Extraction method influences dissolved organic matter from invasive Japanese knotweed-derived biochar and environmental implications

Valorization of invasive plant biomass through pyrolysis into biochar offers opportunities for waste management and resource recovery. However, the release of biochar-derived dissolved organic matter (BDOM), which influences carbon dynamics and pollutant mobility, remains poorly understood. This study investigated BDOM from Japanese knotweed biochar (500 °C) using seven extraction methods, covering mild aqueous conditions (water and CaCl 2 ), salt effects (NaCl), standardized acidic leaching tests (the synthetic precipitation leaching procedure, SPLP, and the toxicity characteristic leaching procedure, TCLP), and strong chemical extraction conditions (HCl and NaOH). NaOH and HCl maximized dissolved organic carbon (DOC) release (75–183 mg L - ¹) while the resulting BDOM exhibited low aromaticity (SUVA 254 : 1.8–5.3) and relatively lower molecular weight (E 2 /E 3 > 0.5). In contrast, mild extraction (water and CaCl 2 ) released less DOC (5–15 mg L - ¹) but preserved high-aromaticity components with larger molecular weights. Fluorescence analysis identified four distinct BDOM components: (1) a terrestrial humic-like substance (C4) preferentially extracted by water, (2) a fulvic-like component (C2) dominant in NaCl and SPLP extracts, (3) a protein-like component (C1) most abundant in NaCl extract, and (4) a transitional component (C3) that decreased under acidic conditions. The results demonstrate that extraction method influences BDOM quantity, optical characteristics, and compositional features, providing a useful framework for understanding BDOM behavior and informing the environmental management of invasive plant.