Soils supporting the invasive annual, cheatgrass, are different from soils with other stable vegetative cover. Differences have been observed in soil microfauna, soil nitrogen pool sizes, and net mineralization and nitrification rates. Several mechanisms are potential drivers of these differences. To determine the processes involved, a ¹⁵N tracer was used to quantify plant derived differences in nitrogen (N) cycling in soils supporting cheatgrass and crested wheatgrass in greenhouse mesocosms. Half the mesocosms were exposed to a low-water treatment to determine how drying would affect variables of interest. The cotton wick method was used to deliver ¹⁵N urea to aboveground plant tissues, and appearance of ¹⁵N in extractable mineral, organic, and microbial biomass N pools were quantified. Gross N mineralization rates were measured using ¹⁵N pool dilution. Above and below ground plant tissue was also analyzed for ¹⁵N enrichment. In high root density zones of well-watered mesocosms, the amount of extractable ammonium was lower in cheatgrass than crested-wheatgrass (p=0.013) but did not differ in drier mesocosms (p=0.087). Extractable nitrate was lower in cheatgrass soils in both high and low root density sections of the low-water treatment (p=0.046, high root; p=0.035, low root) but did not differ from crested wheatgrass in the well-watered mesocosms. Further analysis will reveal whether plant inputs are significantly affecting N pool sizes and gross N mineralization rates, or if most differences are due to soil drying effects.