Dose-dependent molecular composition of dissolved organic matter in biochar-amended manure composts

Biochar (BC) amendment is a promising strategy for enhancing compost humification, yet its dose-dependent effects on the molecular-level characteristics of dissolved organic matter (DOM) remain unclear. This study systematically investigates the molecular composition of DOM in compost amended with 0 % (CK), 1 % (T1), 2 % (T2), and 5 % (T5) BC. Elemental analysis revealed a progressive increase in N content from 3.46 % (CK) to 4.56 % (T5), indicating enhanced N retention. Spectroscopic analysis indicated an increase in humification and enrichment of O-containing groups, particularly C-O, related to carbohydrate-like substances. High-performance size exclusion chromatography showed that BC additions increased the abundance of medium molecular weight (0.5-8.0 kDa) while reducing high molecular weight (>8 kDa) fractions, resulting in a slight decrease in weight-average molecular weight (Mw) from 23.76 kDa to 20.32-22.35 kDa. High-resolution mass spectrometry revealed that BC amendment, particularly at 5 %, led to lower oxidation, reduced aromaticity, and a higher proportion of saturated, aliphatic CHO compounds. Molecular-level analysis showed substantial degradation of lignins/CRAM compounds (39 %-56 %) and CHON species (33 %-41 %), alongside the formation of carbohydrates (21-33 %) and CHO species (53-79 %). Overall, this study highlights that BC dosage significantly alters DOM molecular composition, providing mechanistic insights into humification enhancement. The findings offer valuable implications for improving compost quality and suggest that even low BC dosages could serve as a cost-effective and scalable strategy in composting practices.