A reevaluation of the magnitude and impacts of anthropogenic atmospheric nitrogen inputs on the ocean

Abstract

We report a new synthesis of best estimates of the inputs of fixed nitrogen to the world ocean via atmospheric deposition and compare this to fluvial inputs and dinitrogen fixation. We evaluate the scale of human perturbation of these fluxes. Fluvial inputs dominate inputs to the continental shelf, and we estimate that about 75% of this fluvial nitrogen escapes from the shelf to the open ocean. Biological dinitrogen fixation is the main external source of nitrogen to the open ocean, i.e., beyond the continental shelf. Atmospheric deposition is the primary mechanism by which land-based nitrogen inputs, and hence human perturbations of the nitrogen cycle, reach the open ocean. We estimate that anthropogenic inputs are currently leading to an increase in overall ocean carbon sequestration of~0.4% (equivalent to an uptake of 0.15 Pg C yr À1 and less than the Duce et al. (2008) estimate). The resulting reduction in climate change forcing from this ocean CO 2 uptake is offset to a small extent by an increase in ocean N 2 O emissions. We identify four important feedbacks in the ocean atmosphere nitrogen system that need to be better quantified to improve our understanding of the perturbation of ocean biogeochemistry by atmospheric nitrogen inputs. These feedbacks are recycling of (1) ammonia and (2) organic nitrogen from the ocean to the atmosphere and back, (3) the suppression of nitrogen fixation by increased nitrogen concentrations in surface waters from atmospheric deposition, and (4) increased loss of nitrogen from the ocean by denitrification due to increased productivity stimulated by atmospheric inputs.

Atmospheric water-soluble organic nitrogen (WSON) in the eastern Mediterranean: origin and ramifications regarding marine productivity

“…Depending on the season, limitation by nitrogen or co-limitation by nitrogen and phosphorus in the eastern Mediterranean has been reported (Yücel, 2013(Yücel, , 2017, and references therein). Based on molar N / P ratios in the atmospheric input (order of magnitude higher than that of Redfield; Markaki et al, 2003Markaki et al, , 2010Koçak et al, 2010) and riverine fluxes (at least 1.8 times larger than that of Redfield; Ludwig et al, 2009;Koçak et al, 2010) it has been suggested that the eastern Mediterranean receives excessive amounts of dissolved inorganic nitrogen and that this unbalanced input may result in even more phosphorus deficiency (Ludwig et al, 2009;Koçak et al, 2010), whilst the atmospheric deposition of reactive nitrogen may cause accumulation of nitrogen in the water column (Jickells et al, 2017). Very little research has focused on the importance of water-soluble organic nitrogen input to marine productivity in the eastern Mediterranean (Mace et al, 2003a;.…”

“…The laboratory experiments performed by Seitzinger and Sanders (1999) demonstrated the production of coastal marine bacteria and phytoplankton, which are stimulated by the addition of water-soluble organic nitrogen, 45-75 % being bioavailable. From the mid-1800s to 2000, as a result of anthropogenic activities, reactive nitrogen and reactive anthropogenic organic nitrogen increased by almost 3-and 5-fold, respectively, leading to a significantly modified global nitrogen cycle (Jickells et al, 2017). This in turn has impacted marine nitrogen biogeochemical cycling (Galloway and Cowling, 2002;Galloway et al, 2008;Duce et al, 2008;Jickells et al, 2017).…”

Multi-model study of HTAP II on sulphur and nitrogen deposition

“…Dentener et al (2006) the Unites States is found to deviate with inferential dry deposition data. Kanakidou et al (2016) used the ACCMIP simulation results under historical, RCP6.0 and RCP 8.5 emission scenarios to 90 estimate the changes in N deposition driven by human activity in the past (1850), present (2005) and future (2050).…”

“…Elevated S and N deposition are also associated with a host of environmental issues such as acidification and eutrophication of the terrestrial system (Bouwman 60 et al, 2002), loss of ecosystem biodiversity (Bobbink et al, 2010), harming the heterotrophic respiration and disturbing the soil decomposition process (Janssens et al, 2010), although some studies found increasing N deposition could benefit the carbon uptake by land processes (Reay et al, 2008;Holland et al, 1997). Similar to the terrestrial system, over-richness of S and N deposition is also a threat to the aquatic system by acidification (Doney et al, 2007) and 65 eutrophication of the ocean (Bergstrom and Jansson, 2006;Jickells, 2006;Jickells et al, 2017).…”

“…Elevated N deposition can cause exceedance of N critical loads on ecosystems Sun et al, 2017). 11% of 55 the world's natural vegetation has already received N deposition that exceeds the critical load in 2000 (Dentener et al, 2006). The most affected regions are Eastern Europe (80%), South Asia (60%) and East Asia (40-50%).…”