Turmeric (
Curcuma longa L.) which belongs to the family Zingiberaceae is a plant with historical connotations and a cultural reputation, widely used as a dye, condiment and medicine. Turmeric is believed to have originated in southeast Asian countries such as India, China and Vietnam, but is now cultivated globally in the tropics and subtropics
(Fuloria et al., 2022). The plant grows extensively throughout the country, mainly in states including Telangana Andhra Pradesh, Odisha, Tamil Nadu, West Bengal, Assam, Maharashtra, Bihar, Karnataka and Kerala
(Mirjanaik et al., 2020). One of the most expensive plants on the world market is the Curcuma plant which plays a major role in India’s spice export and economy
(Nair, 2019), India is the global leader in turmeric production, accounting for approximately 80% of the annual global output. Additionally, India excels in the export of turmeric and value-added turmeric products
(Srinivasan et al., 2016; Nair, 2013).
Turmeric output and land have increased during the last ten years as demand in international markets has risen
(Aarthi et al., 2020; Srinivasan et al., 2016). As a result, individuals are employing various management approaches to boost production, including the use of chemical fertilizers (conventional nutrient management), which may compromise the soil’s long-term viability. The quality of turmeric grown with chemical and inorganic fertilizers is currently compromised for export value and organic turmeric is gaining popularity in international markets with premium prices. Soil fertility influences both the quality and productivity of turmeric, which can be increased by effective agronomic management measures
(Nandhini et al., 2023).
Bacteria, fungi, archaea and other microorganisms in the rhizosphere play crucial parts in nutrient cycling, organic matter breakdown, disease prevention and plant growth promotion
(Kumar et al., 2022). These interactions have a major effect on turmeric growth and yield. Different management approaches in turmeric production have emerged as a key aspect of long-term agricultural operations. Effective fertilizer management not only ensures optimum crop yields but also has an enormous effect on the diversity and function of the microbial community in the turmeric rhizosphere
(Nandhini et al., 2023). The impact of agricultural activities on soil quality has received international attention and it is vital to highlight the importance of measuring changes in soil quality as a result of these operations. A soil amendment treatment’s performance is determined by its ability to directly supply and encourage the synthesis of metabolites that are advantageous to the host plant, consequently improving nutrient absorption, disease resistance and yield
(Berihu et al., 2023). Different management regimes such as crop rotation, fertilization, irrigation and pest control profoundly impact the microbial properties of the rhizosphere and overall soil health and agricultural productivity. Within the field of soil quality, biochemical metrics representing microbial processes such as microbial biomass carbon and soil enzyme activities occupy center stage due to their sensitivity and importance, because biologically mediated processes in soils are vital to their ecological roles
(Dick, 1992).This review focuses on understanding how different management approaches affect soil biology and microbial diversity in turmeric, which are key components towards a sustainable agricultural future.
Effect of management practices and their impacts on plant growth and soil health parameters
Turmeric thrives on a wide range of soils; nevertheless, because it is a nutrient-intensive crop, it requires appropriate organic manures and fertilizers to produce optimal outcomes. Maximum absorption of nutrients occurs during the active development phase (fourth and fifth months after planting), emphasizing the importance of timely delivery of nutrients such as nitrogen (N), phosphorus (P) and potassium
(Srinivasan et al., 2016). India has seen a significant shift towards organic farming practices in recent decades. This shift has led to a growing interest in understanding the impact of different management practices on soil biological processes and microbial diversity in turmeric cultivation. Though India leads in turmeric production, average productivity is exceedingly low due to an unequal and improper addition of chemical fertilizers and organic manures
(Kandiannan and Chandaragiri, 2008).
Organic agricultural practices rely on renewable resources and assure long-term environmental sustainability. Organic farming affects the soil’s physical, chemical and biological traits, improving its fertility. In comparison to chemical fertilizers, it is less expensive, less polluting and a sustainable practice.
Kumar et al., (2018) study on turmeric indicated that applying farmyard manure, vermicompost and neem cake was the most effective nutrient management approach. Plant growth metrics including plant height, leaf area, tiller count and stem girth have all increased with the use of organic fertilizer. The use of organic amendments was shown to be effective in terms of yield, which might be attributed to the nutrient’s ease of access and improved plant growth parameters. A comparable study conducted by
Chamroy et al., (2015) demonstrated that a combination of 50% N (urea) + 50% N (poultry manure) organic fertilizer was more suitable in terms of turmeric growth, yield and quality when compared to the application of chemical fertilizers (60: 50: 120 kg NPK/ha).
Kale et al., (1992) discovered that the application of vermicompost boosted the activity of beneficial microorganisms such as N2 fixers and mycorrhizal fungi colonization; they play a major part in N
2 fixation and Phosphate mobilization, resulting in better nutrient uptake by the turmeric plant, which in turn leads to higher yield.
Nandhini et al., (2023) conducted a study on a 28-year-long organic nutrient management field and found that there is a 48% increase in turmeric yield and a 62% rise in profitability in terms of net return. Furthermore, the study revealed considerable increases in soil biological characteristics, including higher soil enzyme activities and increased soil microbial biomass, linked to the long-term application of sustainable organic nutrient management strategies. Similar results were reported by
Sahu et al., (2024).
The use of biofertilizers such as
Rhizobium sp.,
Azotobacter sp. and
Azospirillum sp.
(Macik et al., 2020) or changes in nutrient profiles has a considerable impact on soil microbes in the turmeric rhizosphere
(Virk et al., 2024). For example, using biofertilizers can introduce beneficial bacteria into the soil, increasing nutrient availability and stimulating plant development. This can cause a shift in the makeup of soil microbiota, favoring beneficial microbes that improve the general health and productivity of turmeric plants. Furthermore, fertilizing strategies might modify the nutrient requirements and preferences of soil microbial communities, affecting their quantity and activity in the turmeric rhizosphere.
Roy and Hore (2011) found that the use of
Azospirillum aids in improved plant development due to their nitrogen fixing abilities and they were able to produce plant growth-enhancing hormones such as Indole Acetic Acid and Gibberellic Acid
(Satyanarayana et al., 2013). Tariq et al., (2024) reported that plant defense molecules such as polyphenols and phenols were higher in bioinoculant-treated plants, especially in
Pseudomonas putida treated plants.
Integrated nutrient management (INM) involves a combination of organic manures, bio-fertilizers and chemical fertilizers to improve crop yield and quality while preserving soil health. INM strives to provide balanced nutrient availability, encourage sustainable farming and reduce environmental impact. To minimize losses due to leaching and volatilization, Integrated Nutrient Management (INM) strategically integrates nutrient sources to guarantee that nutrients are accessible in optimal proportions at the right times
(Paramesh et al., 2023). This approach enhances nutrient use efficiency and mitigates environmental pollution. Reduced reliance on chemical fertilizers decreases the risk of groundwater contamination and lowers greenhouse gas emissions associated with fertilizer production and application
(Brodt et al., 2011). While the initial setup costs for composting units or bio-fertilizers may be high, the long-term benefits and reduced dependence on expensive chemical fertilizers can ultimately lower overall production costs
(Ahmed et al., 2023; Nongbet et al., 2022; Nandhini et al., 2023; Hajam et al., 2023; Kantwa et al., 2023). The recommended dose of fertilizer supplies only two or three major nutrients, whereas the combined use of organic manure and fertilizer can provide an adequate amount of all essential nutrients
(Tripathi et al., 2021). This encourages healthy plant growth and has a direct impact on crop productivity.
Dinesh et al., (2010) demonstrated that the administration of organic manures (such as farmyard manure, vermicompost, neem cake and ash) and biofertilizers, in conjunction with chemical fertilizers, positively impacted microbial biomass, carbon content, nitrogen mineralization, soil respiration and enzyme activities.
Srinivasan et al., (2016) reported that integrated nutrient management enables a reduction of up to 50% in the application of inorganic nitrogen fertilizers, significantly mitigating the adverse environmental effects of high-analysis chemical fertilizers.
Conventional nutrient management (CNM) or modern farming practices rely heavily on chemical fertilizers to improve plant growth and yield and applying various doses of N, P and K has been shown to significantly boost growth traits and total dry matter output
(Prasath et al., 2019). However, it is crucial to understand that this comes at a cost, such as detrimental effects on the environment and possible risks to human health
(Thakur et al., 2020). According to
Wang et al., (2012) and
Nandini et al., (2023), conventional farming methods are linked to a number of environmental problems, such as nitrate leaching, soil erosion, pollution and a decrease in soil organic matter, all of which eventually compromise soil quality. Ammonium fertilizers have the potential to lower soil pH because they increase the release of protons, which is a byproduct of improved nitrification processes and ammonium uptake by plants. This soil acidification can lead to nutrient deficiencies, reduced crop yields and deterioration of soil fertility
(Barak et al., 1997) in addition to which acidification of the soil can suppress the viability and activities of the rhizosphere bacterial community
(Kunito et al., 2024; Rousk et al., 2010). A study on nutrient management systems in turmeric by
Srinivasan et al., (2016) found that CNM registered lower levels of soil organic carbon, which in turn decreased soil microbial biomass carbon
(Dlugosz and Piotrowska-Długosz, 2023) and activities of enzymes such as acid phosphatase, β-glucosidase and dehydrogenase.
Rahman et al., (2021) and
Prasath et al., (2019) reported that soils exclusively treated with inorganic fertilizers exhibited a limited microbial community due to the reduced availability of organic substrates. The impact of different management practices on soil biology and rhizosphere microbial community of turmeric (
Curcuma longa L.) is shown in Fig 1.
The role of soil microorganisms in turmeric cultivation
The microbial colonization in plant organs is influenced by a range of biotic and abiotic stresses. Significant effects are played by variables such as root exudates, moisture content, pH, salinity, type of soil, texture, structure, organic matter,
(Kumawat et al., 2022; Saeed et al., 2021; Yadav et al., 2018; Jacoby et al., 2017; Zilber-Rosenberg and Rosenberg, 2008) and factors such as altered nutrient composition which include soil carbon, soil available N, P, K, C : N ratio
(Kumawat et al., 2022; Bulgarelli et al., 2013, Peiffer et al., 2013), presence of Phyto-pathogens and environmental conditions.
Donn et al., (2015) reported that microbial communities in the rhizosphere and rhizoplane are influenced by factors such as plant age, plant species and environmental conditions,undergoing dynamic modifi-cations over time. In contrast, microbial communities in bulk soil remain relatively stable
(Kong et al., 2024). This finding highlights that microbial communities surrounding the plant root system are influenced by plant-released metabolites, allowing the plant to preferentially support beneficial organisms. Studies on sugarcane, potato and model plants such as Arabidopsis
(Kumawat et al., 2022) have demonstrated genotype-dependent variability in the rhizosphere’s microbial population.
Vinayarani and Prakash (2018) isolated
Bacillus cereus from the rhizosphere of
Curcuma longa. This isolate exhibited several beneficial properties, including the production of indole-3-acetic acid (IAA) and hydrogen cyanide (HCN)
(Rana et al., 2023), phosphorus solubilization, siderophore production and both cellulase and protease activity. Furthermore,
Bacillus cereus promoted greater plant height while reducing rhizome rot caused by
Pythium aphanidermatum and leaf blight caused by
Rhizoctonia solani.
Kumar et al., (2019) found nine bacterial strains in the
Curcuma longa rhizosphere to investigate their role in plant growth and health. All isolates exhibited plant growth-promoting characteristics such as phosphate solubilization, indole-3-acetic acid (IAA) synthesis and ammonia production. Furthermore, practically all strains demonstrated antifungal activity against
Aspergillus niger and
Alternaria alternata.
The rhizosphere is the most intricate out of several niches due to its significant influence on plant growth and nutrition
(Bandyopadhyay et al., 2017; Lakshmanan et al., 2014). It is the zone of soil surrounding plant roots, hosts a diverse community of microorganisms, including bacteria, fungi and archaea. This zone is characterized by complex interactions and a diverse community structure
(Kumawat et al., 2022; Saeed et al., 2021). Turmeric rhizomes interact with numerous microbial species in the rhizosphere, some of which enter plant tissue as endophytes. Rhizospheric and endophytic species play a direct or indirect role in plant development and disease control and regulate factors such as morphology of growth, secondary metabolite production, curcumin concentration and antioxidant capabilities,
etc.
(Tripathi et al., 2022; Kumar et al., 2017).Three groups of PGPR mainly, α, β and γ-Proteobacteria were found to be prevalent in the turmeric rhizosphere. Dominant species include
Pseudomonas,
Klebsiella,
Agrobacterium,
Azotobacter and
Burkholderia, representing nearly two-thirds of the Proteobacteria isolates
(Kumar et al., 2017). Additionally, bacterial endophytes like
Penibacillus,
Klebsiella,
Bacillus and
Pseudomonas (Vinayarani and Prakash, 2018) are significant inhabitants of the turmeric rhizome
(Khan et al., 2023). Beneficial microbes enhance plant growth by improving nutrient uptake through the solubilization of essential minerals such as phosphorus (P), potassium (K) and zinc (Zn) and by nitrogen fixation. They also produce plant growth promoting hormones such as auxin [(indole-3-acetic acid (IAA)], cytokinins and gibberellic acids
(Khan et al., 2023; Hakim et al., 2021; Suman et al., 2022). PGPRs contribute to the biocontrol of phytopathogens by producing antibacterial, antibiotic and antifungal compounds.
Kumar et al., 2018 reported that bacterial strains, including
B.
subtilis CL1,
Bacillus sp. CL3,
Klebsiella sp. CL6,
B.
cereus CL7,
P.
putida CL9 and
P.
fluorescens CL12, exhibit significant antibacterial and antifungal properties, they produce bioactive compounds such as Ecomycins, Munumbicins, Pseudomycins and Kakadumycins that inhibit pathogenic bacteria and fungi
(Christina et al., 2013). These isolates formed zones of inhibition against fungi like
A.
niger,
A.
alternata and
B.
fulva and inhibited
E.
coli, except for
A.
tumefaciens CL5 and
A.
chroococcum CL13. Additionally, the isolated PGPRs produced iron-chelating siderophores, stimulated systemic resistance and alleviate biotic and abiotic stresses, enhancing the accumulation of secondary metabolites like curcuminoids in turmeric rhizomes. By understanding and leveraging these microbial interactions, it is possible to optimize turmeric cultivation practices for better yield and plant health
(Khan et al., 2023).
Apart from conventional methods, molecular techniques such as the multi-omics approach have provided new opportunities to study the rhizosphere microbiota mostly the unculturable population and investigate its impact on plant growth and health
(Hellal et al., 2023; Sharma et al., 2020). Metagenomic sequencing methods highlight the dynamics of microbial community modifications, allowing exploration of their functional impacts
(Berihu et al., 2023). These techniques allow researchers to identify and quantify microbial populations, measure processes in the rhizosphere and simulate natural conditions in the laboratory
(Azubuike et al., 2016). By merging molecular approaches with traditional methodologies, we can acquire a better knowledge of the biotic and abiotic variables that shape the rhizosphere microbiota, as well as the complicated mechanisms that govern its impact on plant development, yield and disease resistance
(Fadiji et al., 2023). Understanding the soil microbiota is critical since it is a key component in attaining sustainable agriculture. This information enables us to limit the usage of artificial fertilizers and pesticides, improving soil quality for future generations. By understanding the natural interactions within the soil microbiome, we can promote healthier, more robust agricultural systems that are less reliant on synthetic inputs and more environmentally sustainable
(Saeed et al., 2021).
Understanding soil microbiota in turmeric rhizosphere
The rhizosphere has long been a source of great scientific interest, as it plays an important role in plant health, nutrient cycling and ecosystem functioning. Recent advances in the study have provided new light on the intricate dynamics of this critical microhabitat. One key area of advancement is the understanding of root exudates and their impact on the rhizosphere microbiome
(Ansari et al., 2023; Afridi et al., 2023). Root exudates, a diverse array of organic compounds or metabolites released by plant roots, have been shown to play a pivotal role in shaping the microbial communities that thrive in the rhizosphere
(Sharma et al., 2023; Olanrewaju et al., 2019). These exudates serve as a source of energy and nutrients for the microbiome, while also acting as signaling molecules that can influence the composition and activity of the microbial community
(Pantigoso et al., 2022; Sun et al., 2021). Understanding the dynamic metabolic fluxes in root exudates can be exploited to create a plant prebiotic in which substrates or precursors of biosynthetic pathways are added to boost the growth of specific microbial species or augment microbial activities that benefit the plant host
(Song et al., 2021a). Most environmental microbes (>95%) are unculturable
(Singh et al., 2009), indicating that only a small fraction of potentially beneficial microorganisms can be cultivated and engineered for agricultural applications. By utilizing metagenomic sequencing techniques, it is possible to reveal the workings of microbial community transitions and investigate their functional consequences about modifications in management regimes. Additionally, this will make it possible to apply the strategy to different agricultural systems and regions
(Berihu et al., 2023). Understanding the metabolite-driven successional trajectories within the soil microbial communities would allow for more targeted manipulation of the native soil microbiome to improve plant health. Therefore, leveraging the inherent capabilities of the extensive indigenous plant microbiome allows for the identification and utilization of novel and enhanced microbial functions
(Qiu et al., 2019).
Salas-Gonzalez et al. (2021) demonstrated that root diffusion barriers in the endodermis of Arabidopsis regulate microbiota assembly and homeostasis, while also being influenced by plant-associated microorganisms. The plant endodermis comprises Casparian strips and suberin, which restrict the diffusion of microbe-associated molecular patterns, essential for establishing the root microbiome and nutrient uptake. Suberin deposition varies with the plant’s nutritional status and is regulated by hormones such as ethylene and abscisic acid
(Barberon et al., 2016). Microbes can interact with abscisic acid signals to modify root diffusion barriers, thereby enhancing plant stress tolerance.
Pacheco-Moreno et al. (2024) discovered that the plant genotype influenced the recruitment of microbial populations into the rhizosphere of barley cultivars, resulting in varied microbiome composition and
Pseudomonas abundance driven by root exudate secretion. Later a small number of loci in barley were found to be associated with the differential recruitment of taxonomically varied rhizosphere bacteria, with an NLR-like gene serving as one of the primary drivers of this phenomenon
(Escudero-Martinez et al., 2022). Another recent study demonstrated how FERONIA, a previously acknowledged receptor kinase
(Guo et al., 2018; Duan et al., 2010 Stegmann et al., 2017), regulates basal levels of reactive oxygen species to adversely affect
P.
fluorescens colonization in
Arabidopsis thaliana (Song et al., 2021b). According to
Wang et al., (2021), it is observed that certain microorganisms and a given host’s genetic makeup have an interrelationship.
Advancements in rhizosphere biological research
The rhizosphere microbiome is a complex network and strategically engineering microbial populations to enhance or optimize specific functions remains a substantial problem due to its lack of predictability
(Afridi et al., 2022; Thakur et al., 2023; Kaur et al., 2022). However, advancements in technologies such as genomics and metagenomics are rapidly advancing our understanding of microbial community structure and dynamics
(Satam et al., 2023). By integrating these technologies with metabolic modeling approaches, we can systematically elucidate the architecture of ecosystems
(Berihu et al., 2023). Through this enhanced understanding, beneficial microbial communities can be encouraged to colonize the rhizosphere. Additionally, plant genetic makeup can be manipulated, particularly the genes involved in attracting beneficial microbes, to promote the aggregation of beneficial species in the rhizosphere, thereby improving plant growth and productivity
(Backer et al., 2018).Genomic-based algorithms can help to develop testable hypotheses for strategically manipulating the rhizosphere microbiome. By finding specific substances that may operate as selective modulators of microbial communities, these algorithms can help design ecologically sound techniques to re-establish functional microbiomes in agricultural and other settings
(Berihu et al., 2023).
Nanofertilizers, composed of nanoparticles carrying macro- and micronutrients, offer a promising alternative to chemical fertilizers. These engineered materials facilitate targeted nutrient delivery and controlled release within the plant rhizosphere thereby minimizing nutrient loss through leaching and runoff and reducing degradation and volatility. Encapsulation within nanomaterials enables slow nutrient diffusion into the soil, thereby enhancing soil fertility and crop productivity over time
(Nongbet et al., 2022). Due to their high surface area-to-volume ratio and superior penetration ability, nanofertilizers improve crop productivity by promoting seed germination, nitrogen metabolism, photosynthesis, protein and carbohydrate synthesis and stress tolerance
(Rautela et al., 2021).
Rhizosphere engineering is a novel approach to encouraging beneficial microbe establishment and restoration of agro-ecosystems. Considerable evidence shows that plant growth, productivity and health are strongly influenced by plant-microbe inter-relationship
(Hakim et al., 2021; Masood et al., 2020; Htwe et al., 2019; Adesemoye and Egamberdieva, 2013) enhancing these relationships by engineering the rhizosphere can improve plant health and productivity, while also protecting against both biotic and abiotic stresses. This represents an eco-friendly and sustainable approach to agricultural production
(Hakim et al., 2021).In terms of rhizosphere engineering,
Song et al., (2021a) examined numerous ways to create a habitat for beneficial microbiomes. A significant advancement in this field is the development of versatile and scalable synthetic microbial community (Syn Com) systems. Synthetic microbial communities (Syn Coms) are made up of three or more well-defined and trackable microbial strains designed to tackle specific research objectives. These consortia are critical for validating hypotheses and providing a practical insight into the interactions among various microbes, plants and environmental factors and helping to close existing knowledge gaps
(Marin et al., 2021). These systems involve the assembly of defined and controlled microbial communities from a broad collection of microbes, facilitating the study of microbial interactions within various natural and artificial microbiomes, including those of hosts
(Berg et al., 2020; McCarty et al., 2019). The goal is to simplify microbial community complexity by monitoring and tracking changes in microbial inoculants in the environment. This strategy seeks to achieve reproducibility in field applications, boosting robustness in colonization, crop yield and resilience against biotic and abiotic challenges in agriculture
(Chen et al., 2022; Mishra et al., 2022). Recruitment of microbiota varies with plant variety.
Zhang et al., (2019) discovered differences in nitrogen-use efficiency between two rice varieties,
Indica and
Japonica. Through microbiome profiling and genetic analysis, they identified the nitrogen transporter
NRT1.
1B as key to recruiting
indica-enriched bacteria. Based on this, they designed SynComs with operational taxonomic units enriched in either
indica or
japonica (Marin et al., 2021). The
indica-enriched SynCom significantly enhanced rice growth.Furthermore, research has shown that multistrain microbial inoculants, such as SynComs, boost plant growth more consistently and effectively. For instance, SynComs provided longer-lasting plant protection compared to individual strains
(Lee et al., 2021).
