Authors: Sunanda Biswas1 and Bharat Hanamant Gawade2
1Division of Soil Science and Agricultural Chemistry, ICAR-IARI, New Delhi-110012
2Plant Quarantine Division, ICAR-NBPGR, New Delhi-110012
*Corresponding author, email: sunandabiswas13@gmail.com
Introduction
Soil health can be defined as the continued capacity of soil to function as a vital living system, within ecosystem and land-use boundaries, to sustain biological productivity and maintain their water quality as well as plant, animal, and human health. Soil is living because below our feet there is an immensely complex web of life that includes the smallest bacteria and fungi to burrowing mammals. Soil microorganisms perform a wide range of functions: they decompose organic matter, release nutrients into plant-available forms and degrade toxic residues; they also form symbiotic associations with plant roots, act as antagonists to pathogens, influence the weathering and solubilization of minerals and contribute to soil structure and aggregation. The majority of vascular plants are associated with arbuscular mycorrhizal or ectomycorhizal fungi and benefits from an increased capacity to extract phosphorus and other nutrients from the soil. Mycchorizal fungi thus have an important role in plant community development, nutrient cycling and the maintenance of soil structure. Being an integral part of soil and as most of function of soil is microbial mediated process; soil organisms play an important role in maintaining soil health and sustainable production year after year.
Important groups of organisms commonly present in soil
• Macro fauna (Termites, Ant, Earthworms)
• Mesofauna (Collembolan, Acari)
• Microfauna (Nematodes, Protozoa)
• Macroflora (Root of Higher Plants)
• Microflora (Bacteria, Fungi, Actinomycetes, Algae)
Termites
Termites are major decomposers in most tropical terestrial ecosystems, responsible for the mineralization of up to 30% of net primary production (mostly as CO,) in some systems and the breakdown of up to 60% of litterfall. Subterranean termites enhance macroporosity and infiltration with beneficial effects on soil water storage and primary productivity.
Ant
Ant species diversity declines with increasing latitude, altitude, and aridity. Soil ants (including mound builders) are representatives of predators, herbivores (granivores) and bioturbators, bringing about important changes in the physical and chemical properties of soils, as well as dispersing plant propagules. Networks of galleries and chambers increase the porosity of the soil, increasing drainage and soil aeration and reducing bulk density.
Earthworms
Bouche classifed earthworms as epigeic, endogeic and anecic, depending on whether they inhabit litter, soil or both. Each group has particular morphological and behavioral adaptations, which in turn produce different pedological effects. Earthworm cast is a rich source of nutrients, particularly N, P, Ca, etc. and contains more bacteria and organic matter. Dead tissues of earthworm decompose faster due to their high protein content. Geophagous species of earthworms ingest material per day which is 5-36 times of their body weight. Casting rates of tropical earthworms are reported to be as high as 2600t/ha/yr. Earthworms do intimate mixing of organic matter with mineral matter which increases the stabilization of clay bound carbon, depending on soil type. Earthworm worked soils generally have high porosity, increased water holding capacity, higher water infiltration rate, more water-stable aggregates and increased availability of plant nutrients. These organisms may also affect microbial population as they ingest microbes along with soil and organic matter.
Collembolan
Collembolan or springtails are small wingless insects. They are well differentiated into ecomorphological groups occupying different soil horizons. Most are highly specialized feeders on soil microbiota (fungi, bacteria, actinomycetes, algae). Some mix small mineral particles with dead organic matter in their guts and contribute by their faecal pellets to soil microstmctures.
Nematodes
Soil nematodes are microscopic (about 1-1.5 mm) roundworms that live in water films around soil particles. Nematodes are a major component of all soil food webs and thus comparisons of abundance, biomass and community structure can be made across ecosystems. Functional groups are based on morphology and known feeding habits of a few species, and in most soils include plant parasites and plant grazers, bacterivores, fungivores, predators, and omnivores. Plant parasites and plant grazers are the best known of soil nematodes, due to the damage they cause to agricultural crops, i.e. decreasing plant production, disrupting plant nutrient and water transfer, and decreasing fruit and tuber quality and size. Soil disturbance, weather pollution, erosion, pesticides, or water quality, affects nematode species composition. For this reason they are used as indicators of soil health. Besides plant parasite nematodes there are some beneficial species also which play an important role in essential soil processes like nitrogen mineralization and distribution of biomass within the plants.
Protozoa
Protozoa are microscopically small, unicellular organisms. It is assumed that only 10% of soil protozoans are known. Vickerman suggested that the total number of species is close to 40 000. Protozoa are, with nematodes, the principal microbial grazers in terrestrial systems. By classifying protozoa based on feeding preferences (bacterial or fungal), habitat preferences (acidophilic or neutrophilic) or ecological weightings, it may be possible to relate changes in diversity and/or biomass to ecosystem functioning.
Root of Higher Plants
Plant roots which have immense role in soil formation, its fertility and productivity, can also be considered as soil microorganisms. Plant roots exercise tremendous influence on soil properties. However, being the primary source of organic matter in soil, plants and their roots supply food and energy to saprophytic soil microorganisms and maintain soil biological activity. By producing different chemicals as root exudates, mucilages, mucigel and lysate, root influence the chemical and biological environment of soil around them. Root rhizospheres have specific kind of niche for the proliferation of microbes.
Bacteria
Have diverse metabolic capacities: They have diverse metabolic capacities that allow them to exploit the wide range of energy sources in soil.
Primary agents of biogeochemical transformations: They mainly take part in C, N, P and S cycles and the decomposition of dead plant and animals which recycle nutrients for plant growth. They produce extracellular enzymes for the decomposition of polymeric substances.
Nutrient mobilizers: They also act as nutrient mobilizer, for example, a wide range of soil bacteria, Pseudomonas, Bacillus are involved in solubilization of inorganic P and they are also involved in the transformation of metal in soils, e.g. Chemolithotrophic oxidation of Fe2+ under acidic condition is carried out by Thiobacillus ferroxidans.
Plant growth promotory hormones: They are also known to produce some hormones like IAA, auxin etc. which promote plant growth.
Bio-control agents: Being bio-control agents they help to protect the plant from pollutants and plant pathogens. There are also some soil bacteria which directly feed on other bacteria and maintain a biological equilibrium in soil by keeping a check on the growth of other bacteria, for ex. Bdellovibrio, Myxococcus etc.
Fungi
Have predatory role: Certain fungi are predators which attack protozoa and nematodes in soil and play important role in biological equilibrium in soil, for ex. Rhizopus, Mucor, Penicilium.
Main decomposer of cellulose in acid soil: In acid soil fungi are main decomposer of cellulose as bacteria become inactive ex. Penicillium, Trichoderma etc.
Decompose protein, lignin and form humus: They also play important role in the decomposition of protein through proteolytic enzymes and ammonification, ex. Fusarium, Mucor. Basidiomycetes are capable of decomposing lignin, but the rate of breakdown is very slow. Certain sp. of fungi, Alternaria, Aspergillus etc. are important in the synthesis of soil humus.
Mobilizer of less mobile nutrients: The role of fungi in the oxidation of elemental and reduced forms of inorganic S is well known. Similar to micorrhizal symbionts, some free living fungi (Aspergillus, Penicillium) also excrete organic acids and Fe siderophores that solubilize insoluble forms of phosphate.
Some micorrhizal fungi that penetrate roots and form specialized structures, such as vesicles and arbuscles within the cortex are called vesicular- arbuscular micorrhizae (VAM). For ex. Glomus, Endogene have been reported to increase the uptake of P and biomass production and to enhance resistance against drought and certain root- infecting pathogens.
Actinomycetes
They are next to bacteria in abundance in soil. They are aerobic, heterotrophic, slow growing, and highly adaptive to degrade wide range of organic substances. Actually they start functioning when easily decomposable fractions like sugars, starches etc. are already used up by bacteria and fungi. They are also responsible for the synthesis of humus and are reported to produce a no. of colour pigments contributing dark colour of soil humus.
Algae
Estimates show that populations of algae in soil vary between 10 and 106 per gram of soil. Valuable nitrogen inputs to soil are made by the blue green algae due to their capacity to fix nitrogen, for ex. Anabaena associated with aquatic fern, Azolla growing in rice field can produce total biomass of 20 tonnes/ ha. Because of their photosynthetic capacity, they contribute to the organic carbon input of soil, and also produce extracellular polymers that may help to conserve soil structure.
Some beneficial microorganisms
Nitrogen fixing organisms
They are potent N2 fixer. They fix atmospheric N2 by two ways-
• Symbiotic N2 Fixation: Rhizobium sp. in association with root nodules of leguminous plants, fix molecular N2 from atmospheric air. They also reduce atmospheric N2 and make it available to plants.
• Nonsymbiotic N2 Fixation: Azotobacter, Azospirillium, Clostridium etc. are capable of fixing molecular N2 from air without any symbiosis.
Through biological nitrogen fixation, 180 x 106 tones nitrogen per year is being added to the soil but still it is not sufficient to replace completely the use of chemical fertilizers. Various N fixing systems shares this global fixation and the estimate of contribution of each component is given in table 1.
Table1. Nitrogen fixing system with their contribution
| Nitrogen fixing system | Estimated contribution (kg/h/year) |
| Free living | 15 |
| Cyanobacteria | 7-80 |
| Aossciative Bacteria | 36 |
| Azolla/Anabaena | 4.5-450 |
| Frankia | 2-362 |
| Rhizobium -legume | 24-585 |
(Prasad et al.,1990)
Phosphate solubilizing microorganisms
Phosphorous in soil is available chiefly as orthophosphate and can generally be categorized as soil solution P, insoluble inorganic P or insoluble organic P. Besides fertilization, the availability of P can be achieved by two pathways (a) the enzymatic decomposition of organic P compounds and inorganic P compounds and (b) the non enzymatic solubilization of different rock phosphate and inorganic phosphorous sources. Many fungi, bacteria, actinomycetes and cyanobacteria are potential solubilizers of bound phosphate in soil (Banik & Dey, 1983; Singh & Kapoor, 1992; Vazquez et al., 2000). Some important P solubilizing fungi, bacteria and actinomycetes are listed in table 2.
Table 2. List of some important phosphate solubilizing microorganisms.
| Group | Members |
| Fungi | Aspergillus niger, A. nidulans, Cephalosporium sp |
| Bacteria | Pseudomonas, Bacillus |
| Cyanobacteria | Anabaena, Calothrix, Nostoc, Scytonema |
(Banik & Dey, 1983)
font color="green" size="3">Cellulolytic microorganisms
Cellulolytic capacity is found in large number of microorganisms (table 3). Those organisms have in common the capability of elaborating extracellular hydrolytic enzymes that attack the cellulose polymer. Fungi are most active ones with respect to cellulose degradation and cellulase production. Fungi possessing the necessary enzymes for the cell free degradation of crystalline cellulose generally belong to Ascomycetes and Deuteromycetes groups or to the white-rot Basidiomycetes . (Saha et al.2009).
Table 3. Most widely studied cellulolytic microorganisms
| Fungi | Bacteria |
| White-rot | Aerobic |
| Phaenerochaete chrysosporium | Pseudomonas fluorescens |
| Coriolus versicolor | Bacillus subtillis |
| Brown-rot | Anaerobic |
| Poria placenta | Clostridium thermocellum |
| Lanzitus trabeum | Acetovibrio cellulolyticus |
| Soft-rot | Actinomycetes |
| Fusarium oxysporum | Thermoactinimyces curvata |
| Aspergillus niger | Streptomyces flavogriseus |
| (Saha et al.,2009) |
Conclusion
To cope up with ever-increasing population rate, declining productivity and degrading land resources it is necessary to maintain soil health for sustainable agriculture system. Current agricultural practices reduce soil biodiversity, mainly as a result of the overuse of chemicals, leading to compaction or other disturbances and hence irreversible adverse ecological alterations, resulting in loss of agricultural productivity. As biodiversity is important for soil functioning and as living organisms are reliable indicators of environmental quality, providing the best reflection of the actual fitness of their habitat and ecological changes therein, diversity, abundance and activity of soil organisms may indicate the degree of sustainability of soil management. There is a need for a holistic consideration of soil health as well as transdisciplinary soil management approaches that integrate biological, chemical, and physical strategies to achieve soils supporting a sustainable agriculture.
References
1. Banik, S & Dey, BK. (1983). Phosphate solubilizing potentiality of the microorganisms Capable of utilizing aluminium phosphate as a sole phosphate source. Zentral Microbiol 138, 17-23.
2. Bouche, M.B. (1972). Lombriciens de France. Ecologie et systematique. Ann. Soc. Ecol.
3. Prasad, B., Sharma, S.N., Singh, S. & Prasad, M. (1990). Nitrogen Management. In soil Fertility and fertilizer Use. Vol. IV Marketing Division, IFFCO, New Delhi.
4. Saha , N and Mukherjee, D.(2009). Cellulolytic Bacterial Diversity-Its Importance in Agriculture. In: Agriculturally Important Microorganisms-II. Chapter-7, George,K. Arora,D.K. and Srivastava,A.K.(eds.). World Academic International, Varanasi, India.
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WORKING AS A SCIENTIST IN ICAR-IARI