Reply on RC2

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Marine diazotrophs convert dinitrogen (N₂) in seawater into bioavailable nitrogen (N), contributing approximately half of the external input of bioavailable N to the global ocean. A global oceanic diazotroph database was previously published in 2012. Here, we compiled version 2 of the database by adding 23,095 <em>in situ</em> measurements of marine diazotrophic abundance and N₂ fixation rates published in the past decade, increasing the number of N₂ fixation rates and microscopic and qPCR-based diazotrophic abundance data by 140 %, 26 % and 443 %, respectively. Although the updated database expanded spatial coverage considerably, particularly in the Indian Ocean, the data distribution was still not uniform and most data were sampled in the surface Pacific and Atlantic Oceans. By summing the arithmetic means of the N₂ fixation rates in each ocean basin, the updated database substantially increased the estimate of global oceanic N₂ fixation from 137 &plusmn; 9 Tg N yr^-1 using the old database to 260 &plusmn; 20 Tg N yr^-1 (mean &plusmn; standard error). However, using geometric means instead, the updated database gave an estimate of global oceanic N₂ fixation (60 Tg N yr^-1) similar to that estimated from the old database (62 Tg N yr^-1), while the new estimate had a larger uncertainty (confidence intervals based on one standard error: 47 &ndash; 107 Tg N yr^-1 versus 52 &ndash; 73 Tg N yr^-1), mostly attributable to elevated uncertainties in the Pacific Ocean. An analysis comparing N₂ fixation rates measured at the same months and location (1&deg; &times; 1&deg; grids) showed that the new ¹⁵N₂ dissolution method obtained N₂ fixation rates higher than the conventional ¹⁵N₂ bubble method in 65 % of cases, with this percentage increasing when the N₂ fixation rates were high (&gt; approximately 3 &mu;mol N m^-3 d^-1 using the ¹⁵N₂ dissolution method). With greatly increased data points, this version 2 of the global oceanic diazotrophic database can support future studies in marine ecology and biogeochemistry. The database is stored at the Figshare repository (<a href="https://doi.org/10.6084/m9.figshare.21677687" target="_blank" rel="noopener">https://doi.org/10.6084/m9.figshare.21677687</a>) (Shao et al., 2022).