A nutrient cycle (or ecological recycling) is the movement and exchange of inorganic and organic matter back into the production of matter. Energy flow is a unidirectional and noncyclic pathway, whereas the movement of mineral nutrients is cyclic. Mineral cycles include the carbon cycle, sulfur cycle, nitrogen cycle, water cycle, phosphorus cycle, oxygen cycle, among others that continually recycle along with other mineral nutrients into productive ecological nutrition.
Overview
The nutrient cycle is nature's recycling system. All forms of recycling have feedback loops that use energy in the process of putting material resources back into use. Recycling in ecology is regulated to a large extent during the process of decomposition. Ecosystems employ biodiversity in the food webs that recycle natural materials, such as mineral nutrients, which includes water. Recycling in natural systems is one of the many ecosystem services that sustain and contribute to the well-being of human societies.
There is much overlap between the terms for the biogeochemical cycle and nutrient cycle. Most textbooks integrate the two and seem to treat them as synonymous terms. However, the terms often appear independently. The nutrient cycle is more often used in direct reference to the idea of an intra-system cycle, where an ecosystem functions as a unit. From a practical point, it does not make sense to assess a terrestrial ecosystem by considering the full column of air above it as well as the great depths of Earth below it. While an ecosystem often has no clear boundary, as a working model it is practical to consider the functional community where the bulk of matter and energy transfer occurs. Nutrient cycling occurs in ecosystems that participate in the "larger biogeochemical cycles of the earth through a system of inputs and outputs."
Complete and closed loop
Ecosystems are capable of complete recycling. Complete recycling means that 100% of the waste material can be reconstituted indefinitely. This idea was captured by Howard T. Odum when he penned that "it is thoroughly demonstrated by ecological systems and geological systems that all the chemical elements and many organic substances can be accumulated by living systems from background crustal or oceanic concentrations without limit as to concentration so long as there is available solar or another source of potential energy" In 1979 Nicholas Georgescu-Roegen proposed the fourth law of entropy stating that complete recycling is impossible. Despite Georgescu-Roegen's extensive intellectual contributions to the science of ecological economics, the fourth law has been rejected in line with observations of ecological recycling. However, some authors state that complete recycling is impossible for technological waste.
Ecosystems execute closed loop recycling where demand for the nutrients that adds to the growth of biomass exceeds supply within that system. There are regional and spatial differences in the rates of growth and exchange of materials, where some ecosystems may be in nutrient debt (sinks) where others will have extra supply (sources). These differences relate to climate, topography, and geological history leaving behind different sources of parent material. In terms of a food web, a cycle or loop is defined as "a directed sequence of one or more links starting from, and ending at, the same species." An example of this is the microbial food web in the ocean, where "bacteria are exploited, and controlled, by protozoa, including heterotrophic microflagellates which are in turn exploited by ciliates. This grazing activity is accompanied by excretion of substances which are in turn used by the bacteria so that the system more or less operates in a closed circuit."
Ecological recycling
An example of ecological recycling occurs in the enzymatic digestion of cellulose. "Cellulose, one of the most abundant organic compounds on Earth, is the major polysaccharide in plants where it is part of the cell walls. Cellulose-degrading enzymes participate in the natural, ecological recycling of plant material." Different ecosystems can vary in their recycling rates of litter, which creates a complex feedback on factors such as the competitive dominance of certain plant species. Different rates and patterns of ecological recycling leave a legacy of environmental effects with implications for the future evolution of ecosystems.
Ecological recycling is common in organic farming, where nutrient management is fundamentally different compared to agri-business styles of soil management. Organic farms that employ ecosystem recycling to a greater extent support more species (increased levels of biodiversity) and have a different food web structure. Organic agricultural ecosystems rely on the services of biodiversity for the recycling of nutrients through soils instead of relying on the supplementation of synthetic fertilizers.
The model for ecological recycling agriculture adheres to the following principles: Protection of biodiversity. Use of renewable energy. Recycling of plant nutrients.
Where produce from an organic farm leaves the farm gate for the market the system becomes an open cycle and nutrients may need to be replaced through alternative methods.
Influence of trophic interactions on nutrient cycling in freshwater systems Trophic interactions also play a crucial role in regulating nutrient cycling, especially in freshwater systems. They not only alter how nutrients, such as nitrogen (N) and phosphorus (P), are distributed and the rate at which they are recycled in these systems, but also can determine which species are most dominant by doing so. Predators can indirectly influence nutrient availability through top-down control of herbivores, thereby affecting primary production and nutrient uptake. One of the more important mechanisms linking foodwebs to nutrient cycling is the trophic relationship of planktovorous fish, zooplankton, and phytoplankton. When zooplankton populations are high, they consume phytoplankton at a greater rate, thereby limiting the incorporation of nitrogen and phosphorus into organic matter. As a consequence, inorganic forms of nitrogen, such as ammonium (NH₄⁺) and dissolved phosphorus in the form of phosphate (PO₄³⁻), are kept at a higher concentration in the water column. Under these conditions, organisms such as nitrogen-fixing cyanobacteria will be less competitive since inorganic nitrogen is already abundant, favoring species that depend on direct uptake of ammonium or nitrate. Meanwhile, increased inorganic phosphorus in the water favors those plankton species that are more efficient at uptaking this limiting nutrient. If fewer predators are feeding on zooplankton in the system, species such as Daphnia will have a competitive advantage because of their larger size and higher phosphorus requirement for growth. In contrast, if the zooplankton population is regulated by an increase in planktovorous fish numbers, phytoplankton can more easily proliferate and assimilate large amounts of nitrogen and phosphorus into their biomass, reducing the availability of these nutrients in their bioavailable forms. Consumers such as zooplankton and fish also excrete ammonium as a waste product and phosphate during metabolic processes. These can be directly taken in by primary producers, making this consumer-driven internal cycling of nutrients another important source of the redistribution of nutrients in freshwater systems.
Ecosystem engineers
The persistent legacy of environmental feedback that is left behind by or as an extension of the ecological actions of organisms is known as niche construction or ecosystem engineering. Many species leave an effect even after their death, such as coral skeletons or the extensive habitat modifications to a wetland by a beaver, whose components are recycled and re-used by descendants and other species living under a different selective regime through the feedback and agency of these legacy effects. Ecosystem engineers can influence nutrient cycling efficiency rates through their actions.
Earthworms, for example, passively and mechanically alter the nature of soil environments. The bodies of dead worms passively contribute mineral nutrients to the soil. The worms also mechanically modify the physical structure of the soil as they crawl about (bioturbation) and digest on the molds of organic matter they pull from the soil litter. These activities transport nutrients into the mineral layers of soil. Worms discard wastes that create worm castings containing undigested materials where bacteria and other decomposers gain access to the nutrients. The earthworm is employed in this process and the production of the ecosystem depends on their capability to create feedback loops in the recycling process.
Shellfish are also ecosystem engineers because they: 1) Filter suspended particles from the water column; 2) Remove excess nutrients from coastal bays through denitrification; 3) Serve as natural coastal buffers, absorbing wave energy and reducing erosion from boat wakes, sea level rise and storms; 4) Provide nursery habitat for fish that are valuable to coastal economies.
Fungi contribute to nutrient cycling and nutritionally rearrange patches of ecosystem creating niches for other organisms. In that way, fungi growing on dead wood allow xylophages to grow and develop, and in turn, affect dead wood, contributing to wood decomposition and nutrient cycling in the forest floor.
History
Nutrient cycling has a historical foothold in the writings of Charles Darwin, particularly in reference to the decomposition actions of earthworms. Darwin wrote about "the continued movement of the particles of earth". Even earlier, in 1749, Carl Linnaeus wrote: "the economy of nature we understand the all-wise disposition of the creator in relation to natural things, by which they are fitted to produce general ends, and reciprocal uses", in reference to the balance of nature in his book Oeconomia Naturae. In this book, he captured the notion of ecological recycling: "The 'reciprocal uses' are the key to the whole idea, for 'the death, and destruction of one thing should always be subservient to the restitution of another;' thus mould spurs the decay of dead plants to nourish the soil, and the earth then 'offers again to plants from its bosom, what it has received from them.'" The basic idea of a balance of nature, however, can be traced back to the Greeks: Democritus, Epicurus, and their Roman disciple Lucretius.
Following the Greeks, the idea of a hydrological cycle (water is considered a nutrient) was validated and quantified by Halley in 1687. Dumas and Boussingault (1844) provided a key paper that is recognized by some to be the true beginning of biogeochemistry, where they talked about the cycle of organic life in great detail. From 1836 to 1876, Jean Baptiste Boussingault demonstrated the nutritional necessity of minerals and nitrogen for plant growth and development. Prior to this time, influential chemists discounted the importance of mineral nutrients in soil. Ferdinand Cohn is another influential figure. "In 1872, Cohn described the 'cycle of life' as the "entire arrangement of nature" in which the dissolution of dead organic bodies provided the materials necessary for new life. The amount of material that could be molded into living beings was limited, he reasoned, so there must exist an "eternal circulation" (ewigem kreislauf) that constantly converts the same particle of matter from dead bodies into living bodies." These ideas were synthesized in the Master's research of Sergei Vinogradskii from 1881–1883.
Variations in terminology