Land use and management intensity shape nitrogen cycling and microbial functions, driving environmental impacts in French urban soils

Urbanization profoundly alters soil 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 public urban land uses —including showcase gardens, parks, tree-covered areas, roadsides, residential zones, sports fields and unused areas—influence nitrogen dynamics and microbial functions. We used a multifaceted approach combining biogeochemical measurements (NH₄ + , NO₃ − , total N), functional assays (potential mineralization, potential nitrification, potential denitrification), and molecular techniques (functional gene abundances and bacterial community profiling using Illumina MiSeq). Results showed that land use significantly influenced nitrogen cycle processes. Showcase garden areas were characterized by elevated nitrate levels, driven by fertilization and higher nitrification activity, whereas denitrification and related gene abundances were higher in parks and roadside soils. Structural equation modeling (SEM) revealed that soil organic carbon was a major driver of denitrification in most of the land uses, and pH positively influenced the abundance of ammonia-oxidizing archaea ( AOA ), whereas soil water retention, often reduced in compacted soils, was negatively correlated with most nitrogen variables. In terms of bacterial communities, this study revealed that bacterial functions linked to the nitrogen cycle varied according to land use. Lightly managed areas harboured more nitrogen-fixing bacteria, while intensely managed areas showed a predominance of bacteria involved in nitrification. Furthermore, the absence of any significant correlation between the multiple stages of the nitrogen cycle (mineralization, nitrification, denitrification) in intensively managed areas suggested a breakdown in nitrogen cycling, probably linked to excessive aeration, tillage and low organic matter accumulation or to differences in organic 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.