Could invisible effects of land use and management on soil microbial communities durably impact carbon and nitrogen cycling in urban soils?
Résumé
Urbanization profoundly alters nitrogen and carbon cycling through changes in land use and management intensity, affecting both soil functions and microbial communities. These shifts can degrade soil fertility and disrupt ecosystem processes. This study investigates how different urban land use types—including showcase gardens, parks, tree-covered areas, roadsides, residential zones, sport fields and unused areas—influence nitrogen dynamics and microbial functions. We used a multifaceted approach combining biogeochemical measurements (NH₄⁺, NO₃⁻, total N and C), functional assays (mineralisation, nitrification, potential denitrification), organic matter characterization (via Rock-Eval analysis), and molecular techniques (functional gene abundance and bacterial community profiling using Illumina MiSeq). Results show land use significantly influences nitrogen processes. Showcase garden areas showed elevated nitrate levels, driven by fertilization and higher nitrification activity, whereas denitrification potential and related gene abundances were higher in parks and roadside soils. Structural equation modelling (SEM) revealed that soil organic carbon is a major driver of denitrification, and pH influences the abundance of ammonia-oxidizing archaea (AOA). Water retention, often reduced in compacted soils, negatively affected the nitrogen cycle. In terms of bacterial communities, the study reveals that bacterial functions linked to the nitrogen cycle vary according to land use. Lightly managed areas harbour more nitrogen-fixing bacteria, while intensely managed areas show a predominance of bacteria involved in nitrification. Furthermore, the absence of any significant correlation between the different stages of the cycle (mineralisation, nitrification, denitrification) in intensively managed areas indicates a breakdown in the nitrogen cycle, probably linked to excessive aeration, tillage and low organic matter accumulation or to difference in organics matter decomposition and stability. These findings highlight the sensitivity of nitrogen cycling to urban land management. Crucially, the role of organic matter dynamics in this context requires deeper investigation. Future analysis using Rock-Eval will be key to understanding the quality, stability, and thermal reactivity of soil organic matter and its influence on microbial nitrogen transformations. This approach can reveal how different land uses affect organic matter decomposition and its capacity to support sustained microbial activity.
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