Biological fluxes dominate the global nitrogen cycle

Nitrogen (N) plays a key role in biological processes as a constituent of proteins and enzymes, and it is one of the resources that most commonly limits primary production, as we saw in Concept 20.2.

The largest pool of N (>90%) is atmospheric dinitrogen gas (N₂) (FIGURE

25.7). This form of N is very stable chemically and cannot be used by most organisms, with the important exception of nitrogen-fixing bacteria, which are able to convert it to more chemically usable forms, as described in Concept 22.1. These fixed chemical compounds are referred to as reactive N because, unlike

N₂, they can participate in chemical reactions in the atmosphere, soils, and water. Terrestrial N₂ fixation by bacteria provides approximately 128 teragrams (1 Tg = 10¹² g) of reactive N per year (Cleveland et al. 1999; Galloway et al. 2004) and supplies 12% of the annual biological demand (Schlesinger and Bernhardt 2020). The remaining 88% is met by uptake of N from the soil in forms released by decomposition. Oceanic N₂ fixation contributes another 120 Tg to the biosphere annually. Geologic pools associated with sediments containing organic matter represent a much smaller fraction of global N than of global C, but some N-rich sedimentary sources may be important in some sites (Morford et al. 2016).

FIGURE 25.7 TheGlobalNitrogenCycle Boxes represent major pools of N, measured in teragrams (1 Tg = 10¹² g). Arrows represent major fluxes of N, measured in teragrams per year; anthropogenic fluxes are shown in orange. The percentage of the total atmospheric N pool made

up of reactive N is minuscule (it is also difficult to quantify because it is very dynamic).

Given its small size, why is the reactive pool of N of such great interest?

(After W. H. Schlesinger and E.S. Bernhardt. 2013. Biogeochemistry: AnAnalysis of Global Change, 1st and 3rd eds. Academic Press: Cambridge, MA; F. S. Chapin et al. 2002. Principles of Terrestrial Ecosystem Ecology. Springer-Verlag: New York. Data from various sources including C. Cleveland et al. 1999. Global Biogeochem Cycles 13: 623-645; J. N. Galloway et al. 2004. Biogeochemistry 70: 153-226.) View larger image

Although the pools of N at land and ocean surfaces are relatively small, they are very active biologically, and they are held tightly by internal ecosystem cycling processes. Fluxes from these pools are small relative to the rates of internal cycling, usually less than 10% (Chapin et al. 2002). The natural flux of N between terrestrial and oceanic pools that occurs via rivers is tiny, but it plays an important biological role by enhancing primary production in estuaries and salt marshes. Denitrification, a microbial process that occurs in anoxic soils and in the ocean (described in Concept 22.2), results in movement of N (as N₂ and as N₂O, a greenhouse gas, also known as laughing gas) from terrestrial and marine ecosystems into the atmosphere. Oceanic and terrestrial ecosystems also lose N through burial of organic matter in sediments and through burning of biomass.

Human activities have altered the global N cycle tremendously—even more than they have altered the global C cycle. Anthropogenic fluxes are now the dominant components of the N cycle (Galloway et al. 2004; Canfield et al. 2010) (FIGURE 25.8). The rate of fixation of atmospheric N₂ by humans now exceeds the rate of natural terrestrial biological fixation. Emissions of N associated with industrial and agricultural activities are causing widespread environmental changes, including acid precipitation, as we'll see in Concept 25.3. Three major processes account for these anthropogenic effects.

FIGURE 25.8 Changes in Anthropogenic Fluxes in the Global Nitrogen Cycle Increases

in fertilizer production through the Haber-Bosch process, the growing of nitrogen-fixing crops, and combustion of fossil fuels have all contributed to the tremendous increase in biologically available (reactive) N. (After J. N. Galloway et al. 2004. Biogeochemistry 70: 153-226.) View larger image