Home lawns are a dominant cover type in urban and suburban ecosystems and there is concern about their impacts on water quality. However, recent watershed-level studies suggest that these pervious areas might be net sinks, rather than sources, for nitrogen in the urban environment. A 15N pulse-labeling experiment was performed on lawn and forest plots in the Baltimore metropolitan area to test the hypothesis that lawns are a net sink for nitrogen and to compare and contrast mechanisms of N retention in these vegetation types. A pulse of 15N-NO3-, simulating a precipitation event, was followed through soils, roots, Oi-layer/thatch, aboveground biomass, microbial biomass, inorganic nitrogen and evolved N2 gas over a one-year period. Gross rates of production and consumption of NO3- and NH4+ were measured to assess differences in internal nitrogen cycling under the two vegetation types. Rates of nitrogen retention in forests and lawns were similar during the first 5 days of the experiment, with lawns showing higher nitrogen retention than forests after 10, 70, and 365 days. Lawns had larger pools of available NO3- and NH4+; however, gross rates of mineralization and nitrification were also higher, leading to no net differences in NO3- and NH4+ turnover times between the two systems. Levels of 15N remained steady in forest soils from days 70 to 365 (at 23% of applied 15N), but continued to accumulate in lawn soil organic matter (SOM) over this same time period, increasing from 20% to 33% of applied 15N. The dominant sink for nitrogen in lawn plots changed over time; abiotic immobilization in soils dominated immediately (1 day) after tracer application (42% of recovered 15N), plant biomass dominated the short (10 days) term (51%), thatch and SOM pools together dominated the medium (70 days) term (28% and 36% respectively), while the SOM pool alone dominated long (1 year) term retention (70% of recovered 15N). These findings illustrate the mechanisms whereby urban and suburban lawns under low to moderate management intensities are an important sink for nitrogen.