Urban lawns are the largest irrigated crop in the U.S., and offer ecosystem services such as carbon sequestration and cooling via transpiration. However, they are also associated with disservices such as emissions of nitrous oxide (N₂O), a powerful greenhouse gas, and significant irrigation water use. Thus, it is important to quantify the magnitude and drivers behind nitrous oxide losses, and potential changes in response to climate change. In addition, it is important to understand the amount and fate of irrigation water applied to lawns, and the management controls on lawn water use. To elucidate these uncertainties, we have made a variety of measurements in experimental lawns including chamber measurements of N₂O and evapotranspiration (ET), plant and soil C and N content and isotopic composition, soil moisture, irrigation and runoff collection. We sought to understand (1) how warming influences nitrous oxide emissions and community composition, and (2) how management influences ET and irrigation efficiency (ET/applied irrigation water) in urban lawns.

Results/Conclusions

In southern California, there is experimental evidence that N₂O fluxes can serve as a positive feedback to global warming in lawns, due to a positive response of N₂O fluxes to warming. Soil moisture is also positively correlated with N₂O fluxes, and irrigation as well as fertilization leads to higher N₂O fluxes. In addition, warming may exacerbate weed invasions, which may require more intensive management, e.g. herbicide application, to manage species composition.  With regard to lawn water use, we define the irrigation efficiency of lawns as the amount of applied water that is allocated to plant transpiration. In a controlled study of the complete water budget of three experimental lawns, we found that irrigation efficiency varied from 16-44%, depending on management practices.  At least half of the water applied to these sites was drained below the rooting zone. In addition, the “reference ET” method of calculating evapotranspiration of lawns from meteorological measurements greatly over-estimated actual ET.  Recommended rates of water application based on reference ET calculations may lead to over-irrigation.  Our results show that there is potential for climate change mitigation in lawn management with practices that reduce N₂O emissions, and for climate change adaptation with practices that reduce lawn water use in response to predicted future water scarcity and drought.