Long-chain n-alkanes ranging from C21 to C33 are the dominant constituents of the aliphatic fractions in all plant samples, showing a strong odd-over-even predominance (Fig. 2). The total concentrations of the C21 to C33 n-alkanes in the leaf samples vary greatly, ranging from 46.2 to 476.0 μg/g dry weight (Table 1). In general, G. pretense growing in both habitats (dominated by C31 and C33) and M. hupehensis growing in the non-peatland habitat (dominated by C29 and C31) have a higher concentration of n-alkanes than Q. aliena (dominated by C25 and C27) and C. wilsonii (dominated by C29). The three tree species all show higher n-alkanes concentrations in non-peatland than peatland habitats, while the herb species (G. pretense) displays comparable n-alkane concentrations in both conditions (Fig. 2).
Figure 2. Histograms showing the distribution patterns of n-alkanes in four plant species growing in peatland (wet) and non-peatland (relatively dry) conditions. The standard deviation in (a) was calculated on duplicate analyses. (a) Q. aliena; (b) G. pretense; (c) M. hupehensis; (d) C. wilsonii.
Sample Habitat type Concentration (μg/g) Sum (μg/g) ACL CPI C21 C23 C25 C27 C29 C31 C33 Q. aliena Peatland 0.3 4.15 10.4 24.3 21.2 4.25 0.3 74.9 27.3 6.0 Non-peatland 0.5 4.1 66.4 56.3 33.4 3.9 0.3 188.4 26.6 6.7 G. pretense Peatland 0.4 0.7 4.0 27.5 65.2 168.8 173.6 476.0 31.2 9.9 Non-peatland 4.7 16.3 10.0 31.6 61.3 151.6 157.3 459.5 30.6 13.1 M. hupehensis Peatland 0.8 2.5 3.5 5.1 14.7 9.1 0.5 46.2 28.3 3.5 Non-peatland 0.5 2.2 3.0 7.1 176.9 215.5 19.6 451.8 30.1 15.4 C. wilsonii Peatland 0.2 1.1 3.0 5.1 33.2 8.4 1.0 61.5 28.8 5.4 Non-peatland 0.8 4.4 9.7 13.9 67.9 10.9 1.3 131.4 28.3 4.8
Table 1. Concentration of odd-numbered n-alkanes and the total n-alkanes (C21–C33), and the results of alkane ratios
The above distribution differences between habitats are also clearly revealed by the values of average chain length (ACL) and carbon preference index (CPI). The ACL values of
the plant samples in this study range from 26.6 to 31.2, with the highest ACL value occurring in G. pretense growing in the peatland condition (Table 1, Fig. 3). The plant species except M. hupehensis have relatively higher ACL values in the wet conditions than in the dry conditions (Fig. 3), although the difference is not significant for the quite small dataset (t-test, p > 0.05). The CPI values in the plant samples vary from 3.5 to 15.4 (Table 1, Fig. 3). G. pretense and M. hupehensis have higher CPI values in the dry environment than in the wet one. In contrast, the CPI values of Q. aliena and C. wilsonii are similar in both conditions.
Figure 3. Comparisons of ACL and CPI vales in the species growing in the peatland and non-peatland habitats. Filled symbols refer to peatland, while unfilled symbols refer to non-peatland. Distribution of ACL and CPI in the different types of plants collected from the Dajiuhu peatland. The ACL and CPI values were calculated as follows
In the peatland habitat, the δ13Calk values of the four species fall into the range previously reported in the same peatland (Zhao et al., 2018). M. hupehensis has been investigated in both studies, and shows quite similar δ13Calk values of C29 and C31 n-alkanes, ranging from -32.3‰ to -33.2‰ (Zhao et al., 2018; Table 2, Fig. 4). The difference in δ13Calk values among individual homolog in the same species is less than 1‰ (Fig. 4). The M. hupehensis specimen has the least negative δ13Calk values, while the δ13Calk values of G. pretense are near 4‰ more negative relative to those of M. hupehensis. In our previous study, the life-form averaged δ13Calk values of C29 and C31 n-alkanes are similar for forb, graminoid and shrub plants (Zhao et al., 2018). For the four species, the δ13Calk values of long-chain n-alkanes are comparable between the peatland and non-peatland habitats (Figs. 4a and 4b). Only the δ13Calk value of C27 n-alkane in M. hupehensis growing in the non-peatland is >1‰ negative than that in the peatland habitat (Fig. 4). Moreover, the plant n-alkanes in this study show a slight decrease in their δ13C values with longer chain lengths, although limited by the small number of samples. This trend has also been found in previous studies (Zhao et al., 2018; Smith et al., 2007). In summary, the habitats do not affect the δ13Calk values in the four species investigated in this study.
Sample Habitat type Carbon number C25 C27 C29 C31 C33 Q. aliena Peatland -33.4±0.5 -36.0±0.2 -33.9±0.4 Non-peatland -33.2±0.7 -33.0±0.5 -32.6±0.3 G. pretense Peatland -36.0±0.4 -35.6±0.1 -36.0 -36.0±0.9 Non-peatland -35.8±0.4 -35.5±0.4 -37.0±0.2 M. hupehensis Peatland -32.0±0.4 -32.5 -32.3±0.5 -33.0 Non-peatland -34.3 -32.6±0.4 -33.8±0.6 C. wilsonii Peatland -32.4±0.1 Non-peatland -32.1±0.2 -32.4±0.2
Table 2. Individual carbon isotope values of odd-numbered n-alkanes (‰, VPDB) in the plant samples.
Different from δ13Calk, the δ2Halk values are distinctive between the two habitats. The δ2Halk values of three of the four species are more negative in the non-peatland habitat than their counterparts in the peatland (Figs. 4c and 4d). Taking G. pretense as an example, the differences of the δ2Halk values of C29 and C31 n-alkanes between the two habitats are as large as near 50‰ (Table 3). For Q. aliena, the δ2Halk values of the predominant C27 and C29 n-alkanes are 15‰–20‰ more negative in the non-peatland habitat than those in the peatland. Different from the other three species, the M. hupehensis species show less negative δ2Halk values in the non-peatland habitat than in the peatland (Fig. 4c and 4d). Notably, the δ2Halk values among the species growing in the peatland habitat vary in a quite narrow range; however, the δ2Halk values in the non-peatland habitat are scattered. Such a finding is consistent with the results in Zhao et al. (2018), which reports similar life-form averaged δ2Halk values between forb and shrub plants. In the non- peatland habitat, the forb species (G. pretense) has more negative δ2Halk values (C31, -227‰; C33, -242‰) than the other three tree species, and is also more negative than the life-form averaged δ2Halk values for forb plants in the Dajiuhu (Zhao et al., 2018). Different from δ13Calk values, the relation of the chain lengths with δ2Halk values is scattered, especially for plant samples collected from non-peatland.
Sample Habitat type Carbon number C25 C27 C29 C31 C33 Q. aliena Peatland -159±3 -167 -162±3 Non-peatland -187±5 -177±1 G. pretense Peatland -174±6 -181±2 -185±2 Non-peatland -228±1 -218 -227 -242±1 M. hupehensis Peatland -141±1 -158±4 -169 -173±1 Non-peatland -159±1 -142 C. wilsonii Peatland -177±1 -158±4 Non-peatland -179±3 -202 -178±1 -160±3
Table 3. Individual hydrogen isotope values of odd-numbered n-alkanes (‰, VSMOW) in the plant samples