Increasing biodiversity and ecosystem functions along an age gradient of urban greenspaces

• Urban greenspace soils show substantial ecological development over time, despite persistent urban constraints. • Soil organic matter is a key indicator of development and encourages diverse, complex microbial communities. • Shifts in microbial communities enhance potential metabolic activity and traits, aligning with natural ecosystems. • Management should prioritize long-term ecological health and soil organic matter over short-term aesthetics. Urban greenspaces are constructed on soils disturbed by human activities, raising concerns about their capacity to sustain biodiversity and ecosystem functions. Uncertainty over whether these soils can recover toward conditions observed in natural ecosystems, as well as over the rates, mechanisms, and limits of soil development, has reinforced the perception that urban soils are irreversibly degraded. Here, we examined soil properties, microbial communities, and functional attributes across 30 urban greenspace sites spanning an age gradient in Seoul, Republic of Korea, using nearby natural ecosystems as references. Soil conditions improved with greenspace age, reflected in the accumulation of soil organic matter. These improvements were associated with higher microbial diversity and microbial gene copy numbers, along with increased potential metabolic capacity and shifts in functional attributes. Older sites harbored more plant-associated microbes and complex carbon decomposers, whereas taxa associated with pathogens and pollutants declined. Functional profiles showed increased ectomycorrhizal and saprotrophic functions, together with a broader potential for carbon metabolism in older sites. These findings identify soil organic matter as a key driver of ecological development in urban greenspaces and underscore the need to shift management from short-term, aesthetic-focused maintenance toward long-term strategies that enhance and maintain soil organic matter to support ecosystem services and resilience.