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The diversity of microbial populations in the soil ecosystems are far much more than the eukaryotic organisms. In one gram of soil there may be ten billion microorganisms of numerous different species. When studying both the structure and function of an ecological unit, the evaluation of microbial communities must take into consideration not only the distribution and abundance of species but also the redundancy present in microbial community and functional diversity. Gastron () defined functional diversity as the number of distinct processes (thus functions) that can potentially be performed by a community where as functional redundancy is measured as the number of different species within the functional groups present in a community.
To apprehend the character of microbial communities in different soil environments, it is an essence to have knowledge of community function and functional diversity. These implies actual catabolic action expressed by the microorganisms and its potential activity that is the capability of the community to acclimatize metabolism (catabolism) or the relative composition and size of integral populations to varying abiotic conditions such as microclimate and added substrate.
The population dynamics of microorganisms in soil are extremely difficult to assess due to the complex nature of the soil environment. Also the diverse nutritional requirements of microorganisms in soil may not be easily estimated hence isolation of the soil microorganisms and studying them in the laboratory as pure or mixed culture cannot be easily attained. In the case the soil microorganisms can be isolated and brought to the laboratory, there may be not in the same physiological and morphological state as in the soil. In the soil microorganisms interact with other abiotic and biotic entities in the soil so in laboratory cultures it is often difficult to manipulate such conditions hence the ecological balance is not attained.
As a result indirect methods have been developed to study the microbial populations and communities in soil. Soil respiration response is an example of such methods that are widely used. Total population is estimated by measuring the total respiration of the population using soil incubated in jars and carbon dioxide traps. This procedure is based on the fact that all soil microorganisms respire and during respiration carbon dioxide concentration changes. In the case when soil is altered with a compound, the soil microflora responds by either using the compound as a substrate or if it is toxic they may die from it and in such case they do not tackle the compound. When the compound is used as a substrate, the response often results in elevation in the soil population hence increase in soil respiration. But when the compound is toxic, decline in the population is noted by decrease in soil respiration measured by the carbon dioxide released. In recent studies soil respiration have been employed to measure the biomass of microorganisms in soil but fewer studies have been conducted on the population dynamics of specific microbial populations, an example being the microorganisms responsible for biodegradation of toxic compounds in soil. Chemoheterotrophic bacteria for example differ in the specific organic substrates they use as a carbon and energy source for their growth. The considerable diversities in the substrates that are biodegradable and the capability of individual species to catabolize specific substrates have been used for many years to identify and characterise the microorganisms. In this study the effects of different soil amendments on microbial populations were assessed by soil respiration.
Materials and methods
Table 1: Materials and reagents used in the study
-100ml and 50ml beakers
-2L mason jars
-Blank (to measure CO2 in the atmosphere)
-Soil only (control)
-Soil + saw dust (2.5g)
-Soil + paper (2.5g)
-Soil + glucose (2g)
-Soil + ammonium nitrate (1g)
-Soil + engine oil (5ml)
-Soil + roundup (xenobiotic compound)(0.02g)
-Soil + chicken manure (2.5g)
200g of fresh soil was measured into the 2L Mason jars and water was added to bring the moisture holding capacity to 60% and mixed well. To each jar the corresponding treatment to the soil was added and mixed well. Then 25ml of 0.5N NaOH was measured using a burette into a 50ml beaker and placed into the Mason jar. Also approximately 5ml of tap water was poured into a test tube and the tube placed into the Mason jar as a way of maintaining relative humidity. The jars were then tightly sealed and incubated at room temperature for a week.
After a week of incubation, the beaker containing NaOH was removed from the jar and to it drops of BaCl2 was added to precipitate the excess carbonate as BaCO3. Then few drops of phenophthanein indicator were added. Using a burette 0.5N HCl was titrated to the unneutralised alkali until an end point was reached (change from pink colour to clear milky white). The amount of the acid used was recorded for each treatment. After titration the beakers were then washed and another fresh 25ml of 0.5N NaOH was added and then the jars were reincubated. The amount of carbon dioxide evolved during the week was then calculated using the formula CO2= (B-V)NE where V is the volume of acid used in titration, B volume of acid used to titrate the blank, N normality of the acid and E is the equivalent weight ( if data is expressed in terms of carbon E is 6 and if expressed as CO2 E is 22)
Following another week of incubation, beakers of NaOH were removed from the jars and then titration was carried out following the same procedure as the past week. The same procedure was also duplicated in reincubation of the jars, the only modification was the introduction of the Rossy cholodyney slides which were buried in the soil according to the procedure of their preparation.
The following the Rossy cholodyney slides were removed. Heat fixation was carried out and the slides were stained with crystal violet and methylene blue and kept for microscopic observation. The NaOH containing beakers were also removed from the jars and titration with the acid was carried out as before. Reincubation was also done but now the slides were not included.
Finally after the last incubation titration was carried out. The stored Rossy cholodyney slides were then observed using the microscope. On a final step all glassware were cleaned and the soil treatments were disposed off in plastic bag. Results obtained were then analysed by ANOVA.
Results and analysis