||While lawn management practices have altered the capacity for urban lawns to act as Nitrogen (N) sinks, there have been few studies of the effects of organic vs. inorganic fertilizer additions to urban lawns. We evaluated how foliar and soil N content and isotopic N composition (δ15N) varied as a result of different fertilizer treatment (inorganic, organic or control). We also evaluated differences in lawn above ground net primary productivity (ANPP) among fertilization treatments. We hypothesized that (1) lawn plots managed with organic fertilizer would exhibit lower leaf %N than inorganic fertilizer treatments due to N cycling and losses via leaching; (2) organic fertilizer would exhibit more enriched δ15N in plant and soil tissue due to trophic level effects of animal derived N; (3) the fertilization effect on %N of grasses and soil would be strongest immediately following fertilization and taper off throughout the growing season due to N mineralization losses and immobilization; (4) if soils are N-limited prior to fertilization, fertilized plots - regardless of form (inorganic or organic) -- would have higher ANPP compared with control plots. We found that lawn plots managed with organic fertilizer had lower leaf %N than inorganic fertilizer plots, and that all fertilized plots showed a declining treatment effect on %N over time. However, treatment effects on δ15N varied over time. We found that organic fertilizer plots had significantly lower foliar δ15N than inorganic and control throughout the growing season. Finally, we found no differences in ANPP across treatments for plots. In this study, fertilizer treatments did not stimulate ANPP of grasses, suggesting that these lawns were not strongly N-limited prior to fertilization.