Abstract

Urban green infrastructure (UGI) often relies on constructed soils, yet it remains unclear how Technosol composition and irriga-tion regulate soil organic carbon (SOC) fractions and microbial functioning under semi-arid conditions. An 18-month mesocosmexperiment in Central Chile compared four Technosol mixtures differing in excavated subsoil (ExS) content under two irrigationregimes with lavender plantations. SOC fractions (POM and MAOM), microbial respiration (BR, SIR), substrate-use patterns(CLPP), qCO2 and enzymatic activities were assessed over time. SOC and MAOM were progressively higher with increasing ExScontent (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 res-piration was consistently higher in ExS-rich mixtures and intensified over time. Similarly, SIR increased over time (27.4 ± 8.3 to50.1 ± 22.4 μg CO2–C g−1 h−1), whereas qCO2 declined over time. Substrate-use patterns in ExS mixtures shifted toward greaterutilization of carboxylic acids, while enzymatic activities showed limited responsiveness, except for urease. Technosol composi-tion was the primary driver of early SOC fraction dynamics and microbial functioning, whereas irrigation within realistic rangesplayed a secondary role. Selecting an ExS-rich mineral matrix represents a key design strategy to promote early organo-mineralcarbon association and sustain microbial functioning in semi-arid UGI soils.