7.3: Nutrient stoichiometry

Stoichiometry essentially means ratio. Thus, nutrient stoichiometry simply refers to the ratios or proportions of nutrients in an environment. Nutrient stoichiometry is a major control on structure and function of ecosystems (Cleveland and Liptzin, 2007; Schlesinger and Bernhardt, 2013). A microorganism’s nutrient demand is determined by the proportions of nutrients in its biomass relative to the availability of those nutrients in the environment (Sinsabaugh et al., 2009). If one nutrient is not available in sufficient supply relative to others required by the organism, it may be a growth limiting factor. For example, if an organism’s biomass has a \(\text{N}:\text{P}\) ratio of \(16:1\), but the environment’s \(\text{N}:\text{P}\) ratio is \(32:1\), then phosphorus supply may limit that organism’s growth.

As an example of this relationship, in marine environments, biological productivity is often limited by nitrogen, which is supplied by nitrogen fixation. However, rates of nitrogen fixation are limited by the availability of iron, a critical ingredient needed to make nitrogenase, the enzyme that organisms use to fix nitrogen (Section 5.2.1). As a result, over large areas of the ocean, iron that blows in from the land controls marine productivity. In general, productivity can sometimes be increased by adding limiting nutrient itself or some trace element that helps the organism obtain the limiting nutrient from environment (Schlesinger and Bernhardt, 2013).

In addition to overall productivity, nutrient stoichiometry can influence which species are able to grow because different species have different biomass stoichiometries. Moreover, different species may vary in their preferred forms of nutrients. Some groups may prefer nitrogen delivered as ammonium, for example, whereas others prefer nitrate. As an example study, Andersen et al. (2020) examined impacts to algal communities of seasonal variation in \(\text{N}:\text{P}\) ratios and nitrogen form (nitrate vs ammonium) in a hyper-eutrophic reservoir. Their results showed that algal growth in the reservoir was limited by phosphorus in the spring fall, but nitrogen was limiting during the summer. During periods of phosphorus limitation, the relative abundances of chlorophytes and diatoms increased within the algal community. However, during periods of nitrogen limitation, the proportion of cyanobacteria in the community increased. Moreover, while nitrogen limited, study results indicated that chlorophytes and cyanobacteria were more strongly stimulated by ammonium addition whereas diatoms were more strongly stimulated by nitrate addition.

Changes in nutrient proportions in an environment can occur in response to natural processes and human activities. As an example of natural processes, widespread weathering of pyrite and other sulfide minerals has been proposed to have increased fluxes of phosphorus, iron, and sulfur to the oceans between about \(2.22\) and \(2.06 \ \text{Ga}\) (Bekker and Holland, 2012). According to the hypothesis, increased availability of these nutrients stimulated growth of cyanobacteria, which led to anomalously high burial of organic matter in marine sediments. Similarly, nutrients supplied by windblown volcanic ash inputs have also been proposed as a mechanism for stimulating marine biological productivity and organic matter burial during the Cretaceous (Lee et al., 2018). An example of human impacts to nutrient availability is the massive application of fertilizer to crop areas, much of which makes its way to the oceans (Gruber and Galloway, 2008).

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