Biodiversity and Biological Degradation of Soil

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1 GENERAL I ARTICLE Biodiversity and Biological Degradation of Soil Upasana Mishra and Dolly Wattal Dhar Upasana Mishra and Dolly Wattal Dhar are working at the Centre for Conservation and Utilization of Blue Green Algae, IARI, New Delhi. Upasana Mishra has done research on the taxonomy and biology of Rivulariaceae. Dolly Wattal Dhar has wide experience of research in biodiversity, conservation and characterization of blue green algae. Soils contain enormous numbers of diverse living organisms assembled in complex and varied communities. Microscopic examination of a soil sample reveals the presence of billions of organisms like nematodes, protozoa, fungi, algae, actinomycetes, bacteria and cyanobacteria. These diverse organisms interact in the ecosystem, forming a complex web of biological activity. Environmental factors, such as temperature, moisture and acidity, as well as human activities such as agricultural and forestry management practices, affect soil biological communities and their functions. Soil biology is an interesting area of soil research and has yielded considerable information that is used in soil fertility management. Soil organisms playa vital role in the sustainable functioning of ecosystems. They act as the primary driving agents of nutrient cycling, regulate the dynamics of soil organic matter, soil carbon sequestration and greenhouse gas emissions, modify soil physical structure and water regimes, and influence plant health. Microbially Mediated Soil Processes Carbon, which enters the soil from plant sources (e.g., as cellulose) is usually released as carbon dioxide or methane. Disturbance in soil carbon cycling, which has been observed under field conditions, is the result of several environmental factors. Keywords Microbial biodiversity, soil science, biogeochemical cycles, sustainable agriculture, ecology. Nitrogen cycling includes processes like mineralization, immobilization, nitrification, denitrification, and nitrogen fixation, of which nitrogen fixation and nitrification are most easily disrupted. Nitrifying bacteria (e.g., Nitrosomonas and Nitrobacter) are reported to be sensitive to acidic environment and require aerobic conditions. Waterlogged soils can become anoxic and -2A ~~ R-ES-O--N-A-N-C-E-I--Ja-n-u-a-rY

2 GENERAL I ARTICLE thus may not support nitrification. Symbiotic nitrogen fixation by bacteria in root nodules of legumes is known to be a delicate and complicated phenomenon, and can be disrupted by some kinds of pollutants. Phosphorus cycling can be affected by processes that interfere with mycorrhizal fungi. Cycles involving sulfur, iron, manganese and other elements also depend upon the significant roles played by microorganisms. Indicators of Biological Degradation of Soil Community diversity can be used to assess perturbation of soil biology. Bacteria and fungi can be evaluated by 'viable count' methods; most involve plating on nutrient-rich agar. These methods are, however, controversial and sometimes provide erroneous results. Biodiversity of soil microbial communities can also be assessed by various mathematical indices that emphasize richness (number of species), equitability (evenness of allocation of individuals among the various species), or combinations of these two aspects. Soil organisms playa vital role in the sustainable functioning of ecosystems. They act as the primary driving agents of nutrient cycling, regulate the dynamics of soil organic matter, soil carbon sequestration and greenhouse gas emissions, modify soil physical structure and water regimes, and influence plant health. Nutrient cycling can be indexed by measurement of soil enzymes, various components of the nutrient cycle, cellulose or wood degradation, and respiration. Accumulation of pollutants. Toxins may accumulate if soil microbial life is degraded. The soil's ability to dechlorinate organic compounds can be impaired, especially in sulfate-rich anaerobic environments. Heavy application of animal wastes to low-ph soils can lead to buildup of ammonium ions and a concomitant reduced functioning of Nitrobacter, which would lead to nitrite accumulation. Redox status. Anaerobic conditions can arise as a result of compaction or water logging. Oxygen diffusion is only 1/10,000 as fast through water-filled soil pores as it is through air-filled pores. Production of large amounts of methane would indicate the strongly reducing conditions associat~d with anoxia. -R-ES-O-N-A-N-C-E--I-J-a-nu-a-r-Y ~

3 GENERAL I ARTICLE Organic pesticides show less dramatic effects on soil microbiology than do other classes of toxic organics. Factors Affecting Biological Degradation of Soil Toxic substances Organic compounds are usually more speedily and readily degraded in warm rather than cold climates. Toxic metals, on the other hand, can remain in soils and cause long-term damage to soil microbial communities, even in warm climates. Pesticides can influence soil microbial activity, at times paradoxically. For example, glyphosate or diquat + paraquat application can cause a buildup of Gaeumannomyces graminis var. tritici, the causal agent of 'take-all disease' of wheat, as well as reductions in CO 2 production, cellulose degradation, and nitrogenase activity. Nitrification and symbiotic nitrogen fixation are sensitive to pesticides, probably due to the small numbers of microbial genera involved. At normal application rates, Amitrole, 2,4-DB, and diallate can inhibit nitrification for at least 8 weeks, whereas atrazine, bromacil, picloram, and simazine may inhibit the process for shorter periods. Some degradation products of these substances may also be inhibitory. For processes like symbiotic nitrogen fixation, denitrification, and ammonification, there are reports regarding inhibition, no effect, and stimulation effects of these compounds. Soil respiration is relatively insensitive to pesticide application, but antimicrobial compounds like fungicides, can suppress the process. Toxic organic and inorganic pollutants can result from chemical synthesis, coal mining, and petroleum processing. Organic pesticides show less dramatic effects on soil microbiology than do other classes of toxic organics, probably because the former are screened to avoid such effects. Oil spills in cold regions cause long-term damage to soil microbial populations, including adverse effects on carbon (e.g., cellulose degradation) and nitrogen cycling (especially nitrification and nitrogen fixation). Contamination by heavy metals (e.g., Cd, Cu, Ni, Pb, Zn) can cause long-term suppression of carbon cycling, microbial biomass, nitrogen fixation, nitrification, dehydrogenase activity, and ~~ R-ES-O--N-A-N-C-E-I--Ja-n-u-a-rY

4 GENERAL I ARTICLE mycorrhizal incidence. Toxic metals can be abundant in some sewage sludges. Microbes can mobilize and increase the toxicity of cadmium, perhaps by producing water-soluble ligands or otherwise changing soil properties. Acidific'ation can occur when excavation of ores containing iron sulfides leads to oxidative production of sulfates. Discharge waters from mine spiels may have ph values of 1-2, and can result in severe problems related to soil biology. Waters draining from coal mines and associated spiels can be high in potentially toxic metals like Zn, Cu, Ni, or Mn. Effects of Land Management In addition to the effects of pesticides and other toxic materials, soil biology may also be affected through various land management practices, including forest management and crop production, In addition to the effects of pesticides and other toxic materials, soil biology may also be affected through various land management practices, including forest management and crop production. Burning of forests can lead to increased soil ph and enhanced availability of Ca and Mg. These hot fires may also lead to loss of S, P, and B, and destruction of surface structure resulting in lowered infiltration and increased runoff and erosion. Cool fires would cause formation of hydrophobic surface layers that decrease infiltration and consequent soil moisture. Fires may also inhibit certain fungi whose mycelia previously imparted a hydrophobic character to the soil surface. Under these conditions, infiltration can be increased. If fire darkens the soil surface and increases insolation by opening the forest canopy, soil temperatures can be increased. Effects of fire on soil microbiology are usually transitory, and mainly involve the surface strata. Cutting of hardwood deciduous or coniferous forests leads to a reduction of litter inputs, increased microbial breakdown of litter, and a consequent reduction in forest-floor biomass, decreased shading, and increased soil temperature and ph. The latter two phenomena result in increased nitrification, which in turn may lead to nitrate-rich runoff and pollution of nearby streams. Under such conditions, increased release of nitrous oxide, a significant greenhouse gas, may also occur. -R-ES-O-N-A-N-C-E--I-J-a-nu-a-r-Y ~~

5 GENERAL I ARTICLE Soil erosion is particularly deleterious to organic matter, which is less dense than other soil solids available. The clay fraction of soil, to which organic matter is often adsorbed, is vulnerable to erosion. Microbial communities appear to be tolerant to normal farming practices, but may not meet the nitrogen requirements of crops. Fallowing would lead to reduced microbial biomass, mainly through reduction of fungi. The addition of manures would increase carbon and nitrogen levels in soils without affecting the population of bacteria, fungi, or protozoa. Soil erosion is particularly deleterious to organic matter, which is less dense than other soil solids available. The clay fraction of soil, to which organic matter is often adsorbed, is vulnerable to erosion. Vesicular-arbuscular mycorrhizae evam) can be adversely affected even by mild soil erosion. Survival of rhizobia in eroded soils may depend on their tolerances to ph of deeper soil horizons. Erosion can lead to decreased microbial activity due to loss of organic matter and exposure of inhospitable deeper horizons. Tillage, particularly of virgin lands, can lead to immediate and short-term increases in microbial biomass and metabolism. On the other hand, soil meso fauna may decline. Earthworms are often adversely affected by tillage practices. Incorporation of crop residues disperses microbial activity to a greater depth than is seen in reduced-tillage systems. The latter show aerobes, facultative anaerobes, and nitrifers mainly concentrated in the ~urface strata. Reduced-tillage CRT) practices supply less N than that of conventional tillage. Long-term changes under tillage appear to be related to gradual decline in soil organic matter, rather than to cultural practices per se. Tillage of virgin soils results in accelerated degradation of organic matter and consequent deterioration of soil structure with the increased threat of erosion. Under a given farming system, stabilization may occur with time, but reversal of the initial degradation is unlikely. Restoration of soil microbial communities may occur after cultivation ceases, but the recovery time is unknown ~ R-E-S-O-N-A-N-C-E--1 -Ja-n-u-a-rY

6 GENERAL I ARTICLE Soil Health Maintenance and Organic Agriculture Reaping the benefits of soil biological activity for agricultural production depends upon the application of several ecological principles. Different soil organisms occupy different niches and favour different substrates and nutrient sources. Most soil organisms rely on organic matter for food. Thus, a rich and varied supply of organic matter in the soil will generally support a wider variety of organisms. Crops should be mixed as far as possible and their spatio-temporal distribution should be varied, to create a greater diversity of niches and resources that stimulates soil biodiversity. Diverse habitats support complex mixes of soil organisms, and crop rotation or inter-cropping, therefore, encourages the establishment of a range of soil organisms and improves nutrient cycling. Soil biodiversity can also be nurtured by improving soil living conditions, such as aeration, temperature, moisture, and nutrient quantity and quality. In this regard, reduced soil tillage and minimized compaction and refraining from the use of chemicals is of particular significance. Along with effective water and crop management, the optimal use and management of soil fertility and soil physical properties is important for sustainable agriculture. Enhancing soil biological activity is crucial to building up long-term soil productivity and health. The practice of organic agriculture has been shown to significantly enhance soil biological activity and biodiversity. Organic agricultural practices such as manipulation of crop rotations and strip-cropping, use of green manuring and organic fertilizer (animal manure, compost, crop residues), minimum tillage and avoidance of pesticides and herbicides are known to improve soil fertility status and nutrient cycling as summarized below. Organic agriculture increases the abundance of beneficial arthropods (insects, spiders, etc.) living above ground, as well as earthworms. Mycorrhizal colonization of roots is highest in crops in unfertilized systems, followed by organic systems. Reaping the benefits of soil biological activity for agricultural production depends upon the application of several ecological principles. Crops should be mixed as far as possible and their spatio-temporal distribution should be varied, to create a greater diversity of niches and resources that stimulates soil biodiversity. -R-ES-O-N-A-N-C-E--I-J-a-nu-a-r-Y ~

7 GENERAL I ARTICLE Mycorrhizae are fungal associations with the root systems of plants that greatly enhance nutrient uptake by the plants. Conventional crops have mycorrhizal colonization levels that are substantially lower. Mycorrhizae are fungal associations with the root systems of plants that greatly enhance nutrient uptake by the plants. In organically managed soils, the activity of microorganisms like fungi and bacteria is high and organic soils therefore, not only mineralize more actively, but also contribute to the build up of stable soil organic matter. Thus, nutrients are recycled faster and soil structure is improved, with little light fraction material remaining undecomposed in soils managed organically. The total mass of microorganisms in organic systems is 20-40% higher than in the conventional systems with manure, and 60-85% higher than in the conventional systems without manure. The ratio of microbial carbon to total soil organic carbon is higher in organic system as compared to conventional systems of agriculture. Thus, organic management promotes soil microbial carbon and as a result soil carbon sequestration potential is enhanced. Organic practices also enhance biodiversity. Large organic fields (over 15 ha area) exhibit occurrence of wild flora upto six times more abundant than conventional fields. In organic grassland, the average number of herbs is 25 percent more than in conventional grasslands. Vegetation structure and plant communities in organic grasslands are more even than in conventionally managed systems. In particular, field margin strips of organic farms and semi-natural habitats conserve weed species that are endangered or at risk of extinction. Animal grazing or rooting activity (e.g. by pigs) is also important in enhancing plant species composition in any system. In organic orchards, weeds are sown in strips to reduce the incidence of aphids that can influence the diversity and abundance of arthropods. Many flowering weeds are particularly beneficial to pollinators. Organic land management allows the development of a relatively rich weed-flora as compared to conventional systems. Some 'accompanying plants' of a crop are desired and considered useful in organic management. The presence of versatile flora attracts beneficial herbivores and other air-borne or above ~~ R-ES-O--N-A-N-C-E-I--Ja-n-u-a-rY

8 GENERAL I ARTICLE ground organisms. Their presence improves the nourishment of predatory arthropods. When comparing diversity and the demand of energy for microbial maintenance, it becomes evident that diverse populations need less energy per unit biomass. A diverse microbial population may divert a greater part of the available carbon to microbial growth rather than maintenance. In agricultural practice this may be interpreted as an increased turnover of organic matter with a faster mineralization and delivery of plant nutrients. Finally, more organic matter is diverted to build-up of soil humus. Suggested Reading [1] NCBrady, TheNatureand Properties o/soils, 10th ed., Macmillon, New York, [2] A Burges and FRaw, Soil Biology, Academic Press, London, pp.532, Organic soil management also improves soil structure by increasing soil activity and thus, reduces erosion risk. Organic matter has a positive effect on the development and stability of soil structure. Silty and loamy soils are benefited from organic matter by an enhanced aggregate structure. Organic matter is adsorbed to the charged surfaces of clay minerals. The negative charge decreases with increasing particle size. Silt is very susceptible to erosion since it is not charged, but organic matter layers on the silt surface favor aggregates with silt too. Conclusion Increasing population pressure, unsustainable use of natural resources, continuing loss of farm land for non-agricultural uses and continuing destruction of ecosystems rich in biological diversity are all leading to a situation where ensuring food security may become a serious challenge. Strip mining and metal contamination are serious threats to soil microbial communities, as is the application of pesticides. Remediation of damage caused may require long time, and damaged sites that return to relatively healthy status may not return to their pristine states. Injudicious use of natural resources, mining for exploitation of earth reserves and industrialization is depleting soil organic matter which in turn may be eroding soil biodiversity and in long term affecting soil fertility and agriculture productivity. Address for Correspondence Upasana Mishra and Dolly Wattal Dhar National Centre for Conservation and Utilisation of Blue-Green Algae Indian Agricultural Research Institute New Delhi , India. -R-ES-O-N--A-N-C-E-I-J-a-n-u-a-ry ~