Quality and Biodegradation Process of Dissolved Organic Carbon in Typical Fresh-Leaf Leachate in the Wuhan Urban Forest Park

The plant litter in urban forest parks was considered to be an important non-point source of carbon and nutrient substances for urban water bodies [1,2]. The leachates from leaves and litter carry substantial dissolved organic matter, infiltrating the soil environment and participating in the forest’s nutrient cycling [3]. The study of leaves became increasingly important for understanding the input of organic matter in ecosystems. However, less attention was given to the topic.

Until now, researchers have studied the DOM (dissolved organic matter) (all abbreviations and symbols in this article could be found in Table S1) leached from trees. Regarding the whereabouts of litter DOC (dissolved organic carbon), it was confirmed that most of it remained in the soil environment, with a small part being mineralized [4]. There were significant differences in the production and degradation of DOC from different plant litter sources, and the structural and physiological changes in plants also had significant effects on DOC production [5]. Under the condition of global warming, Hagedorn et al. found that, with the increase in atmospheric CO₂, the initial DOC leaching of forest litter increased, while the biodegradable part decreased [6]. Additionally, the initial carbon quality of plant litter also affected the decomposition and carbon sequestration of litter [7]. Regarding studying the leaching and degradation of litter, Xu et al. [8] found that the DOM leaching amount in leaves was more than that in other organs, and the biodegradability of DOM leaching was low, which was affected by DOM aromatics and nitrogen (N) availability. Others demonstrated that the composition and molecular weight of DOC could also impact microbial decomposition and the biodegradability of DOC [9,10,11]. During the period of DOM biodegradation, components such as protein-like, sugars, and organic acids were broken down into substances like fulvic acid and humic acid [12]. Microorganisms preferentially degraded DOM molecules with smaller molecular weights [13]. Easily degradable DOM in the early stages degraded more rapidly, while later-stage refractory DOM underwent slower degradation. Indicators of HIX (humification index), BIX (biological index), and S_R are commonly used for studying patterns in DOM degradation [14,15]. It was important to note that microbial metabolism significantly influenced ultraviolet and fluorescence spectra during degradation [16]. Research has demonstrated that the degradation of DOM primarily occurs within the initial seven days, accounting for 51% to 88% of the total degradation over 28 days [17]. To obtain the quality of leaf organic matter input to the ecosystem and evaluate the biodegradability of DOC, constant temperature culture experiments had to be carried out. Previous studies focused more on the study of leachate DOC of litters, while our study focused on the study of leachate DOC degradation of fresh leaves. Moreover, our study area was an urban forest park, which had different management modes and influencing factors from ordinary forests. It was thus an important supplement to studying the biodegradation of leachate DOC of fresh leaves in an urban park.

By the end of 2023, China’s forest area accounted for about 51.20% of the total land area [18]. Among which urban forest parks were concentrated in cities. Under the influence of urbanization, forest parks were built by planting trees, shrubs and grasses, and attention had to be paid to the contribution of leaf leachate to DOC. Wuhan city was located in the middle and lower reaches of the Yangtze River Basin, characterized by a subtropical monsoon climate with the widespread distribution of rivers and lakes, providing rich forest resources. The study of the biodegradation of dissolved organic carbon in fresh-leaf leachate from Wuhan Urban Forest Park has significant implications. It improves our understanding of carbon dynamics of urban park ecosystems, supporting sustainable urban planning. It also contributes to urban ecosystem protection, guides water quality management, and fosters community awareness for broader social impact. This research largely promotes practical actions for urban carbon management. This excessive fresh leaves and litters understandably affected soil organic matter content, water nutrient concentrations, and biogeochemical cycling. Understanding of DOM production and degradation processes from fresh tree-leaf leachates is essential in unraveling biogeochemical cycling of DOM, whilst this topic is not well understandable. This study aims to (1) reveal the concentration and spectral characteristics of DOM in leaf leachate, (2) investigate the influencing factors of DOC biodegradability at different temperatures, and (3) explore the generation and degradation process of DOM in leaves of different types of fresh trees.