Carbon Allocation in Mojave Desert Plant-Soil Systems as Affected by Nitrogen and Water Availability

Changes in atmospheric nitrogen (N) deposition due to increased urbanization and precipitation due to climate change are likely to affect carbon (C) allocation in plants and soils in arid ecosystems in the Southwestern United States where net primary production is often limited by N and water availability. We conducted a greenhouse study to determine the effects of N and water availability on one year old creosote (Larrea tridentata) plants, the dominant shrub in the Mojave Desert. In our greenhouse study we employed two N levels (0 and 40 kg ha-1) and two soil moisture levels (7% and 15%). We grew creosote seedlings in PVC columns filled with topsoil from the Mojave Global Change Facility at the Nevada Test Site. The columns were covered and sealed at the base of the plant to separate the above- from belowground plant compartment. Plants were distributed over two growth chambers receiving ambient light while day/night temperatures were set at 25° C/15° C. In one chamber plants were labeled once a week with 13C-enriched CO2 while a second chamber acted as an unlabeled control. Throughout the six month study we measured soil CO2 concentrations, respired CO2 as well as their isotopic signatures. At the end of the study plants were harvested and we measured plant above- and belowground biomass and isotopic composition of the vegetation. In addition, we measured isotopic composition of soil organic and inorganic C. Increased N availability stimulated stem weight and decreased total C losses through soil respiration. Other plant and soil parameters including isotopic composition were not affected by changes in N availability. Increased soil moisture stimulated plant biomass mainly due to an increase in leaf weight while root biomass tended to decrease. Soil CO2 concentrations increased with increasing water availability despite a reduction in root biomass. The isotopic data showed that net new C uptake increased mostly in leaves, soil organic matter and soil respiration but C allocation to stems decreased. The enhanced net C uptake at higher water availability was due to an increase in leaf biomass as well as an increase in specific photosynthetic activity. In both treatments soil CO2 was derived from soil organic and inorganic sources but none of the treatments caused significant changes in carbonate-C. Overall, changes in soil moisture are more likely to affect C allocation in these arid ecosystems than changes in N availability. However, changes in soil moisture may be short-lived (episodic) while increased N deposition may represent a more chronic change.