# Material cycle

In ecology, a cycle of materials is a periodic transformation of chemical compounds , in the course of which - after a series of chemical reactions - the starting material is created again. There are various material cycles in ecosystems , for example a carbon cycle , a nitrogen cycle , a sulfur cycle and a phosphorus cycle .

## Factors

Living things need a number of chemical elements and their compounds for the structure of organic tissues and the metabolism. The supply and replenishment of these nutrients, like the supply of energy, can limit the production of an ecosystem . The production of many terrestrial systems is limited by a lack of nitrogen , in aquatic systems phosphorus is often limiting, in marine systems a lack of iron can limit productivity. The nutrient elements can be replenished from outside the ecosystem, depending on the element through weathering of rock, through water transport or through supply from the atmosphere. However, large quantities of the necessary nutritional elements are exchanged within the system. This allows the same nutrients to be used multiple times within the system, thereby maintaining its productivity. Through these internal material cycles, consumers and destructors can indirectly control the productivity of the ecosystem.

A nutrient cycle within an ecosystem can only enter into elements that (in addition to being stored in the organisms themselves) have inorganic storage. Gaseous compounds in the atmosphere, such as carbon dioxide, cannot build up a cycle in the system; they are exchanged with the entire reservoir of the atmosphere. Substances dissolved as ions can be retained in the soil matrix or in the water body of a body of water and reabsorbed. Since the earth is an (almost) closed system for substances, all nutrient cycles must inevitably be closed on a global level. This does not necessarily apply within the biosphere , which is only a subsystem. Exchange processes with deeper layers such as the Earth's mantle are small in the short term compared to sales within the biosphere, but can have enormous effects on evolutionary and geological time scales.

For the consideration of the material cycles, in addition to the cyclical processes themselves, the material exchange with abiotic stores, especially the earth's atmosphere and the soils and sediments, is important.

## cause

Material cycle triangle
Extended material cycle diagram

Biological material cycles are driven by living things . Living beings that build organic matter from inorganic substances are known as producers . Besides some photo- or chemoautotrophic bacteria, these are exclusively plants. The biomass formed in this way is mineralized back into inorganic substances by destructors (decomposers). Destructors are predominantly heterotrophic bacteria and fungi . The simplest conceivable ecosystem therefore consists of a producer and a destructor.

In addition to these groups, almost all ecosystems also have consumers , who use organic substances to generate energy. Consumers partly use the ingested biomass to build up their own body tissue, the rest is given off in the feces and other waste materials. The consumption rate, i.e. H. In most terrestrial ecosystems, the proportion of primary production consumed by herbivores (primary consumers) is in the order of 10%; in exceptional cases it can be higher (up to 90%), e.g. B. in savannah ecosystems grazed by ungulates. The rest of the production goes directly into the destructive food chain (as plant litter, in the forest e.g. as dead wood and fallen leaves). Even with low sales shares, the indirect influence of consumers on the substance flows can be considerable.

Ultimately, consumer biomass will also be mineralized by destructors. Via the food web, the substances get from the producers partly to the consumers and everything finally to the destructors. Consumers accelerate the turnover of substances and thus the cycles in relation to the destructive chain.

Since many animals migrate, they also transport and distribute the nutrients. This leads to a networking of the material cycles of different ecosystems. The displacement of nutrients by animals is known as translocation .

## Effects

In many near-natural terrestrial ecosystems, the material cycles are largely closed, the input and output of substances are low, with the exception of carbon. Through effective storage, ecosystems can accumulate nutrients that represent deficiency factors such as nitrogen and phosphorus from the small supplies from outside and thus increase the productivity of the system enormously. The destruction of the memory such. B. the biomass or the humus reserves of the soil can thereby drastically reduce productivity and destroy the system temporarily or permanently. People strive to increase the productivity of the agro-ecosystems they maintain by adding nutrients as fertilizer. These substances, which are supplied in large quantities, can be lost to the system not only as desired through the goods produced, but also through leaching, erosion or outgassing. The cycles of the elements involved, such as B. strongly influenced carbon and nitrogen. Natural ecosystems are changed by the resulting supplies, which can influence their stability. The impact is also significant at a global level. For example, the nitrogen supply to the entire terrestrial biosphere has roughly doubled due to human influences.

## Mathematical description

Referred the matter content at the level of the producers, those at the level of consumers and those at the level of decomposers or freely available shares, shows a very simple material cycle following form: ${\ displaystyle P}$${\ displaystyle K}$${\ displaystyle D}$

${\ displaystyle {\ frac {\ mathrm {d} P} {\ mathrm {d} t}} = aP-bPK}$
${\ displaystyle {\ frac {\ mathrm {d} K} {\ mathrm {d} t}} = bPK-cK}$
${\ displaystyle {\ frac {\ mathrm {d} D} {\ mathrm {d} t}} = cK-aP}$

For reasons of symmetry:

${\ displaystyle {\ frac {\ mathrm {d} P} {\ mathrm {d} t}} + {\ frac {\ mathrm {d} K} {\ mathrm {d} t}} + {\ frac {\ mathrm {d} D} {\ mathrm {d} t}} = {\ text {const.}}}$

Taking advantage of this relationship, it is possible to substitute what leads to and for a system with only two variables. ${\ displaystyle D}$${\ displaystyle P}$${\ displaystyle K}$

## swell

1. James F. Kitchell et al. : Consumer regulation of nutrient cycling. In: BioScience , 29 (1), 1979, pp. 28-34.
2. Stephen R. Carpenter, James F. Kitchell, James R. Hodgson: Cascading trophic interactions and lake productivity. Fish predation and herbivory can regulate lake ecosystems. In: BioScience, 35 (10), 1985, pp. 634-639.
3. SJ McNaughton, FF Banyikwa, MM McNaughton: Promotion of the Cycling of Diet-Enhancing Nutrients by African grazers. In: Science, New Series, 278 (5344), 1997, pp. 1798-1800.
4. John Pastor, Robert J. Naiman: Selective Foraging and Ecosystem Processes in Boreal Forests. In: The American Naturalist , 139 (4), 1992, pp. 690-705.
5. ^ GE Likens, FH Bormann, RS Pierce, WA Reiners: Recovery of a deforested ecosystem. In: Science, 199, 1978, pp. 492-496.
6. Peter M. Vitousek et al. : Human alteration of the global nitrogen cycle: Sources an consequences. In: Ecological applications , 7 (3), 1997, pp. 737-750. doi : 10.1890 / 1051-0761 (1997) 007 [0737: HAOTGN] 2.0.CO; 2