How Does the Rhizosphere Effect Influence Plant Growth and Soil Health?

• The rhizosphere is the soil surrounding the plant root and soil interface.
• Root exudates, controlled by plant species, functional type, root traits, and biomes, can determine the biology, physics, and chemistry of the rhizosphere effect.
• The rhizosphere effect can enhance nutrient availability and defense at the plant level; carbon and nutrient cycling and possible bioremedial effects are important at the ecosystem level.
• The rhizosphere, characterized by increased biotic activity, interactions, and nutrients, can be manipulated to enhance crop productivity and environmental quality.

Rhizosphere and Rhizosphere Effects

The soil 2 to 10 mm around the root surface is called the rhizosphere. Though it is not a part of the plant physically, it influences plant nutrient chemistry, growth, and defense abilities.

Plants secrete 5 to 21 percent of the photosynthates as root exudates, a complex mix of carbohydrates, amino acids, proteins, vitamins, lipids, enzymes, nucleosides, sugars, and inorganic ions and molecules.

The biological, physical, and chemical changes occurring in soil due to the root exudates are called the rhizosphere effect.

• Biological processes: The root exudates stimulate and maintain microbial populations, which boost plant growth, fitness, and productivity. The result is a soil micro-ecosystem, which the plant and microbial processes influence. The specific interactions can be beneficial or harmful to plants.
• Physical changes: The organic compounds in the root exudates can bind soil particles into water-stable aggregates. The fungi attracted to the rhizosphere produce hyphae to bind the soil and improve soil structure. These processes strengthen soil anti-erodibility.
• Chemical changes: The root exudates and root and microbial dead biomass are decomposed to add new compounds to the soil. The exudates can directly also change soil particles and rocks.

The rhizosphere is considered a crucial region of the plant ecosystem and one of the most dynamic interfaces in nature, vital for individual plants. Due to their contribution to soil bio-physicochemical properties, the rhizosphere effect is felt also at the ecosystem level.

Factors Affecting the Rhizosphere Effect

Several factors shape the rhizosphere effect (RE), such as plant species, root exudates, soil properties, and climates, and these are discussed below.

Plant Species and Type

Plant species are crucial for determining the rhizosphere microbial communities. The root traits and exudates, as are the RE they produce, are specific to each species.

Roots traits that influence the microbial communities and RE are:

• Specific root length
• Root diversity
• Root density
• Root length density
• Branching

For example, highly branched roots usually secrete more exudates.

Microbial Biodiversity

The microbe communities are part of the rhizosphere effect but also influence other aspects of the RE, so they have to be considered as a factor. Moreover, the microbial communities that thrive in the rhizosphere are also determined by soil type and their ability to interact with the plant host.

The effect of plant and microbe signaling in the rhizosphere is understudied. Still, we know that the complex mix of compounds in the root exudates signal, build, and interact with signals by microbes to produce the following effects:

• The plant secretes root exudates to attract microbe species that are beneficial to it and destroy pathogens.
• The specific composition of root exudates attracts more copiotrophic microbes, like bacteria, or oligotrophic ones, like fungi, in the rhizosphere compared to bulk soils.
• The microbe community is more diverse and specific to the plant species due to RE.

The rhizosphere microbes called rhizobiome help plants adapt to their niche, nutrient acquisition, and plant defense.

Mycorrhiza

Mycorrhiza, a symbiosis between plant roots and fungi, is a RE factor, as it influences plant productivity, fine root traits, nutrient availability, carbon inputs and outputs in soil, and microbial communities.

The two main types of mycorrhizal fungi –arbuscular mycorrhizal (AM) and ectomycorrhizal fungi, produce distinct root morphology, nitrogen acquisition, carbon sink, and nutrient cycling properties, soil organic matter composition, and support dissimilar microbial communities to influence RE.

The importance of the Rhizosphere effect for both the plants and the ecosystem comes from its role in nutrient cycling, carbon cycling, and plant defense, which are discussed in the following sections, see Figure 1.

Figure 1. The changes brought about by rhizosphere plant-growth-promoting fungi in biotic and abiotic factors impacting crop potential, Adedayo and Babalola, 2023. (Image credits: https://doi.org/10.3390/jof9020239)

Rhizosphere Effect on Nutrient Cycling

RE is crucial since rhizobiomes microbes are involved in the geochemical nutrient cycling of nitrogen, phosphorus, and micronutrients like manganese, copper, zinc, and iron, and determine availability to plants and other microbes. Since plant nutrients can constrain plant growth, especially in the tropics where soil fertility is low, the rhizosphere nutrient cycling processes are an area of research focus nowadays.

• Crops: For individual plants, RE enhances productivity by increasing nutrient availability and producing growth hormones, as plants absorb nutrients in the rhizosphere at the root and soil interface. Scientists are trying to leverage the RE by changing agronomic practices or breeding to improve crop yield.
• Natural ecosystems: The entire nutrient cycling is crucial at the ecosystem scale. Less is known of RE effects on soil biogeochemical cycling in forests. A meta-analysis of 185 woody plants in over a hundred sites shows that RE increased carbon and nitrogen mineralization by 38 and 40 percent, respectively, and was influenced by soil properties and not plant attributes.

Rhizosphere Effect on Carbon Cycling

Underground carbon cycling is a vital ecosystem process. RE significantly contributes to carbon sequestration by stimulating carbon decomposition in fresh exudates, roots, and microbial detritus.

The REs with high levels of root exudates supported more microbial biomass per unit of soil than bulk soil. The increased respiration and decomposition of higher microbial biomass added more carbon to the soil. Even if microbial biomass doesn’t increase, RE increases respiration. All this leads to higher carbon decomposition and soil organic carbon in the soils. These effects are reported across terrestrial ecosystems, which is a global phenomenon.

Rhizosphere Effects Strengthen Plant Defense

REs attract microbes that are beneficial for the plant and inhibit or eradicate pathogens and spoilage microbes through competition and antagonism. This property is essential for individual plants.

Competition: Beneficial microbes compete for nutrients to reduce pathogen and spoilage microbes.

Antagonism: Microbes in the rhizosphere are also actively antagonistic to harmful microbes and kill them by producing antibiotics and microbial predation. This RE property is used for biocontrol of known crop pathogens.

Beneficial pathogens use biochemical defense, induced resistance, and defense signaling to strengthen plant defense; see Figure 2.

Figure 2: “The mechanisms by which PGPF and other microbial communities induce resistance and initiate defense signaling in crop plants,” Adedayo and Babalola, 2023. (Image credits: https://doi.org/10.3390/jof9020239)

Rhizosphere Effect on Human Health

Besides the beneficial microbes, potential human pathogens also thrive in the rhizosphere. These can contaminate food and pose a public health hazard. It is one of the few negative REs reported so far.

RE Remedial Measures Against Heavy Metal

Soil can get contaminated by heavy metals through pollution, mining activities, gas exhausts, sewage sludge, or the addition of fertilizers containing the metals. The heavy metals must be removed as they harm people’s health and the environment. One of the methods being tried is adding the microbes that inhabit the rhizosphere by the following methods:

• These microbes immobilize the metals by using various chemicals of root exudates.
• Fungal cell walls also absorb these metals from the soil.

Thus, rhizobiome communities can be bioremedial measures against copper, lead, and zinc.

Applications in Agriculture

Understanding RE and its possible applications for agriculture can enhance crop productivity.

Indiscriminate fertilizer applications without understanding RE has sometimes stunted plant growth.

RE diversity arising from different root systems is being manipulated by genotype choice and agronomy to increase crop yields to apply all the rhizosphere effects:

• Nutrient availability
• Biocontrol of pathogens and weeds
• Form stable soil aggregates
• Bioremediation of contaminated soils

These effects can be achieved by using different methods:

• For current crops: Scientists manipulate roots and exudates to change RE to benefit the crop. It can be done by breeding and choosing a genotype with the desired root architecture to change exudate concentrations that would influence microbial community diversity and numbers.
• For succeeding crops: Crops will leave remnants of roots, exudates, and microbes, which can be used to benefit succeeding crops. The legume and oilseed crop rotation use this principle. Scientists also want to focus on the remnant RE of cereal crops in rotation. For example, a wheat cultivar out of six tested genotypes promoted disease-suppressive Pseudomonas strains. The effects are expected to depend on soil type and agricultural practices.

Importance of Roots in RE

Root traits are critical in influencing rhizosphere effects in managed and natural environments. Though RE has been known for many decades, its applications to enhance plant productivity are new and need more study. Moreover, there are gaps in our knowledge of some aspects of RE, especially in natural ecosystems. Research in RE will involve root system studies, requiring non-destructive, precision tools like the Minirhizotrons system with Root Imagers, like the CI-600 In-Situ Root Imager and CI-602 Narrow Gauge Root Imager, for single-case and long-term studies. Modern technology can speed up research and allow for investigations that were challenging some years ago.

Sources

Adedayo, A.A., & Babalola, O.O. (2023). Fungi That Promote Plant Growth in the Rhizosphere Boost Crop Growth. J. Fungi. 9, 239. https://doi.org/10.3390/jof9020239

 

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Gan, D., Feng, J., Han, M., Zeng, H., & Zhu, B. (2021). Rhizosphere effects of woody plants on soil biogeochemical processes: A meta-analysis. Soil Biology and Biochemistry, 160, 108310. https://doi.org/10.1016/j.soilbio.2021.108310

 

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