Nitrate and ammonium uptake for single- and mixed-species communities grown at elevated CO2

Sustained increases in plant 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 the amount of available CO2. We combined our physiological estimates of nitrogen uptake with measurements of root biomass to assess whole root-system rates of nitrogen 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 nitrogen uptake in elevated CO2 increased for ammonium in nutrient-rich soil (P<0.05) and were unchanged for nitrate (P>0.80). Root-system rates of nitrogen 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 nitrogen. In research to date, there appears to be a fairly general increase in root biomass with elevated CO2, and little evidence of up-regulation in root physiology.