Oluwadara Pelumi Omotayo, Olubukola Oluranti Babalola

Food Security and Safety Niche, Faculty of Natural and Agricultural Science, North-West University, Mmabatho, 2735, South Africa

Keywords:Food security Land use activity Soil microbial community Soil degradation Sustainable agriculture

ABSTRACT The role of the rhizosphere microbiome remain crucial to human existence and can therefore not be over-looked, particularly with respect to plant growth, development and yield. The rhizosphere is the‘home’ for several genes, some of which are responsible for the various trait and features exhibited by plants. Numerous microbes have been identified to inhabit the rhizosphere, some of which are the proteobacteria, nitrogen fixing bacteria, plant growth promoting rhizobacteria, mycorrhizal and mycoparasitic fungi. Likewise, chemical compounds such as amino acids (crystathionine and homoserine),which inhibits nematodes, carbohydrates (fructose), which enhance the growth of microorganisms, and sterols (sitosterol), which regulate plant growth, reside in the plant rhizosphere. Several factors contributes to the existence and survival of these microbes, one being good agricultural practices, as some farming techniques negatively affects the soil microbial content. This study therefore focused on the agricultural and health importance of the soil organisms, as well as the various techniques that can be employed to maintain the rhizosphere microbiome, despite the rise and intensification of various harmful land use activities.This will enable a better understanding of the soil rhizosphere conservation,restoration and sustainable use,specifically of degraded land.This will avoid depleting or alternating the soil microbial content while aiming to ensure a healthy ecosystem, biodiversity, maximal crop productivity, food security and all round sustainability, as targeted by the United Nation’s Sustainable Developments Goals.

1. Introduction

The exponential growth in the world’s population has created an increase in the demand for food and energy,thereby intensifying agricultural land use. Together with urbanization, poor plant cultivation and soil maintenance practices have resulted in adverse consequences on agricultural soils, ranging from a reduction in to the loss of the soil microbial content,and the extinction/migration of important soil organisms (Furtak & Gajda, 2018). Within the upper soil profile,the rhizosphere houses numerous microbes that are responsible for plant productivity and sustainability (Li et al.,2014; Liang et al., 2019). However, with the increase in inappropriate land use and varying agricultural practices,the quality of this soil region depreciates and the microbes becomes incapacitated(Bajsa et al., 2013; Schmidt et al., 2019).

A number of authors have indicated that various techniques utilized in agricultural land use and farming practices can alter and reduce soil fertility, with a resulting decrease in food production(Bajsa et al.,2013;Jiang et al.,2019).This occurs due to the interruption in the activities of soil enzymes and microorganisms,as well as the disruption of the basal soil respiration,all of which are important tools to evaluate the quality and health of the soil (rhizosphere)(Niemeyer et al., 2012). It has also been reported that the resultant effect on the rhizosphere microbial community inevitably puts the soil’s healthinjeopardy,asits microbiota islinked to its wellbeing and vitality(Mendes et al.,2013).Due to an increase in land use practices,changes in the layout and composition of the rhizosphere microbiota has been reported(Bajsa et al.,2013),particularly in the rural African communities as a result of changing agricultural practices caused by growing populations(Calicioglu et al.,2019;CNBC,2018;FAO 2019).

Land users therefore need to be informed about the effects that various inappropriate practices are having on the health of agricultural soil. In addition, the degeneration of agricultural land will prevent the achievement of the sustainable developmental goals(SDG goals 15 and 2) by 2030 on land and soil conservation,improving food security and the income of small-scale farmers,sustainable food production systems and buoyant/resilient agricultural practices.

The productive capacity of agricultural land in developing nations, particulary in Africa, will not experience positive changes if land users do not adopt better strategies to maintain the health of the soil.This will require the promotion of sustainable agricultural practices that can increase food productivity. Given the economic and agricultural benefits of soil biodiversity with respect to sustainability, it is essential to improve people’s knowledge and understanding about maintaining a healthy soil microbial community.

While much has been written about soil and soil microbes(Bertin et al., 2003; Hiltner, 1904; Mahrous et al., 2019; Moreau et al., 2019), it is important to establish the status of the resident soil (rhizosphere) microbiomes and ecology from a wide perspective,so as to optimize suitable land use methods which will enhance the achievement of the Sustainable Development Goals.Also, urgent attention is needed to motivate for developing and adopting non-hazardous land use strategies.

Herein, the limitations to soil health, the targets for soil health improvement,and suitable methods which can be used to achieve the long-term sustainability of the targeted ecosystems are identified. Moreover, this study appraised the optimization of effective land use techniques which are essential and will enable maintenance and preservation of the quality and health of the rhizosphere and its microbial community. This will enhance sustainable agriculture and increase food security in communities that rely on the land for their livelihood.

2. Methodology of literature search

As illustrated in Fig.1, literature search on the soil rhizosphere microbiome’s ecological dynamics and conservation was conducted on materials published from 1987 to 2019 (specifically 2014 to 2019) through various scholarly search engine: Science Direct,Scopus,Web of Science,PubMed and Google Scholar.The focus was on the key words of this article (biodiversity, ecosystem, food security, sustainability, soil microbes, soil conservation), with 334 research articles being downloaded for review. A conscious, purposive and careful selection of the referenced articles yielded 106 articles (32% of the total research exploits) that were included in this review. Owing to the anticipated differences in the methods,data and view in the explored articles, proper synthesis of the retrieved information was ensured. Standardised and original chemical structures, images and pictures were adopted.

3. The rhizosphere, its role and mechanism

The rhizosphere is also known as the‘root zone’(Hiltner,1904;York et al., 2016), being regarded as the area within a few millimetres from the roots as shown in the following Fig.2.This zone is characterised by intense biological activities due to exudate released from the roots that serve as nutrients for soil microorganisms.This makes it an important area for plant growth,nutrient recycling and disease resistance (De Luna et al., 2005).

Two types of food chain occurs in the rhizosphere,the first being detritus based(dead plant matter),which fuels either the bacterial or fungal carbon channel, depending on the labile or recalcitrant resources, respectively (Ruess and Ferris, 2004), and affects the energy and nutrient flows.The second type is based on living plant roots, which are the carbon sources of living plant tissue and root exudates (Ruess and Ferris, 2004). These are essential for plant growth and health as it assists in nutrient uptake and protects the plant from diseases (Berendsen et al., 2012). The activity of the microbes in this region determines the nutrients available to the plants and their rooting pattern,therefore affecting the quality and quantity of root exudates (Berendsen et al., 2012).

Fig.1. Flow diagram for article selection.

4. Resident rhizosphere microbiome’s and ecological dynamics

The rhizosphere soil consists of various classes of microbes, and includes fungi, bacteria and archaea, actinomycetes, protozoa and nematodes, as illustrated in Fig. 2. The classes of fungi include saprophytic,mutualistic and pathogenic.The bacteria group include decomposers and lithotrophs, while the archea consists of euryarchaeota and crenarchaeota. Norcadia are the main class of actinomycetes, while bacteria and fungal feeders are present as nematodes.and the Amoeboid,flagellated,and ciliatedprotozoans,as well as sporozoans, are the main classes of protozoans. These organisms permiate this region of soil and form the rhizosphere’s microbiome (Mendes et al., 2013). In addition to the soil microbes,some chemical compounds and allelochemicals have been reported to be present (Table 1), which are responsible for the growth and development of plants,and the maintenance of the ecosystem(Bertin etal.,2003).Thesenotonlyinfluencetheconsciouslycultivatedplants but also have an impact on the surrounding weeds and grasses.

4.1. Fungi, bacteria and archaea communities in the rhizosphere

According to Bu′ee et al.(2009),microbes in the rhizospere do not escape the‘Big plate Anomaly’,this referring to the large difference in the number of microbes in a soil sample and that which can be cultured,with approximately 90%remaining unculturable.A handful ofsoilcontainsbillions ofmicrobes(Bhat,2013),whichincludes fungi,bacteria and archeae, which varies according to root zone, plant specie,growth phase/stage as well as occurrence of stress and disease(Rovira,1965;Hinsinger et al.,2009;Marschner et al.,2011).

Due to their ability to adopt various forms in response to harsh and unfavourable conditions,fungi thrives in soil(Sun et al.,2015).The class of fungi present in the rhizosphere are the saprophytic,mutualistic and pathogenic fungi, while Arbuscular mycorrhizal fungi(AMF)are also important as they promote crop yield,stimulate nutrient cycling,improve soil structure and plant tolerance to stress.

The main bacteria groups found in the rhizosphere areBacillus subtillis, proteobacteria, acidobacteria and actinobacteria (Mendes et al., 2011; Weinert et al., 2011). The proteobacteria are regarded as the most abundant in this zone due to their ability to respond to labile carbon, therefore showing rapid growth and adaptation to various plant rhizospheres (Bulgarelli et al., 2013; Chaparro et al.,2012;Peiffer et al.,2013).Acidobacteria are the next most abundant group and play a vital role in the carbon cycle, as they degrade lignin and cellulose (Ward et al., 2009). The plant growthpromoting rhizobacteria are also an important group, as they enhance growth and the uptake of nutrient,and control pathogens(Lagos et al., 2015; Mendes et al., 2011).

Archeaeforms the third domain of life,the major classes being euryarchaeota and crenarchaeota. According to Buee et al., 2009,they possess the ability to survive in environments low in oxygen making it easy for them to dwell in the soil. They can also recycle carbon, sulfur and nitrogen which are vital for the function of the ecosystem (Offre et al., 2013). Summarized in Fig. 3 below are the groups of microbes found in the soil and their roles, while Table 1 shows the chemical compounds present in the maize rhizosphere and their influence on the ecosystem.

Fig. 2. Soil section showing the rhizosphere region.

5. The role of soil microbes in nutrient availability

Soil microbes play an important role in sustaining plants growth by decomposing and recycling nutrients in organic matter,assisting in accessing minerals in rocks, and refining nitrogen from the atmosphere for plant use. Soil microorganisms can be classified into three groups based on their function, namely:

1. Recyclers: such asFrankia,PseudomonasandBacillus subtilis(NSW, 2005)

2. Miners: for example, litotrophic bacteria such asAcidithiobacillus ferrooxidans(Jon, 2014)

3. Refiners: likeAzotobacter,Azospirillum,RhizobiumandClostridum(NSW, 2005).

5.1. Recyclers

These are also known as‘decomposers’,as they turn waste into raw materials for the plant by breaking down dead plant and animal matter (Caitlin, 2018; Buchan et al., 2014). In the course of recycling, basic components, such as nitrogen, phosphorus,potassium and other nutrients are released for plant health and growth enhancement(Buhan et al., 2014).

5.2. Miners

These work on the rocks and minerals and not on organic matter,and produce exudates in form of a sludge/slush(Jon,2014).According to Caitlin(2018),they have the ability to bind and extract nutrients such as phosphorus, potassium and calcium, which are important for healthy plants and crop yields.The mycorrhizal fungi fall under this category and connect with the plant root to provide access to nutrient,and in return obtains energy rich carbon,which aids in their growth(Caitlin, 2018).

5.3. Refiners

These are able to take nitrogen from the atmosphere,process it,and convert it into plant-available ammonia and other nitrogen organic compounds.

6. Degradation of the rhizosphere community: causes and recommendations

The soil is prone to the loss of nutrients and degradation through various human activities, such as soil erosion, leaching,overgrazing and mining,which affects the soil layers,structure and nutrients. Various agricultural and other land use practices significantly alter the proportion of microbes in the rhizosphere by changing the physical and chemical environment (Babin et al.,2018; Bajsa et al., 2013). These activities reduce the quality and quantity of plant species and residues, thus reducing food for soil organisms (Babin et al., 2018; Wilson & Tisdell, 2001). Habitual agricultural actions, which have been reported to alter the rhizosphere microbiome,can be generally classified into two categories:

1. Chemical practices: pesticides, chemical fertilizers and sewage sludge amendment

2. Mechanical practices: various forms of tillage.

Table 1 Chemical compounds present in the rhizosphere.

Table 1 (continued)

Table 1 (continued)

Fig. 3. Soil microbes and their roles.

6.1. Chemical practices

Pest and diseases are a common phenomenon in agriculture that must be guided against, this often being achieved with the use of pesticides to protect cultivated crops. Herbicides are also used on plants and crops to eradicate weeds,being regarded as effective,fastacting and easy to obtain (Sharifzadeh et al., 2018). The wide acceptability and optimization of chemicals, such as pesticides and herbicides to control and eliminate pests,weeds and plant diseases,has proven to be effective in achieving maximum yield and productivity (Sharifzadeh et al., 2018). However, the constituents of these various chemical additives contributes negatively to the rhizosphere microbiome,leading to its deterioration(Aktar et al.,2009).

6.1.1. Pesticides

References(Pajares &Bohannan, 2016;Pell et al., 1998)Effects on soil inhibits the soil bacteria that converts ammonia to nitrate Chemical formular C7H4Cl3NO3 eight Molecular W 256,46 Chemical structure Table 2 Pesticides and their effects on soil.Pesticides/Herbicides Triclopyr Van et al. (2018)reduces the proliferation and activity of nitrogen fixing bacteria C3H8NO5P 169,07images/BZ_139_619_2017_633_2029.pngGlyphosate(Tozum-Calgan and Sivaci-Guner,1993).Goodwin and Beach(2001)Reduces nitrogen fixation & the growth&activity of nitrogen fixing blue-green algae.Inhibits coversion of ammonia into nitrates by soil bacteria Damages Mycorrhizal fungi that grow with plant roots and helps in the uptake of nutrients C8H6Cl2O3 C12H18N406S 221,03 346,36 2,4 Dichlorophenoxyacetic acid Oryzalin:4-(dipropylamino)-3,5-dinitrobenzenesulfonamide Moorman, 1989 Reduces the quantity of mycorrhizal fungal spores C15H18Cl2N2O3 345,22 Oxadiazon

Altman &Campbell, 2003 Damages Mycorrhizal fungi that grows with the roots of plants C13H16F3N3O4 335,28 Trifluralin Sanders et al.(1996)Significantly reduces soil microbial biomass C9H13BrN2O2 261,12 Bromacil

According to the Food and Agricultural Organization (FAO,2013),“Pesticide may be descibed as any substance or combination of substances designated to prevent, ruin or control pest which include vectors of plant, that causes harm during production or those that interferes with the production,storage,transportation,or marketing of agricultural commodities”.The word pesticide is a broad name that covers a wide range of natural or chemical related/synthetic compounds, including insecticides, fungicides, bactericides and herbicides (Aktar et al., 2009). Examples of natural pesticides include floating row covers,(insect barrier),pheromone traps(insect bait to attract mostly male insects), sticky traps, oil sprays (used to suffocate insects), organisms such asBacillus Thuringiensis(soil dwelling bacterium poisonous to certain pests) and parasitic nematodes (Cavoski et al., 2011).

Examples of chemical related pesticides are organophosphate,carbamate, sulfonylurea herbicides and pyrethroid (Nicolopoulou-Stamati et al., 2016). Due to their rapid efficacy and ease of application(spraying),especially in large scale and commercial farming,there has been an increase in their use (Wilson & Tisdell, 2001).However, while the use of the natural pesticides poses little or no threat to the soil ecosystem and the environment, the chemical substances have been reported to affect the soil and the rhizosphere microbiome, some of their effects being discussed further.

6.1.1.1. Effect of pesticides on the rhizosphere microbiome.Pesticides undergo some non-biological processes upon application, and are therefore converted to transformation products(Michael et al., 2014), which interferes with the soil and the environment in various ways. The community of microbes present in the soil can be altered or damaged by the unguided use or overuse of these chemicals (Aktar et al., 2009; Bharagava & Chowdhary,2019). Although pesticides are targeted at a specific pests or group of pests, they tend to harm more than the intended population due to their broad-spectrum toxicity (Aktar et al., 2009).

The excessive application of pesticides to plants/crops can lead to the deterioration or death of the soil microorganisms(Bharagava& Chowdhary, 2019), particularly the beneficial ones, thereby causing a setback in productivity. For example, plants rely on various soil microorganisms to transform nitrogen into nitrates for plants use, with common pesticides (herbicides) distrupting this process (Potera, 2007),examples being shown in Table 2.

The consequences of the unguided use of chemical pesticides on rhizosphere organisms has been likened to the effects of the misuse/abuse of antibiotics in humans (Aktar et al., 2009), with resistance developing in certain insects. In his report from studies conducted in the laboratory, Estok et al. (1989)reported that even at lower concentrations than those found in the soil, Roundup pesticide was observed to be toxic and damaging to mycorrhizal fungi (Estok et al.,1989).

6.1.2. Use of chemical fertilizers

Despite the availability of organic/natural fertilizers, their use has decreased since the 1970s due to the availability and accessibility of chemical fertilizers (Yan & Gong, 2010). Moreover, the former use of manure as an organic fertilizer (especially in small scale farming which is most often integrated with animal husbandry)is not effective and suitable in recent agricultural practices.This is as a result of the shift and expansion in agriculture to large scale/commercial farming, thus making manure application less appropriate and limited as it is not evenly distributed by animals on farmlands (Kantikowati et al., 2019). In other to maximize crop productivity and avoid worldwide food shortage, the use of chemical fertilizers is widespread, especially among commercial and large scale farmers (Nkoa, 2014; Sun et al., 2015). The application of chemical fertilizers, such as ammonium sulphate and phosphate, is an accepted technique for increasing crop yields in Africa in particular(Zhu&Chen,2002).However,uncontrolled use of these has been reported to cause the loss of soil carbon, severe soil degradation, and a reduction in soil organic matter (Horrigan et al., 2002; Nkoa, 2014; Sun et al., 2015). The long-term use of these products have been shown to cause soil acidification, rhizosphere micro-ecological environment degeneration, nutritional imbalance and an increase in the heavy metal ions in the soil(Horrigan et al.,2002;Nkoa,2014;Sun et al.,2015).According to Lin et al. (2019), the long-term application of chemical fertilizers caused a declination in the soil pH and a degeneration in the chemical properties of the rhizosphere.

6.1.3. Sewage sludge

The sludge is an unavoidable and organic carbon-rich semi-solid material that is produced as a by-product in the process of sewage treatment of industrial or domestic wastewater (Eid et al., 2020).Pathogens, micro-pollutants (e.g. ammoniacal nitrogen), heavy metals and other hazardous materials make up the sludge, which also consist of a significant amount of nutrients, such as carbon,nitrogen and phosphorus (Aislabie et al., 2013). It has also been observed to contain high levels of soluble salts, heavy metals and trace elements, which are essential for plant growth (Wong et al.,1998).

However,the application of sewage sludge in its raw form is not encouraged due to it high pollutant load (including heavy metals,organic compounds, pharmaceuticals), which might be poisonous for the rhizosphere microorganisms and the soil generally. Treatment and application of sludge at recommended rates, causes increase in microbial activity (Eid et al., 2020).

When sewage sludge is treated, it can be used in agriculture as fertilizers to enhance plant growth and productivity, although the pH of the soil may decrease due to nitrification and the production of organic acid (Wong et al., 1998). In their report, Chander and Brookes (1993) noted that excessive modification of sewage sludge may also restrain the activity of the microbes and nutrient mineralisation of the soil.This makes it necessary to appropriately modify and regulate the quantity and quality of sludge applied to land.

According to their studies in 1998,Wong et al.(1998),observed that during incubation process, sewage sludge modifications caused a significant increase in soil pH.It also led to an increase in the electrical conductivity of sandy soils,raising the possibility that this might be due to a high soluble salt content in the sludge, and the change in the equilibrium of the salt and ionic species in the soil.He also noted that the modification process of the sludge led to an increase in NH4+-N composition of the soil.

Fig. 4 below illustrates the various chemical application practices that leads to degradation of land as earlier mentioned in this section.

Fig. 4. Chemical Activities leading to the loss of Soil Microbes.

6.2. Mechanical activities

6.2.1. Tillage

This is an agricultural procedure carried out for soil preparation through various types of mechanical processes, such as digging,mixing and overturning,being classified into primary or secondary.The former is done to attain a reasonable depth of soft soil, aerate the soil, incorporate soil residues and eliminate weeds. The later entails a shallower depth, being done to incorporate fertilizers,level the soil surface or reduce the soil to a finer tilth.

This pervasive practice as shown in Fig.5 below,which involves overturning the top soil (15-25 cm) in preparation for planting,loosens and aerates this layer.However,as studies have indicated,if done frequently, reduces the water retention capacity. It has also been reported to reduce the organic matter in the soil,destroy the soil aggregates and alter the microbial content of the rhizosphere and surrounding soil (Drinkwater & Snapp, 2000). This practice,which not only alters the soil pore structure,has been noted to also disrupt the soil environment, change the pore volume, reduce the vertical stratification and cause the dispersal of the microbial communities (Bajsa et al., 2013; Drinkwater & Snapp, 2000).

Fig. 5. Mechanical Activities leading to soil Degradation.

Table 3 Types and description of tillage practices.

According to the Conservation Tillage Information Centre and FAO 2019, two major types of tillage have been identified which are:

· Conventional tillage

· Conservational tillage

a. Conventional tillage

This practice invert the soil and buries crop residues(Bajsa et al.,2013), often being done mechanically through the use of plough and harrows or more traditionally,with the use of cutlass and hoe.

b. Conservational tillage

This leaves the residue from the previous year’s crop(Hydbom,2017), for example, wheat stubble on fields before and after planting the next crop so as to diminish soil erosion and runoff,which helps with carbon sequestration.The plant residues can also serve as nutrient for the soil microbes,and increases the amount of fungi,particularly Arbuscular mycorrhizal fungi(AMF),which may have a positive effect on crop nutrition and soil stabilization(Bhat,2013;Hydbom,2017).Five types have been identified in this group,namely no-tillage (NT), mulch tillage (MT), strip tillage (ST), ridge tillage (RT) and reduced tillage (RDT) as shown in the following Table 3 (Khursheed et al., 2019; Hydbom, 2017).

6.2.1.1. Effects of tillage on the soil microorganisms.Microbes present in the rhizosphere (Li et al., 2014) have been reported to be affected by tillage. As observed by Benites and Ofori (1993),following tillage,the soil tends to settle such that when compared to its conditions before tilling, the porosity is reduced due to the pressure exerted on the soil environment.In his study,Kabiri et al.(2016)found that the less disturbed soils had more microorganism than soils tilled using conventional method.He speculated that this may be due to improved environmental conditions as a result of the optimised tillage method, as the soil organic matter was not affected.

Gosai et al.,2009 noted that tillage intensity and frequency can have an impact on soil’s biological characteristics and processes.He further suggested that conservation tillage can boost substrate availability and the accumulation of microbial nutrient rate throughout the crop’s growing season.Soil samples from NT and ST samples across four depths were compared, with the microbial biomass carbon being higher for NT at 0-5 cm depth. Hydbom(2017) described how the long-term use of conventional tillage can result in degradation of soil quality due to the increased decomposition and mineralisation of soil organic matter.

In addition,conventional tillage practices have been linked with increase in soil salinity which has affected many croplands globally,thus leading to shortage in food production and insecurity (Botha,2013). According to the Red River Basin Commission (2014),tillage practices (with the use of implements such as cultivators,disks or deep tillers)causes salt trapped in deeper layers of the soil to rise to the surface. This occurs simultaneously with the rise of water in the soil(caused by tillage practices),thereby saturating the topsoil. Tilling the soil continuously can therefore result to declination in the plant growth promoting activities of soil microbes,leading to poor plant growth and consequently reduced crop production.

Moreover, Botha, 2013 in his study compared the effect of various types of tillage practices on soil salinity, he therefore observed that the no-tillage procedure proved to be beneficial, as salt was leached out and not trapped in the soil. This implies that application of conservational tillage procedure will not only help in preserving the soil microbes but also prevent soil salinity.This will therefore promote achievement of the “Land degradation reverse”and “Land restoration” aim as indicated by Sustainable Development Goal 15, of the United Nations.

7. Rhizosphere maintenance methods

Considering the various effects exerted on the rhizosphere through common land cultivating methods, the following strategies are recommended to maintain the rhizosphere/soil microbiota.This will help to avoid reducing or depleting soil microorganisms,and enhance food productivity and security for the growing world population.

a. Discourage pesticide/herbicide

Table 4 Importance of the soil and its microbes(Aislabie et al., 2013).

While the use of pesticides is effective and easy, they are reported to alter the soil microbiota(Bharagava&Chowdhary,2019).However biological control measures which have little or no adverse effects need to be used to control and manage pests and diseases. This depends on natural mechanisms such as predation and parasitism,and involves a human management role.According to the FAO (2019), three basic strategies are involved in the biological control of pests and weeds:

· Importation:introduce a weed/pest enemy

· Augmentation:multiply these enemies and administer them for quick pest control

· Conservational:ensure the maintenance of the natural pest enemies through frequent re-establishment by providing them with suitable conditions.

Biological control agents (natural enemies of pests) include parasitoids (Colmenarez et al., 2018) (mainly wasps), pathogens and competitors. Antagonists can be introduced to control plant disease and seed predators, while herbivores can also be used to control weeds instead of herbicides. Biological control agents are environmentally friendly, as they create little or no pollution, are safe for the soil (does not disturb or alter the soil), and are selfsustaining and cost effective. In controlling plant diseases, several organisms have been found to be useful and efficient, such as:

1.Streptomycetes lydicuswhich have been successfully used to reduce root and seed rot in tomatoes (Goudjal et al., 2016;Weiland, 2014), and in control ofPythiumandFusariumchallenge in spinach(Weiland,2014).Streptomycesandgriseoviridishave been known to significantly reduce black scurf(rhizoctonia solani)(Shahrokhi et al.,2005)and common scab(streptomyces scabies) (Hosny et al., 2014).

2.Bacillus subtilis: a plant growth promoting rhizobacteria has been found to significantly diminish the incidence of seed and root rot in peas (Dicklow et al., 1993). These and many other organisms can be used to manage and eradicate plant diseases,as some not only control disease but also enrich the soil and promote plant growth.The use of biological pest controls, such as pheromones, should also be encouraged, and genetic engineering can be used to breed pest resistant crops.

b. Increase the availability and use of organic fertilizers: Agricultural land users should be educated on organic fertilizers and their advantages over chemical fertilizers,As they may increase crop productivity and limit changes to the microbial composition of the rhizosphere microbiome.

c. Treat and correct/guided sewage sludge application:Before the application of sludge,treatments should be performed to reduce its biodegradability and thereby lessen the release of labile compounds(Eid et al.,2020).The volatile compounds should be regulated or removed, and sludge should be appropriately applied in suitable quantity for effective results.Additional post treatments, such as composting or thermal drying, should also be done to stabilize its organic matter.

d. Education on conservation tillage options: Awareness on conservation tillage,which has been noted to increase the quantity of fungi in the soil, should be made to land users (whether subsistence or commercial)to enable it or replace conventional tillage methods.

8. Agricultural and economic importance of soil microorganisms

A gram of a healthy soil is home to many different microbes,which performs an important role in the ecosystem, as indicated below. It is also shown in Table 4, the benefits of the soil and the importance of its microbes.

1. Promote plant growth by maintaining soil fertility: certain soil organisms produce a variety of substances that promote plant growth, including auxins and gibberellins, for example, Mycorrhizal fungi and Actinomycetes (Alori & Babalola, 2018; Bargaz et al., 2018; Jacoby et al., 2017)

2. Act as Bio-pesticides: these kill only the disease causing pests and are not harmful to humans and other plant, for example,.Bacillus, TrichodermaandPseudomonas(Hashem et al., 2019;Santoro et al., 2014),

3. Act as Bio-fertilizers: Organisms likeRhizobium, Azotobacter,Azospirillumincreases crop productivity and yield when used as biofertilizers e.g.(Adegboye&Babalola,2016;Wani et al.,2016;Zaim et al., 2017; Igiehon et al., 2019)

4. Fixes atmospheric nitrogen: Bacteria that carries out nitrogen fixation includeAzotobacterandAzospirilium(Shridhar, 2012;Van Dommelen & Vanderleyden, 2007)

5. Decompose organic waste:This is carried out by organisms such asLactobaciliandRhizobium(Cyprowski et al., 2018)

6. Recycle plant nutrients:Examples of microbes that perform this activity areAchromobacter, Azotobacter, PseudomonasandActinobacter(Azco′n-Aguilar & Barea, 2015; Jnawali et al., 2015)

7. Production of antibiotics and vaccines: e.g.BacillusandStreptomyces(Adegboye & Babalola, 2013; Kacem et al., 2009;Sharma & Diwan, 2011)

8. Carbon Cycling: this is achieved by decomposing plant and animal matter leading to the oxidization of carbohydrates for energy and the release of stored carbon into the atmosphere as carbon (iv)oxide e.g cyanobacteria (Gougoulias et al., 2014)

9. Sewarage disposal by decomposition: this assists with environmental sanitation, e.g.LactobacilliandMethanococcus(Cyprowski et al., 2018).

9. Stylized facts on the soil rizosphere maintenance and the UN’s SDG, 2030 vision

The rhizosphere has been noted to contain chemical compounds and microbes,which enhance plant growth,crop development and food productivity (Li et al., 2014; Trouvelot et al., 2014). The rhizosphere habitat is under intense pressure due to an increase in agricultural and other land use activities to supply food for the ever increasing global population.However,effort to maintain a healthy soil are being neglected, as agricultural land users only aim to achieve an abundant crop at the expense of long-term soil sustainability, specifically SDGs 2 and 15.

In an effort to end the loss of biodiversity, prevent starvation,hunger, land degradation and other soil related challenges, and to enhance food security, the United Nations has put in place some goals and targets which will help achieve a more sustainable agricultural system, and restore soil health and quality (Omer and Noguchi, 2020). According to the SDG 2, sustainable food production systems, resilient agricultural practices that increase food productivity, and improved land and soil quality need to be ensured. This will give rise to food security, eradicate hunger and malnutrition, and double agricultural productivity and incomes of small scale farmers. This will require maintaining soil vitality (and the rhizosphere), as a degraded soil cannot maximize crop yields that are needed for food security. Therefore, the soil microbial community must be preserved(specifically the beneficial microbes in this ecosystem) and rejuvenated where necessary.

Complementing SDG 2 is Goal 15,which is aimed at halting and reversing land degradation and biodiversity loss (Omer and Noguchi, 2020). It is targeted at taking urgent and significant actions to reduce the deterioration of natural habitats, restore degraded land and soil, and protect and prevent the extinction of soil threatened species.This will help to inculcate habits to promote the restoration of a soil quality is that will promote sustainable agricultural production. To achieve more sustainable food production system, improved agricultural methods that do not distrupt the rhizosphere habitat and will help to maintain and progressively ameliorate the already damaged soil and rhizosphere quality,must be used. This will lead to improved food production and food security to enable hunger to be eliminted by 2030, and for all living being to have access to nutritious and nourishing foods.

10. Improving agricultural productivity through the rhizosphere microbiome

Soil microbes possess the ability to enhance crop growth, contributes to nutrient uptake and resist plant pathogens and diseases.The use of these potentials will bring about improvement in the quality of food produced and an increase in crop yield which can also be sustained giving rise to increase in food availability, food security and other economic benefits. In other to maximize the prospects of the community of microorganisms in the rhizosphere to achieve the above, there is need for follow up on the important microbes present in this soil region. That is, the organisms that contributes to the excellent performance of this habitat. So as to understand how they influence their ecosystem in other to implement strategies that will strengthen and increase these microbes for the sake of enhancing agricultural productivity. Highlighted below are means by which the rhizosphere organisms can be engaged and optimised for improved crop productivity:

· Soil and seedling innoculation

· Transgenesis

· Breeding

Soil and seedling innoculationhas been established to influence crop productivity positively(Kumar et al.,2017).For example,the innoculation of legumes withrhizobia spp.reduced the external demand of nitrogen by the plant due to nitrogen fixation byrhizobia spp. (Vargas et al., 2014). It also increases the nitrogen fixing bacteria present in the soil as a result of exudate release from the root. Introducing beneficial microbes will modify the rhizosphere microbiome by initiating beneficial traits with the end result of promoting plants performance, productivity and yield.

Transgenesiscan also be helpful in developing stress resistance cultivars that are important for crop improvements (Bender et al.,2016). For example, the introduction ofGluconacetobacter diazotrophicusPAL5 into sugar cane has led to production of drought tolerance plant according to Vargas et al.,2014.This method can be used for the integration of plant and rhizosphere organism behaviour. For example, introducing more carbon excreting crops has the ability to increase the rhizosphere organisms and activity,thus influencing crop growth(Chapparro et al.,2012).Introduction of microbial products through transgenesis have demonstrated 10-20%of increase in crop productivity on economically important crop(Trivedi et al., 2017).

Moreover, Catriona and Singh established in 2014 that, the activity of microbes in the rhizosphere can be manipulated to increase nutrient availability in this region for plant use. This is achieved through development of crops with excellent root features capable of penetrating different soil zones and extracting nutrient, water and beneficial organisms like the nitrogen fixing bacteria, plant growth promoting bacteria and so on. These will have direct and positive impacts on farm productivity leading to increase crop yields.

11. Conclusion and recommendations

Extensive research has demonstrated that inappropriate land use practices jeopardize the community of microbes present in the rhizosphere microbiome, leading to the loss or extinction of organisms and poor agricultural yields.In ignorance,agricultural land users adopt and practice various cultivation procedures, such as pesticide/herbicides and chemical fertilizers to control pest control and boost productivity.Conventional tillage practices that are used to prepare land for farming activities have been shown to only produce positive results in the short-term, while tampering with the living organisms in the soil in the long-term.

Considering the beneficial role of the rhizosphere,the microbes in the soil ensure its health,enable food production and maximum crop yields,while the damaging effects of the various agricultural practices threaten food security and soil sustainability.A global intervention is needed to preserve the health of soil to achieve sustainable agriculture and SDGs 2 and 15 by 2030. International awareness and endeavours are required to address the inappropriate land use practices,especially in the agricultural sector. Soil scientists, research organisations,policy makers and governments are therefore encouraged to create awareness about the harm being done to the soil rhizosphere microbiome. A shift in the use of biological control agents, biofertilizers and other methods that are not chemically based,but are equally effective as pest, diseases and weed controls, should be encouraged.This will help to preserve the health of agricultural soil for the sake of both current and future generations.

Authors’ contributions

O.P.O under the supervision of O.O.B wrote the manuscript.Both Authors agreed on the final version of the manuscript before submission.

Declaration of competing interest

None.

Acknowledgements

O.P.O.appreciate the North west University,South Africa,for the enabled environment provided for the research. O.O.B acknowledged the National Research Foundation for grant(ref:UID123634)which supports several research activities in her laboratory. We also appreciate the effort, contribution and support of Dr A.O Omotayo(for assisting with the original chemical structures used in this article),the effort of peakpase printing and publishing Nigeria limited for the Corel draw figures in this work is equally appreciated. Finally, the Professional English Language editing service rendered by Ms Carrin Martin is appreciated.