**4. Discussion**

Different components with hierarchical decomposability have been widely used to assess chemical changes during litter decomposition [6]. Labile components are highly decomposable, but a clear boundary is absent in soil organic matter studies and Earth system models [1]. Dissolved organic C has been considered as a labile component and is rich in fresh plant litter [4]. In this study, the initial contents of water-soluble substances (extracted at 80 ◦C for 1 h) were 24%, 14%, 30% and 10%, respectively, for fresh fir, cypress, larch and birch litters (Table 1). However, the initial contents of acid hydrolysable components were only 19%, 19%, 21% and 16% for fresh fir, cypress, larch and birch litters, respectively, which were similar with the content of water-soluble substances but lower than the contents of acid-soluble substances (extracted with 72% H2SO4) (Table 1). These results sugges<sup>t</sup> that the acid hydrolysable components under the conditions of this study (extracted by 2.5 mol/L H2SO4) are just parts of the soluble components that can be dissolved in hot water.

Acid hydrolysable components have been thought to release during the early or intermediate periods of litter decomposition in both field and laboratory experiments [4,28], and this hierarchical pattern of decomposability is similar with dissolved organic C [1]. For example, in a 13C tracer experiment, Kammer et al. [3] found that approximately 80% of the annual dissolved organic C was leached from the litter layer during the first winter of decomposition after litter fall. Our results showed that 38%–55% of acid hydrolysable components were released during the first 41 days, suggesting that the acid hydrolysable components are highly soluble and release rapidly. However, another peak for the release of acid hydrolysable components was observed in the fourth year of decomposition, possibly due to the large increase in litter water content (Figure 3). These results sugges<sup>t</sup> that the acid hydrolysable components are different with dissolved organic C and could be released at a later period than previously expected. In addition, a significant release of acid hydrolysable components occurred during the growing season of the first year of decomposition. These releases were in accordance with the increase in gravimetric water content in the decomposing litter, implying that water content may be the dominant driver for the release of acid hydrolysable components. The path analysis results also indicated that gravimetric water content had a strong effect on the release of acid hydrolysable components (Figure 5), suggesting that hydrological leaching strongly controls the release of these labile components during litter decomposition.

However, we also found that a high level of acid hydrolysable components was maintained over a long term after a rapid release and even increased during snowmelt (Figure 6). Several reasons can explain the high level of acid hydrolysable components during litter decomposition, particularly during snowmelt. First, some acid hydrolysable are intermediate decomposable carbohydrates [2], and these components were decomposed to smaller carbohydrates [6], increasing the proportion of acid hydrolysable components measured under the conditions of this study. Our previous studies also found that cellulose content greatly decreased during the snow coverage and snowmelt stages in the first year of decomposition at these sites [29]. Moreover, we found that repeated soil freezing and thawing during winter strongly contributes to C decay during litter decomposition in this alpine forest [19]. In addition, earlier studies found that cold-tolerant microbes maintain high activities at temperatures above −4 ◦C [30] but die during snowmelt due to C starvation over winter and their inability to adapt to an increase in ambient temperature [31]. Our previous research conducted in this alpine forest also found that the microbial biomass was higher during snow cover than that during snowmelt [32], suggesting that dead microbial necromass (i.e., glucosamine and muramic acid) may contribute to the increase in acid hydrolysable components during snowmelt. Our results sugges<sup>t</sup> that acid hydrolysable components do not consistently decrease as decomposition proceeds, and winter snow exerts an important influence on these labile components in plant litter during snow formation, snow coverage and snowmelt in this alpine forest.

Reduced snow cover increases soil freezing (Figure 1d) [16] and alters soil microbial processes in cold areas [33,34]. In this study, a reduction in snow depth decreased the

gravimetric water content, which had a negative effect (*r* = −0.25, *p* < 0.001) on the content of acid hydrolysable components, and this effect was greater than that from the change in temperature driven by the shift in snow depth (*r* = −0.11, *p* < 0.01). These results indicated that a decline in winter snow cover may primarily reduce leaching and, thus, decrease the release of acid hydrolysable components from plant litter [4]. A reduction in snow cover has also been demonstrated to strongly decrease the microbial biomass [35], which was positively correlated with the content of acid hydrolysable components. This result suggests that microorganisms can either degrade the intermediate decomposable compounds from plant-derived carbohydrates [36] or contribute the dead biomass to microbial-derived fractions [37] that can be measured by acid hydrolysis.
