Growth rate of bacteria is affected by soil texture and extraction procedure

doi:10.1016/S0038-0717(02)00254-7

Soil Biology and Biochemistry

Volume 35, Issue 2, 1 February 2003, Pages 217-224

https://doi.org/10.1016/S0038-0717(02)00254-7 Get rights and content

Abstract

Effects of soil texture on the extraction efficiency of bacteria from soils and on biosynthetic activity of the extracted bacteria were studied. Bacterial extracts were prepared from three soils of different texture by homogenization (ultrasonication and mixing) or by homogenization–centrifugation at different speeds. Bacterial biosynthetic activity was estimated using thymidine and leucine incorporation techniques. In each step of the extraction procedure, a higher extractability of bacteria was obtained in finer soils than in coarse soil. Also cell-specific growth rates of bacteria were higher in the finer soils than in the coarse soil. However, in all soils, the extracted bacteria always had significantly lower cell-specific thymidine and leucine incorporation rates than the bacteria in soil slurries and thus did not represent so well the bacterial growth in the original soils. The total declines in cell-specific incorporation rates caused by the extraction were larger in fine soil (96–98%) than in coarse soil (90%), but bacteria in the coarse soil were more responsive to only minor intervention. The homogenization–centrifugation method eliminated the differences in bacterial biosynthesis found when working with soil slurries. Therefore, we recommend using of soil slurries or, optionally, soil suspensions to compare bacterial biosynthetic activity among soils of different textures.

Introduction

Bacterial growth reflects biosynthetic activity of a cell. It is the ability of the cell to create new biomass and to divide (Pollard and Greenfield, 1997). The biosynthetic activity characterizes an actual physiological state of the bacterial population and its ability to survive and develop under the given environmental conditions. Bacterial biosynthetic activity can be expressed as growth rate, biomass production g⁻¹ of soil, generation time, or turnover time. Techniques of labeled thymidine (TdR) and leucine (Leu) incorporation by bacteria offer a good possibility of measuring the gross bacterial growth rate in soil, which is not distorted by cell death and predation.

The TdR and Leu incorporation by soil bacteria can be measured directly in a soil or in a soil extract obtained after homogenization–centrifugation of a soil sample. When working with soil, labeled TdR and Leu are added to a soil slurry, therefore, the activity of the entire soil bacterial community is taken into account (Christensen et al., 1989, Bååth, 1990, Michel and Bloem, 1993). The use of the soil extract, i.e. the homogenization–centrifugation method (Bååth, 1992), deals only with an extracted portion of the original soil bacterial community. Although it has been demonstrated that the bacterial community obtained with the homogenization–centrifugation method is not representative of the total bacterial community of the soil (Bååth, 1996), the method has been routinely used in soil microbiology because of its clear advantages. These include the following: (i) the extract contains fewer soil particles than the slurry, causing higher sample homogeneity and lower sorption of TdR and Leu to soil particles, which increases the accuracy of the measurement; (ii) the isotope dilution is lower in the extract than in the soil slurry and, therefore, the actual concentration of available labeled substrate is higher and blank controls are very low; and (iii) the method is markedly less time-consuming (Bååth, 1992).

The homogenization–centrifugation method selects only a certain part of original bacterial community (Bååth, 1996). If bacteria for subsequent TdR or Leu incorporation measurement are extracted from differently textured soils, one should also consider, whether soil texture can notably affect the extraction efficiency of bacteria from soils and especially their physiology. It is generally accepted that soil texture affects bacterial movement and that bacteria can be bound to soil particles in different ways and with different strengths (Tan et al., 1991, Huysman and Verstraete, 1993, Mayr et al., 1999). Bacteria that are more tightly bound to soil particles have higher cell-specific growth rates than bacteria readily released to an extractant by gentle shaking or mixing a soil sample (Bååth, 1996). It suggests that soil texture can affect not only the extraction efficiency of bacteria from soil by homogenization–centrifugation but also the resulting growth rates of the extracted bacteria. The extraction of bacteria prior to an incorporation measurement might not be appropriate, if it is necessary to compare bacterial growth rates among soils of different texture.

Our goal was to test: (1) if soil texture affects the extractability of bacteria and bacterial growth rates and (2) whether homogenization–centrifugation is usable for soils of different textures. We used the TdR and Leu incorporation techniques to measure total and cell-specific biosynthetic activity of bacteria in soil slurries and soil extracts prepared from three soils of different textures using different treatments (homogenization or homogenization–centrifugation at different speeds).

Section snippets

Soils

Three soils of different texture were used in the experiment (Table 1). Soils 1 and 2 were meadows, soil 3 was a forest soil. The soils were collected in different parts of South Bohemia (Czech Republic), at depths of 5–20 cm, in October 2000. The samples did not contain litter material. The fresh soils were sieved (2 mm) and stored in polyethylene bags at 4 °C until used.

Sample treatment

Each soil was divided into six samples: a soil slurry, a soil suspension and four soil extracts. Soil slurries were prepared by

Direct counts of bacteria

The highest direct counts of bacteria (6.3×10⁹ cells g⁻¹ dry soil) were found in the sandy loam, which coincided with the highest C content. Loamy sand and loam contained 2.8×10⁹ and 3.0×10⁹ cells g⁻¹ dry soil, respectively. Bacterial C (Cbact) made up 1.5–3.5% of C content in the studied soils (Table 1).

The soil-sample treatment considerably decreased bacterial counts in soil suspensions and extracts compared with the bacterial abundance in soil slurries (Fig. 1). Extraction efficiencies that

Effect of soil texture

Soil texture significantly affected extraction efficiencies of bacteria by the homogenization–centrifugation method as well as bacterial biosynthetic activity, measured by TdR and Leu incorporation. In each extraction step, higher extraction efficiencies were obtained in finer soils, i.e. in the loam and the sandy loam, than in the coarse-textured loamy sand, as found by Ramsay (1984). She showed that ultrasonication released bacteria not only from coarse particles but also from finer soil

Conclusions

Soil texture affects the extraction efficiency of bacteria from soils and also their biosynthetic activity. The finer the soil the higher the extraction efficiency of bacteria and the higher the biosynthetic activity and turnover of bacteria.

Soil texture in combination with the homogenization–centrifugation cause a distortion of cell-specific TdR and Leu incorporation rates of bacteria. The finer the soil texture, the higher the underestimation of biosynthetic activity of the extracted

Acknowledgements

We thank T. Picek for valuable suggestions and K. Edwards for correcting the English language. The study was supported by the grants structure and functioning of the ecosystem water–soil–plant from MŠMT ČR and MIDAIR from EU.

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Growth rate of bacteria is affected by soil texture and extraction procedure

Abstract

Introduction

Section snippets

Soils

Sample treatment

Direct counts of bacteria

Effect of soil texture

Conclusions

Acknowledgements

Soil Biology & Biochemistry

Soil Biology & Biochemistry

Soil Biology & Biochemistry

Soil Biology & Biochemistry

Soil Biology & Biochemistry

Advances in Microbial Physiology

Geoderma

Journal of Microbiological Methods

Soil Biology & Biochemistry

Water Research

Soil Biology & Biochemistry

Soil Biology & Biochemistry

Soil Biology & Biochemistry