Nitrogen cycling in sediments with estuarine populations of Potamogeton perfoliatus and Zostera marina

Abstract

Rates of nitrogen transformations and concentrations of extractable NH4+ and NO3- (plus NO2-) were measured in estuarine sediments vegetated with the submersed macrophytes Potamogeton perfoliatus and Zostera marina, and in adjacent bare sediments, 3 or 4 times during the growing season. Nitrification and denitrification potentials were measured in substrated-amended sediment slurries at 5 depth intervals to provide a measure of bacterial activity. In general, rates were significantly higher in vegetated compared to bare sediments. It appears that both plant species affected nitrogen transformations through several similar mechanisms, while the microbial community, in turn, regulated nitrogen available for plant growth. In P. perfoliatus beds, ammonification and potential nitrification rates were correlated. Both exhibited summer maxima coinciding with peak plant biomass and productivity. Although vertically integrated (0-12 cm) ammonification rates were about twice as high in vegetated than in bare sediments, NH4+ pools were significantly lower, probably due to high plant nitrogen demand. In contrast, denitrification rates were highest in spring when NO3- concentrations peaked, and were significantly correlated to nitrification rates in both spring and fall. Denitrification was only about 20% of total NO3- reduction, suggesting that NH4+ production from NO3- may be important in conserving nitrogen within the grassbed. In sediments with Z. marina, rates of ammonification, and nitrification and denitrification potentials each exhibited a distinct seasonal cycle, indicating that rates were not as tightly coupled as in P. perfoliatus beds. High ammonification rates exceeded plant demand leading to NH4+ accumulation. Potential nitrification rates were highest in vegetated sediments during fall. Denitrification rates, which were also greater in vegetated than in bare sediments, were highest in spring when NO3- concentrations were high. Potential denitrification rates comprised about 10% of total NO3- reduction, indicating that NO3- reduction to NH4+ dominated. The microbial communities responsible for key nitrogen transformation in the sediments were enhanced by both P. perfoliatus and Z. marina ammonification by inputs of organic nitrogen; nitrification by release of O2 by plant roots; and denitrification by production of NO3-.