Soil nutrient availability plays a key role in ecosystem ecology and may be directly impacted by global change. This study used ion exchange resin bags to measure nitrate (NO₃^-) and ammonium (NH₄⁺) availability in a California annual grassland under four global change treatments: warming, elevated CO₂, increased precipitation, and nitrogen deposition. The only significant treatment effect averaged across the entire growing season was nitrogen deposition, which increased both nitrate and ammonium availability by approximately 50%. However, many treatment effects showed seasonal variation. At the end of the growing season, increased precipitation significantly increased availability of both nitrate and ammonium. Warming had a positive effect on ammonium availability at the end of the growing season, with a significant interaction between warming and CO₂. Elevated CO₂ alone had no effect on ammonium availability and caused a slight decrease in nitrate availability. Our results suggest that the overall consequence of anthropogenic global change will be to increase soil nitrogen availability in this ecosystem, at least temporarily, but with complex seasonal and treatment interactions.