Stoichiometry of Limiting Elements

The earliest application of stoichiometric reasoning in ecology probably was that in the work by Justus von Liebig (1803-73), who was concerned with factors influencing crop growth yield. His studies led to what we now call 'Liebig's law of the minimum'. One way to phrase Liebig's law is that growth is controlled not by the total of all resources available, but by the one scarcest resource. This is a direct analogy to the concept of a limiting reagent in a chemical reaction producing a stoichiome-trically fixed product. 'Scarcest' is a relative term and refers to the availability of a resource in the environment relative to biological demand. For example, chemical resources needed in large quantities such as C or N might be 'scarce' in this context even if they are at higher concentration than elements such as Fe or Mo needed in much smaller quantities. Liebig's law does not apply to all possible resources. Ones that fit it most precisely are referred to as 'nonsubstitutable' resources, ones which organisms cannot trade one for another. Individual elements are largely nonsubstituable with each other whereas many (though not all) organic molecules have substitutable alternatives.

If one knows the stoichiometry of resources and consumers, one can make predictions about which resource will run out and therefore become the 'limiting reagent', regulating growth and production. We see this reasoning in such recently studied phenomena as the Southern Ocean, high-nutrient/low-chlorophyll (HNLC) region. This portion of the ocean is notable because of not only a simultaneous occurrence of low algal biomass but also high levels of the nutrient elements N and P. In most marine and freshwaters, one or both of N and P become exhausted and become limiting. However, in the HNLC regions, it is usually Fe that becomes exhausted first. Iron limitation in HNLC regions has been confirmed in small-scale bottle experiments, and in some spectacular, large-scale open-ocean fertilization experiments where Fe was introduced from a ship and the subsequent development and decay of a high algal patch was followed. Fe-limitation has also been described in near-shore oceans and even in lakes.

Liebig's law is most often thought of in terms of plant population dynamics, where individual elements form the set of possible resources. There are, however, also circumstances where Liebig's law has been applied to animal growth. Liebigian dynamics are strongly suggested, for instance, in the section below on stoichiometry and animal growth.

The stoichiometry of limiting factors varies considerably in space and time. Over long timescales associated with soil development during primary succession on a mineral soil, the N:P balance shifts because P is present in the original parent material, but N derives largely from biological processes including N-fixation. Thus, a new mineral soil will contain P but almost no N, and N will be limiting to plants growing on that site. N-fixers are favored. As N in the soil builds up over successive production/decay cycles, N comes into approximate balance with P. Over long timescales, highly weathered soils can have most of their P removed or bound into inaccessible forms, and P can become limiting. Soil development fitting this general pattern has been observed on the Hawaiian islands. Ancient tropical soils are often highly deficient in P, and there is a general pattern of an increased N:P in the foliage of coniferous trees, grasses, herbs, shrubs, and trees as one goes from the poles to the tropics.

When one seeks to apply Liebig's law in any individual case, complications often arise. For example, different species of an assemblage may be limited by different nutrients simultaneously, meaning multiple resources might limit a community. This form of multiple resource limitation is very well studied in theoretical models and has been the foundation of some recent elegant experimental studies as well; theory and experiment together suggest that the number of elements that are simultaneously limiting to different species has a large influence on the biodiversity of a community. Where multiple resources become limiting, biodiversity is higher. In this form of multiple-element limitation, one might potentially see the community as a whole respond to single additions of more than one resource. The largest response is expected where multiple resources are added, because this will stimulate the greatest number of species. In other cases, due to biochemical reasons, availability of one element might aid in the acquisition of another element. For example, the enzyme that allows cells to make use of organically bound P contains Zn; thus, potentially adding or subtracting Zn might be functionally similar to adding or subtracting P. Ecologists today do not have a comprehensive, widely accepted terminology for dealing with cases of multiple-nutrient limitation.

Liebig's law appears today in work referred to as resource competition theory (RCT). RCT provides a predictive, mechanistic framework for studying how community structure relates to resource availability. Good competitors for a resource are defined as those that require relatively small amounts of a resource in order to maintain themselves in a community in the face of mortality losses. Stochiometrically, good competitors often are those that themselves have low nutrient contents or high C:P or C:N ratios. However, both differential resistance to mortality and differing ability to acquire nutrients can also play strong roles in determining competitive ability.

Various tests of this theory have been performed and support for RCT predictions have come from the laboratory and field in experiments involving microorganisms (bacteria, cyanobacteria, algae) and vascular plants and even metazoan animals. For example, resource conditions with high Si:P ratios tend to favor diatoms (which have relatively low P-requirements but require silica), while other algal taxa (which have no Si-requirement) dominate over diatoms when Si:P ratios are low. Coexistence is predicted, and observed, when Si:P ratios are intermediate; in such a situation, the diatoms are limited by Si while its competitor would be limited by P. Similarly, the relative abundance of cyanobacteria (and especially N-fixing cyanobacteria) is often higher when environmental nutrient supplies occur at low N:P ratios. Recent studies have also shown that the nonlinear resource utilization functions of species allow for multi-species coexistence due to the complex, and perhaps chaotic, dynamics that ensue when multiple resources (light, nutrient elements) can be limiting to multiple species ('coexistence by chaos').

Nutrient limitation by N or P interacts with the carbon cycle in interesting ways. Ecosystems that are strongly nutrient limited are often observed to have primary producers with elevated C:N or C:P ratios. Those nutrient-limited ecosystems can be said to have high nutrient-use efficiency. They make much biomass with each unit of nutrient acquired. As discussed above, in some systems it is the overall availability of light relative to nutrients that controls C:N or C:P ratios. No matter which terminology or thought process is used to study these relationships, the functional outcome is that growth rate and stoichiometry are related in nonhomeostatic organisms such as autotrophs.

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