Urbanization can accelerate climate change by increasing soil N2 O emission while reducing CH4 uptake.

Abstract

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 (N2 O) and methane (CH4 ) fluxes, (2) quantify direct N2 O emission factors (EFd ) of fertilized urban soils used, for example, as lawns or forests, and (3) identify the key drivers leading to flux changes associated with urbanization. On average, urbanization increases soil N2 O emissions by 153%, to 3.0 kg N ha-1 year-1 , while rates of soil CH4 uptake are reduced by 50%, to 2.0 kg C ha-1 year-1 . The global mean annual N2 O EFd of fertilized lawns and urban forests is 1.4%, suggesting that urban soils can be regional hotspots of N2 O emissions. On a global basis, conversion of land to urban greenspaces has increased soil N2 O emission by 0.46 Tg N2 O-N year-1 and decreased soil CH4 uptake by 0.58 Tg CH4 -C year-1 . Urbanization driven changes in soil N2 O 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 N2 O 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 by restoring native ecosystems in urban landscapes.