In fire-influenced savanna ecosystems, native and invasive exotic plants may use different abilities to coexist and compete for nutrients available in post-fire soil. The availability and uptake of nitrate by two C4 grasses (Urochloa brizantha (exotic invasive) and Axonopus siccus (native)) were characterized in an unburned area and in a recently burned area in the Cerrado of Southeastern Brazil. Ecophysiologic tests were performed to measure Nitrate Reductase Activity (NRA) and nitrate content in leaves and roots. Soil fertility was also analyzed. In the unburned area, organic matter (25.3 ± 2.4 g kg^–1), phosphorus (19.0 ± 2.0 mg dm^–3), and nitrate (19.9 ± 7.4 mg dm^–3) levels were higher than in the recently burned area, which had 27% less cations in the soil. In comparison with soils from the recently burned and unburned areas, ashes of the recently burned area had higher pH, electrical conductivity and nitrate, potassium, and calcium contents. In both areas, the aboveground part of U. brizantha was more responsive to nitrate compared to A. siccus and both had low belowground NRA. This confirms the working hypothesis that the two species undergo distinct ecophysiologic processes and indicate the adaptive potential of U. brizantha to bypass soil nutrient limitation.