**1. Introduction**

The release of labile components from plant litter is one of the fastest pathways for carbon (C) turnover in forest ecosystems due to its high decomposability [1]. These labile fractions, such as water soluble and acid hydrolysable components, are primarily leached during early decomposition [2] and are rapidly consumed by soil decomposers [3]. However, "labile" is defined relative to "recalcitrant", and our current knowledge of the entire fraction of "labile" components is a nebulous concept in litter decomposition studies [1]. Most studies have focused on dissolved organic carbon (DOC), but other forms of labile fractions, such as acid hydrolysable components, are also released from plant litter during early stages of decomposition and regulate some important ecological processes [4]. These acid hydrolysable components decomposed from fresh litter supply the C that is available for soil microorganisms, which in turn contribute to litter breakdown and C and nutrient cycling [5].

Acid hydrolysable components in plant litter are consumed or released in different stages of decomposition [2]. In the early stage, acid hydrolysable components are released rapidly from senesced litter because fresh litter contains large amounts of soluble compounds, including some polysaccharides, hemicellulose, cellulose and some soluble lignin [6]. Additionally, the snowmelt stage may be another peak of acid hydrolysable components release because higher microbial activity under a warmer environment facilitates

**Citation:** Liao, S.; Yue, K.; Ni, X.; Wu, F. Acid Hydrolysable Components Released from Four Decomposing Litter in an Alpine Forest in Sichuan, China. *Forests* **2022**, *13*, 876. https:// doi.org/10.3390/f13060876

Academic Editor: Sadanandan Nambiar

Received: 6 April 2022 Accepted: 1 June 2022 Published: 3 June 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the degradation of hemicellulose and cellulose, and this decomposed detritus could be leached by snowmelt water [7]. Moreover, repeated soil freezing and thawing during winter causes previously stabilized organic matter to be available as labile C and nutrients [8]. These previous findings sugges<sup>t</sup> that shifts in the ambient environment during different decomposition periods strongly control the release of acid hydrolysable components from plant litter.

Snow cover in the Northern Hemisphere has decreased by 7% and is projected to decrease by 25% by the end of this century [9]. This reduction has a profound influence on litter decomposition and nutrient cycling in many cold ecosystems [10,11]. Notably, a reduction in snow depth directly decreases water availability due to the decreased snowmelt water supply [12]. Furthermore, some acid hydrolysable components in plant litter are water soluble and primarily released by leaching [13]; thereby, a decline in winter snow depth may directly decrease the release of acid hydrolysable components from decomposing litter materials [14]. Additionally, reduced snow cover has been demonstrated to reduce soil microbial biomass and extracellular enzymatic activity, and this effect could decrease the depolymerization of acid hydrolysable components during litter decomposition [15]. However, reduced snow cover also leads to more fluctuations of soil freezing and thawing [16], which can physically impact litter breakdown and therefore promote the release of acid hydrolysable components [17].

Here, we hypothesize that (1) the decline in snow depth decreases the release of acid hydrolysable components from plant litter, and (2) the formation, coverage and melting of winter snow cover alters the release of acid hydrolysable components. We conducted a litterbag experiment in a high-elevation alpine forest using four dominant tree litter species and measured the contents and release patterns of acid hydrolysable components during four years of decomposition from 2012 to 2016. Our objectives were as follows: (1) to follow the long-term release pattern of acid hydrolysable components, which have been thought to release rapidly during early periods of litter decomposition; and (2) to assess the potential influence of variations in snow depth and timing on the release of acid hydrolysable components in decomposing litter in this alpine forest.

#### **2. Materials and Methods**
