Assessing the impact of lawn management practices on the US carbon and water budgets

Despite the ubiquity of turf grasses in the United States, the large-scale functioning of these ecosystems has been largely understudied. Although we know that turf grass systems are sequestering carbon at the expense of a large amount of freshwater resources, a continental carbon and water budget has yet to be attempted. The limited existing information on the total extent and spatial distribution of turf grasses and the variability in management practices are the major factors complicating this assessment. In this study, relating turf grass area to fractional impervious surface area, it was estimated that potentially 165,000 km2 (± 31,500 km2) of land are cultivated with turf grasses in the continental United States, an area three times larger than that of any irrigated crop. Using the Biome-BGC ecosystem process model, the growth of warm season and cool season turf grasses was modeled for 865 sites across the 48 conterminous states under different management scenarios, including either removal or recycling of the grass clippings, different nitrogen fertilization rates and two alternative water irrigation practices. The simulations portray potential carbon and water fluxes as if the entire turf surface was to be managed like a well-maintained lawn. The results indicate that well watered and fertilized turf grasses act as a carbon sink, even assuming removal and bagging of the grass clippings after mowing. The potential carbon sequestration that could derive from the total surface under turf (up to 17 Tg C/yr with the simulated scenarios) would require a 30% to 70% increase in current domestic and commercial water use, depending on the modeled water irrigation practices. Landscaping water conservation practices such as xeriscaping and irrigation with recycled waste-water may need to be extended as municipalities continue to face increasing pressures on freshwater resources due to continued population growth.