Understanding the diagenetic processes of organic matter (OM) within mangrove forests is essential to grasp ecosystem dynamics and exchanges with adjacent ecosystems. This study investigated the influence of urban runoff on leaf litter degradation in a New Caledonian mangrove forest, subjected to urban rainwater for over 50 years. Focusing on and , our objectives were to determine factors affecting leaf litter degradation, assess urban runoff effects on decay rates and element concentrations, and understand OM changes at the molecular level. Litterbags containing senescent leaves placed on the mangrove floor were collected after 7, 14, 28, 56, and 72 days. C and N along with their stable isotopes were evaluated during degradation as well as lignin and neutral carbohydrates contents. Despite lower initial N content, exhibited faster degradation (t of 36 ± 3 and 28 ± 2 days) than (t of 43 ± 9 and 33 ± 4 days), emphasizing the critical role of species position within the mangrove forest regarding tidal immersion. Urban runoff, submerging the urban site, intensified leaf litter degradation, influencing both mass loss and molecular changes in OM. After 72 days of leaf litter degradation, the loss of rhamnose and glucose was more pronounced at the control site (13 % and 75 % for rhamnose and 46 % and 53 % for glucose for and , respectively) compared to the urban site. The study also exposed molecular tendencies during mangrove leaf litter degradation. The content of ferulic acid in the leaf litter decreased at all stands after 72 days of degradation (between −25 % and −58 %), while total syringyl phenols increased (between + 64 % and + 232 %). This research exposed the global implications of urban runoff, indicating accelerated leaf litter degradation in mangrove forests, potentially disrupting C sequestration dynamics and threatening ecosystem services.

中文翻译：

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城市红树林退化过程中物种和林分位置对凋落物同位素和分子组成的影响

了解红树林内有机质（OM）的成岩过程对于掌握生态系统动态以及与邻近生态系统的交换至关重要。本研究调查了新喀里多尼亚红树林中城市径流对落叶层退化的影响，该红树林已遭受城市雨水 50 多年。我们的目标是确定影响落叶层退化的因素，评估城市径流对腐烂率和元素浓度的影响，并了解分子水平上的有机质变化。 7、14、28、56 和 72 天后收集放置在红树林地面上的含有衰老叶子的垃圾袋。在降解过程中评估了 C 和 N 及其稳定同位素以及木质素和中性碳水化合物含量。尽管初始氮含量较低，但表现出比（t为43±9和33±4天）更快的退化（t为36±3天和28±2天），强调了红树林内物种位置对于潮汐浸没的关键作用。城市径流淹没了城市场地，加剧了落叶退化，影响有机质的质量损失和分子变化。经过 72 天的凋落物降解，与城市地点相比，对照地点鼠李糖和葡萄糖的损失更为明显（鼠李糖分别为 13% 和 75%，葡萄糖分别为 46% 和 53%）。该研究还揭示了红树林落叶退化过程中的分子趋势。降解 72 天后，所有林分的凋落物中阿魏酸含量均下降（-25% 至 -58% 之间），而总紫丁香基酚增加（+ 64% 至 + 232% 之间）。这项研究揭示了城市径流的全球影响，表明红树林落叶退化加速，可能破坏碳固存动态并威胁生态系统服务。