Experimental Additions And Deletions In Soil Biodiversity Studies

Additional studies of biotic roles of soil fauna and bacteria and fungi have been approached in two ways. One approach is by gamma irradiating sieved soils and inoculating them with suspensions of full-strength, 102, 104, and 106 dilutions of soil organisms (Griffiths et al., 2001). The other approach subjects unsterile soils to chloroform fumigation and incubation (Griffiths et al., 2000), and tracks the subsequent changes in functional variables such as ammonium, nitrate, soil respiration, etc., in relation to microbial biomass and diversity, as measured by DNA patterns on denaturing gel electrophoresis (DGGE). Results were divergent in the two studies, with the chloroform fumigation simplification of community biomass and species diversity having a direct impact on the functional stability, as measured by the physiological response variables. In contrast, although there were progressive declines in biodiversity of the soil microbial and protozoan populations, there were no consistent changes in functional parameters. Some functions showed no trend (thymidine and leucine incorporation, nitrate accumulation, respiratory growth response), some a gradual increase with increas ing dilution (substrate induced respiration), some declined only at the highest dilution treatment (short-term respiration from added grass, potential nitrification rate, and community level physiological profile), while others varied even more idiosyncratically. At no stage were any of the physiological functions eliminated completely. The final commentary on this by Griffiths et al. (2001) is that within any realistic sort of range of changes in biodiversity to be experienced by soils, there will be no direct effect on any soil functional parameters measured. Other authors, for example, Wardle et al. (1999) suggested that it is possible to overcome selective species effects by (1) measuring the effects of all species in monoculture and (2) by species removal experiments. Neither of these approaches is feasible with current technology, so this problem awaits the attention of a future generation of soil ecologists.

Another approach to microcosm studies was taken by the large group working in the Ecotron controlled-environment facility at Silwood Park in the United Kingdom. Constructing analogs of a temperate, acid, sheep-grazed grassland in northern Britain, Bradford et al. (2002) established terrestrial microcosms of graded complexity, with soil, plant, and microorganisms, and then assemblages of microfauna, micro-and mesofauna, and then micro-, meso-, and macrofauna. This functional group approach provided a range of metabolic rates, generation times, population densities, and food size. The microcosms were maintained in the Ecotron for a period of 8.5 months. Bradford et al. (2002) found significant increases in decomposition rate in the most complex faunal treatment, but both mycorrhizal colonization and root biomass were less abundant in the macrofauna treatments. Interestingly, plant growth was not enhanced in these treatments, despite higher nutrient (nitrogen and phosphorus) availability. Contrary to initial hypotheses, neither aboveground net primary production (NPP) (plant biomass) nor net ecosystem production (net CO2 uptake) were enhanced in the most complex microcosms. Bradford et al. suggested that respiration was most likely buffered by the combined stimulatory effect of both meso-fauna and macrofauna on microbes (see Chapters 4 and 5), which served to maintain microbial activity at a level equivalent to that in the microfauna and mesofauna communities. This study has served as a benchmark in large-scale microcosm studies, but as Bradford et al. (2002) note, it is not a substitute of longer-term in situ field studies, as difficult to conduct and interpret as they may be.

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