Protocol for the Derivation of Environmental and Human ... - CCME

Appendix A
McGill and Cole (1981) reviewed patterns of cycling of carbon, nitrogen, sulfur, and phosphorus in
soils. They concluded that nitrogen and to a lesser extent, sulfur transformations in soil were closely
linked to the energy needs of heterotrophs searching for organic carbon. Organic phosphorus and a
portion of organic sulfur, on the other hand, is mineralized by extracellular enzymes (phosphatases and
sulfatases) partly in response to biological demand for this element. As a result of these different
mechanisms, phosphatase and sulfatase activity varies with the phosphorus and sulfur status of the soil
(Spiers and McGill 1979, Maynard et al. 1984), and is a source of variability in any potential bioassay.
Moreover, some enzymatic activity may be stabilized in soil outside the microbial cell (Stewart and
Tiessen 1987), adding further uncertainty to the interpretation of sulfur and phosphorus mineralization
rates or estimates of phosphatase and sulfatase activity. Mineralization rates of phosphorus and sulfur
are therefore not good candidates to report on soil quality in relation to contaminants. Carbon and
nitrogen transformations, however, are well suited to report on soil quality in relation to contaminants.
Rates of decomposition (mass loss of organic substrates) and respiration (CO 2 evolution) are common
measures of carbon cycling and are potentially useful for assessing contaminant effects. These measures
integrate the activities of soil heterotrophs and therefore report on community-level performance. This
integration indicates a high degree of ecological relevance. However, respiration also reflects functional
redundancy since many heterotrophic organisms carry out the same process during catabolic energy
generation (Paul and Clark 1989). Biological redundancy in respiration and decomposition is
responsible for relatively high levels of resistance and resilience to stressors such as contaminants.
Therefore, at contaminant levels sufficient to inhibit respiration or decomposition, it is likely that
significant impacts have occurred in at least a segment of the heterotrophic community. Conversely,
such impacts are likely to be mitigated by compensating actions by resistant, re-colonizing or physicallyprotected
organisms. Respiration and decomposition are therefore expected to have limitations as
indicator processes for contaminant effects.
Nitrogen cycling in soils is complex and involves a broad range of soil organisms. Some nitrogen cycling
processes such as mineralization, immobilization and denitrification (Figure A.1) are carried out
incidentally by generalists during the catabolism of carbon-rich substrates (McGill and Cole 1981).
These processes share the same limitations for respiration and decomposition. Other nitrogen cycling
processes are carried out by more specialized organisms with narrower ecological amplitudes.
Nitrogen fixation (Figure A.1), the process by which nitrogen is added to soils from atmospheric N 2 , is
carried out only by a very limited range of specialized bacteria. The root nodule symbionts Rhizobium
species and Frankia species are particularly important. Nitrification, the oxidation of ammonium to
nitrate, is performed by only a few genera of chemoautotrophic bacteria. It is important in soil because
the mobility of nitrate allows plants to acquire large amounts of nitrogen — poorly mobile in other
chemical forms — in the mass flow of water to roots. In a given soil, often only two species of nitrifiers
are involved.
The nature of the energy metabolism of both nitrogen-fixers and nitrifiers make them susceptible to
stressors. Nitrogen-fixers require large amounts of energy, in a micro-anaerobic
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