Urbanization can accelerate climate change by increasing soil N₂O 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 (N2O) and methane (CH4) fluxes, (2) quantify direct N2O 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 N2O 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 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 year−1 and decreased soil CH4 uptake by 0.58 Tg CH4‐C year−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 by restoring native ecosystems in urban landscapes.

A global meta‐analysis of yield‐scaled N2O emissions and its mitigation efforts for maize, wheat, and rice

“…In addition, the positive effects of climate warming on plant growth in high latitude regions and its strong negative effects in low latitude regions may play a role (Challinor et al, 2014;Iizumi et al, 2021). This explanation is further supported by our results that cereal yields are lowest in the tropics (Figures S4 and S6), with lowincome countries and farmers using insufficient amounts of fertilizer for crop production (Leitner et al, 2020;Smerald et al, 2023), and by the significant negative correlation between cereal yields and MAT (Figures S8 and S10 Zhan et al, 2023). For upland crops (maize and wheat), high MAP levels generally stimulated yield-scaled N 2 O emissions, with possible reasons including rapid vegetative growth and high plant nutrient demand, but large soil N losses along hydrological and gaseous pathways (e.g., NO − 3 leaching and N 2 O emissions) and consequent N-limitation of crop yields associated with high rainfall (Armour et al, 2013;Zhao et al, 2022).…”

“…The MAP is another important climatic factor controlling variations in yield‐scaled N 2 O emissions on a global scale, as MAP potentially affects crop growth and N 2 O emissions by modulating soil moisture, oxygen availability, redox potential and soil microbial activities (Li et al., 2022; Zhan et al., 2023). For upland crops (maize and wheat), high MAP levels generally stimulated yield‐scaled N 2 O emissions, with possible reasons including rapid vegetative growth and high plant nutrient demand, but large soil N losses along hydrological and gaseous pathways (e.g., $$$ {\mathrm{NO}}_3^{-} $$$ leaching and N 2 O emissions) and consequent N‐limitation of crop yields associated with high rainfall (Armour et al., 2013; Zhao et al., 2022).…”

Urbanization associated changes in biogeochemical cycles

“…In view of the severe lack of available data and attention devoted to the topic thus far, the novel meta‐analysis by Zhan et al (2023) presented in this issue of Global Change Biology is a welcome step towards enhancing our understanding of the impacts of urbanization on non‐CO 2 GHG emissions. As the authors point out, the observations available for analysis in this synthesis article were made primarily in North America, with a few studies in Europe, Australia, or Asia.…”

A distinctive latitudinal trend of nitrogen isotope signature across urban forests in eastern China

“…In addition, the correlations between leaf δ 15 N and soil δ 15 N may vary among different functional types due to their different N utilization strategies (Craine et al, 2009; Franche et al, 2009; Hobbie & Högberg, 2012; Tang et al, 2013). Compared to natural forests, anthropogenic activities in urban forests, such as litter removal, irrigation and fertilization, may also interfere the correlation between leaf δ 15 N and soil δ 15 N (Sayer, 2006; Xu et al, 2013; Zhan et al, 2023). However, the coupling between leaf δ 15 N and soil δ 15 N in urban forests remain unexplored in the context of high N inputs with distinctive 15 N abundances.…”