Uptake of nitrogen from compound pools by the seagrass Zostera noltii

In nature, seagrasses are confronted with a compound pool of low concentrations of inorganic and organic nitrogen-containing substances of varying bioavailability. Nevertheless, the majority of research on nitrogen acquisition by seagrasses has been largely limited to studies addressing a single nitrogen substrate at a time. Using a combination of one of 15N-labeled substrates and one 14N-labeled background substrate, we investigated how the rate of nitrogen uptake by the seagrass Zostera noltii varies with nitrogen background. Leaf and root mediated uptake were studied separately for different combinations of inorganic (ammonium and nitrate) and organic substrates (urea and glycine). Ammonium uptake rates were higher than for the other substrates. However, substrate uptake was not dependent on the background nutrient. Similar patterns and uptake rates were found for above- and belowground plant parts. The dependence of uptake rate on substrate type, combined with an independence of nutrient background is explained as difference in uptake capacity, rather than substrate preference. For the dual labeled (15N and 13C) urea and glycine, strong relationships existed between nitrogen and carbon uptake, but with deviations from expectations under complete uptake of the molecules. Overall, this study indicates that at realistically low ambient concentrations, seagrasses can simultaneously use inorganic and organic sources for their nitrogen needs, and do not distinguish between substrates. In other words, they take up whatever is available.