Lake sediments provide records of metal and nitrogen deposition over time. In Rocky Mountain National Park (RMNP), studies have shown that upslope weather events transport nitrogen from the eastern Colorado plains to the park. Steps like the RMNP Early Warning System have been implemented on a voluntary basis to reduce nitrogen emissions from crop and livestock producers during such weather events. However, Poudre Lake in RMNP is less than 10 meters from Trail Ridge Road, and therefore likely receives substantial nitrogen and metal loadings from vehicle emissions and road run-off. This study uses sediment core analysis to reconstruct the past 300 years of metal contamination and nitrogen deposition in Poudre Lake. Copper, zinc, lead, cadmium, and total nitrogen fluxes increase dramatically after the construction of Trail Ridge Road and the accumulation of metals correlates positively with RMNP visitor count. Potential ecological impacts of sediment contamination are explored by characterizing the diversity and composition of sediment microbial communities and potential nitrogen cycling functions. Increases in nitrogen cycling genes nirS and nosZ start around 1930 CE and then drop back down in the 1970s. Sediment microbial communities play a key role in nitrogen removal, yet little is known about how combinations of metal and nitrogen contaminants influence microbial community structure and nitrogen cycling activities. Interestingly, bacteria and archaea found in relatively high abundance in Poudre Lake sediments are also commonly found in urban pond sediments and wastewater treatment facilities. Results indicate that roadside lakes in RMNP are impacted by metal and nitrogen contamination from vehicle exhaust and non-exhaust emissions, and that contamination may alter the nitrogen cycling potential of sediment microbial communities over time.