Overlooked runaway feedback in the marine nitrogen cycle: the vicious cycle

Abstract

The marine nitrogen (N) inventory is thought to be stabilized by negative feedback mechanisms that reduce N inventory excursions relative to the more slowly overturning phosphorus inventory. Using a global biogeochemical ocean circulation model we show that negative feedbacks stabilizing the N inventory cannot persist if a close spatial association of N₂ fixation and denitrification occurs. In our idealized model experiments, nitrogen deficient waters, generated by denitrification, stimulate local N₂ fixation activity. But, because of stoichiometric constraints, the denitrification of newly fixed nitrogen leads to a net loss of N. This can enhance the N deficit, thereby triggering additional fixation in a vicious cycle, ultimately leading to a runaway N loss. To break this vicious cycle, and allow for stabilizing negative feedbacks to occur, inputs of new N need to be spatially decoupled from denitrification. Our idealized model experiments suggest that factors such as iron limitation or dissolved organic matter cycling can promote such decoupling and allow for negative feedbacks that stabilize the N inventory. Conversely, close spatial co-location of N₂ fixation and denitrification could lead to net N loss

Water column biogeochemistry of oxygen minimum zones in the eastern tropical North Atlantic and eastern tropical South Pacific oceans

“…This coupling which seems to exist in OMZ waters may in fact have far-reaching consequences: while N loss may pro- vide a niche for N 2 fixation, model studies suggest that denitrification of N 2 fixation-derived organic matter may lead to a net N loss that further stimulates N 2 fixation, because 120 mol of nitrate per mole of phosphorus is used to remineralize Redfield organic matter via denitrification (Landolfi et al, 2013). In contrast, N 2 fixation fixes only 16 mol of N (per mole of P).…”

“…In contrast, N 2 fixation fixes only 16 mol of N (per mole of P). Because of those stoichiometric constraints, any addition of fixed N to the surface ocean only exacerbates the problem (Canfield, 2006) unless the corresponding primary production is prevented from being remineralized in the underlying OMZ (Landolfi et al, 2013). Indeed, Lipschultz et al (1990) stated that N loss in the ETSP OMZ is high enough to respire all produced organic material.…”

“…This preferential P release was present in the water column and was further increased under O 2 -depleted, reduced conditions. Although the quantitative contribution to the N deficit in the ETSP is not yet entirely clear, it may act as a factor decoupling the "vicious" cycle between N 2 fixation and N loss (Landolfi et al, 2013), because it may shift the abovementioned stoichiometric constraints.…”

Microzooplankton Stoichiometric Plasticity Inferred from Modeling Mesocosm Experiments in the Peruvian Upwelling Region

“…These areas are known as oxygen minimum zones (OMZs), defined by oxygen concentrations less than 20 µmol L −1 at depths between ∼100 and 900 m (Stramma et al, 2008). OMZs strongly influence the marine biogeochemical cycles of carbon (C), nitrogen (N), and phosphorus (P) and therefore primary production (Deutsch et al, 2007;Landolfi et al, 2013). OMZs are sites of denitrification and anaerobic ammonium oxidation (anammox), the major fixed-nitrogen-loss processes in the global ocean (Helly and Levin, 2004;Galán et al, 2009).…”

Ocean dinitrogen fixation and its potential effects on ocean primary production in Earth system model simulations of anthropogenic warming

“…High P:N ratios tend to favour diazotrophs over other phytoplankton groups, which may result in strong spatial coupling between surface DNF and subsurface denitrification (Tyrrell et al, 1999; Deutsch et al., 2007;Landolfi et al, 2013). Denitrification removes DIN and potentially enhances DNF over OMZs in five of the six models (CESM, GFDL, IPSL, MPI, and UVic), all of which have P dependence of DNF.…”