Urbanization can accelerate climate change by increasing soil N 2 O emission while reducing CH 4 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 (N 2 O) and methane (CH 4 ) fluxes, 2) quantify direct N 2 O emission factors (EF d ) 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 N 2 O emissions by 153%, to 3.0 kg N ha -1 yr -1 , while rates of soil CH 4 uptake are reduced by 50%, to 2.0 kg C ha -1 yr -1 . The global mean annual N 2 O EF d of fertilized lawns and urban forests is 1.4%, suggesting that urban soils can be regional hotspots of N 2 O emissions. On a global basis, conversion of land to urban greenspaces has increased soil N 2 O emission by 0.46 Tg N 2 O-N yr -1 and decreased soil CH 4 uptake by 0.58 Tg CH 4 -C yr -1 . Urbanization driven changes in soil N 2 O emission and CH 4 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 N 2 O emissions and reduces the role of soils as a sink for atmospheric CH 4 . These effects can be mitigated by avoiding soil compaction, reducing fertilization of lawns, and by restoring native ecosystems in urban landscapes.