1 Department of Bioscience, Science and Technology, Aarhus University2 Department of Bioscience - Plant Biology, Department of Bioscience, Science and Technology, Aarhus University3 Department of Bioscience, Science and Technology, Aarhus University
Polluted waters and waste water often contain elevated levels of salt in hot and dry regions because of the inherent high evapotranspiration rates. Constructed wetlands must therefore be planted with species that tolerate saline water. The perennial sedge, Cyperus laevigatus L., is predominantly growing in brackish wetlands and on wet alkaline and mineral-rich soils. Hence, this species may be a candidate species for use in constructed wetland systems in hot and dry regions. We studied the nitrogen nutrition of C. laevigatus in order to determine if the plant prefers NO3- over NH4+, which could be expected based on the species’ natural distribution on alkaline and mineral-rich soils. A preference for NO3- over NH4+ would suggest that the species is less suitable for use in constructed wetland systems because the predominant form of nitrogen in waste water is NH4+. The growth, N-uptake kinetics and tissue carbon and nitrogen contents of C. laevigatus were studied in hydroponically grown plants supplied with 0.5 mM nitrogen as NH4+or NO3- alone or in combination (NH4NO3). The relative growth rate (RGR) of NO3-–fed plants (RGR = 0.10 ± 0.02 g g-1 d-1) was significantly (P = 0.0002) lower than the RGR’s rates of NH4+ and NH4NO3-fed plants (RGR = 0.13 ± 0.01 g g-1 d-1 in both treatments) after the first three weeks. But after five weeks there was no significant difference among the N-nutrition treatments (mean RGR = 0.08 ± 0.03 g g-1 d-1). The uptake kinetics of NH4+ and NO3- were also not affected by the N-nutrition. However, the maximum uptake rate (Vmax) of NH4+ was four times higher than that of NO3- (mean = 99 ± 17 and 24 ± 9 µmol g-1 root dry weight h-1, respectively). Nitrogen content did not differ among treatments with mean N concentrations of 4.0 ± 0.5 and 1.9 ± 0.3 % dry weight in roots and leaves, respectively. On the other hand, the carbon content of NO3- - fed plants (43.0 ± 0.3 % dry weight) was significantly (P<0.05) lower than the carbon content of NH4NO3 and NH4+-fed plants (43.8 ± 0.3 and 44.1 ± 0.1 % dry weight, respectively). The results show that C. laevigatus has a higher capacity to take up NH4+ than NO3- at equimolar concentrations, but the growth of the plants seems to be indifferent to the form of nitrogen taken up. We conclude that C. laevigatus may have potential for use in constructed wetland systems receiving nitrogen-rich waters, but its tolerance to higher concentrations of particularly NH4+ needs to be clarified.