The anthropogenic perturbation of the marine nitrogen cycle by atmospheric deposition: Nitrogen cycle feedbacks and the ¹⁵N Haber‐Bosch effect

Abstract

Over the last 100 years, anthropogenic emissions have led to a strong increase of atmospheric nitrogen deposition over the ocean, yet the resulting impacts and feedbacks are neither well understood nor quantified. To this end, we run a suite of simulations with the ocean component of the Community Earth System Model v1.2 forced with five scenarios of nitrogen deposition over the period from 1850 through 2100, while keeping all other forcings unchanged. Even though global oceanic net primary production increases little in response to this fertilization, the higher export and the resulting expansion of the oxygen minimum zones cause an increase in pelagic and benthic denitrification and burial by about 5%. In addition, the enhanced availability of fixed nitrogen in the surface ocean reduces global ocean N2 fixation by more than 10%. Despite the compensating effects through these negative feedbacks that eliminate by the year 2000 about 60% of the deposited nitrogen, the anthropogenic nitrogen input forced the upper ocean N budget into an imbalance of between 9 and 22 Tg N yr−1 depending on the deposition scenario. The excess nitrogen accumulates to highly detectable levels and causes in most areas a distinct negative trend in the δ15N of the oceanic fixed nitrogen pools—a trend we refer to as the 15N Haber‐Bosch effect. Changes in surface nitrate utilization and the nitrogen feedbacks induce further changes in the δ15N of NO3−, making it a good but complex recorder of the overall impact of the changes in atmospheric deposition.

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

“…The effects of atmospheric deposition have been evaluated previously using global ocean biogeochemical models [ Krishnamurthy et al ., , , ; Suntharalingam et al ., ; Yang and Gruber , ] or based on various scaling procedures by Duce et al . [].…”

“…[], Suntharalingam et al . [], Yang and Gruber [], and Krishnamurthy et al . [] have recently provided estimates of atmospheric inputs of fixed nitrogen to the world oceans and the impact of this deposition on ocean biogeochemistry.…”

Strong and Dynamic Benthic-Pelagic Coupling and Feedbacks in a Coastal Upwelling System (Peruvian Shelf)

“…The function has been coupled to Earth System Models to predict benthic denitrification in the modern ocean (e.g., Somes et al, 2013;Yang and Gruber, 2016). The function is also dynamic in the sense that N loss responds instantaneously to changes in the boundary conditions, but does not consider the storage of solutes and particulates in sediments.…”

Reviews and syntheses: Processes and functional genes involved in nitrogen cycling in marine environments

“…Indirect alterations include activities that increase the atmospheric concentration of greenhouse gases, leading to ocean warming, acidification and deoxygenation (Hutchins and Fu, 2017). The scale and impact of such anthropogenic perturbation of the marine N cycle remain highly uncertain (Gruber, 2016;Hutchins and Fu, 2017). 25 Estimated anthropogenic release of N into the global environment (~160 Tg N yr -1 ) is now of similar magnitude to all natural global ocean N 2 fixation (~140 Tg N yr -1 ), and may increase along with growing global population (Eugster and Gruber, 2012;Gruber and Galloway, 2008).…”

“…Modeling studies have shown that the predicted anthropogenic increase in atmospheric N deposition could initiate a series of strong negative feedback in the marine N cycle. There could be a pronounced decrease in global ocean N 2 fixation and an increase in denitrification, which would compensate for much of the enhanced N input 30 and limit the impact of atmospheric N deposition on marine productivity (Somes et al, 2016;Yang and Gruber, 2016 Ocean warming enhances water stratification and decreases oxygen solubility, causing a major loss of O 2 from the future ocean (Dybas, 2005). Worldwide expansion of dead zones and OMZs will have large impacts on microbially mediated N processes in marine ecosystems.…”