Differential responses of root uptake kinetics of NH4+ and NO3- to enriched atmospheric CO2 concentration in field-grown loblolly pine

The nitrogen requirement of plants is predominantly supplied by NH4+ and/or NO3- from the soil solution, but the energetic cost of uptake and assimilation is generally higher for NO3- than for NH4+. We found that CO2 enrichment of the atmosphere enhanced the root uptake capacity for NO3-, but not for NH4+, in field-grown loblolly pine saplings. Increased preference for NO3- at the elevated CO2 concentration was accompanied by increased carbohydrate levels in roots. The results have important implications for the potential consequences of global climate change on plant- and ecosystem-level processes in many temperate forest ecosystems.