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