GROWTH-RESPONSES OF 2 CONTRASTING UPLAND GRASS SPECIES TO ELEVATED CO2 AND NITROGEN CONCENTRATION

Growth parameters of Agrostis capillaris L. and Nardus stricta L. were measured in relation to ambient and elevated concentrations of CO2 (340 and 550 mul CO2 l-1, respectively) and at low and high concentrations of nitrogen (0.8 and 3 mm NH4NO3, respectively). After 60 d of growth A. capillaris had attained approx. four times the total dry weight of N. stricta in all treatments, which was attributed to the greater leaf area ratio of the former. A. capillaris grown at the low nitrogen concentration attained 30% of the total dry weight of plants grown at high nitrogen. Over the 60 d period, destructive harvests (seven in total) showed the growth of N. stricta to be less sensitive than that of A. capillaris to the concentration of nitrogen, but in both species growth analysis showed the lower total dry weight at low nitrogen to be attributable to lower unit leaf rate. There was a differential response of both species to elevated concentrations of CO2 which was nitrogen dependent. A. capillaris grown at elevated CO2 attained a greater total dry weight than at ambient CO2 and this response was proportionately greater at low nitrogen (78% increase) than at high nitrogen (58% increase). In contrast, in N. stricta there was no effect of CO2 concentration on the total dry weight at low nitrogen whilst at high nitrogen plants grown at elevated CO2 had a greater total dry weight after 48 d of growth. Calculation of the allometric coefficient (K) relating root growth to shoot growth indicated that the effect of the lower nitrogen concentration was to increase partitioning to the roots while the higher CO2 concentration did not alter partitioning.