Effects of inorganic carbon supply on the nitrogen requirement of two submerged macrophytes, Elodea canadensis and Callitriche cophocarpa

Effects of inorganic carbon supply on nitrogen requirement and critical nitrogen concentration was examined for the submerged macrophytes E. canadensis and C. cophocarpa. The plants were grown in a factorial setup of four inorganic nitrogen (0.001-0.1 mM N as NH4 NO3) and two CO2 concentrations (17 and 430 mu M) at an alkalinity of 0.85 meqv l(-1). The two species are morphologically similar, but differ in their ability to use bicarbonate in photosynthesis. E. canadensis has a high affinity for bicarbonate whereas C. cophocarpa is restricted to use CO2. The growth rate was higher and the tissue-N concentration lower at high compared to low CO2 for both species. Further, the tissue-N concentration needed to saturate growth of E. canadensis was higher at low than at high CO2, 2.0 and 1.1 mmol N g(-1) DW, respectively. For C. cophocarpa growth was saturated at 1.3 mmol N g(-1) DW at high CO2, At low CO2, growth was suppressed by the low carbon availability but was independent of nitrogen availability. Growth rate and the efficiency of nitrogen use expressed as growth rate per unit tissue-N, was higher for E. canadensis than for C. cophocarpa at low CO2. In contrast, at high CO2, C. cophocarpa grew faster and had a higher growth rate per unit tissue-N than E. canadensis. This difference was greater at low nitrogen availability suggesting that C. cophocarpa may have a competitive advantage over E. canadensis when growing under conditions with limited nitrogen supply but ample CO2. Growth per unit tissue-N was higher at high than at low CO2 for both species. For E. canadensis the higher growth per unit N counterbalanced the lower tissue-N concentration at high CO2 and as a result the nitrogen requirement of E. canadensis was similar at high and low CO2 For C. cophocarpa the requirement was higher at high CO2 where growth was substantially higher than at low CO2. This suggests that depending on species and relative change in inorganic carbon concentration, higher growth rates can be expected in systems with higher inorganic carbon supply rates even without a concomitant increase in nitrogen load. (C) 1998 Elsevier Science B.V. All rights reserved.