Estimating nitrate, dissolved organic carbon and DOC fractions in forest floor leachates using ultraviolet absorbance spectra and multivariate analysis

Multivariate regressions on ultraviolet (UV) absorbance (210–300 nm) data were tested as a potential tool to estimate concentrations of total dissolved organic carbon (DOC), DOC in the XAD-8-adsorbable (hydrophobic) fraction (HoDOC), and dissolved nitrate (NO3-N) in forest floor leachates. Partial least square (PLS) regressions were established for 20 zero-tension lysimeter solutions sampled in plots of coniferous and deciduous forest stands from May 2002 to January 2003. These regressions were validated against data from 24 other zero-tension solutions sampled in the same stands. The root mean square error of prediction (RMSEP) was 2.0 mg/l for DOC and 1.6 mg/l for HoDOC. These errors were 39% (DOC) and 20% (HoDOC) smaller than the errors obtained with regressions using only absorbance at 280 nm. For NO3-N, RMSEP was 0.12 mg/l (estimation by single wavelength regression is not possible). DOC and HoDOC covaried strongly and could not be estimated independently based on absorbances; HoDOC varied only between 61% and 81% of the DOC. Partly for this reason, an attempt to estimate the HoDOC/DOC ratio based on specific absorbance spectra (absorbance units per mg/l of total DOC) was little successful. Partly, however, this problem resulted from the fact that both HoDOC and HiDOC fractions had a considerable UV absorptivity, which was not consistent between samples.