Simultaneous measurements of nitrogen fixation in different plant tissues of the seagrass Posidonia oceanica

The lack of simultaneous measurements of N2 fixation associated with different plant tissues in seagrasses led us to investigate the temporal (seasonal and diurnal) variability of N2 fixation rates associated with the different plant tissues (leaves, rhizomes and roots) of Posidonia oceanica along the Mallorcan coast (Mediterranean Sea) in areas with differing local nutrient regimes. Additional measurements were taken to quantify the activities of endophytic N2 fixers in the roots. N2 fixation in the different portions of the plants showed variability through the seasons, with generally higher activity associated with the leaves during summer (reaching up to 0.29 ± 0.00 µg N g-1 dry weight [DW] h-1 [mean ± SD] during the day) and higher activity associated with the roots (reaching up to 0.12 ± 0.02 µg N g-1 DW h-1) during the day and night in winter. Root endophytic N2 fixers also showed maximal rates (0.11 ± 0.07 µg N g-1 DW h-1) during winter. N2 fixation associated with the rhizomes remained low throughout the seasons (<0.01 µg N g-1 DW h-1). N2 fixation associated with the leaves can contribute up to 86% of total plant N2 fixation during summer, while during the colder months, the belowground tissue parts play a more significant role in the plant’s N2 fixation. The rhizomes can contribute up to 67% of total N2 fixation of the plant in autumn, because of their high biomass, while the roots can contribute up to 30% in winter.