Nitrate and ammonium uptake for single- and mixed-species communities grown at elevated CO2
Authors: Jackson, RB, HL Reynolds
Sustained increases in production in elevated CO2 depend on adequate belowground resources. Mechanisms for acquiring additional soil resources include increased root allocation and changes in root morphology or physiology. CO2 research to date has focused almost exclusively on changes in biomass and allocation. We examined physiological changes in nitrate and ammonium uptake in elevated CO2, hypothesizing that uptake rates would increase with CO2. We combined our physiological estimates of N uptake with measurements of root biomass to assess whole root-system rates of N uptake. Surprisingly, physiological rates of ammonium uptake were unchanged with CO2, and rates of nitrate uptake actually decreased significantly (p<0.005). Root biomass increased 23% in elevated CO2 (p<0.005), but almost all of this increase came in fertilized replicates. Rates of root-system N uptake in elevated CO2 increased for ammonium at high nutrients (p<0.05) and were unchanged for nitrate (p>0.80). Root-system rates of N uptake were more strongly correlated with physiological uptake rates than with root biomass in unamended soil, but the reverse was true in fertilized replicates. We discuss nitrogen uptake and changes in root biomass in the context of root nutrient concentrations (which were generally unchanged with CO2) and standing pools of belowground plant N. In research to date, there appears to be a fairly general increase in root biomass with elevated CO2, and little evidence of upregulation in root physiology.