Exploratory Characterization of Dissolved Organic Matter Released from Composite Leaf Litter Samples Representing Five Deciduous Tree Species Under Controlled Laboratory Conditions

Leaf litter decomposition is a key pathway for carbon transfer from forest ecosystems to soils and surface waters. Dissolved organic matter (DOM) released during early-stage leaching represents a potentially reactive fraction of this carbon pool; however, its molecular composition and short-term reactivity remain insufficiently characterised. This study provides a comparative characterisation of DOM released from composite leaf litter samples representing five common deciduous tree species (Betula pendula, Carpinus betulus, Alnus glutinosa, Populus tremula, and Quercus robur) under controlled laboratory conditions. Leaf material collected from multiple trees per species was pooled to obtain a single composite sample; therefore, replicate leaching experiments represent procedural rather than biological replication. DOM was isolated using solid-phase extraction (SPE) and analysed by gas chromatography–mass spectrometry (GC–MS) following trimethylsilyl (TMS) derivatisation, while chemical oxygen demand (COD) and biochemical oxygen demand (BOD₅) were used as indicators of oxidative reactivity and short-term biodegradability. The applied analytical approach captures a selective and operationally defined fraction of DOM, primarily low-molecular-weight and derivatisable compounds; therefore, the results are interpreted as semi-quantitative compositional fingerprints. Carbohydrates, phenolic compounds, and low-molecular-weight organic acids dominated the detected fraction of DOM, with differences observed among composite samples. The composite samples representing A. glutinosa and P. tremula contained higher relative proportions of carbohydrate-related compounds, whereas the composite samples representing B. pendula and C. betulus showed higher relative contributions of aromatic compounds. Apparent differences in BOD5 were observed among composite samples; however, these observations likely reflect procedural variability rather than independent biological effects. The results indicate variability in DOM composition and apparent reactivity among composite litter samples under controlled laboratory conditions. Due to the lack of biological replication and the selective nature of the analytical approach, the findings should be interpreted as exploratory and not as evidence of generalised tree-species effects.