Abstract
Proximate analysis (PA) is widely used to assess foliar, litter, and wood quality. The acid-unhydrolyzable residue (AUR) of PA, originally known as Klason lignin from wood analysis, is often assumed to be entirely lignin-derived, but the AUR of much plant material also includes contributions from condensed tannins (CT) and cutin or suberin. To improve understanding of chemical changes throughout the PA procedure, we characterized seven foliar litters and their sequential PA fractions (nonpolar and hot-water extracted, AUR). Changes in total C and N, extractable and insoluble CT (as detected by butanol/HCl hydrolysis), δ13C values and solid-state 13C NMR spectra were consistent with loss of carbohydrates and protein after acid hydrolysis, and support previous studies that the AUR residue includes lignin, cutin and CT, all of which are depleted in δ13C. Hot-water extraction removed the bulk of extractable plus insoluble CT. Only trace levels were detected in the AUR, although 13C NMR shows that these are likely underestimates. The assumption of lignin-AUR equivalence still causes misinterpretation of PA results for many sample categories. It is time for the scientific community to limit use of “lignin” to chemically meaningful contexts?
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