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Keywords:

  • carbon dioxide (CO2);
  • grassland;
  • climate change;
  • N cycle;
  • nitrogen-use efficiency;
  • N productivity;
  • competition

Summary

  • • 
    The aim of this study was to evaluate whether the responsiveness of mixed C3 grass species to elevated CO2 is related more to nitrogen uptake or to N-use efficiency.
  • • 
    Nitrogen uptake and whole-plant N-use efficiency were investigated with two binary mixtures: Lolium perenne was mixed either with Festuca arundinacea or with Holcus lanatus. The swards were grown on sand with or without CO2 doubling, and subjected to two cutting frequencies. A C20 alcohol was used as a marker to determine species proportion in the total root mass of the mixtures. The mean residence time of N was calculated from that of 15N-labelled fertilizer.
  • • 
    Lolium perenne took up significantly more N per unit root mass than its grass competitors, but its N-use efficiency was lower. Elevated CO2 significantly reduced the N uptake of the three grass species.
  • • 
    A trade-off between N capture and use was found, as N-use efficiency and N-uptake rate were negatively correlated. A high N-use efficiency, and conversely low N uptake appeared to favour the responsiveness to elevated CO2 of the infrequently cut grasses.