Urbanization can accelerate climate change by increasing soil N2O emission while reducing CH4 uptake

Urban land use change has the potential to affect local to global biogeochemical carbon (C) and nitrogen (N) cycles and associated greenhouse gas (GHG) fluxes. We conducted a meta-analysis to 1) assess the effects of urbanization-induced land-use conversion on soil nitrous oxide (N2O) and methane (CH4) fluxes, 2) quantify direct N2O emission factors (EFd) of fertilized urban soils used e.g., as lawns or forests, and 3) identify the key drivers leading to flux changes associated with urbanization. On average, urbanization increases soil N2O emissions by 153%, to 3.0 kg N ha-1 yr-1, while rates of soil CH4 uptake are reduced by 50%, to 2.0 kg C ha-1 yr-1. The mean annual N2O EFd of fertilized lawns and urban forests is 1.4%, suggesting that urban soils can be regional hotspots of N2O emissions. On a global basis, conversion of land to urban greenspaces has increased soil N2O emission by 0.46 Tg N2O-N yr-1 and decreased soil CH4 uptake by 0.58 Tg CH4-C yr-1. Urbanization-driven changes in soil N2O emission and CH4 uptake are associated with changes in soil properties (bulk density, pH, total N content and C/N ratio), increased temperature, and management practices, especially fertilizer use. Overall, our meta-analysis shows that urbanization increases soil N2O emissions and reduces the role of soils as a sink for atmospheric CH4. These effects can be mitigated by avoiding soil compaction, reducing fertilization of lawns, and restoring native ecosystems in urban landscapes.