Hose for collecting leachate water
FIGURE 7.4. A scheme of the lysimeter used with forest soils (from Liiri et al., 2002).
intervals, or alternatively run through drought cycles, and leachates drawn off from the collecting bottle underneath to analyze for inorganic nitrogen and organic carbon in them. The experiment was run for 152 weeks, or nearly 3 years. The authors followed microbial community composition using phospholipid fatty acid analysis (PLFA) and BIOLOG to differentiate between bacteria and fungi, and sampled periodically for nematodes, enchytraeids, and microarthropods. They varied pH regimes by applying wood ash to some microcosms and not to others. They observed significant decreases in microarthropod numbers in the first year, followed by gradual increases in numbers of organisms with small body sizes. Enchytraeid numbers followed similar patterns. Nematodes had ready access to all microcosms, and were quite numerous, ranging from 67 to 191 g soil-1 in the controls, and from 98 to 545 g-1 soil in the ash-treated microcosms. The ash had significant effects on the microbial community makeup inside, but not in the soil outside the microcosms. The main effects on pine seedling growth and nutrient dynamics were governed by the abiotic factors of pH and availability of water. These seemed to govern the overall dynamics, in spite of the functional complexity of the soil biota. It would be most instructive to see this experimental design repeated in other habitats and biomes to ascertain the generality of the findings. In addition, it would be useful to compare the fauna at least to the family or genus level to see if finer-grained responses to the experimental manipulations occurred during this nearly 3-year-long experiment.
In an extensive comparison of seven food webs of native and agricultural soils, de Ruiter et al. (1998) modeled energetics and stability, evaluating the roles of various groups of organisms and their interactions in energy flow and community stability. They measured feeding rates, interaction strengths, and impacts of the interactions on food web stability arranged according to trophic position in seven belowground food webs: one from Central Plains, Colorado, in the United States; two tillage manipulations at Lovinkhoeve in the Netherlands; two tillage manipulations at Horseshoe Bend, Athens, Georgia; and no fertilizer and fertilizer additions at Kjettslinge in southern Sweden. De Ruiter et al. (1998) found that only a fraction of the species manipulations had a strong effect on food web structure. Also there was an absence of correlation between the impacts on stability and feeding rates, meaning that interactions representing a relatively low rate of flow of materials can have a large impact on stability, and interactions having a high rate of material flow can have a small impact. Thus the higher-level predatory mites and nematodes had an impact far out of proportion to their biomass, and the contrary was true of the high biomass organisms, namely bacteria and fungi. De Ruiter et al. (1998) urge that future research be focused on the energetic properties of the organisms forming the basis of the patterning of interaction strengths. This is a big order, and one that will require innovative experiments under both laboratory and field conditions. The stakes are high, however, because these studies should help to provide further insights into the nature of biodiversity and ecosystem function.
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