Influence of the submersed plant, <i>Potamogeton perfoliatus</i>, on nitrogen cycling in estuarine sediments

Abstract

Effects of Potamogeton perfoliatus on N transformations in sediments were examined with 15 N 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 15 NH 4 + and the appearance of 15 N over time in overlying water, pore water, and plant tissues was measured. Most of the 15 N added was recovered. Rates of bacterial NH 4 + utilization (including NH 4 + assimilation and nitrification) were similar to gross ammonification rates in vegetated and bare sediments, although rates were highly variable. Root (and rhizome) uptake represented ~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 15 N‐N 2 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 ~16% of total denitrification. In vegetated sediments, ~75% of the 15 N 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 NH 4 + and NO 3 − and by indirect mechanisms leading to enhanced nitrification and denitrification.