Hydrochar post-treatment and biochar impose differential selective pressures shaping rhizosphere microbiomes and plant performance

Hydrochar and biochar are increasingly proposed as sustainable soil amendments within circular economy strategies, yet their comparative effects on rhizosphere bacterial communities and plant performance remain poorly understood, particularly for hydrochar. This study evaluated the effects of fresh hydrochar (FHC), washed hydrochar (WHC), and biochar (BC) applied at rates of 1–5% on soil bacterial communities and plant responses in tomato and quinoa. Bacterial community composition, alpha- and beta-diversity, and their relationships with soil physicochemical properties and plant traits were analysed. FHC induced the strongest microbial restructuring, particularly at the highest dose, significantly reducing alpha diversity and promoting Gammaproteobacteria and Actinobacteria enrichment. In contrast, WHC produced less pronounced changes, whereas BC caused limited, dose-dependent taxonomic shifts while largely preserving microbial diversity and community stability. Microbial community variation was strongly associated with amendment-induced changes in soil chemistry, particularly elemental composition. Plant responses followed a similar pattern: FHC reduced tomato germination and growth and was associated with stress-related bacterial assemblages, BC enhanced plant biomass and was associated with rhizosphere taxa commonly linked to plant growth promotion, and WHC showed intermediate responses. Overall, the intensity of microbial restructuring followed the gradient FHC > WHC > BC, demonstrating that hydrochar washing effectively mitigates the impact of fresh hydrochar on soil bacterial communities while maintaining plant performance. These findings highlight hydrochar post-treatment as a promising strategy to improve the sustainability and agronomic suitability of char-based soil amendments.