More carbon with fewer roots: Diversity-rich spontaneous groundcovers outperform lawns in urban soil carbon storage

Urban greenspaces are increasingly promoted as nature-based solutions for climate mitigation, yet urban management still relies largely on lawns, whose soil‑carbon benefits remain uncertain relative to lower-input spontaneous groundcovers. We compared 13 typical urban groundcover communities (six spontaneous and seven lawn types) at an experimental site in northwestern China and quantified soil organic carbon (SOC), root biomass, species richness, and functional group richness in the 0–10, 10–20, and 20–30 cm soil layers over one year, alongside taxonomic and Functional Group Richness. Across the 0–30 cm profile, spontaneous communities had, on average, about 40% higher SOC than lawns, with the difference increasing to approximately 60% in the 0–10 cm layer. By contrast, lawns had approximately 60% greater root biomass overall, although root biomass did not differ significantly between the two groups in the 10–30 cm layer, there was no statistically significant difference in root biomass distribution; SOC was positively associated with species richness and functional group richness ( P < 0.01), whereas root biomass was negatively associated with Shannon diversity and functional group richness in the 0–10 cm layer. These patterns indicate that, in this experimental system, spontaneous groundcovers were associated with higher SOC concentrations despite lower root biomass than lawns. However, because vegetation type and associated management co-varied in this study, these results should be interpreted as system-level contrasts rather than evidence of a pure diversity effect.Furthermore, Partial Least Squares Path Modeling (PLS-PM) was employed as a multivariable framework to disentangle the direct and indirect drivers of SOC accumulation. • Spontaneous urban groundcovers stored about 40% more soil organic carbon than lawns across the 0–30 cm profile. • Lawns had about 60% greater root biomass than spontaneous groundcovers at all soil depths. • Soil organic carbon increased with species richness and functional group richness. • Root biomass declined with taxonomic and functional diversity.