Bacterial and fungal growth in soil: The effect of temperature and substrate addition
Author
Summary, in English
Bacteria and fungi are the main agents in decomposition of soil organic matter. Their activity is determined by the availability and quality of substrate, especially its content of carbon, but also nitrogen and phosphorus. Other environmental factors, which affect the activity of the microbial community in soil, are temperature, moisture and pH. In this thesis the focus was on how temperature and substrate addition affected the growth of the bacterial and fungal communities in soil.
To study the effects of environmental factors on bacteria and fungi in soil with enough sensitivity to allow for a high time resolution, it is necessary to measure their growth rate and not only rely on biomass changes. I have used methods to measure the bacterial and fungal growth rate that rely on incorporation of tracer amounts of radioactive substrates during a short time. With these methods I have investigated:
(i) how addition of different glucose-C concentrations affects the simultaneous development of total respiration and bacterial and fungal growth,
(ii) how different temperatures affect the balance of bacterial and fungal growth when different substrates are available as a carbon source,
(iii) the adaptation of bacterial growth and mineralisation to new temperature regimes,
(iv) the temperature sensitivity of the bacterial community in a hot desert soil.
The main findings of this thesis were:
• Respiration and bacterial growth rate did not coincide in their development over time after glucose-C addition. After glucose addition an initial lag period, with increased respiration but constant bacterial growth, was followed by an exponential phase in both, followed by a drastic decrease in respiration and a much slower decrease in bacterial growth. These findings suggest that only during the exponential period the increase in respiration can be used to estimate the bacterial growth rate.
• With the addition of increasing glucose-C concentrations to the soil the community responded by changing from mainly bacterial growth to mainly fungal growth on the added substrate. Respiration measurements could not detect this switch between these two microbial groups.
• A threshold glucose-C concentration higher than 200 µg per g soil was needed to induce bacterial growth. This was not reflected in the respiration measurements.
• By adding between 500 and 1000 µg glucose-C, no exponential phase in respiration was found, and respiration decreased to low values within 24 h. Still bacterial growth was evident, showing that during periods with constant or decreasing respiration, added glucose-C was not only used for maintenance.
• Temperature strongly affected the rate of development of bacterial and fungal growth after adding substrate, but there were only minor differences in the relative importance of the two groups at different temperatures.
• A two month exposure of soil to a new temperature led to temperature adaptation of the bacterial community when the incubation temperature was above Topt for growth (30 °C). Incubation below 30 °C did not affect the temperature adaptation.
• Even though hot desert soil is characterized by high and seasonally fluctuating temperatures, the temperature sensitivity of the bacterial community in the soil did not change between seasons. Periods with warm temperatures were more important in determining the temperature response of the bacterial community than cold periods. By comparing our Tmin and Topt values with a study from a cold area (Antarctica) we were able to suggest a range of possible values of these cardinal temperatures for soil bacterial communities worldwide.
These results show that direct growth measurements of the main agents in the soil, bacteria and fungi, are important to make predictions on the potential of climate change. Respiration alone cannot give this information how substrate availability and substrate quality will affect the development of the microbial community in soil and their effect on higher trophic levels of the food web.
To study the effects of environmental factors on bacteria and fungi in soil with enough sensitivity to allow for a high time resolution, it is necessary to measure their growth rate and not only rely on biomass changes. I have used methods to measure the bacterial and fungal growth rate that rely on incorporation of tracer amounts of radioactive substrates during a short time. With these methods I have investigated:
(i) how addition of different glucose-C concentrations affects the simultaneous development of total respiration and bacterial and fungal growth,
(ii) how different temperatures affect the balance of bacterial and fungal growth when different substrates are available as a carbon source,
(iii) the adaptation of bacterial growth and mineralisation to new temperature regimes,
(iv) the temperature sensitivity of the bacterial community in a hot desert soil.
The main findings of this thesis were:
• Respiration and bacterial growth rate did not coincide in their development over time after glucose-C addition. After glucose addition an initial lag period, with increased respiration but constant bacterial growth, was followed by an exponential phase in both, followed by a drastic decrease in respiration and a much slower decrease in bacterial growth. These findings suggest that only during the exponential period the increase in respiration can be used to estimate the bacterial growth rate.
• With the addition of increasing glucose-C concentrations to the soil the community responded by changing from mainly bacterial growth to mainly fungal growth on the added substrate. Respiration measurements could not detect this switch between these two microbial groups.
• A threshold glucose-C concentration higher than 200 µg per g soil was needed to induce bacterial growth. This was not reflected in the respiration measurements.
• By adding between 500 and 1000 µg glucose-C, no exponential phase in respiration was found, and respiration decreased to low values within 24 h. Still bacterial growth was evident, showing that during periods with constant or decreasing respiration, added glucose-C was not only used for maintenance.
• Temperature strongly affected the rate of development of bacterial and fungal growth after adding substrate, but there were only minor differences in the relative importance of the two groups at different temperatures.
• A two month exposure of soil to a new temperature led to temperature adaptation of the bacterial community when the incubation temperature was above Topt for growth (30 °C). Incubation below 30 °C did not affect the temperature adaptation.
• Even though hot desert soil is characterized by high and seasonally fluctuating temperatures, the temperature sensitivity of the bacterial community in the soil did not change between seasons. Periods with warm temperatures were more important in determining the temperature response of the bacterial community than cold periods. By comparing our Tmin and Topt values with a study from a cold area (Antarctica) we were able to suggest a range of possible values of these cardinal temperatures for soil bacterial communities worldwide.
These results show that direct growth measurements of the main agents in the soil, bacteria and fungi, are important to make predictions on the potential of climate change. Respiration alone cannot give this information how substrate availability and substrate quality will affect the development of the microbial community in soil and their effect on higher trophic levels of the food web.
Department/s
Publishing year
2013
Language
English
Full text
Document type
Dissertation
Publisher
Department of Biology, Lund University
Topic
- Biological Sciences
Keywords
- Bacterial growth
- Fungal growth
- Respiration
- Carbon loading
- Glucose mineralisation
- Temperature adaptation
- Temperature sensitivity
- Temperature dependence
Status
Published
Research group
- Microbial Ecology
Supervisor
ISBN/ISSN/Other
- ISBN: 978-91-7473-612-0
Defence date
27 September 2013
Defence time
10:00
Defence place
Blue Hall
Opponent
- Serita Frey