*2.1. Study Site*

This study was conducted at the Sanming Research Station of Forest Ecosystem, Fujian Province, China (26◦19 N, 117◦36 E). This area is dominated by low mountains with an average elevation of approximately 300 m and slopes of 25–45◦. The soil is red soil with a thickness of more than 1 m. The climate is a maritime subtropical monsoon climate, with a mean annual temperature and precipitation of 19.3 ◦C and 1610 mm. Approximately 80% of rainfall occurs between March and August. The natural vegetation is a subtropical evergreen broadleaved forest. The nature *Castanopsis carlesii* is the dominant species, with *Schima superba* Gardn. et Champ., *Castanopsis fissa* (Champion ex Bentham) Rehder et E. H. Wilson, *Litsea elongate* (Wall. ex Nees) Benth. et Hook. f., and *Neolitsea aurata* (Hay.) Koidz. in the forest canopy and *Ormosia xylocarpa* Chun ex L. Chen, *Itea chinensis* Hook. et Arn., and *Ilex pubilimba* Merr. et Chun in the shrub layer.

#### *2.2. Foliar Litter Decomposition Experiment*

In January 2021, the fresh senescent foliar litter from *C. carlesii* was collected from the forest floor at the sample sites. The initial dry mass was obtained by measuring litter samples that were oven-drying (65 ◦C, 48 h) after being air-dried for more than 2 weeks at room temperature. Samples of air-dried foliar litter were placed inside nylon mesh bags (20 cm × 20 cm; 1.0 mm mesh size nylon mesh of bag; 10.00 g per bag) [25]. On 28 March 2021, litterbags were placed in three different habitats, including forest floor, headwater stream, and intermittent stream, respectively.

Three natural *C. carlesii* forest plots were selected for setting a 3 m × 3 m homogeneous quadrat in each site as three sample repeated plots. Before the formal experiments, we investigated the annual accumulation of litter per unit area one year and loaded required weight according to the area of the litter bag. Litter bags were placed on the forest floor after removing plants and litter from the soil surface of the quadrat. An interval of at least 2 cm was placed between each litterbag to avoid mutual disturbance upon collection. At a distance of about 200 m from the natural forest sample plot, three representative intermittent streams with basically the same environmental conditions, about 200 m apart and concentrated natural litter, were randomly selected as three repeated sample plots to place litter bags. The withered leaf bag is fixed to the root of the main branch of the shrub in the sample belt through the safety rope to prevent scouring and falling off. A stream about 100 m away from the natural forest sample was selected. Three points are randomly selected from the upstream to the downstream as repeated sample plots. Each sample plot is 1–2 km away from each other. The safety rope tied with litter bags is placed in the water body in the center of the stream, which is similar to the distribution of litter under natural conditions and can fluctuate with the flow of water. The safety rope is fixed at the main root of tall trees in the sample zone. As a result, a total of 98 litterbags (3 samples × 12 sampling dates (a whole year) × 3 replicates) were prepared in each habitat. All litter bags in terrestrial and aquatic ecosystems were carefully placed for avoiding overlap among samples.

After placing in the plots, three litter bags of each site were sampled and immediately brought back to the laboratory for refrigeration at the end of each month from April. However, due to the fast decomposition rate of the headwater stream and the intermittent stream, the litter is decomposed within 4 months. At the same time, we also chose to analyze the litter decomposition on the forest surface for four months. The specific sampling date is the 28th, 65th, 92nd, and 119th days after decomposition. Foreign materials, such as roots, soil debris, lichen, and bryophyte, were carefully removed from the litterbags. We freeze-dried (Wizard 2.0 hot plate vacuum freeze dryer VirTisUSA) all the collected samples until constant weight (about 3–4 days), which is used for the determination of residue rate. After that, three bags of litter in the same sample plots were mixed and ground and placed at −80 ◦C for the determination of PLFA (*n* = 3).

#### *2.3. Monitoring Environmental Conditions*

At the beginning of the experiment, rainfall buckets and thermohygrometers were set up in the sample plot to record the rainfall and atmospheric temperature every 1 h and 0.5 h respectively. The headwater stream water temperature was also monitored. Water samples from the headwater stream and intermittent stream were collected to determine the pH value. At the same time, according to rainfall data and water samples collected in intermittent streams in the same period, we found that the hydrological regimes of intermittent stream showed seven times alternating flowing during litter decomposition (during 31 March 2021–3 April 2021, 8–11 April 2021, 14 April 2021–7 July 2021, and 23–24 July 2021, there was water flow for 93 days in total; during 4–7 April 2021, 12–13 April 2021, and 8–23 July 2021, there was no water flow for 21 days in total).
