Submitted to: Oecologia
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/6/2010
Publication Date: 4/15/2010
Citation: Jin, V.L., Evans, R.D. 2010. Elevated CO2 increases root uptake of organic and inorganic N in the desert shrub, Larrea tridentata. Oecologia. 163(1):257-266. Interpretive Summary: Plant growth usually increases under elevated concentrations of carbon dioxide (CO2) in the atmosphere. Increased plant growth can only occur when enough soil nutrients are available. An important soil nutrient for plant growth is nitrogen (N). Plants take up mineral forms of nitrogen (ammonium, nitrate) for plant growth, but it is unclear whether plants use organic nitrogen (amino-N) as well. To date, there have been no studies examining plant organic N uptake by desert plants or how organic N uptake affects ecosystem processes under increasing atmospheric CO2. We examined organic N uptake by creosotebush (Larrea tridentata), the dominant desert shrub of southwestern U.S. deserts. We grew creosotebush seedlings in soils from the Mojave Desert under laboratory conditions (i.e., a controlled environmental growth chamber) and tested these plants for the uptake of organic N compared to inorganic N. Overall, exposure to elevated CO2 increased root uptake of all N forms. We also found that creosotebush took up organic N sources from the soil. Organic N uptake rates were similar to inorganic N uptake rates. This is the first time that a plant species from an arid ecosystem has been shown to use organic N. Further, our results suggest that elevated CO2 will alter the availability of N sources in the soil. Plant species distributions in arid ecosystems are strongly controlled by competition for soil N resources. Water, however, is the most limiting resource in desert ecosystems. The long-term effects of elevated atmospheric CO2 concentrations in the Mojave Desert will ultimately depend on how global climate changes affect rainfall and the resulting interactions between soil nutrient availability and plant growth.
Technical Abstract: We quantified the effect of elevated atmospheric CO2 on root nitrogen (N) uptake and leaf carbon (C) turnover in the desert shrub, Larrea tridentata. We also examined the impact of elevated CO2 on soil N fluxes and plant-soil feedbacks. Seedlings of L. tridentata were grown in reconstituted Mojave Desert soils that had been continuously fumigated for eight years with ambient or elevated atmospheric CO2 at the Nevada Desert FACE Facility. Pots were simultaneously labeled with 15N (glycine, NH4+, or NO3-) and fumigated with 13CO2, followed by sequential destructive harvests (0, 2, 10, 24, and 49 days). Elevated CO2 enhanced glycine and NH4+ uptake, with approximately 52% of added glycine taken up as intact amino acid. Elevated CO2 did not affect NO3- uptake which remained high in both CO2 treatments. Up-regulation of root uptake of organic N and NH4+ under elevated CO2 was accompanied by lower mean residence times (MRT) of 13C in leaves, but N treatments did not affect plant 13C MRT under ambient CO2. Under elevated CO2, soil NH4+ concentrations decreased with no change in gross NH4+ fluxes, but soil NO3- concentrations increased with faster gross NO3- consumption rates. Significant correlations between root biomass and gross rates of mineralization and NO3- consumption reflect the tight coupling between organic matter availability and microbial N cycling in these low organic, resource-poor Mojave Desert soils.