*2.1. Study Site*

Fresh tree tissues were collected from plantations in the long-term Forest Restoration Experimental Station of Jiangxi Agricultural University (26◦5516 N, 114◦4814 E) located in the Luoxi Town, Taihe County, Jiangxi Province, southern China. The study site has a humid subtropical monsoon climate. Mean annual temperature is 18.6 °C, and mean annual precipitation is 1726 mm. These plantations have been established in 1991 and the main afforestation species are *L*. *formosana*, *S*. *superba*, *P*. *massoniana*, and *P*. *elliottii*. These selected four tree species have been widely planted for restoring degraded lands and maintaining ecosystem services in subtropical regions of China. For example, the area of *P*. *massoniana* plantations has reached 1.00 × 10<sup>7</sup> hm<sup>2</sup> according to the eighth National Forest Resource Survey in China. Mean tree height in the *L*. *formosana*, *S*. *superba*, *P*. *massoniana*, and *P*. *elliottii* plantations was 10.7 m, 9.8 m, 8.7 m, and 11.6 m, respectively, and mean diameter at breath height (DBH) in the *L*. *formosana*, *S*. *superba*, *P*. *massoniana*, and *P*. *elliottii* plantations was 16.2 cm, 20.3 cm, 17.9 cm, and 26.4 cm, respectively. The detailed description of the study site is shown in Xu et al. [28].

#### *2.2. Plant Sampling and Measurement*

In the study site, we randomly established six plots (10 m × 10 m) in the *L. formosana*, *S. superba*, *P. massoniana*, and *P. elliottii* plantations in June 2019. In each plot, we selected three individuals with similar tree height and DBH and then harvested thirty fully expanded and healthy leaves and ten twigs from the canopy of each targeted individual. For each organ type, plant materials collected from the same plot were mixed and divided into two subsamples. The first subsample was oven-dried at 65 ◦C to determine the initial moisture content and then ground to pass through 0.15 mm sieves for chemical analyses, and the second subsample was used for the leaching experiment. Plant organic C and total N concentrations were measured on a FlashSmart CHNS/O Elemental Analyzer (Thermo Fisher Scientific, Bremen, Germany), total P concentration was colorimetrically determined on an autoanalyzer (AA3, Seal Analytical, Germany) after acid digestion, total polyphenols were measured with the Folin–Ciocalteau method [29], soluble sugars and starch were

measured by the anthrone colorimetry method [30], and tissue density was assessed by the procedures of Pérez-Harguindeguy et al. [31]. The initial chemical properties of tree tissues are shown in Table 1.

Tree tissue-leached DOM was extracted with a short-term leaching experiment [32]. For each tree organ, 3 g of fresh plant material was placed in 500 mL glass jars and soaked in 200 mL of deionized water in the dark at room temperature (about 20 ◦C) for 48 h. Subsequently, the leachates were filtered through 0.7 μm Whatman ™ GF/F glass microfiber filters (Little Chalfont, Buckinghamshire, UK) and used to measure DOM parameters. In the leachates, DOC and DTN concentrations were measured on a TOC analyzer (multi N/C 2100S, Analytik Jena, Jena, Germany), DTP concentration was colorimetrically measured on an autoanalyzer after peroxodisulfate oxidation [33], and the ultraviolet absorbances of DOM at 254 nm and 350 nm was measured with an ultraviolet-visible spectrophotometer (UV600SC, Jinghua Instruments, Shanghai, China). Plant-leached DOC, DTN, and DTP productions were obtained from the total amounts of DOC, DTN, and DTP in the leachates and the dry mass of fresh tree tissues. The specific ultraviolet absorbance at 254 nm (SUVA254) and 350 nm (SUVA350) were calculated by dividing the ultraviolet absorbance at 254 nm and 350 nm by the DOC concentration, respectively [26,27].

Plant-leached DOM biodegradability was assessed with a 28-day standard laboratory incubation experiment [17]. Prior to incubation, DOC concentration in the leachates was diluted to about 20 mg L−<sup>1</sup> to avoid excessive microbial growth. Meanwhile, microbial inoculums were obtained by placing 5 g of fresh forest soils in 1000 mL of deionized water. Subsequently, 100 mL of diluted leachates per treatment was placed in a 500 mL glass jar, and 5 mL of microbial inoculums was added to the jar. In addition, six glass jars with 100 mL of deionized water and 5 mL of microbial inoculums was established as blanks. All glass jars were sealed and aerobically incubated in the dark at 20 °C for 28 days. By the end of incubation, DOC concentration in the leachates was determined on a TOC analyzer. Plant-leached DOM biodegradability was calculated from the difference between the initial and final DOC amounts and was expressed as the proportion of the initial DOC amount (%).
