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Decision makers in urban areas actively pursue strategies to decrease carbon dioxide (CO2) emissions and other greenhouse gases. Lawns dominate urban lands in the U.S. and require intensive management, including frequent mowing, which may influence CO2 emissions from both biogenic and anthropogenic sources. We tested whether different lawn mowing frequencies (every one, two or three weeks) affected soil respiration (i.e., biogenic CO2 emissions), by changing soil moisture and temperature, and the gasoline emissions associated with lawn maintenance via lawn mowing (i.e., anthropogenic CO2 emissions). Sixteen yards in Springfield, Massachusetts USA were assigned a mowing frequency for two seasons (2013-2014). We measured grass height, air and soil temperature, soil moisture, soil CO2 flux, lawn mower emissions, tree canopy coverage and precipitation. We used a mixed effects modeling approach to test how these variables interacted with each other and responded to mowing frequency. Lawn-mowing frequency did not influence soil temperature, moisture, or biogenic soil CO2 fluxes. Soil microclimate and soil respiration varied more with ambient climatic fluctuations and tree canopy cover. By contrast, anthropogenic emissions increased with more frequent mowing due to emissions associated with the mower. When scaled to the entire mowing season, biogenic CO2 fluxes far exceeded the anthropogenic fluxes, thus requiring consideration for accurate accounting of urban greenhouse gas emissions. The interplay between biogenic (e.g., increasing tree canopy in lawn-dominated yards) and anthropogenic (i.e., mowing less frequently) methods of reducing CO2 emissions in cities highlights the need for more rigorous accounting processes for cities to meet climate action goals.