Changes in organic components for fallen logs in old-growth Douglas-fir forests monitored by ¹³C nuclear magnetic resonance spectroscopy

Abstract

13C cross-polarization magic-angle spinning nuclear magnetic resonance (CPMAS NMR) spectroscopy was used to characterize heartwood from decaying fallen boles of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco), western hemlock (Tsugaheterophylla (Raf.) Sarg.), and western red cedar (Thujaplicata Donn). The sample decay classes I to V had been previously assigned based on field observations. Solid-state 13C CPMAS NMR spectra were analyzed to determine the proportion of C of the following chemical types: carbohydrate, lignin, aliphatic, and the sum of carboxyl plus carbonyl. For both Douglas-fir and western hemlock, the proportion of carbohydrate C increased slightly in the early stages of decay. This was followed by a substantial increase in lignin C, while carbohydrate C declined to about 10% of total C. By contrast, the spectra for western red cedar generally showed little change with increasing decay class. One exceptional sample of western red cedar class IV was highly decomposed, indicating complete loss of carbohydrate C, and some loss of lignin side-chain C. For all three species, signals from alkyl and carbonyl C were weak, but tended to increase slightly with decomposition, most likely because of the selective preservation of waxes and resins (alkyl C), and oxidation. Accumulation of chitin was not observed, and there was little evidence for lignin decomposition or for formation of humic polymers. 13C CPMAS NMR offers a simple and information-rich alternative to wet chemical analyses to monitor changes in organic components during decomposition of woody litter.

Decomposition, δ13C, and the “lignin paradox”

“…They were then adjusted using correction factors in Davis et al (1994) developed for wood samples run with 1 ms contact time. A similar approach was used in Preston et al (1998) to compare results obtained at 0.5 ms contact time with those from a previous NMR study of CWD (Preston et al 1990) using 1 ms contact time. The quantitative reliability of CP NMR spectra is a complex issue (e.g., Davis et al 1994;Preston 1996;Smernik and Oades 2000), and the relative areas presented here should not be regarded as absolutely correct, as discussed previously (Preston et al 1998).…”

“…One exception occurs with high inputs and slow decomposition of coarse woody debris (CWD), especially from gymnosperms, as found in old-growth coastal rainforests of the Pacific Northwest (Preston et al 1990(Preston et al , 1998(Preston et al , 2002Means et al 1992;deMontigny et al 1993). The CWD is a conspicuous feature of these forests, and a defining characteristic of oldgrowth (Sollins et al 1987;Means et al 1992;Daniels et al 1997;Trofymow et al 2003;Sun et al 2004).…”

Influence of coarse wood and fine litter on forest organic matter composition

“…5, Table 3) can be interpreted from previous studies of lignin (Hatcher, 1987), CW (Preston et al, 1990(Preston et al, , 1998 and CW-derived organic horizons (de Montigny et al 1993;Preston et al 2002). The CW materials are similar to the guaiacyl lignin-rich residues that tend to dominate the later stages of CW decomposition.…”

“…Areas of the FA spectra obtained at the higher field were corrected for the spinning sidebands of the carboxyl signal by assuming equal areas for the upfield and downfield sidebands (Preston et al, 2002). For the woody materials, proportions of lignin and carbohydrate C were determined as described in Preston et al (1990). The validity of this procedure was assessed by determining the ratio of CW aromatic to phenolic lignin C, which should be approximately two.…”

“…Although lignin decays more slowly than cellulose and hemicellulose, certain white-rot fungi are capable of degrading both lignin and cellulose material (Crawford 1981). In the early stages of wood decay, white-rot fungi tend to be predominant, which results in the loss of lignin at similar or faster rates as other components (Preston et al 1990(Preston et al , 1998(Preston et al , 2002.…”