Response of free-living soil protozoa and microorganisms to elevated atmospheric CO2 and presence of mycorrhiza

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Abstract

Possible interactions between mycorrhiza, atmospheric CO2, free-living soil microorganisms and protozoa were investigated in pot experimental systems. Pea plants (Pisum sativum L. cv. Solara) were grown under ambient (360 μl l−1) or elevated (700 μl l−1) atmospheric CO2 concentration with or without the presence of the arbuscular mycorrhizal (AM) fungus Glomus caledonium. It was hypothesised that (1) the populations of free-living soil protozoa would increase as a response to elevated CO2, (2) the effect of elevated CO2 on protozoa would be moderated by the presence of mycorrhiza and (3) the presence of arbuscular mycorrhiza would affect soil protozoan numbers regardless of atmospheric CO2. After 3 weeks growth there was no difference in bacterial numbers (direct counts) in soil, but the number of free-living bacterial-feeding protozoa was significantly higher under elevated CO2 and was significantly reduced in the mycorrhizal treatments. These effects on protozoa disappeared after 5 and 9 weeks. Neither mycorrhiza nor CO2 concentration had any substantial effect on the microbial community structure as evaluated by phospholipid fatty acid analysis. The increased protozoan populations under elevated CO2 suggest increased bacterial production, whereas the lower populations in response to presence of mycorrhiza suggest a depressing effect on bacterial production by AM colonisation.

Introduction

Increased atmospheric CO2 concentrations generally increase plant photosynthetic rates (Stitt, 1991, Curtis and Wang, 1998) and plant growth (Rogers et al., 1994). An increase in the allocation of carbon to the root system has also been frequently reported (Rattray et al., 1995, Cotrufo and Gorissen, 1997), probably as a result of nutrient or water limitation caused by the increased growth under elevated CO2 (Stulen and den Hertog, 1993). This is likely to result in a larger input of organic carbon into the soil through an increase in rhizodeposition (carbon loss from actively growing roots, including exudates, secretions, lysates and sloughed off root cells) under elevated CO2. This provides additional substrate for the microbial community in the soil and may result in a larger microbial production and biomass.

Several studies have actually shown an increased microbial biomass under elevated CO2 (Dı́az et al., 1993, Zak et al., 1993, Dhillion et al., 1996, Cotrufo and Gorissen, 1997). The root symbionts are also likely to be affected by the altered carbon allocation pattern under elevated CO2 (Diaz, 1996) but the response of the arbuscular mycorrhizal (AM) symbiosis is not well understood. Whereas some studies have reported an increase in percentage root length colonised with hyphae (Monz et al., 1994, Dhillion et al., 1996) or an increase in colonisation intensity (Rillig and Allen, 1998), other studies have shown no effect of atmospheric CO2 on mycorrhizal development and functioning (Staddon and Fitter, 1998, Staddon et al., 1999, Gavito et al., 2000).

Mycorrhizal fungi can act as a substantial plant carbon sink and increase the amount of C allocated below-ground (Jakobsen and Rosendahl, 1990); hence, they could play an important role for the response of plants and saprotrophic soil organisms to elevated CO2. Mycorrhizal fungi may alter the quantity and quality of root exudation (Schwab et al., 1984, Bansal and Mukerji, 1994), soil pH and chemical composition (Bago and Azcón-Aguilar, 1997) and release substances into the soil (Wright et al., 1996) that can directly affect soil fungi and bacteria (Filion et al., 1999). Mycorrhizae have induced changes in population composition of soil microorganisms in the mycorrhizosphere (Linderman, 1988, Christensen and Jakobsen, 1993) and have interacted in the root-free mycosphere with bacteria (Ravnskov et al., 1999) and saprotrophic fungi (Larsen et al., 1998, Green et al., 1999). Hence, mycorrhiza may modify the effect of elevated CO2 on the free-living rhizosphere organisms by altering carbon release from the roots and by interacting differentially with the free-living soil microorganisms. These direct and indirect interactions may be important for the response of terrestrial ecosystems to elevated CO2. It has been suggested that an increase in carbon release into the rhizosphere may lead to an increase in nutrient immobilisation by the microbial biomass, resulting in nutrient limitation on plant growth (Diaz et al., 1993). This would act as a negative feedback mechanism on the ability of the plant communities to respond to the increased atmospheric CO2 concentration with increased growth. An alternative hypothesis on the effect of an increase in the input of carbon substrates to soil was put forward by Zak et al. (1993). They observed an increased N mineralisation in soil under Populus grandidentata grown under elevated CO2 and suggested that this could be due to a larger and more active microbial biomass. It is likely that both of these mechanisms (immobilisation due to increased microbial biomass and faster turnover of nutrients due to a more active biomass) are operating simultaneously.

The balance between immobilisation and mineralisation is affected by other components of the soil food web. The protozoa are important predators of bacteria in soil and protozoan grazing often regulates the size of bacterial populations and enhances C and N mineralisation (Ekelund and Rønn, 1994). An increase in protozoan abundance and activity as a response to increased C input to soil might therefore partly counteract the enhancing effect this would have on the size of the microbial biomass and immobilisation of nutrients. There are, however, only relatively few studies of the effect of increased atmospheric CO2 on soil biota which have included protozoa. Lussenhop et al. (1998) observed an increase in protozoan population density in soil under poplar seedlings grown under elevated CO2, and Treonis and Lussenhop (1997) found a shift in the relative abundance of flagellates and amoebae towards fewer amoebae in soil under Brassica nigra grown under elevated CO2.

Our purpose was to investigate the possible interactions between elevated CO2, mycorrhiza and the free-living soil microfauna. However, there are some methodological difficulties with this kind of studies. AM fungi cannot grow without a host plant and they cannot be eliminated selectively from soils without affecting other microorganisms. In order to obtain non-mycorrhizal controls and colonised plants, it is common to use sterilised soil or a propagule-free substrate and then reintroduce mycorrhizal inoculum. Mycorrhizal inoculum, as a rule, also contains other microbes. These unavoidable procedures are likely to result in different microbial communities from those found in field soils. Usually, an attempt to restore similar microbial activity in all treatments is made by adding a mycorrhiza-free filtrate obtained from either fresh field soil or the mycorrhizal inoculum, but generally its success is not tested. We conducted an experiment using partial sterilisation–mycorrhizal inoculation–filtrate inoculation procedures typically used in experiments requiring mycorrhizal and non-mycorrhizal systems and studied the effect of mycorrhizal inoculation and two concentrations of atmospheric CO2 on bacteria, protozoa and nematode numbers.

Section snippets

Experimental design

The experiment was set up as a two-factor experiment with half of the pots exposed to ambient atmospheric CO2 and the other half to elevated CO2. Half of the plants at each CO2 concentration were inoculated with a mycorrhizal fungus and half were left uninoculated. Each treatment combination had six replicates.

Soil

The soil (49.9% sand, 31.8% silt, 16% clay, 1.36% O. M.) was collected from the arable layer at an organic cropping site in Denmark. Soil was air-dried, sieved (8 mm) and mixed with quartz

Mycorrhizal development

Roots inoculated with G. caledonium were more than 50% colonised 5 weeks after planting, with no differences between the two CO2 concentrations. Uninoculated plants had no mycorrhizae until 5 weeks after planting. Some pots became contaminated with another mycorrhizal fungus (fine endophyte) towards the end of the experiment but colonisation in contaminated pots was very low and all pots, except one, had less than 5% colonised root length. Roots of all plants were extensively nodulated (Gavito

Establishment of populations in the soil

The irradiation procedure used for partial sterilisation of the soil is relatively mild and several types of microorganisms survive the procedure (Jakobsen and Gavito, unpublished results). Previously, we observed that a low number of naked amoebae and flagellates also survived this dose of irradiation (Rønn, unpublished results). These surviving microorganisms and protozoa were present in the soil before the mycorrhizal and non-mycorrhizal treatments were reinoculated with mycorrhizal inoculum

Acknowledgements

This work was supported by the Danish Agricultural and Veterinary Research Council. We thank Peter Holter for comments on the manuscript.

References (56)

  • R Rønn et al.

    Optimizing soil extract and broth media for MPN-enumerations of naked amoebae and heterotrophic flagellates

    Pedobiologia

    (1995)
  • P.L Staddon et al.

    Does elevated carbon dioxide affect arbuscular mycorrhizas?

    Trends in Ecology and Evolution

    (1998)
  • R.N Ames et al.

    Rhizosphere bacterial population responses to root colonization by a vesicular-arbuscular mycorrhizal fungus

    New Phytologist

    (1984)
  • G Andrade et al.

    Bacteria from rhizosphere and hyphosphere soils of different arbuscular-mycorrhizal fungi

    Plant and Soil

    (1997)
  • B Bago et al.

    Changes in the rhizospheric pH induced by arbuscular mycorrhiza formation in onion (Allium cepa L.)

    Zeitschrift für Pflanzenernaehrung und Bodenkunde

    (1997)
  • M Bansal et al.

    Positive correlation between AM-induced changes in root exudation and mycorrhizosphere mycoflora

    Mycorrhiza

    (1994)
  • H Christensen et al.

    Reduction of bacterial growth by a vesicular-arbuscular mycorrhizal fungus in the rhizosphere of cucumber (Cucumis sativus L.)

    Biology and Fertility of Soils

    (1993)
  • M.F Cotrufo et al.

    Elevated CO2 enhances below-ground C allocation in three perennial grass species at different levels of N availability

    New Phytologist

    (1997)
  • P.S Curtis et al.

    A meta-analysis of elevated CO2 effects on woody plant mass, form and physiology

    Oecologia

    (1998)
  • J.F Darbyshire et al.

    A rapid micromethod for estimating bacterial and protozoan populations in soil

    Revue d'Ecologie et de Biologie du Sol

    (1974)
  • S Dı́az

    Effects of elevated [CO2] at the community level mediated by root symbionts

    Plant and Soil

    (1996)
  • S Dı́az et al.

    Evidence of a feedback mechanism limiting plant response to elevated carbon dioxide

    Nature

    (1993)
  • S.S Dhillion et al.

    Assessing the impact of elevated CO2 on soil microbial activity in a Mediterranean model ecosystem

    Plant and Soil

    (1996)
  • C.C Doncaster

    A counting method for nematodes

    Nematologia

    (1962)
  • N.J.E Dowling et al.

    Phospholipid ester-linked fatty acid biomarkers of acetate oxidizing sulphate reducers and other sulphide-forming bacteria

    Journal of General Microbiology

    (1986)
  • M Filion et al.

    Direct interaction between the arbuscular mycorrhizal fungus Glomus intraradices and different rhizosphere microorganisms

    New Phytologist

    (1999)
  • A Frostegård et al.

    Microbial biomass measured as total lipid phosphate in soils of different organic content

    Journal of Microbiological Methods

    (1991)
  • A Frostegård et al.

    Dynamics of a microbial community associated with manure hot spots as revealed by phospholipid fatty acid analyses

    Applied and Environmental Microbiology

    (1997)
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