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

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Abstract
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.