**1. Introduction**

Litter decomposition, a crucial component of carbon and nutrient cycles, plays an indispensable role in regulating ecosystem structure and function in forests [1]. The litter produced by plants in terrestrial ecosystems most reaches the forest floor. Meanwhile, there are abundant headwater streams and intermittent streams in forest ecosystems. These aquatic ecosystems can receive substantial inputs of terrestrially-derived leaf litter. As a result, streams reconnect terrestrial and aquatic ecosystems by flows of organic matter that represent a grea<sup>t</sup> contribution to the global carbon cycle [2]. Some microbial decomposer communities, including fungi and bacteria, drives leaf litter decomposition which controls the flow of energy and nutrients in forest ecosystems [3,4]. Due to the different sensitivity of bacteria and fungi, they respond differently to relatively rapid shifts in microenvironmental conditions, such as temperature, precipitation, dissolved oxygen, or pH which affects their relative contribution to leaf decomposition [5,6]. After entering the ecosystems, habitats differ in properties such as nutrient availability, major environmental conditions, processes, and dynamics, which altogether influence the microbial abundance and community composition, and ultimately contribute to the decomposition process of litter [7,8]. Unfortunately, studies on microbial communities in litter decomposition have often been focused only on individual terrestrial or aquatic ecosystems, and only

**Citation:** Guo, H.; Wu, F.; Zhang, X.; Wei, W.; Zhu, L.; Wu, R.; Wang, D. Effects of Habitat Differences on Microbial Communities during Litter Decomposing in a Subtropical Forest. *Forests* **2022**, *13*, 919. https:// doi.org/10.3390/f13060919

Academic Editor: Bartosz Adamczyk

Received: 10 April 2022 Accepted: 8 June 2022 Published: 13 June 2022

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a handful of studies have dealt with the differences between them. This hinders our understanding of the impact of habitat differences on microbial community changes during litter decomposition.

The microenvironment in which microbial decomposers survive varies greatly between terrestrial and aquatic systems, due to the contrasting spatial and temporal scales at which litter is decomposed in different habitats [9]. These differences can control microbial decomposition by regulating microbial decomposers. Specifically, on the forest floor, the litter is in direct contact with the soil surface. At this time, the surface temperature and rainfall are important environmental factors to regulate the activities of microbial decomposers in terrestrial ecosystems [10]. Meanwhile, the microtopography, exposition, animal activities, or plant presence will also lead to the variations of temperature and humidity at regional scales of the forest floor [11] and then affect the activities of microbial decomposers. Compared with terrestrial ecosystems, the headwater stream is typified by relatively stable temperature, sufficient water source, low oxygen, and strong scour, which regulate microbial community composition [12,13]. When entering a stream, leaf litter will be promptly colonized by a diverse array of microbial decomposers, particularly aquatic fungi [14]. The hydrological regimes of intermittent streams are characterized by alternating flowing, nonflowing, and dry phases [13,15]. The microbial decomposer in litter decomposition in intermittence streams could be similar to headwater streams during flowing phases while it may be similar to the forest floor during dry phases. When it comes to the dry phases, the litter will be exposed to solar radiation, and its temperature and humidity will change dynamically [16,17]. Once the dry phase finishes, gradual rewetting promotes the onset of microbial decomposition [18]. As a result, microbial decomposition processes and rates dramatically change among these different habitat types but need further and unified understanding.

The phospholipid fatty acids (PLFAs) analysis method has been used for more than 42 years and is still popular as a means to characterize microbial communities in a diverse range of environmental matrices [19]. It is considered that degradation of PLFA biomarkers proceeds rapidly after cell death, providing robust information on the living microbial biomass and community structure [20]. Thus, the approach is advantageous there in that it can identify living microbial biomass and is more sensitive in detecting shifts in the microbial community compared to DNA-/RNA-based methods [21]. In addition, PLFA could provide more information on such a variety of microbial characteristics (both functional and structural) in a single analysis than other methods [19].

South China is rich in hydrothermal resources and breeds the world's largest humid subtropical mountain forest, which is typified by fast material circulation, rich biodiversity, and high productivity [22]. There are dense headwater streams hidden between mountains and gullies, and abundant intermittent streams on the forest surface in the rainy season [23]. Under the influence of water collection, a large number of forest litter gather in intermittent streams and headwater streams [24]. Microorganism plays a vital role in the biological decomposition of leaf litter in aquatic and terrestrial ecosystems. Due to its sensitivity to environmental conditions, microbial biomass and community composition may be different, and they ultimately influence the decomposition process of litter. Therefore, we hypothesized that the unique environmental conditions in different habitats will change the microbial community structure and influence the decomposition process of litter. To address this hypothesis, we collected leaf litter of the typical tree species *Castanopsis carlesii* (Hemsl.) Hayata. and use the litter bag decomposition method in the natural forest floor, headwater stream, and intermittent stream of the subtropical area. The intention of this study is not only to focus on the changes of microbial community in the process of litter decomposition in a single habitat but to focus on the main differences between habitats and its influence. This may help to improve our understanding of the abiotic drivers (microbial decomposers) on litter decomposition in forest ecosystems.

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