DETECTION OF LIMITING OR SURPLUS NITROGEN IN ALGAE AND AQUATIC WEEDS(1)

The rate of NH4 +-N absorption by algae and aquatic weeds in the dark has been shown to be 4-5 times greater for plants which are N-limited as compared to plants with sufficient available N. Eight species of green algae, 2 blue-green algae, 2 diatoms, and 3 aquatic weeds were used to demonstrate the usefulness of the test in determining if available N was in surplus or limited supply in a particular environment. The test was shown not to differentiate between blue-green algae capable of fixing N (4 species) from media with NO3 -N or without combined nitrogen. The factors influencing the results of NH4(+) -N absorption tests have been investigated. In order to differentiate between plants with sufficient available N and those which are N limited, the rate of NH4 (+) -N absorption (0.1 mg N) over 1-hr incubation in the dark by 10-20 mg of algae or aquatic weed tissues is measured. The relatively simple analysis for NH4 (+) -N in the samples makes it very easy to follow the changing N nutrition of plants in cultures with a limited N supply or in the presence of possible N sources.