Fig. 1 soil properties: a-e; FT-IR: (f) fourier-transformed infrared spectra of soil organic matter mixtures (mixtures were prepared with replicate three samples). Mean values are shown with standard error (n = 3). Y and D denote the effects of different ages and soil depth, respectively (only significant effects based on two-way ANOVA are shown)
There were some differences in the concentrations of DOM quantity and SOC in soil layers of the sampled stands along the L. gmeliniiafforestation along the chronosequence. SOC and TN content in the 0–10 cm soil layers was significantly higher than the 10–40 cm soil layers in each afforestation (p < 0.05; Figs. 1a and 1c), and the SOC and TN content in the chronosequence were significantly different, mainly in the deep layer 10–40 cm. DOC/SOC ratio significantly increased with increasing forest age (p< 0.05). NH4+ content ranged from 5.68 to 15.88 and increased with increasing forest age (p< 0.05) but did not affect the NO3- contents (p > 0.05; Table S1). The quality of DOM (FI and HIX index) in the soil changed with increasing forest age (Figs. 1d and 1e)). The FI index of different forest ages fluctuated around 1.5. FI index in subsoil significantly decreased over time, while in the topsoil, the change was not significant (p > 0.05). There were no significant differences in HIX index between soil layers in the younger stand (10 years old), but the HIX index was significantly higher in topsoil than in subsoil at the older stands (30 and 50 years old) (p < 0.05). In comparison with the results of previous studies (Gao et al., 2017), it can be inferred that the DOM of different forest ages may have similar sources in the L. gmelinii .
To investigate the structure of the soil organic matter, FT-IR spectroscopy was used to examine, whereas the spectra are generally similar between samples, the relative intensity of particular bands differs, revealing the SOC chemical changes during in afforestation (Hartman et al., 2017) (Fig. 1f). With increasing forest age polysaccharides [1030-1080 cm−1] weaken. 10 years plantations had more abundant polysaccharides, abundant lignins aromatics [1600–1650 cm−1], and less abundant clay minerals [3691 cm−1] than plantations aged 30 and 50 years. It indicated more cellulose (O-alkyl-C) plant material in the younger stand vs. more organic–mineral compounds in the older stands.