**1. Introduction**

Dissolved organic matter (DOM), a small but labile fraction of soil organic matter, plays a crucial role in regulating key ecological processes in forests [1,2]. For example, DOM provides the main sources of energy and nutrients for heterotrophic microbes [3–6], and thus drives greenhouse gas emissions and soil organic matter formation in forests [7–9]. Moreover, DOM is an important vector controlling the transport of carbon (C) and other elements from terrestrial to aquatic systems [1,10,11]. Therefore, knowledge about DOM characteristics is necessary to fully understand forest ecosystem structure and function.

In forests, the leaching of soluble organic matter from plant litter, especially in the early stage of decomposition, is considered the primary source of DOM in soils [12,13]. It is well acknowledged that litter-leached DOM amounts are controlled by a variety of physical and chemical traits such as water holding capacity [14], soluble carbohydrates [15], and lignin concentration [16]. Likewise, litter-leached DOM biodegradability is co-regulated

**Citation:** Xu, J.-W.; Ji, J.-H.; Hu, D.-N.; Zheng, Z.; Mao, R. Tree Fresh Leaf- and Twig-Leached Dissolved Organic Matter Quantity and Biodegradability in Subtropical Plantations in China. *Forests* **2022**, *13*, 833. https://doi.org/10.3390/ f13060833

Academic Editors: Fuzhong Wu, Zhenfeng Xu and Wanqin Yang

Received: 29 April 2022 Accepted: 23 May 2022 Published: 27 May 2022

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by C quality [3,17] and stoichiometric ratios between C and nutrients [18–21]. However, these previous studies have concentrated on senesced litter [4,16,22], and little is known about the production and biodegradability of DOM leached from fallen fresh plant tissues. In forests, extreme weather events such as hurricanes and storms often destroy the trees, increasing the input of fresh plant tissues into soils [23–25]. For example, typhoon-induced fine litterfall ranges from 2.21 to 5.12 Mg ha−<sup>1</sup> year<sup>−</sup><sup>1</sup> in subtropical pine plantations [24]. Due to the substantial differences in physical and chemical properties between fresh and senesced plant tissues, the controls on amounts and biodegradability of fresh organ-leached DOM are unclear in these ecosystems. These knowledge gaps will limit our understanding of plant-mediated C and nutrient cycles in forests.

Here, we sampled the fresh leaves and twigs of four common afforestation species (broadleaf trees: *Liquidambar formosana* and *Schima superba*; coniferous trees: *Pinus massoniana* and *Pinus elliottii*) from subtropical plantations in southern China, and then conducted a 48 h leaching experiment to investigate the amounts of fresh tissue-leached dissolved organic carbon (DOC), dissolved total nitrogen (DTN), and dissolved total phosphorus (DTP). In this study, we used the amount of DOC in the leachates to indicate DOM production and assessed DOM aromaticity by measuring the specific ultraviolet absorbances at 254 nm (SUVA254) and 350 nm (SUVA350) [26,27]. Subsequently, we measured DOM biodegradability using a 28-day standard incubation experiment [17]. The main objectives of this study were to: (1) determine the effects of tree species and organ type on fresh tissue-leached DOM production and biodegradability, and (2) uncover the factors determining the variations of fresh tissue-leached DOM quantity and biodegradability in subtropical plantations.

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