Growth and Root NO₃- and PO₄3- Uptake Capacity of Three Desert Species in Response to Atmospheric CO₂ Enrichment

Abstract

In a phytotron experiment, we examined growth and rates of NO^-₃ and PO^3-₄ uptake in seedlings of two desert C₃ shrubs (Larrea tridentata and Prosopis glandulosa) and a desert C₄ perennial grass (Bouteloua eriopoda) grown under CO₂ partial pressures of 35 or 70 Pa. Plants were grown in soil but uptake studies were conducted on roots of intact seedlings placed in nutrient solutions containing both NO^-₃ and PO^3-₄. Elevated CO₂ increased total biomass by 69 and 55% in Larrea and Prosopis seedlings and by 25% in Bouteloua. NO^-₃ and PO^3-₄ uptake rates were more than doubled in Bouteloua at high compared to ambient CO₂. In contrast, CO₂ enrichment inhibited root NO^-₃ uptake capacity in Larrea by about 55% without a significant effect on PO^3-₄ absorption rate; rates of NO^-₃ and PO^3-₄ and uptake in Prosopis were insensitive to CO₂ treatment. Elevated CO₂ enhanced the proportion of biomass allocated to the fine roots in Bouteloua but markedly reduced this fraction in Larrea and Prosopis. Foliar N concentration of both shrubs decreased in response to elevated CO₂, but was unaffected in Bouteloua. We suggest that compensatory changes in root size and activity are critical in determining interspecies variation in plant nutrient relations under high CO₂.