Management impacts on nitrous oxide flux and water loss from urban ecosystems

Urban ecosystems are heavily fertilized and irrigated, which causes emissions of nitrous oxide (N2O), a powerful greenhouse gas, and consumption of water resources. Thus, it is important to (1) quantify the magnitude and drivers of N2O losses, and their potential responses to climate change, (2) understand the amount and fate of irrigation water, including evapotranspiration (ET), (3) distinguish transpiration losses from evaporation, and (4) understand the water sources of urban plants, which may utilize irrigation water or groundwater for transpiration. To reduce these uncertainties for urban ecosystems in southern California, I made a variety of measurements including flux chamber measurements of N2O and ET in experimental lawns, as well as the isotopic composition of plant, soil, water and atmospheric vapor, and soil moisture, irrigation and runoff in lawns, urban forests, and an urban wetland. This research has provided experimental evidence that N 2O fluxes can serve as a positive feedback to global warming in lawns, due to a positive relationship with temperature. In addition, warming may exacerbate weed invasions, which may require more intensive management to manage species composition. I found that the irrigation efficiency (ET/applied irrigation) of urban lawns in this region may be very low (16-43%). The fate of more than half of the applied irrigation water at my study sites was drained below the rooting zone. Subsequently, the "reference ET" method of calculating ET of lawns from meteorological measurements likely over-estimates actual ET, and may lead to over-irrigation. I also found that despite frequent irrigation, urban trees tap into groundwater at some sites. However, mature urban trees may also develop very shallow roots, as I detected uptake of soil water above 30 cm depth. Finally, stable isotope techniques applied at an urban marsh provided support for primary water vapor loss by transpiration. A mass balance approach based on isotope measurements of standing water was more robust than a mass balance based on measurements of water vapor, due to a large influence of marine background vapor. The isotopic methods used in this research were powerful tools for understanding urban ecosystem nitrogen and water losses, especially when combined with flux measurements. My results show great potential for reducing N2O emissions and water use in urban ecosystems in response to future climate change and water scarcity.