Determination of total N (2) fixation rates in the ocean taking into account both the particulate and filtrate fractions

Using the N-15(2) tracer method and high-sensitivity delta N-15 analytical systems, we determined N-2 fixation rates for ocean samples by dividing them into particulate (> 0.7 mu m) and filtrate (< 0.7 mu m) fractions. While N-2 fixation in the filtrate fraction had been ignored in previous studies, we found a significant N-2 fixation rates in the filtrate fraction in our study. The areal N-2 fixation rates in the western North Pacific Ocean estimated from the particulate fraction varied from < 1 to 160 mu mol N m(-2) d(-1), and those rates estimated from the filtrate fraction ranged from < 0.5 to 54 mu mol N m(-2) d(-1). Thus, N-2 fixation in the filtrate fraction accounts for on average 50% (ranging from < 10% to 84%) of the total N-2 fixation rates. If these results are confirmed generally in the ocean, the new total N-2 fixation flux, which includes fixation in the filtrate fraction, possibly doubles the original estimates; therefore, the revised influx may reduce the imbalance in the global oceanic fixed nitrogen budget.