Elevated CO2 increases plant uptake of organic and inorganic N in the desert shrub Larrea tridentata

Resource limitations, such as the availability of soil nitrogen (N), are expected to constrain continued increases in plant productivity under elevated atmospheric carbon dioxide (CO2). One potential but under-studied N source for supporting increased plant growth under elevated CO2 is soil organic N. In arid ecosystems, there have been no studies examining plant organic N uptake to date. To assess the potential effects of elevated atmospheric CO2 on plant N uptake dynamics, we quantified plant uptake of organic and inorganic N forms in the dominant desert shrub Larrea tridentata under controlled environmental conditions. Seedlings of L. tridentata were grown in the Mojave Desert (NV, USA) soils that had been continuously exposed to ambient or elevated atmospheric CO2 for 8 years at the Nevada Desert FACE Facility. After 6 months of growth in environmentally controlled chambers under ambient (380 mu mol mol(-1)) or elevated (600 mu mol mol(-1)) CO2, pots were injected with stable isotopically labeled sole-N sources (C-13-[2]-N-15 glycine, (NH4)-N-15 (+), or (NO3)-N-15 (-)) and moved back to their respective chambers for the remainder of the study. Plants were destructively harvested at 0, 2, 10, 24, and 49 days. Plant uptake of soil N derived from glycine, NH4 (+), and NO3 (-) increased under elevated CO2 at days 2 and 10. Further, root uptake of organic N as glycine occurred as intact amino acid within the first hour after N treatment, indicated by similar to 1:1 M enrichment ratios of C-13:N-15. Plant N uptake responses to elevated CO2 are often species-specific and could potentially shift competitive interactions between co-occurring species. Thus, physiological changes in root N uptake dynamics coupled with previously observed changes in the availability of soil N resources could impact plant community structure as well as ecosystem nutrient cycling under increasing atmospheric CO2 levels in the Mojave Desert.