The researchers studied how different levels of nitrogen and types of nitrogen fertilizer, along with elevated carbon dioxide levels, affect the growth and photosynthesis of two plant species. They found that higher nitrogen levels generally led to increased plant growth. The type of nitrogen fertilizer used affected the total dry weight of the plants, with ammonium-grown plants showing greater growth in one species. Elevated carbon dioxide levels had varying effects depending on nitrogen levels, with positive growth responses at higher nitrogen concentrations. Plants grown in elevated CO2 had lower photosynthetic capacity at lower nitrogen levels but not at higher nitrogen levels.