Determination of total N &amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fixation rates in the ocean taking into account both the particulate and filtrate fractions

U Konno,F Nakagawa,D D Komatsu,Y.-J Eum,T Matsui,K Suzuki,U Tsunogai,A Tsuda,S Daita

doi:10.5194/bg-7-2369-2010

Determination of total N &lt;sub&gt;2&lt;/sub&gt; fixation rates in the ocean taking into account both the particulate and filtrate fractions

U Konno, F Nakagawa + Show 7 more

Open Access

https://doi.org/10.5194/bg-7-2369-2010

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Abstract

Abstract. Using the 15N2 tracer method and high-sensitivity δ15N analytical systems, we determined N2 fixation rates for ocean samples by dividing them into particulate (>0.7 μm) and filtrate (<0.7 μm) fractions. While N2 fixation in the filtrate fraction had been ignored in previous studies, we found a significant N2 fixation rates in the filtrate fraction in our study. The areal N2 fixation rates in the western North Pacific Ocean estimated from the particulate fraction varied from <1 to 160 μmol N m-2 d−1, and those rates estimated from the filtrate fraction ranged from <0.5 to 54 μmol N m-2 d−1. Thus, N2 fixation in the filtrate fraction accounts for on average 50% (ranging from <10% to 84%) of the total N2 fixation rates. If these results are confirmed generally in the ocean, the new total N2 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.

Highlights

The areal N2 fixation rates estimated from the particulate fraction varied from
We found significant N2 fixation rates in the filtrate fraction (
The abundance of nifH gene copies determined by the quantified PCR method indicated that the large N2 fixation rates observed in the filtrate fraction at high latitudes could be attributed to active secondary N release processes for filtrate fraction from recently fixed nitrogen by Trichodesmium

Summary

Introduction

Over the last three decades, the global budget of oceanic fixed nitrogen (NO−3 , NO−2 , NH+4 , particulate organic nitrogen (PON), and dissolved organic nitrogen (DON)) has been extensively studied as one of the representative nutrients that regulate primary production in the ocean (e.g. Wada et al, 1975; Codispoti and Christensen, 1985; Gruber and Sarmiento, 1997; Brandes and Devol, 2002). The total fixed nitrogen is predominantly controlled by the total influx of fixed nitrogen through N2 fixation and by the total efflux of fixed nitrogen through denitrification (Codispoti et al, 2001; Brandes and Devol, 2002; Deutsch et al, 2004). The estimated values of both fluxes obtained in different studies are highly variable (e.g., Wada et al, 1975; Liu, 1979; Codispoti and Christensen, 1985; Gruber and Sarmiento, 1997; Codispoti et al, 2001; Brandes and Devol, 2002; Capone and Knapp 2007; Naqvi et al, 2008). Recent studies on denitrification in sediments and water columns have revealed a considerably large efflux of 275–450 Tg N yr−1

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