Nitrogen cycling in inland waters accelerated three to six times in the 20th century

According to a new publication, biogeochemical processes in global inland waters that break down, transform, or bury nitrogen in sediment are occurring three to six times faster compared to 110 years ago. This can be attributed to a warmer climate, dam building and the frequently increased use of fertilisers in the 20th century. These results allow a more accurate assessment of the impact of environmental policy on nitrogen reduction. The paper was published last month in Nature Water.
Nitrogen is an essential nutrient to organisms and has prehistorically been limiting biomass and production. However, biologically available (a.k.a. fixed or reactive) nitrogen has increased because of numerous causes, including (among others) combustion of fossil fuels, cultivation of crops that promote biological nitrogen, and industrial-scale fertiliser production and nitrogen fixation (the famous Haber-Bosch process). 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. Most of 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. This means that upstream soil nutrient management can have implications for the occurrence, spatial extent and frequency of harmful algal blooms 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, the same research 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. These 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
This paper presents changes in the overall global inland-water nitrogen budget based on simulation with the IMAGE-Dynamic Global Nutrient Model, which resolves nitrogen transformation 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).
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. 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, this 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.
Publication
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