Carbon Sequestration and Greenhouse Gas Emissions in Urban Landscaped Soils.

Urban soils, particularly lawns, are thought to have high organic matter densities relative to unmanaged or agricultural soils.  However, these soils may be subject to greater management-related carbon dioxide emissions and emissions of nitrous oxide and methane that are similar or greater than agricultural soils, decreasing or negating the overall ability of green spaces to mitigate urban greenhouse gas emissions.  We have measured carbon sequestration rates and greenhouse gas emissions from urban soils in two regions, southern California and the Midwest.  Both regions show a positive relationship of carbon stock versus time in ornamental lawns, indicating that lawn soils can be a positive sink for atmospheric carbon dioxide.  Carbon sequestration rates are higher in warmer southern California, where lawns are photosynthesizing year-round.  We show that minimally managed lawn soils (un-irrigated and un-fertilized) in the Midwest may also have high carbon stocks relative to agricultural soils.  Urban lawns emit nitrous oxide at rates comparable to agricultural soils, and have a reduced ability to consume atmospheric methane.  Overall, we have shown that urban soils will not aid in mitigation of urban greenhouse gas emissions due to increased footprints of nitrous oxide and methane as well as high carbon dioxide emissions from maintenance.