**3. Results**

#### *3.1. Proportions of Organic Components in Newly Shed Foliar Litter*

The fractions of ASE and AUR were the main components among all plant species. In rosa litter, the content of ASE (37.62%) was higher than that of other C fractions, while in fir, spruce and willow litter, the content of AUR was higher than that of other C fractions (fir: 37.05%; spruce: 41.45%; willow: 40.04%) (Table 1). ASE and AUR were significantly

affected by the collection time (ASE: *p* = 0.015; AUR: *p* = 0.003) and plant species (ASE: *p* = 0.020; AUR: *p* < 0.001) (Table 2). Regarding WSE, similar trends were observed between the fir and spruce litter materials, with peaks occurring in April (Figure 1). Regarding AUR, the content in fir, willow and rosa litter generally decreased from August to November (Figure 1).

**Table 1.** Properties of the C quality of newly shed foliar litter for each plant species. Different lowercase letters indicate significant (*p <* 0.05) differences among the various plant species for each C quality aspect (plant species: fir, spruce, willow and rosa; functional C groups: aliphatic C, methoxyl C, O-alkyl C, di-O-alkyl C, aromatic C, phenolic C and carboxyl C; organic components: WSE, water-soluble extractives, OSE, organic-soluble extractives, ASE, acid-soluble extractives, AUR, acidunhydrolyzable residues; HB/HI, (0–45 ppm + 110–165 ppm)/(45–110 ppm + 165–190 ppm), and A/O-A, aliphatic C/O-alkyl C)).


**Table 2.** Results of two-way ANOVA testing for the effects of the collection time (from August 2015 to July 2016) and plant species on the C quality of newly shed foliar litter (plant species: fir, spruce, willow and rosa; functional C groups: aliphatic C, methoxyl C, O-alkyl C, di-O-alkyl C, aromatic C, phenolic C and carboxyl C; organic components: WSE, water-soluble extractives, OSE, organic-soluble extractives, ASE, acid-soluble extractives, AUR, acid-unhydrolyzable residues; HB/HI, (0–45 ppm + 110–165 ppm)/(45–110 ppm + 165–190 ppm), and A/O-A, aliphatic C/O-alkyl C)).


#### *3.2. Proportions of Functional C Groups in Newly Shed Foliar Litter*

Aliphatic C and O-alkyl C were the main functional C groups, while methoxyl C and carboxyl C were less abundant than the other functional groups (Table 1). In rosa litter, the content of aliphatic C (35.44%) was higher than that of the other functional C groups, while fir, spruce and willow litter contained more O-alkyl C (fir: 32.03%; spruce: 35.02%; willow: 32.34%). The plant species (*p* < 0.001) and collection time (*p* < 0.01) remarkably affected the functional C groups (Table 2). Smaller proportions of O-alkyl C in the fir and spruce litter samples were observed in April (Figure 2). The aromatic C and phenolic C contents in the fir and spruce litter samples remained stable with low proportions (Figure 2). The aliphatic C content in willow litter continuously decreased from August to November, while the aliphatic C content in rosa litter increased from August to October (Figure 2).

**Figure 1.** Monthly dynamics of the proportions of organic components (%) (WSE: water-soluble extractives; OSE: organic-soluble extractives; ASE: acid-soluble extractives; AUR: acid-unhydrolyzable residues) in newly shed foliar litter determined via proximate C fraction analysis.

**Figure 2.** Monthly dynamics of the proportions of functional C fractions (%) (aliphatic C, methoxyl C, O-alkyl C, di-O-alkyl C, aromatic C, phenolic C, and carboxyl C) in newly shed foliar litter obtained from the integration of different chemical shifts determined via CP/MAS 13C NMR spectroscopy.

#### *3.3. A/O-A and HB/HI Ratios of Newly Shed Foliar Litter*

The ratios of A/O-A (0.88) and HB/HI (1.15) in rosa litter were higher than those in the litter of the other species, while those in spruce litter were the lowest (A/O-A: 0.43; HB/HI: 0.78) (Table 1). The plant species (*p* < 0.001) and collection time (*p* < 0.001) significantly

influenced A/O-A and HB/HI (Table 2). The ratios of A/O-A in the fir and spruce litter samples were significantly lower than those in the willow and rosa litter samples (Figure 3a). The ratios of HB/HI in the fir and spruce litter samples were more stable than those in the willow and rosa litter samples, respectively (Figure 3b).

**Figure 3.** Monthly dynamics of the ratio of (**a**) aliphatic C to O-alkyl C (A/O-A) and (**b**) the index of hydrophobicity (HB/HI, (0–45 ppm + 110–165 ppm)/(45–110 ppm + 165–190 ppm)) in newly shed foliar litter determined via CP/MAS 13C NMR spectroscopy. The \* indicates significant (*p <* 0.05) differences among the various plant species. \* *p* < 0.05, \*\* *p* < 0.01, and \*\*\* *p* < 0.001.

*3.4. Relationships between the Initial Functional C Groups and C Loss during Litter Decomposition*

Among all the functional C groups, methoxyl C, aromatic C and phenolic C significantly (*p* < 0.05) affected C loss during litter decomposition. Methoxyl C (coefficient: 0.49) and aromatic C (coefficient: −0.54) markedly controlled C loss in the third and first years of litter decomposition, respectively, and phenolic C significantly controlled C loss in both the second (coefficient: −0.51) and fourth (coefficient: −0.47) years of litter decomposition (Figure 4).

**Figure 4.** Results of linear regression analysis between functional C groups and C loss in litter decomposition processes. The \* indicates the significance (*p <* 0.05) between functional C groups and C loss at different decomposition stages in the regression model. \* *p* < 0.05, \*\* *p* < 0.01, and Pearson's r values from linear regression are shown in each panel.
