A comparative study of optical properties of NaOH peat extracts: implications for humification studies

Assessment of the degree of decay of peat (humification) in ombrotrophic mi res has become a standard technique for palaeoclimatic reconstruction, base d on the finding that decay is primarily determined by surface wetness and temperature at the time of peat deposition. Determination of humification i s undertaken by colorimetric measurement of an alkali extract of the peat a t 540 nm. Humification is proportional to the amount of humic matter dissol ved by this extraction process, although few researchers convert results to a quantitative measure of humification expressing results as percentage li ght transmission through the peat. This paper uses luminescence spectroscop y to assess the chemical composition of these extracts. Luminescence excita tion and emission wavelengths suggest that high molecular weight acids ('hu mic acids') are altered by the extraction procedure to form lower molecular weight acids ('fulvic acids'), amino acids and polysaccharides. Percentage transmission is principally related to luminescence emission wavelength an d thus to molecular weight of the compounds present. Luminescence emission shows much more sensitivity to peat composition and demonstrates that diffe rent plant species may be affected to different degrees by the NaOH extract ion process. The findings broadly support the underlying principle of color imetric determination of 'humification' whereby transmission levels decreas e with increasing plant breakdown, but show that it is based on an inadequa te understanding of the chemical processes occurring in peat decay and prep aration procedures. Luminescence spectroscopy provides a technique for reso lving these issues.