Department of Earth System Science

Nitrous oxide (N₂O) is a powerful greenhouse gas increasing in atmospheric mixing ratio and linked to increasing amounts of reactive N in the environment, particularly fertilizer use in agriculture. The consequences of urbanization of agricultural land for global and regional N₂O emissions are unclear, due to high spatial and temporal variability of fluxes from different ecosystems and relatively few studies of urban ecosystems. We measured fluxes and the stable isotope composition (δ ¹⁵N and δ ¹⁸O) of N₂O over 1 year in urban (ornamental lawns and athletic fields) and agricultural (corn and vegetable fields) ecosystems near Los Angeles, California, United States. We found that urban landscapes (lawns and athletic fields) have annual N₂O fluxes equal to or greater than agricultural fields. Fertilization rates of urban landscapes were equal to or greater than agricultural fields, with comparable N₂O emissions factors. δ ¹⁵N and δ ¹⁸O of N₂O varied widely in all ecosystems, and were not consistent with ecosystem type, season, soil moisture, or temperature. There was, however, a consistent response of δ ¹⁵N-N₂O to pulses of N₂O emission following fertilization, with an initial depletion in δ ¹⁵N relative to prefertilization values, then gradual enrichment to background values within about 1 week. Preliminary scaling calculations indicated that N₂O emissions from urban landscapes are approximately equal to or greater than agricultural emissions in urbanized areas of southern California, which further implies that current estimates of regional N₂O emissions (based on agricultural land area) may be too low.