INFLUENCE OF THE SUBMERSED PLANT, POTAMOGETON-PERFOLIATUS, ON NITROGEN CYCLING IN ESTUARINE SEDIMENTS

Effects of Potamogeton perfoliatus on N transformations in sediments were examined with N-15 isotope techniques. Rates of ammonification, nitrification, denitrification, and plant uptake of nitrogen were measured in intact sediments with and without P. perfoliatus twice during the growing season (May and July). Sediments were injected with (NH4+)-N-15 and the appearance of N-15 over time in overlying water, pore water, and plant tissues was measured. Most of the N-15 added was recovered. Rates of bacterial NH4+ utilization (including NH4+ assimilation and nitrification) were similar to gross ammonification rates in vegetated and bare sediments, although rates were highly variable. Root (and rhizome) uptake represented approximately 90% of total N uptake by the plants in May, but only 20% in July. Tissue-specific (root or shoot) uptake rates were similar in May and July, although shoot (leaves plus stems) biomass increased faster than root biomass from May to July. Between 70 and 75% of the N taken up by roots was translocated to shoots. Denitrification rates, calculated from the appearance of N-15-N2, in the overlying and pore water, were significantly greater in vegetated than in bare sediments in July. Nitrification and denitrification were closely coupled, and denitrification in the root zone represented approximately 16% of total denitrification. In vegetated sediments, approximately 75% of the N-15 lost from sediments was denitrified, and 25% was taken up by the plants. P. perfoliatus had a significant influence on sediment N cycling by direct uptake of NH4+ and NO3- and by indirect mechanisms leading to enhanced nitrification and denitrification.