Nutrient Cycling Flux and Dynamics

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The rate of cycling of potentially limiting elements is a critical feature of ecosystems, determining important aspects of their structure and function. This cycling includes movement of elements between abiotic and bio-tic pools as well as among living species. Autotrophs take up resources from their environment, and they may also leak some nutrients back into the surrounding soil or water media; they also produce litter which is broken down, and decays and releases nutrients into abiotic pools. Heterotrophic consumers participate in nutrient cycling when they ingest food and release wastes back to the abiotic pool. If we understand the controls on the rate that nutrients are made available or reavailable for living organisms to take up, we will have a powerful tool to understand and predict many features of ecosystems. Stoichiometric approaches to nutrient cycling are based on the conservation of matter and consider patterns among multiple elements.

Under strict homeostasis, we consider individual species to have fixed stoichiometric coefficients in their chemical makeup. Thus, the chemical flexibility needed to balance a reaction where resources are reactants and organism biomass is a product comes from the composition of waste products, another product in the reaction:

Resources ! Organism biomass + Wastes

If we know both the chemical content of organisms and the chemical content of their resources, we can use the conservation of matter to calculate the chemical composition of wastes. Note that variation in the chemical content of either resources or organism biomass can influence the chemical content of wastes.

One aspect of stoichiometry is merely the use of a set of tools for balancing chemical reactions involved in nutrient cycling and maintaining appropriate relationships among all the elements. However, stoichiometric homeostasis also has a characteristic and somewhat peculiar effect on nutrient cycling. A homeostatic consumer alters the fraction of ingested nutrients that are retained for growth in response to the chemical content of the food. A homeostatic consumer must generally retain most stringently the scarcest element. For example, when ingesting food of relatively low P-content, a homeostatic consumer must retain P with elevated efficiency. In other words, it must retain that substance which already is in scarce supply in the ecosystem while it resupplies the nonlimit-ing elements back to abiotic pools (Figure 3). Nutrients that are found in relative excess tend to be recycled back

10 20 N:P of the food

Figure 3 The ratio of nutrient elements recycling by a feeding homeostatic consumer as a function of the ratios of elements in its food. At low N:P in the food, the homeostatic consumer retains N with high efficiency, recycling excess P, and hence the N:P recycled is very low. In contrast, at high N:P in the food, the homeostatic consumer retains P with high efficiency, releases relatively more N, and thus the N:P recycled is very high. Homeostasis in the consumer causes this relationship to curve and the greater the ability of the consumer to retain the element limiting its own growth, the tighter the bend in the function.

10 20 N:P of the food

Figure 3 The ratio of nutrient elements recycling by a feeding homeostatic consumer as a function of the ratios of elements in its food. At low N:P in the food, the homeostatic consumer retains N with high efficiency, recycling excess P, and hence the N:P recycled is very low. In contrast, at high N:P in the food, the homeostatic consumer retains P with high efficiency, releases relatively more N, and thus the N:P recycled is very high. Homeostasis in the consumer causes this relationship to curve and the greater the ability of the consumer to retain the element limiting its own growth, the tighter the bend in the function.

to the environment while nutrients that are scarce in the food relative to a consumer's requirements are retained for growth. Thus, when put into the context of whole ecosystems, interesting dynamics result from homeostatic consumers.

These processes have been measured in many different situations. Figure 4 shows the effect of chemical variation of both resources and consumers in one set of aquatic organisms. Because natural ecosystems are composed of numerous species all with their own characteristic stoichiometry, natural food webs consist of many different resource-consumer pairs. Some of these will be stoichiometrically similar while others may be stoichiometrically dissimilar.

1000

20 30 Food N:P (molar)

1.75

log body N:P (molar)

log body N:P (molar)

1.75

Figure 4 Two aspects of stoichiometric determination of nutrient cycling, the nutrient content of the resources (a) and the nutrient content of the consumers (b). Both examples consider homeostatic animal consumers feeding upon chemically variable resources. In (a), the N:P ratio of nutrients released by freshwater zooplankton is positively related to the N:P ratio of the food they eat. This example shows that when consuming foods relatively high in either N or P, homeostatic consumers dispose of the excess nutrient and retain the nutrient most limiting to their own needs. Note the log scale of the y-axis, meaning a curvilinear function on linear axes. In (b), the stoichiometric variability of different aquatic vertebrate consumers generates a negative relationship between the N:P ratios of nutrients released and the N:P ratio of the consumers themselves. This example shows that consumers with bodies containing either relatively high N or P must retain that element high in their biomass and further that they will dispose of the element relatively scarce in their own biomass.

We also see a strong stoichiometric control on nutrient recycling in terrestrial systems. Leaves exhibit a wide range in C:N:P ratios, which is largely a function of the species or functional group of the plant, but as we have already seen it also depends on growth conditions and other factors. When leaves die, there is a corresponding wide range in nutrient content of the detritus they form. Due to nutrient resorption prior to leaf abscission, detritus often has a very low N- and P-content, a factor which emphasizes the stoichiometric dissimilarity between resources (detritus) and living consumers (microbial decomposers and detritivorous animals). Ecological stoichiometry is most easily revealed in systems where there is great chemical variability, like in this consideration of detritus and detritivore. Figure 5 shows how strongly the nutrient content of detritus corresponds to the rate that detritus breaks down, at least at this large, cross-ecosystem scale.

This range in mineralization rate results from the stoichiometric match between the chemical composition of the litter and the needs of the organisms feeding on that litter (microorganisms, detritivores), so it combines stoi-chiometric food-quality effects, such as will be described below, with stoichiometric nutrient cycling rates. The highly biodiverse soil food web can grow and metabolize more rapidly when supported by high-nutrient litter instead of low-nutrient litter.

Another example of stoichiometric recycling effects involving plants, soils, and decomposers involves Ca-content. In a long-term (30-year) study where 14 different tree species were raised in monoculture, it was found that soil properties came to strongly depend on the Ca-content of the leaves of the tree species growing on that plot. Soil under tree species with high-Ca leaves, roots, and litter (e.g., maples, basswoods) was higher in pH, higher in C, lower in C:N, and higher in exchangeable Ca than was soil under tree species low in Ca (e.g., oaks, pines). Earthworm biomass also was higher under high-Ca tree species. The chemical parameter Ca was better related statistically to these and other similar relationships than was identity of the tree species themselves or the identity of functional groups like angiosperms versus gym-nosperms, suggesting that it was indeed the stoichiometry of Ca in the litter that was the key factor involved.

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