Biochar-amended constructed soils enhance roadside ecosystem functions with responses shaped by texture and hydrologic conditions

Abstract Purpose Ecosystem services in urban green spaces depend on viable soils, yet many soils of industrialized cities are degraded and infertile: making soils constructed from locally-sourced organic and mineral wastes a sustainable restoration alternative to importing fertile, natural soil. Our study evaluated three ecosystem functions of urban roadside green – vegetative performance, stormwater infiltration and carbon sequestration via rhizodeposition – and assessed their responses to three constructed soil design factors: amendment with high-quality biochar, addition of natural sand material and cultivation of legume versus non-legume perennial forbs. We further examined functional responses under two hydrological regimes representative of roadside systems: 1) verge-like conditions simulated by evenly distributed irrigation and 2) swale-like conditions simulated by flooding and stagnation cycles reflecting future climate scenarios characterized by fewer but more intense storm events. Materials and methods We constructed soils using subsoil excavated from a local construction project and municipally produced green waste compost, promoting sustainability through the reuse of urban byproducts. Following a ten-week greenhouse pot experiment, we quantified three indicator properties representing key ecosystem functions: plant vitality score (PVS) for vegetative performance, unsaturated hydraulic conductivity at field capacity for stormwater infiltration and rhizospheric soil development for carbon sequestration via rhizodeposition. Ecosystem function indicators were analyzed using generalized linear mixed models, with standardized coefficients enabling comparison of treatment effects across functions. Results and discussion Plants under verge-like conditions benefited from biochar amendment, but only in coarser-textured soils where sand addition then increased PVSs by 15%. Under swale-like conditions, PVSs declined and the benefit of the biochar–sand combination was reduced by 52%. Infiltration responses closely mirrored PVS patterns, with biochar and sand increasing conductivity by 88% and swale-like conditions reducing this effect by 132%, indicating potential ecosystem function synergy. In contrast, rhizodeposition-driven carbon sequestration depended primarily on plant functional group, with legumes exhibiting 180% greater rhizosphere development under verge-like conditions and 99% greater development under swale-like conditions; biochar amendment had little influence on this function. Conclusion Our results emphasize that the success of enhancing ecosystem functions in roadside green spaces via constructed soil amendment with biochar is soil particle size dependent, with clear benefits only in coarse-textured soils. Furthermore, intense hydrological conditions, such as flooding and stagnation, can quickly reduce functionality and override any benefits provided by amendments. This highlights the need for tailored management strategies for roadsides and other urban green infrastructure per expected future environmental conditions.