Photosynthesis is the process by which plants absorb atmospheric carbon (C) as they grow and convert it to biomass. However, plants acquire nitrogen (N) and phosphorus (P) only when these are available in the soil solution, which makes these elements the most limiting nutrients to plant growth and productivity in most terrestrial ecosystems. This chapter discusses the C, N, and P reservoirs in soil and plant biomass of two sites (Churince and Pozas Azules) in desert grassland dominated by Sporobolus airoides at the Cuatro Cienegas Valley. We also analyzed the influence of different factors such as soil nutrients, water availability, and microbial nutrient transformations that determine the resource allocation to different pools in this oligotrophic ecosystem. We observed higher aboveground and below-ground biomass in Churince than in Pozas Azules. Additionally, we observed higher C and P contents in roots, higher soil total organic C and organic P at Churince, and higher N concentration in the aboveground grass biomass at Pozas Azules. Nutrient contents showed different patterns between sites. Total carbon, N, and P contents were all higher in Churince than Pozas Azules. At the ecosystem level, organic C and organic P were higher in Churince, but no differences were observed in N. In the two soil types studied, C: N: P stoichiometric ratios were different, suggesting that the same dominant plant species makes different adjustments of nutrient concentrations depending on water and nutrient availability, a response that can affect ecosystem nutrient pools as well as various ecosystem processes.