Experimental Results

In a tallgrass prairie, increases in soil microbial activity have been measured under chambers where the vegetation was exposed to increased [CO2] (Rice et al., 1994). The increases in this case were restricted to dry years. In wet years, the [CO2] had no effect on the microbial activity. The effect of increased [CO2], therefore, seems to depend on the increased soil moisture as a result of decreased transpiration of the grass when [CO2] went up. However, Schortemeyer et al. (1996) observed a doubling of the population of Rhizobium leguminosarum bv. trifolii in the rhizosphere of white clover growing under 600 ppm [CO2] vs. white clover growing under ambient [CO2] (350 ppm). Heterotrophic and ammonium-oxidizing bacteria did not respond to the same treatment. The rhizosphere of perennial ryegrass growing under the same conditions showed no changes in bacterial populations. This indicates that N fixation might increase with increasing [CO2] (Zanetti et al., 1995). Responses of soil fauna to [CO2] have also been observed (Yeates et al., 1997), but results are conflicting (Runion et al., 1994; Ross et al., 1995). Again, indirect effects associated with soil moisture may be the most important factor (Harte et al, 1996; Briones et al., 1997).

Experimental determination of the effects of climatic change on SOM are difficult because of the long times required to reach conclusive results, the perturbations caused by the experimental set-up, and the costs of conducting the experiments. A number of techniques are currently being used

(van de Geijn, 1998), but some of these experiments are so newly established that results are not yet available.

In an experiment with cotton under elevated [CO2] in Arizona, increases in soil C were measured after 3 years of increased biomass production if the crop was irrigated (Wood et al., 1994). In a parallel 2-year experiment with wheat, soil C storage also increased (Prior et al., 1997). However, other studies showed no differences (Ross et al., 1995; Nitschelm et al., 1997; Torbert et al., 1997). These conflicting results may be due to the short observation periods. Observed changes in C isotope signatures (Nitschelm et al., 1997; Torbert et al., 1997) have demonstrated clear changes in processes regulating soil C storage. A possible explanation for the absence of increases in soil C stocks is that the soil has become C-saturated (Hassink and Whitmore, 1997). The C-storage capacity of a soil depends on how much protection can be provided by soil mineral particles. If this storage capacity is exceeded, additional C will be immediately attacked and respired by decomposers.

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