On the influence of “non‐Redfield” dissolved organic nutrient dynamics on the spatial distribution of N2 fixation and the size of the marine fixed nitrogen inventory

Abstract

AbstractDissolved organic nitrogen (DON) and phosphorus (DOP) represent the most abundant form of their respective nutrient pool in the surface layer of the oligotrophic oceans and play an important role in nutrient cycling and productivity. Since DOP is generally more labile than DON, it provides additional P that may stimulate growth of nitrogen‐fixing diazotrophs that supply fixed nitrogen to balance denitrification in the ocean. In this study, we introduce semirecalcitrant components of DON and DOP as state variables in an existing global ocean‐atmosphere‐sea ice‐biogeochemistry model of intermediate complexity to assess their impact on the spatial distribution of nitrogen fixation and the size of the marine fixed nitrogen inventory. Large‐scale surface data sets of global DON and Atlantic Ocean DOP are used to constrain the model. Our simulations suggest that both preferential DOP remineralization and phytoplankton DOP uptake are important “non‐Redfield” processes (i.e., deviate from molar N:P = 16) that need to be accounted for to explain the observed patterns of DOP. Additional non‐Redfield DOP sensitivity experiments testing dissolved organic matter (DOM) production rate uncertainties that best reproduce the observed spatial patterns of DON and DOP stimulate additional nitrogen fixation that increases the size of the global marine fixed nitrogen inventory by 4.7 ± 1.7% compared to the simulation assuming Redfield DOM stoichiometry that underestimates the observed nitrogen inventory. The extra 8 Tg yr−1 of nitrogen fixation stimulated in the Atlantic Ocean is mainly responsible for this increase due to its large spatial separation from water column denitrification, which buffers any potential nitrogen surplus in the Pacific Ocean. Our study suggests that the marine fixed nitrogen budget is sensitive to non‐Redfield DOP dynamics because access to the relatively labile DOP pool expands the ecological niche for nitrogen‐fixing diazotrophs.