site. The spectral lineshapes differed markedly between the needle and litter samples, and showed an increasing overall intensity with increasing extent of humification (increasing depth in the litter layer). At a dissolved organic C (DOC) concentration of 100 g m-3, these effects were accompanied by a general shift in spectral density from lower to higher wavelength such that, in the excitation spectra, there was increasing prominence of a peak at 390 nm. When the DOC concentration was decreased from 100 to 25 g m-3, the overall spectral intensity decreased and the peak at 390 nm in the excitation spectra of the litter samples gave way to a rising peak at 340 nm. Changes in pH from 4 to 5, characteristic of the undiluted litter extracts, produced little effect on the spectra. The addition of Al at 40 mmol m-3 generally produced enhancement of the fluorescence intensity in all three kinds of spectra for the needle and upper litter-layer extracts at 100 g C m-3 and pH 4. Spectral density below 320 nm in the excitation and synchronous-scan spectra of the needle solution (possibly attributable to gentisic acid on the basis of model experiments) was, however, diminished by Al addition. For the lower litter-layer extracts, Al addition decreased the fluorescence intensity in excitation and emission spectra, whereas it increased the intensity in synchronous-scan spectra. These trends indicated that the water-soluble fluorophores in the litter layer differed significantly from those in the fresh needles, and changed with the extent of humification. The 390-nm peak in the excitation spectrum, particularly its behaviour in the presence of added Al, may be useful as a spectral signature of products formed by litter humification processes.