Dose-Dependent Effects of Hydrochar-Derived Dissolved Organic Matter on Soil Bacterial Communities Across Contrasting Soil Types

Hydrochar application to soil inevitably releases hydrochar-derived dissolved organic matter (HDOM), yet its specific impact on soil microbial communities, independent of the hydrochar solid matrix, remains poorly understood. This study investigated, for the first time, the dose-dependent effects of HDOM on bacterial communities in three distinct soil types (red, yellow-brown, and black soils). A concentration gradient, including undiluted stock solution and 10-, 100-, and 1000-fold dilutions with ultrapure water, was established to test for hormesis-like responses. High-throughput 16S rRNA gene sequencing revealed that HDOM induced profound, soil-specific shifts in bacterial community structure. The application of HDOM induced the emergence of numerous specific bacterial taxa, with unique ASVs reaching up to 15,372. However, no significant changes were observed in microbial community richness or evenness (alpha diversity). Drastic shifts in beta diversity were evident only in red soil and yellow-brown soil, and exclusively under the undiluted HDOM treatment. At the phylum level, HDOM application did not alter the dominant bacterial types (top 10 phyla); however, their relative abundances were jointly regulated by both HDOM dose and soil type. Significant HDOM-induced changes in key bacterial biomarkers were primarily detected in red soil (e.g., phylum Elusimicrobia, class Fimbriimonadia, and family Alicyclobacillaceae) and yellow-brown soil (e.g., phylum Proteobacteria, class Alphaproteobacteria, and family Rhizobiaceae), while in black soil, such changes were observed only under the undiluted HDOM treatment (e.g., species Streptomyces rochei). Predictive functional profiling suggested limited impact on major metabolic pathways, with soil type remaining the primary determinant. These findings demonstrate that HDOM exerts a direct, dose-dependent, and soil-specific influence on bacterial communities, providing key insights into the environmental behavior of hydrochar and guiding its safe application.