Nitrogen cascading through inland waters

Nitrogen is an essential nutrient to organisms and has prehistorically been limiting biomass and production. However, in the Anthropocene biologically available (aka fixed or reactive) nitrogen has increased because of combustion of fossil fuels, cultivation of crops that promote biological nitrogen fixation and the Haber-Bosch process that allows industrial-scale nitrogen fixation. This increased fixed nitrogen production has caused accumulation of nitrogen in Earth’s compartments because loss processes (denitrification, Anammox, sediment burial) could not keep pace. This additional nitrogen sometimes has beneficial effects (low-level fertilization increases production of nitrogen-limited ecosystems, e.g. stimulating carbon storage by some ecosystems) but usually the excess nitrogen flows downstream, i.e. along the aquatic continuum from soil and vegetations to inland waters (groundwater, streams, rivers, lakes and reservoirs), and eventually to the sea. During this transfer from soil to sea, reactive nitrogen molecules are involved in multiple processes and impact not only the functioning of the receiving water bodies but also their downstream ecosystems. Galloways’ nitrogen cascading concept articulates the linkages of upstream and downstream ecosystem functioning via nitrogen flows. In other words, upstream soil nutrient management can have implications for the occurrence, spatial extent and frequency of harmful algae or hypoxia in coastal systems.
A first-generation model for nitrogen flow from soil to sea
 The nitrogen cascade has stimulated much research to quantify global inland-water nitrogen budgets using a wide range of approaches. About 10 years ago, our group developed IMAGE-GNM (Integrated Model to Assess the Global Environment-Global Nutrient Model; Beusen et al., 2015) which provided spatially and temporally resolved data on nitrogen and phosphorus supply, retention and export at the global scale while resolving groundwater, streams, rivers, lakes and reservoirs. Our studies revealed that nitrogen and phosphorus supply to inland waters increased, and that export of nitrogen and phosphorus increased as well, but less than expected because of increased retention within inland waters. A recent application published in Nature Sustainability (Liu et al., 2024) documented a strong legacy of groundwater nitrogen on downstream water quality. Although this first-generation model accurately quantifies global river fluxes of total nitrogen, it does not address nitrogen speciation (ammonium, nitrate, organic nitrogen) and uses a spiraling approach for nutrient removal rather than resolving the biological processes involved (primary production, respiration, denitrification, nitrification).
A next-generation nitrogen model for inland waters
In the August issue of Nature Water, Wang et al. (2024) present changes in the overall global inland-water nitrogen budget based on simulation with the IMAGE-Dynamic Global Nutrient Model, which resolves nitrogen transformations processes (nitrification, denitrification, assimilation, regeneration), sediment-water interactions and interactions with other biogeochemical cycles such as that of oxygen. The model was validated not only with available concentration and discharge data, but also with process rates. Simulations with this next-generation model confirmed not only the increase in nitrogen delivery to and export from inland water to the ocean despite increasing retention, but crucially also showed that nitrogen cycling intensified. While nitrogen delivery increased by a factor of 2.5, organic nitrogen mineralization, nitrification and denitrification increased 3, 4 and 6 times, respectively, from 1900 to 2010. One major consequence of this accelerated nitrogen recycling is the steadily increasing production and emission of nitrous oxide, a climate-active gas (Wang et al., 2023).
 
Figure showing nitrate cycling in global inland waters: Nitrification and external nitrate inputs are balanced by export to the ocean, assimilation by primary producers and denitrification to nitrogen gas.
 
Inland-water nitrate cycling has accelerated
The global inland water nitrate budget clearly shows the accelerated recycling of nitrogen This active recycling of nitrogen implies that considering inland waters as a leaky pipe transporting and retaining part of the nitrogen might need revision. Nitrate delivery to inland water and nitrate export to the ocean are presently similar (22 Tg N yr-1). Using Occam’s razor, one might conclude that inland waters act as a passive pipe transporting nitrate from soil to sea. However, one should realize that about 2.5 times more nitrate is produced in inland water (53 Tg N yr-1) than delivered and exported. Moreover, each year denitrification removes 16 Tg N yr-1 and another 36 Tg N yr-1 is assimilated by primary producers, a fraction of which is eventually buried. Accordingly, modelling approaches that scale nitrate removal or nitrous oxide production to external nitrate delivery require revision. More general, our study revealed that inputs and exports of total nitrogen or specific nitrogen species do not follow a simple relationship because of the tight linkages between organic nitrogen, ammonium and nitrate with each other and with other perturbed biogeochemical cycles in inland waters.
 
For more detailed information and access to the code and data, please see https://doi.org/10.1038/s44221-024-00282-x
Wang, J., Bouwman, A.F., Vilmin, L., Beusen, A.H.W., van Hoek, W.J., Liu, X., Middelburg, J.J. (2024). Global inland-water nitrogen cycling has accelerated in the Anthropocene. Nature Water, https://doi.org/10.1038/s44221-024-00282-x
 
Authors of the blog:
Jack J. Middelburg and Junjie Wang, Utrecht University