The fungal loop hypothesis suggests that soil

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...

in arid ecosystems connect the metabolic activity of

plants Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all curr ...

and

biological soil crusts Biological soil crusts are communities of living organisms on the soil surface in arid and semi-arid ecosystems. They are found throughout the world with varying species composition and cover depending on topography, soil characteristics, climate, ...

which respond to different

soil moisture Soil moisture is the water content of the soil. It can be expressed in terms of volume or weight. Soil moisture measurement can be based on ''in situ'' probes (e.g., capacitance probes, neutron probes) or remote sensing methods. Water that enters ...

levels. Compiling diverse evidence such as limited accumulation of

soil organic matter Soil organic matter (SOM) is the organic matter component of soil, consisting of plant and animal detritus at various stages of decomposition, cells and tissues of soil microbes, and substances that soil microbes synthesize. SOM provides numerous b ...

, high phenol oxidative and proteolytic enzyme potentials due to microbial activity, and symbioses between plants and fungi, the fungal loop hypothesis suggests that carbon and nutrients are cycled in biotic pools rather than leached or effluxed to the

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

during and between pulses of

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...

Theory development

The fungal loop hypothesis is similar in concept to the

microbial loop The microbial loop describes a trophic pathway where, in aquatic systems, dissolved organic carbon (DOC) is returned to higher trophic levels via its incorporation into bacterial biomass, and then coupled with the classic food chain formed by phy ...

hypotheses in oceans or soil, more specifically for arid ecosystems. That is because of characteristics specific to arid ecosystems that are not found elsewhere.

Pulse dynamics

In arid ecosystems, there is low total

and high variability in size of rain events (pulses) within and between years. Differences in how plants and decomposers respond to these pulses of precipitation affects

biogeochemical cycling A biogeochemical cycle (or more generally a cycle of matter) is the pathway by which a chemical substance cycles (is turned over or moves through) the biotic and the abiotic compartments of Earth. The biotic compartment is the biosphere and the ...

within the ecosystem. For example,

extracellular enzymes Extracellular enzymes or exoenzymes are synthesized inside the cell and then secreted outside the cell, where their function is to break down complex macromolecules into smaller units to be taken up by the cell for growth and assimilation. These ...

present in the soil become active nearly instantaneously after any moisture pulse, while production in microbes and plants have lag times of various durations, and require pulse events of different sizes.

Patchy distribution of nutrient pools

Arid ecosystems also often have patchy distributions of

vascular plants Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They al ...

with bare patches of

soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...

in between. Such vegetation reduces radiation and wind speed at the soil surface which reduces

evaporation Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...

and thus creates favorable

microhabitats In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...

for other species. In addition, as plants

senesce Senescence () or biological aging is the gradual deterioration of functional characteristics in living organisms. The word ''senescence'' can refer to either cellular senescence or to senescence of the whole organism. Organismal senescence invol ...

and become

litter Litter consists of waste products that have been discarded incorrectly, without consent, at an unsuitable location. Litter can also be used as a verb; to litter means to drop and leave objects, often man-made, such as aluminum cans, paper cups, ...

, this increases carbon and nitrogen contents in the top layers of soil under plant canopies. Together, these effects create “islands of fertility” where plants are distributed. In the bare soil between plants,

are often present. Crust microorganisms can fix carbon and nitrogen from the atmosphere as well as trap nitrogen-rich dust. Therefore, biology soil crusts contribute to carbon content and nutrient resources in soil surfaces where plant cover is low.

The role of fungi in biogeochemical cycling

Litter produced by plants must be broken down by decomposers into nutrients available to organisms. Both

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

and

produce extracellular enzymes to break down large molecules into compounds that can be taken up by plants. However, fungi can

metabolize Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...

at higher temperatures and lower water potentials than bacteria. Therefore, in arid ecosystems where precipitation falls during the hot season, fungi are likely the most important contributors to

nutrient cycling 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 cycli ...

due to their tolerance to temperature and ability to persist during long dry periods. In several sites in the southwestern US,

denitrification Denitrification is a microbially facilitated process where nitrate (NO3−) is reduced and ultimately produces molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products. Facultative anaerobic bacteria perform denitr ...

and

nitrification ''Nitrification'' is the biological oxidation of ammonia to nitrite followed by the oxidation of the nitrite to nitrate occurring through separate organisms or direct ammonia oxidation to nitrate in comammox bacteria. The transformation of amm ...

were shown to be mostly carried out by fungi.

The fungal loop

In arid ecosystems, many primary producers, such as grasses and biological soil crusts, form

symbioses Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...

with fungi.

Mycorrhizal fungi A mycorrhiza (from Greek μύκης ', "fungus", and ῥίζα ', "root"; pl. mycorrhizae, mycorrhiza or mycorrhizas) is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant ...

colonizing plant roots acquire carbon directly from plant roots, provide phosphorus sources to plants, and have been shown to transport water.

Dark septate endophytes Dark septate endophytes (DSE) are a group of endophytic fungi characterized by their morphology of melanized, septate, hyphae. This group is likely paraphyletic, and contain conidial as well as sterile fungi that colonize roots intracellularly or ...

(DSE) are also common in many aridland plants and are hypothesized to perform similar roles as mycorrhizal fungi. Fungi are an integral part of the biological soil crust community, and similar fungal taxa have been found in biological soil crusts and plant root zones, which suggests hyphal connections between these two spatially separate organisms. The fungal loop hypothesis suggests that biological soil crusts and associated microbes are able to become active after smaller water pulses compared to vascular plants, which require more water to become active. However, fungi are able to take up the nutrients produced by biological soil crusts at lower water potentials, and keep them in the biotic pool until larger water pulses allow plants to become active and take up those nutrients. Active plants then are able to contribute excess carbon from

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

to their fungal symbionts. Therefore, root-associated fungi symbiotic with plants and biological soil crusts connect the spatially- and temporally-distinct activities of crusts and plants.

Evidence

Evidence of conditions favorable to the existence of a fungal loop is readily available. However, direct experimental tests of the hypothesis in arid ecosystems are still relatively rare. One study used isotopic labeling to trace where

nitrate Nitrate is a polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zer ...

and

glutamate Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can syn ...

moved when provided to biological soil crust or to grass foliage a distance away. They found that organic and inorganic N could be dispersed up to 100 cm per day bidirectionally between plants and crust. Other research has shown evidence of bidirectional transport in soil-fungal-plant connections in redistributing water in arid ecosystems.Allen, M. F. (2009), Bidirectional water flows through the soil–fungal–plant mycorrhizal continuum. New Phytologist, 182: 290–293.

References

{{reflist Soil biology Symbiosis Fungus ecology