Growth, nitrogen uptake, and metabolism in two semiarid shrubs grown at ambient and elevated atmospheric CO 2 concentrations: effects of nitrogen supply and source

The effect of differences in nitrogen (N) availability and source on growth and nitrogen metabolism at different atmospheric CO 2 concentrations in Prosopis glandulosa and Prosopis flexuosa (native to semiarid regions of North and South America, respectively) was examined. Total biomass, allocation, N uptake, and metabolites (e.g., free NO 3 − , soluble proteins, organic acids) were measured in seedlings grown in controlled environment chambers for 48 d at ambient (350 ppm) and elevated (650 ppm) CO 2 and fertilized with high (8.0 mmol/L) or low (0.8 mmol/L) N (N level ), supplied at either 1 : 1 or 3 : 1 NO 3 − : NH 4 + ratios (N source ). Responses to elevated CO 2 depended on both N level and N source , with the largest effects evident at high N level . A high NO 3 − : NH 4 + ratio stimulated growth responses to elevated CO 2 in both species when N was limiting and increased the responses of P. flexuosa at high N level . Significant differences in N uptake and metabolites were found between species. Seedlings of both species are highly responsive to N availability and will benefit from increases in CO 2 , provided that a high proportion of NO 3 ‐ to NH 4 ‐N is present in the soil solution. This enhancement, in combination with responses that increase N acquisition and increases in water use efficiency typically found at elevated CO 2 , may indicate that these semiarid species will be better able to cope with both nutrient and water deficits as CO 2 levels rise.