Canadian Forest Service Publications

The chemistry of some foliar litters and their sequential proximate analysis fractions. 2015. Preston, C.M.; Trofymow, J.A. Biogeochemistry, 126:197–209

Year: 2015

Issued by: Pacific Forestry Centre

Catalog ID: 36813

Language: English

Availability: PDF (request by e-mail)

Available from the Journal's Web site.
DOI: 10.1007/s10533-015-0152-x

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Proximate analysis (PA) is widely used to assess foliar, litter, and wood quality. The acidunhydrolyzable 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 13CNMR 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?

Plain Language Summary

Plant tissue composition is a factor affecting rates of litter decomposition, a key ecosystem process controlling carbon and nutrient cycling, and is also assessed prior to processing of biomass and forage feedstocks. Proximate Analysis (PA), the measurement of residue masses after sequential extraction of tissues with solvents, hot water, and acid, is a common technique used to characterize plant tissues. The final acid-unhydrolyzable residue (AUR) following PA has been often assumed to be entirely composed of lignin. In this paper sequential PA residues were prepared from seven different foliar litters. Changes in total C, N, δ13CC values and solid-state 13C NMR spectra were consistent with the retention of cutins and condensed tannins (CT) as well as lignin in the AUR fraction. Hot water extraction removed the bulk of the CT, though trace levels of CT were still present in the AUR, though these may be underestimates. The results demonstrate that the casual assumption in many studies that the tissue AUR content is equivalent to lignin content is false, and that the scientific community needs to limit use of “lignin” to chemically meaningful contexts.