Nitrous oxide emissions and isotopic composition in urban and agricultural systems in southern California

Abstract

Nitrous oxide (N 2 O) 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 N 2 O 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 15 N and δ 18 O) of N 2 O 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 N 2 O fluxes equal to or greater than agricultural fields. Fertilization rates of urban landscapes were equal to or greater than agricultural fields, with comparable N 2 O emissions factors. δ 15 N and δ 18 O of N 2 O 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 15 N-N 2 O to pulses of N 2 O emission following fertilization, with an initial depletion in δ 15 N relative to prefertilization values, then gradual enrichment to background values within about 1 week. Preliminary scaling calculations indicated that N 2 O 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 N 2 O emissions (based on agricultural land area) may be too low. Copyright 2011 by the American Geophysical Union.