Aggregation and carbon content in urban soil blends: Insights from a three‐decade chronosequence

Manufactured soil blends are increasingly used for urban landscaping where access to suitable natural topsoil is limited. These mixed sandy soils contain substantial compost but are essentially structureless at installation. Aggregate formation in these blends, critical for water and nutrient retention, remains poorly understood. In addition, although blends initially have very high organic matter contents, the fate of these carbon inputs is unknown. We investigated soil aggregation, carbon content, and related properties of soil blends installed over the past 30 years at University of British Columbia campus, Canada, and examined relationships to vegetation type and time since installation. We analyzed soil samples at 2.5‐ to 7.5‐cm depth from 88 plots across four vegetation types. All blends consistently formed smaller aggregates (10%–53% lower mean‐weight diameter) than nearby natural soils. Soil texture emerged as the dominant factor likely driving soil aggregation. Older blends exhibited greater aggregation, likely due to the relatively finer‐textured mixture specifications (up to 11% clay) rather than time‐dependent development (partial correlation, 95% CI excludes zero [where CI is confidence interval]). Soil carbon was high across all vegetation types (4.55%–6.74%), although it was greatest under trees with shrubs, likely reflecting the influence of vegetation inputs and maintenance practices. Our results provide evidence to refine soil blend design and inform management decisions aimed at improving long‐term soil function in urban landscapes.