Growth, nitrogen uptake, and metabolism in two semiarid shrubs grown at ambient and elevated atmospheric CO2 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 CO2 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 NO3-, soluble proteins, organic acids) were measured in seedlings grown in controlled environment chambers for 48 d at ambient (350 ppm) and elevated (650 ppm) CO2 and fertilized with high (8.0 mmol/L) or low (0.8 mmol/L) N (Nlevel), supplied at either 1 : 1 or 3 : 1 NO3- : NH4+ ratios (Nsource). Responses to elevated CO2 depended on both Nlevel and Nsource, with the largest effects evident at high Nlevel. A high NO3- : NH4+ ratio stimulated growth responses to elevated CO2 in both species when N was limiting and increased the responses of P. flexuosa at high Nlevel. 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 CO2, provided that a high proportion of NO3- to NH4-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 CO2, may indicate that these semiarid species will be better able to cope with both nutrient and water deficits as CO2 levels rise.