Biochar type shapes root architecture and rhizosphere enzyme hotspots

Biochar has been widely promoted as a carbon sequestration strategy in agricultural soils. However, comparatively less attention has been paid to how biochar alters the physical, chemical, and biological properties of soils, particularly within the rhizosphere where plant–microbe interactions are most active.
In this study, we investigated rhizosphere responses of soybean (Glycine max (L.)) to two contrasting biochar type (woody biochar and poultry biochar) applied at different placement strategies (surface vs. mix application). We quantified changes in the forms and amounts of rhizosphere carbon and nitrogen, assessed nodulation as a proxy for nitrogen-fixing bacterial activity, and evaluated rhizosphere enzyme activity.

Our results demonstrate that soybean root morphology responded strongly to biochar type. Poultry biochar, which contains substantial essential nutrients (N and P), promoted a root system characterized by a greater proportion of vertical root components. In contrast, woody biochar induced a wider and more laterally developed root architecture, as confirmed by quantitative root trait analysis. 

Enzyme activities in the rhizosphere (β-glucosidase, N-acetylglucosaminidase, and phosphatase) are currently being analyzed, and preliminary observations indicate consistently higher enzyme activities and greater microbial biomass in the woody biochar compared with the poultry biochar treatment. This pattern is likely associated with enhanced nodule formation under woody biochar, suggesting intensified rhizosphere microbial activity coupled with biological N fixation.

By contrast, biochar placement showed minimal effects on soil biochemical indicators, implying that the primary influence of biochar in this system is mediated through chemical and biological pathways rather than physical modification. It indicates that biochar type, rather than placement, primarily governs soybean rhizosphere responses by reshaping root architecture and associated microbial activity.