Abstract

Materials

Samples were collected from the decayed wooden components in the ancient building of Xichuan Guild Hall in Nanyang City, Henan Province, China (Table 1; Sun et al. 2020, Yang et al. 2020b). Among the wooden component samples, sample No. 3 was obtained from the top surface of a red oak (Quercus sp.; Fagacae) beam and No. 1 was obtained from the side surface of an indoor Schima (Schima sp.) column (Yang et al. 2020b). Undecayed samples of Quercus sp. and Schima sp. were obtained from the Southwest Forestry University. The wood powder needed for FTIR analysis was processed using a grinder (XL-108, Guangzhou Xulang Machinery Equipment Co., Ltd, China) equipped with a 100-mesh screen (Yang et al. 2020a).

Table 1. Information about the materials used.

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Method of FTIR analysis

For FTIR analysis, oven-dried wood powder and spectrum potassium bromide (KBr) were fully mixed in a ratio of 1:150, poured into an agate mortar, and ground into powdered form while keeping the wood powder and KBr in dried condition by using a high-wattage lamp throughout the whole process. The powder mixture was compressed into sliced samples using a powder tablet press machine (FW-4A, Uncommon Technology Development Co., China). To ensure the accuracy of the FTIR spectra, the sliced samples cannot have chipped edges and must have a uniform thickness. FTIR spectra were generated using an FTIR spectrophotometer (ALPHA2, Bruker Inc., Germany) in the range of 4,000 to 400 cm⁻¹ to provide detailed information on the functional groups present in the sample surface (Yang et al. 2015, 2020a). Scans were run at a resolution of 4 cm⁻¹, and the spectra were obtained using attenuated total reflectance. In order to avoid the effect of experimental error, a minimum of five scans per specimen were operated to calculate their average.

Lignin peaks were compared with hemicellulose and cellulose peaks to evaluate relative changes in the composition of wooden components (Pandey and Pitman 2003, Xu et al. 2013).

The ratio of peak heights at 1,374 and 2,900 cm⁻¹ (H1374/H2900) and 1,429 and 897 cm⁻¹ (H1429/H897) was used to determine the cellulose crystallinity index in wood (Monrroy et al. 2011, Tomak et al. 2013).

FTIR analysis of red oak wooden components

Figure 1 shows the absorption peak intensity of the C=O stretching vibration of unconjugated carbonyl groups, acetyl, or carboxylic acid at 1,735 cm⁻¹, which is attributed to hemicellulose (xylans [Pandey and Pitman 2003, Ibrahim et al. 2007, Pandey and Nagveni 2007, Li et al. 2010, Monrroy et al. 2011, Zeng et al. 2012, Tomak et al. 2013, Xu et al. 2013, Liu et al. 2017]), as shown in Table 2. The fungal-decayed wood components were weakly degraded, which indicated a decrease in the small quantity of C=O (i.e., a subsequent reduction of hemicellulose contents). The absorption peak intensity at 810 cm⁻¹ assigned to the mannose structure of hemicellulose was slightly decreased, indicating that a small part of the mannose structure was also degraded by the fungus (Fig. 1).

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Table 2. The Fourier-Transform Infrared Spectroscopy characteristic bands of the wood samples studied.

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The characteristic peaks of 1,374, 1,336, 1,159, 1,039, and 897 cm⁻¹ represent the structural contribution of carbohydrates (Pandey and Pitman 2003, Ibrahim et al. 2007, Pandey and Nagveni 2007, Li et al. 2010, Monrroy et al. 2011, Zeng et al. 2012, Tomak et al. 2013, Xu et al. 2013, Liu et al. 2017; Table 2). However, the absorption peak at 1,039 cm⁻¹ disappeared and was replaced by two small absorption peaks at 1,050 and 1,040 cm⁻¹ (Fig. 1). This finding indicated that the carbohydrates were degraded by fungi, leading to a decrease in the cellulose and hemicellulose contents. Figure 1 shows that the absorption peak intensity at 897 cm⁻¹ was also weakened, also indicating a reduction in cellulose content. The absorption peak intensities at l,374, 1,336, and 1,159 cm⁻¹ in the fungal-decayed wood components also changed relative to those in the nondecayed wood components, indicating that the cellulose was attacked by fungi.

These characteristic peaks of 1,650, 1,600, 1,508, 1,460, 1,424, 1,232, and 1,120 cm⁻¹ are the structural contributions associated with lignin (Pandey and Pitman 2003, Ibrahim et al. 2007, Pandey and Nagveni 2007, Li et al. 2010, Monrroy et al. 2011, Zeng et al. 2012, Tomak et al. 2013, Xu et al. 2013, Liu et al. 2017; Table 2). Their intensities, especially at 1,650, 1,508, 1,460, 1,232, and 1,120 cm⁻¹, were noticeably higher than those in nondecayed wood components (Fig. 1). These values indicated a relative increase in lignin content for the decayed wood components. The elevated absorption peaks of lignin showed the increase in phenolic hydroxyl content. In general, characteristic lignin peaks increase with the progression of decay. A high absorption peak intensity indicates great increase in decay degree (Pandey and Pitman 2003, Pandey and Nagveni 2007).

Brown-rot fungi only degrade carbohydrates and retain lignin, whereas white-rot fungi can consume not only cellulose and hemicellulose, but also lignin (Guo et al. 2010). Based on the analysis of the absorption peak intensities of cellulose, hemicellulose, and lignin, we concluded that cellulose and hemicellulose had decayed, whereas lignin was retained. Hence, we speculated that the red oak wooden components were attacked by brown-rot fungi. This finding is consistent with the microstructures of polarized light and fluorescence (Yang et al. 2020b).

In order to determine the difference of wood powder content and the influence of operation error on the experimental results, the ratios of the absorption peak intensities of lignin and carbohydrates were used to represent the changes in chemical composition (Li et al. 2010, Xu et al. 2013, Liu et al. 2017). The average relative intensities of lignin peaks at 1,649, 1,600, and 1,508 cm⁻¹ against carbohydrate peaks at 1,735, 1,374, 1,159, and 1,039 cm⁻¹ were calculated using the peak heights (Table 3). The changes in lignin content were characterized by the values of H1508/H1735, H1508/H1374, H1508/H1159, H1508/H1039, H1649/H1039, and H1600/H1039; whereas those for carbohydrate content were characterized by the values of H1735/H1508, H1374/H1508, H1159/H1508, and H1039/H1508 (Li et al. 2010, Xu et al. 2013, Liu et al. 2017). The carbohydrate contents were reduced (Table 3). These reductions in H1735/H1508, H1374/H1508, H1159/H1508, and H1039/H1508 ratios were 6.67, 6.32, 23.33, and 55.70 percent, respectively. Meanwhile, lignin contents increased (Table 3). The increase in H1508/H1735, H1508/H1374, H1508/H1159, H1508/H1039, H1649/H1039, and H1600/H1039 was 7.37, 6.67, 29.73, 126.40, 91.74, and 101.64 percent, respectively. These changes indicated that the fungi had a stronger preference to degrade hemicellulose and cellulose than lignin. These findings agree with those reported in previous studies (Li et al. 2010, Xu et al. 2013).

Table 3. The ratio of Fourier-Transform Infrared Spectroscopy characteristic peak height of wood samples studied. Note: +: increase percentage; −: decrease percentage.

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H1374/H2900 decreased from 1.08 in the nondecayed wood components to 0.79 in the fungal-decayed wood components, and H1429/H897 decreased from 0.92 to 0.70 (Table 3). These reductions were 26.85 and 23.91 percent for H1374/H2900 and H1429/H897, respectively. These results showed that both the amorphous and crystalline zones of cellulose were degraded (Monrroy et al. 2011). These reductions would decrease the crystallinity, which would also result in a reduction in physical and mechanical properties of wood. These findings were in good agreement with a previous study (Monrroy et al. 2011).

FTIR analysis of Schima wooden components

Hemicellulose of Schima wood was heavily degraded as a result of fungal decay (Fig. 2). The absorption peak of 1,735 cm⁻¹, which is attributed to hemicellulose, had disappeared as a result of the degradation of glucurono-xylanes connected to the splitting of acetyl groups and 4-O-methylglucuronic acids side units (Solár et al. 2007). This finding indicated a marked reduction of the number of C=O linkages, resulting in a sharp decrease in hemicellulose content. No noticeable changes were found in the absorption peak intensity at 810 cm⁻¹, indicating that the mannose structure was not degraded by the fungi (Fig. 2).

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Peaks at 1,374, 1,336, 1,159, 1,104, 1,039, and 897 cm⁻¹ were attributed to the structural contributions of cellulose and hemicellulose. The absorption peaks of 1,159 and 1,104 cm⁻¹ disappeared in the fungal-decayed wood components, indicating the degradation of celluloses and hemicelluloses with the resulting reductions in their content (Fig. 2). The absorption peak intensities at 1,374, 1,336, 1,039, and 897 cm⁻¹ in the fungal-decayed wood components did not change noticeably compared with those in the nondecayed wooden components, indicating that the cellulose and hemicellulose contents at these absorption peaks did not markedly decrease (Fig. 2).

The characteristic peaks of 1,650, 1,600, 1,508, 1,460, 1,424, 1,260, 1,232, and 1,120 cm⁻¹ are the structural contributions of lignin. The absorption peak of 1,650 cm⁻¹ disappeared in the fungal-decayed wooden components, indicating that lignin was mainly demethylated, depolymerized, and degraded by fungal attack (Irbe et al. 2011), resulting in a reduction in its contents. By contrast, these absorption peak intensities, especially at 1,508, 1,460, and 1,424 cm⁻¹ were higher than those in the nondecayed wood components seen in Figure 2, indicating a relative increase in lignin content. The increase in peak intensities after fungal attack might be related to the degradation of carbohydrates. However, the absorption peak at 1,260 cm⁻¹, which is attributed to guaiacyl-lignin, had disappeared and became two small absorption peaks at 1,270 and 1,232 cm⁻¹, which are attributed to syringyl-lignin. This finding reveals that guaiacyl-lignin was degraded by fungi, resulting in a decrease in guaiacyl units and an increase in syringyl units. A new absorption peak of 1,120 cm⁻¹ was found, indicating an increase in lignin content.

Based on the analysis of absorption peak intensities, we concluded that the cellulose, hemicellulose, and lignin of Schima wood components were attacked by white-rot fungi. This finding is consistent with the microstructures of polarized light and fluorescence (Yang et al. 2020b). White-rot fungi degrade carbohydrates and lignin. Hence, we speculated that the Schima wood components were attacked by white-rot fungi.

Carbohydrate contents decreased as indicated by the values of H1159/H1508 and H1039/H1508, whereas the lignin contents increased as indicated by the values of H1508/H1159, H1508/H1039, H1649/H1039, and H1600/H1039 (Table 3). The reduction in carbohydrate contents indicated that the hemicellulose and cellulose of Schima wood components were more vulnerable to fungal decay. Table 3 also showed that the values of H1508/H1735 and H1508/H1374 decreased from 1.04 to 0.78 and from 1.04 to 1.00, respectively, with 25.00 and 3.83 percent reductions, respectively. This decrease might have been caused by serious lignin degradation resulting from fungal decay. Meanwhile, H1735/H1508 and H1374/H1508 increased from 0.96 to 1.28 and from 0.96 to 1.00, respectively, with associated increases of 33.33 and 4.18 percent, respectively (Table 3). These changes occurred because hemicellulose and cellulose were better able to resist fungal attack relative to lignin. This finding was in agreement with reports by Pandey and Pitman (2003) and Xu et al. (2013).

The H1374/H2900 ratio, representing the cellulose crystallinity index, decreased from 1.02 in the nondecayed wood components to 0.88 in the fungal-decayed wood components. Meanwhile, H1429/H897 decreased from 0.92 to 0.72 (Table 3). The associated reductions were 13.73 and 21.74 percent for H1374/H2900 and H1429/H897, respectively (Table 3). These results showed a decrease in crystallinity and were in good agreement with those of a previous study (Monrroy et al. 2011) and those obtained for red oak wood components discussed in this article.