(SOC) and adding CO2 to the atmosphere through
respiration. But the net impact on greenhouse gas emissions will be reduced by the increased uptake of CO2 from the atmosphere by increased plant growth. However, over the long term, soil reserves of easily decomposed carbon will be depleted by the increase in microbial activity, resulting in increased catabolism of SOC reservoirs, thus increasing atmospheric CO2 concentrations beyond what is taken up by plants. This is predicted to be a particular problem in thawing
permafrost that contains large reserves of SOC that are becoming increasingly susceptible to microbial degradation as the permafrost thaws. Rhizodeposition allows for the growth of communities of microorganisms directly surrounding and inside plant roots. This leads to complex interactions between species, including mutualism, predation/parasitism, and competition.
Predation Predation is considered to be top-down because these interactions decrease the population. Still, the closeness of species interactions directly affects the availability of resources, causing the population to be affected by bottom-up controls. Without soil fauna, microbes that directly prey upon competitors of plants, and plant mutualists, interactions within the rhizosphere would be antagonistic toward the plants. Soil fauna provides the rhizosphere's top-down component while allowing for the bottom-up increase in nutrients from rhizodeposition and inorganic nitrogen. The complexity of these interactions has also been shown through experiments with common soil fauna, such as nematodes and protists. Predation by bacterial-feeding nematodes was shown to influence nitrogen availability and plant growth. There was also an increase in the populations of bacteria to which nematodes were added. Predation upon
Pseudomonas by amoeba shows predators can upregulate toxins produced by prey without direct interaction using supernatant. The ability of predators to control the expression and production of biocontrol agents in prey without direct contact is related to the evolution of prey species to signals of high predator density and nutrient availability. The
food web in the rhizosphere can be considered as three different channels with two distinct sources of energy: the detritus-dependent channels are fungi and bacterial species, and the root energy-dependent channel consists of nematodes, symbiotic species, and some arthropods. Geometrical properties are the density of roots, root diameter, and distribution of the roots. Physicochemical properties are exudation rate, decay rate of exudates, and the properties of the environment that affect diffusion. These properties define the rhizosphere of roots and the likelihood that plants can directly compete with neighbors. Plants and soil microflora indirectly compete against one another by tying up limiting resources, such as carbon and nitrogen, into their biomass. This competition can occur at varying rates due to the ratio of carbon to nitrogen in detritus and the ongoing mineralization of nitrogen in the soil. Mycorrhizae and heterotrophic soil microorganisms compete for both carbon and nitrogen, depending upon which is limiting at the time, which heavily depends on the species, scavenging abilities, and the environmental conditions affecting nitrogen input. Plants are less successful at the uptake of organic nitrogen, such as amino acids than the soil microflora that exists in the rhizosphere. This informs other mutualistic relationships formed by plants around nitrogen uptake. Competition over other resources, such as oxygen in limited environments, is directly affected by the spatial and temporal locations of species and the rhizosphere. In
methanotrophs, proximity to higher-density roots and the surface is important and helps determine where they dominate over heterotrophs in rice paddies. The weak connection between the various energy channels is essential in regulating predator and prey populations and the availability of resources to the biome. Strong connections between resource-consumer and consumer-consumer create coupled systems of oscillators, which are then determined by the nature of the available resources. These systems can then be considered cyclical, quasi-periodic, or chaotic. Plants
secrete many compounds through their roots to serve symbiotic functions in the rhizosphere.
Strigolactones, secreted and detected by
mycorrhizal fungi, stimulate the
germination of spores and initiate changes in the mycorrhiza that allow it to colonize the root. The parasitic plant,
Striga, also detects the presence of strigolactones and will germinate when it detects them; they will then move into the root, feeding off the nutrients present. Symbiotic
nitrogen-fixing bacteria, such as
Rhizobium species, detect compounds like
flavonoids secreted by the roots of leguminous plants and then produce
nod factors that signal to the plant that they are present and will lead to the formation of
root nodules. Bacteria are housed in
symbiosomes in these nodules, where they are sustained by nutrients from the plant and convert nitrogen gas to a form that the plant can use. Non-symbiotic (or "free-living") nitrogen-fixing bacteria may reside in the rhizosphere just outside the roots of certain plants (including many grasses) and similarly "fix" nitrogen gas in the nutrient-rich plant rhizosphere. Even though these organisms are thought to be only loosely associated with the plants they inhabit, they may respond very strongly to the status of the plants. For example, nitrogen-fixing bacteria in the rhizosphere of the
rice plant exhibit
diurnal cycles that mimic plant behavior and tend to supply more fixed nitrogen during growth stages when the plant exhibits a high demand for nitrogen. Beyond symbiotic relationships like mycorrhizae and rhizobia, free-living bacteria in the rhizosphere play a crucial role in plant growth. These communities can be significantly stimulated by organic amendments. The application of organic matter such as sugarcane straw, goat manure, and filtered mud has been shown to enhance the abundance and activity of free-living, plant-growth-promoting bacteria. These stimulated communities, in turn, improve nutrient cycling (such as nitrogen fixation and phosphorus solubilization) and support crop growth, even in the absence of specialized root nodules. In exchange for the resources and shelter plants and roots provide, fungi and bacteria control pathogenic microbes. which plays into the bigger cycles of nutrients that impact the ecosystem, such as biogeochemical pathways. This description has been used to explain the complex interactions that plants, their fungal mutualists, and the bacterial species that live in the rhizosphere have entered into throughout their evolution. Certain species like
Trichoderma are interesting because of their ability to select for species in this complex web.
Trichoderma is a
biological control agent because of evidence that it can reduce plant pathogens in the rhizosphere. Plants themselves also affect which bacterial species in the rhizosphere are selected against because of the introduction of exudates and the relationships that they maintain. The control of which species are in these small diversity hotspots can drastically affect the capacity of these spaces and future conditions for future ecologies. Intensive agricultural management, particularly long-term monoculture, can override these natural selection processes, leading to "soil sickness." This phenomenon is characterized by a shift in pathogenic and beneficial microbial populations, negatively impacting soil fertility and crop yield. For example, continuous sugarcane planting has been shown to reduce soil microbial diversity and alter community composition compared to intercropping systems. Furthermore, the impact of these management practices is not uniform with depth; practices such as straw retention influence topsoil microbial communities differently than those in the subsoil.
Microbial consortium Although various studies have shown that single microorganisms can benefit plants, it is increasingly evident that when a
microbial consortium—two or more interacting microorganisms—is involved, additive or synergistic results can be expected. This occurs, in part, because multiple species can perform a variety of tasks in an ecosystem like the rhizosphere. Beneficial mechanisms of plant growth stimulation include enhanced nutrient availability,
phytohormone modulation,
biocontrol, and
biotic and
abiotic stress tolerance) exerted by different microbial players within the rhizosphere, such as plant-growth-promoting bacteria (PGPB) and fungi such as
Trichoderma and
mycorrhizae. The diagram on the right illustrates that rhizosphere microorganisms like plant-growth-promoting bacteria (PGPB),
arbuscular mycorrhizal fungi (AMF), and fungi from the genus
Trichoderma spp. can establish beneficial interactions with plants, promoting plant growth and development, increasing the plant defense system against pathogens, promoting nutrient uptake, and enhancing tolerance to different environmental stresses. Rhizosphere microorganisms can influence one another, and the resulting consortia of PGPB + PGPB (e.g., a nitrogen-fixing bacterium such as
Rhizobium spp. and
Pseudomonas fluorescens), AMF + PGPB, and
Trichoderma + PGPB may have synergetic effects on plant growth and fitness, providing the plant with enhanced benefits to overcome biotic and abiotic stress. Dashed arrows indicate beneficial interactions between AMF and
Trichoderma.
biological signals and the exchange of information are part of the definition of communication, while the signals themselves are considered as "every structure able to shape the behavior of the organisms". Consequently, the signals can evolve and persist thanks to the interaction between signal producers and receivers. Then, cooperation and fitness improvement are the basis of biological communication. A large number of signals can be exchanged involving the plant itself, insects, fungi, and microbes. This all takes place in a high-density environmental niche. Usually, communication results from chemical responses of cells to signatory molecules from other cells. These signals affect both the metabolism and
transcription of genes, activating several regulatory mechanisms. Nevertheless, a large number of nutrients issued by the plant can be of interest to pathogenic organisms, which can take advantage of plant products for their survival in the rhizosphere. Indeed, because of the chemical signals conveyed by nutrient-rich exudates released by the plant roots, a large variety of microbes can first colonize the rhizosphere and then gradually penetrate the root and the overall plant tissue (
endophytes). Otherwise, they can colonize the host plant establishing a lasting and beneficial symbiotic relationship. To date, numerous investigations on root exudates composition have been performed. However, the knowledge about the earlier steps of rhizosphere colonization, namely the opening line at the root surface, remains poorly characterized. Increasing data have shown the importance of intraspecies and multispecies communications among rhizospheric biotic components for improving rhizobia–legumes interaction. In addition, it has been shown that rhizobia are part of the rhizosphere of a wide variety of non-legume plants. They can be plant growth-promoting components, recovering a central role in the plant core microbiome. == Methods ==