Contrasting responses of soil micro- and macro-food webs to urbanization across an amazonian forest-city gradient

The Amazon biome, one of the world’s major biodiversity hotspots and a key regulator of climate and biogeochemical cycles, is experiencing rapid urban growth and associated land-use change, reshaping biodiversity at the forest–settlement interface. Yet biodiversity in this region remains far less documented than in temperate zones, limiting our ability to understand and anticipate the effects of urbanization. This gap is especially critical for soil biodiversity, a fundamental component of ecosystem functioning but often overlooked in conservation or urban planning. Here, we investigate how urbanization affects size-structured soil food webs across French Guiana, an Amazonian territory offering a clear pristine forest–to–urban gradient, especially along the coasts and rivers where most inhabitants live. We built an integrated eDNA metabarcoding database spanning forests to urban areas across the territory, leveraging citizen-science initiatives for urban soil sampling. Identified soil taxa were classified into 79 trophic guilds and further assigned to either the micro-food web (microbes and microfauna) or the macro-food web (meso- and macrofauna). We hypothesized that increasing urbanization would reduce overall soil food-web complexity, measured as guild richness, and restructure communities by decreasing top predators while favouring generalists, with different mechanisms operating in micro- versus macro-food webs. Contrary to expectations, urbanization increased micro-food-web complexity, driven by positive responses to soil phosphorus and human footprint. This increase was primarily explained by decomposer and plant-symbiont guilds, while richness of higher-trophic-level guilds, including zooparasites and microbivores, remained stable. In contrast, the macro-food web showed no significant change in overall guild richness, although the trend was negative, and richness responded negatively to higher pH, lower C:N ratios, and higher population density, all changes related to the urbanization process. Urbanization also significantly restructured macro-food webs: higher-level consumer guilds declined, whereas herbivore and omnivore guilds peaked at intermediate urbanization levels. Our findings reveal complex, compartment-specific effects of urbanization, with contrasting responses between micro- and macro-food webs. Overall, results indicate a shift toward bottom-heavy, potentially bottom-up-regulated communities, highlighting implications for ecosystem functioning. Using eDNA and citizen-science approaches, this study provides new insights into how urbanization shapes size-structured, multitrophic terrestrial communities in the tropics.