Aggregate-scale carbon dynamics in urban mediterranean soils: Insights from enzyme activities and organic matter distribution

Urbanization is expected to increase, with 68 % of the world's population living in cities by 2050. Urban soils, essential for ecosystem services such as carbon storage and water regulation, are being degraded by human activities, altering their physical, chemical and biological properties. This study investigates the distribution of soil organic carbon and biological activity across urban soil aggregate sizes, hypothesising that aggregate-specific organic matter drives biogeochemical cycling. Soil samples from 12 sites within an urban park in Madrid were analysed, distinguishing between the following aggregate sizes: micro (<0.25 mm), meso (0.25–1 mm), macro (1–2 mm) and mega (>2 mm). The distribution of organic carbon forms (labile and recalcitrant) and enzyme activities related to nutrient cycling (C, N, P and S) were studied. The soils showed a high variability in physical and chemical properties, with low statistical differences in soil organic carbon forms and enzyme activities. Mega-aggregates were the most abundant and had a higher carbon content, but did not show significant differences in carbon fractions or enzyme activities compared to smaller aggregates. Micro- and meso-aggregates showed stronger correlations between soil carbon and enzyme activities and were more closely related to bulk soil data. The results suggest that urban soils exhibit carbon cycling dynamics closer to organically amended soils than natural soils, influenced by nutrient enrichment, contaminants and microbial activity. This research provides a theoretical basis for improving carbon sequestration and biogeochemical balance in urban soils.