A trade-off between nitrogen uptake and use increases responsiveness to elevated CO2 in infrequently cut mixed C3 grasses

Abstract

• 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. © New Phytologist (2005).