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Tayfun Aşkın, Coşkun Gülser, Rıdvan Kızılkaya, Nutullah Özdemir Ondokuz Mayıs University, Faculty of Agriculture, Department of Soil Science, Samsun/TURKEY ABSTRACT In this study, the effects of different numbers of bacterial population on aggregate stability were investigated during the different incubation periods. Same amount of glucose as a substrate was added into each sterilized soil sample. Sterilized soil samples were inoculated with 0, 2, 4, 6 and 8 ml doses of Nutrient Glucose media which included 2x109 bacteria per ml. At the end of 1, 2, 3, 4 and 5 weeks incubation periods, aggregate stability percentages of soil samples were determined using wet sieving method and then used soil samples were placed aside. Generally, 4 and 6 ml inoculation doses of bacteria significantly increased aggregation between soil particles at p < 0.01. Aggregation slowed down after the first three weeks incubation. At the end of the fifth week incubation, probably increase in substrate requirements of bacteria and bacterial attack on products which bind soil particles together resulted in disaggregation in the soil samples inoculated with 6 and 8 ml doses. INTRODUCTION In soils, the primary particles tend to themselves into structural units known as secondary particles or aggregates. Stability of soils against erosion and improvement of the soil physical conditions for plant growth are closely related with aggregation. Aggregation is an important part of soil formation because of influencing the soil behaviors in infiltration, aeration, root penetration, and reducing runoff and erosion. Decreases in soil structures occurs when soil is managed with intensive tillage (Havlin et al., 1990). Soil aggregation and water infiltration increase when cover crops are added to crop rotations (Dormaar and Lindwall, 1989; Meek et al., 1990). Some inorganic materials such as; clay particles, Fe and Al oxides and calcium carbonate can serve as cementing agents within macroaggregates (Hillel, 1982). Biological factors also have a major effect on the development of stable aggregates. The many microbial products such as; polysaccharides, hemiceluloses or uronides, as well as numerous other natural polymers, are attached to clay surfaces by means of cation bridges, hydrogen binding, van der Waals forces and anion adsorption mechanisms. Organic polymers hardly penetrate between the individual clay particles but form a protective capsule around soil aggregates. This, organic products may further promote aggregate stability by reducing wettability and swelling (Hillel, 1982). Stability of microaggregates depends on persistent organic binding agents, whereas macroaggregates are stabilized by plant roots and fungal hype. Soil organic matter is metabolized by a variety of microorganisms to produce polysaccharides that act to bind soil particles into microaggregates (Oades, 1993; Tisdall and Oades, 1982). The objective of this study was to determine the effects of different numbers of bacterial population isolated from the same soil on aggregation at the end of the different incubation periods. MATERIALS & METHODS Soil sample used in this study was taken from 0 to 20 cm soil depth in Merzifon, Turkey. Air dry soil sample was passed through a sieve with 2 mm size opening. Some physical and chemical properties of the soil sample were determined as follows; soil particle size distribution by the hydrometer method (Demiralay, 1993), lime content by Scheibler Calsimeter (Soil Survey Staff., 1993), pH in 1:2,5 (w/v) soil water suspension by pH meter, EC in the same soil suspension by EC meter (Black, 1965); and cation exchange capacity (CEC) according to Bower method (US Salinity Lab. Staff., 1954). Soil organic matter was measured by Walkley-Black method (Kacar, 1994). After the soil sample was added on Nutrient Glucose Agar, grown bacterial colonies were isolated and transferred on Nutrient Glucose Liquid Media which was prepared as including 2x109 total bacteria per ml (Temiz, 1994). 20 g soil sample was transferred into each flask and the flasks were taped with cotton and aluminum foil. The flasks including 20 g soil samples in each were autoclaved at 121 °C for 30 minutes. After 100 mg glucose as a substrate was added to soil sample in each flask, the sterilized soil samples in flasks were inoculated with different numbers of bacteria adding 0, 2, 4, 6 and 8 ml doses of Nutrient Glucose Liquid Media with three replicates. Inoculated soil samples were incubated for 1, 2, 3, 4 and 5 weeks at 25 ± 2 °C. During the incubation periods, soil samples were kept under near the field capacity. At the end of the each incubation period, aggregate stability percentage of the samples were determined using wet sieving method (Kemper, 1965) and placed aside. Soil samples were wet sieved using a sieve with 0,250 mm size opening. An analysis of LSD multiple comparisons test was used to determine the significance of the differences recorded among the total bacterial doses and different incubation periods on aggregate stability percentage (Steel and Torrie, 1980). RESULTS & DISCUSSIONS Soil Properties : Descriptive statistical results for some physical and chemical properties of the soil used in this study are given in Table 1. Soil analyses results can be summarized as; textural class of soil is clay, soil is moderately alkaline in pH, low in organic matter, very high in lime content and non saline according to EC value.  Bacterial Effects on Aggregation : The effects of different inoculation numbers of bacteria on aggregate stability at the end of the incubation periods are given in Table 2. Percent changes in aggregate stability according to average percent aggregation of control (34.74 %) were calculated for every inoculation dose and incubation period and are also given in Table 2. Aggregation in control applications did not show any significant change statistically during the each incubation period at p<0.01. Effect of inoculation doses of total bacteria on aggregation varied for each incubation period. During the five different incubation periods, percent changes in aggregation according to control due to different numbers of bacterial inoculation are given in Figure 1. Aggregation of the samples inoculated with different numbers of bacteria increased during the first three weeks. The higher aggregation for each inoculation dose were obtained significantly at the end of three weeks incubation at p<0.01. The highest aggregation was determined for 6 ml dose (53.91 %) at three weeks incubation. Also, 4 and 6 ml doses showed higher aggregation at the first (42.29 %) and the second (42.46 %) week incubation respectively.  After the first three weeks, these increases in aggregation slowed down during four and five weeks incubation. Aggregate stabilities of all the inoculation doses decreased at the end of five weeks incubation period. Disaggregation or negative percent changes in aggregation according to control were obtained for 6 ml (- 2.85 %) and 8 ml (-9.67 %) of inoculation doses at the fifth week incubation.  In order to see the effects of inoculation of soil with total bacteria on aggregation, percent change in mean aggregate stability of each inoculation dose was calculated according to aggregation in control, regardless of the incubation period differences (Table 2). Percent changes in mean aggregation were ploted versus inoculation doses in Figure 2a. The effect of inoculation doses on aggregation was found significantly higher for 4 and 6 ml doses than for 2 and 8 ml doses at p<0.01. Bacterial activity for aggregation was not enough in inoculation of 2 ml dose due to less binding products by bacterial activity. On the other hand, substance materials in soil were enough to bacterial activity of 4 and 6 ml doses to produce aggregation products between aggregates. However, 8 ml inoculation dose showed a decrease in aggregation. Increasing the number of total bacteria in soil probably decreased aggregation due to more substrate requirements of the bacteria. Because bacteria in soil did not produce enough binding agents between aggregates and probably attacked on some other previous produced products between soil particles.  The addition of energy material results in an increase in the number of microorganisms and their numerous activities (McCalla, 1950). Roberson et al. (1995) found that aggregation was highly correlated with organic C and N. Microbial extracellular polysaccharides due to high C content can be important factors affecting soil aggregation in cultivated soils. Regardless of difference in inoculation doses, percent change in mean aggregate stability for each incubation period was estimated according to aggregation in control (Table 2). Percent changes in mean aggregation were ploted versus incubation periods in Figure 2b. The highest aggregation was obtained at three weeks incubation period. After three weeks incubation, aggregation in soil particles slowed down. Aggregation due to bacterial activity at the fourth week was found less than aggregation of the first three weeks. At the end of five weeks incubation, disaggregation occurred in soil when compared with aggregation of control samples. It may be explained with that consumption of natural binding agents by total bacteria increased with increasing the number of total bacteria after four weeks incubation. The similar results were obtained in the study about the effects of microorganisms on soil aggregation by Aksoy (1973). He also found that increase in aggregation by microorganisms slowed down during the second four weeks of incubation. Incubation effect on aggregation also varied for different soils due to differences in organic matter content. Allison (1947) determined that higher permeability was related with improved aggregation by microorganisms. The lower permeability was due in part to disintegration of soil aggregates. The dispersion was occurred due to attack of microorganisms on the organic materials which bind soil into aggregates. As a results, 4 and 6 ml inoculation doses generally increased aggregation in soil probably due to some bacterial products which bind soil particles together. Aggregation in soil slowed down after the fist three weeks incubation. At the end of the fifth week incubation, disaggregation occurred most probably due to increase in substrate requirements of bacterial population. Many of the soil aggregating substances and the some products by microorganisms are later destroyed by other microorganisms (Waksman, 1952). In conclusion, the inoculation of soil with total bacteria to improve soil structure may be recommended. These inoculation may be effective, if energy materials for bacterial requirements are added to the soil. REFERENCES Aksoy, N. (1973). Mikroorganizmalarla aşılama ve fumigasyonun, muhtelif rutubet seviyelerinde inkubasyona tabi tutulan, bazı Doğu Karadeniz, Doğu Anadolu ve Güneydoğu Anadolu topraklarının agregatlaşmalarına olan etkileri. Atatürk Üniv. Yayınları. No 184. Ziraat Fak. Yayınları No 93. Araştırma No 56. Erzurum. Allison, L.E. (1947). Effect of microorganisms on permeability of soil under prolonged submergence. Soil Sci. 63:439-450. Black, C.A. (1965). Methods of Soil Analysis. Part 1, American Soc. of Agron., No 9. Demiralay, İ. (1993). Toprak Fiziksel Analizleri. Atatürk Üniversitesi Ziraat Fakültesi Yayınları, No:143, Erzurum. Dormaar, J.F., Lindwall, C.W.1989.Chemical differences in dark brown chernozemic Ap horizons under various conservation tillage systems. Can. J. Soil Sci. 69:481-488. Havlin, J.L., Kissel, D.E., Maddux, L.D., Claassen, M.M., Long, J.H. 1990.Crop rotations and tillage effects on soil organic C and N. Soil Sci. Soc. Am. J. 54:448-452. Hillel, D. (1982). Introduction to Soil Physics. Academic Press Limited, 24-28 Oval Road, London. Kacar, B. (1994). Bitki ve Toprağın Kimyasal Analizleri:III, Toprak Analizleri, s:149-165, Ankara Üniversitesi Ziraat Fakültesi Eğitim, Araştırma ve Geliştirme Vakfı Yayınları No:3, Ankara. Kemper, W.D. (1965). Methods of Soil Analysis. Part 1, American Soc. of Agron. , No 9. 511-519. McCalla, T.M., (1950). Studies on the effect of microorganisms on rate of percolation of water through soils. Soil Sci. Soc. Proc. 15:182-186. Meek, B.D., DeTar, W.R., Rolph, D., Rechel, E.R., Carter, L.M. (1990). Infiltration rate as affected by an alfalfa and no-till cotton cropping system. Soil Sci. Soc. Am. J. 54:505-508. Oades, J.M. (1993). The role of biology in the formation, stabilization and degradation of soil structure. Geoderma 56:377-400. Roberson, E.B., Sarig, S., Shennan, C. Firestone, M.K. 1995. Nutritional management of microbial polysaccharide production and aggregation in an agricultural soil. Soil Sci. Soc. Am. J. 59:1587-1594. Soil Survey Staff (1993). Soil Survey Manuel. USDA Handbook No:18, Washington, USA. Steel, R.G.D., Torrie, J.H. (1980). Principles and procedures of statistics.-A biometrical approach. 2nd ed. McGraw-Hill, New York. Temiz, A. (1994). Genel mikrobiyoloji uygulama teknikleri. I. Baskı. Şafak Matbacılık Ltd. Şti. Ankara. Tisdall, J.M., Oades, J.M. (1982). Organic matter and water-stable aggregates in soils. J. Soil Sci. 33:141-163. U.S. Salinity Laboratory Staff (1954). Diagnosis and Improvement of Saline and Alkali Soils. Agricultural Handbook No:60, 1954. Waksman, S.A. (1952). Soil Microbiology. John Willey & Sons. Inc., New York. |