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.