Mineral Matrix Composition Controls the Redistribution of Soil Organic Carbon Fractions and Microbial Functioning in Constructed Technosols Under Semi‐Arid Conditions

Urban green infrastructure (UGI) often relies on constructed soils, yet it remains unclear how Technosol composition and irrigation regulate soil organic carbon (SOC) fractions and microbial functioning under semi‐arid conditions. An 18‐month mesocosm experiment in Central Chile compared four Technosol mixtures differing in excavated subsoil (ExS) content under two irrigation regimes with lavender plantations. SOC fractions (POM and MAOM), microbial respiration (BR, SIR), substrate‐use patterns (CLPP), qCO 2 and enzymatic activities were assessed over time. SOC and MAOM were progressively higher with increasing ExS content (SOC: ~19 g kg −1 in sand‐rich mixtures C and A to 31.9 ± 4.9 g kg −1 in ExS mixture TE; MAOM: ~2.9% to 5.64% ± 0.96%), indicating that mineral matrix composition controlled early organo‐mineral carbon allocation under equal compost inputs. Over time, MAOM increased while POM declined, reflecting a redistribution toward mineral‐associated pools. Microbial respiration was consistently higher in ExS‐rich mixtures and intensified over time. Similarly, SIR increased over time (27.4 ± 8.3 to 50.1 ± 22.4 μg CO 2 –C g −1 h −1 ), whereas qCO 2 declined over time. Substrate‐use patterns in ExS mixtures shifted toward greater utilization of carboxylic acids, while enzymatic activities showed limited responsiveness, except for urease. Technosol composition was the primary driver of early SOC fraction dynamics and microbial functioning, whereas irrigation within realistic ranges played a secondary role. Selecting an ExS‐rich mineral matrix represents a key design strategy to promote early organo‐mineral carbon association and sustain microbial functioning in semi‐arid UGI soils.