Unexpected topsoil nitrogen depletion and nitrogen–phosphorus decoupling in global urban greenspaces

Urban greenspaces are increasingly recognized for their pivotal ecosystem services. Soil nutrients, such as nitrogen (N) and phosphorus (P), are essential to support plant growth, whereas the characteristics of soil N and P concentrations remain unexplored in global urban greenspaces. We compiled global databases of topsoil N and P concentrations, and N to P ratios (N:P, 0–20 cm depth) in (i) urban greenspaces across 175 cities and (ii) natural ecosystems at 277 sites, respectively. Compared with their natural counterparts, urban greenspaces have lower topsoil total N concentrations (1.69 ± 0.06 vs 2.28 ± 0.10 g·kg −1 ), comparable topsoil total P concentrations (0.70 ± 0.04 vs 0.61 ± 0.03 g·kg −1 ), and lower N:P (3.10 ± 0.28 vs 4.71 ± 0.30). Based on additional analyses of observed data on paired urban greenspaces and natural ecosystems, as well as model-predicted N and P concentrations in natural soils at the same sites as the urban greenspaces, we further confirmed the abovementioned differences between urban and natural soils. Topsoil total N and P concentrations both increase significantly toward higher latitudes in urban greenspaces and natural ecosystems, while no significant latitudinal trends are found for topsoil N:P. The global pattern of topsoil total N concentrations in urban greenspaces is significantly associated with climatic, anthropogenic, and vegetational drivers, whereas the topsoil P concentration is strongly correlated with climatic drivers. Unlike natural ecosystems, topsoil total N and P concentrations in urban greenspaces are not correlated significantly. Our findings highlight an unexpected topsoil N depletion and N-P decoupling in global urban greenspaces.