Stages in the breakdown and decay of CPOM

The sequence in the breakdown of CPOM, well documented for autumn-shed leaves in temperate streams (see reviews by Barlocher 1985, Webster and Benfield 1986), is illustrated in Figure 7.3. Leaves fall directly or are windblown into streams, become wetted, and commence to leach soluble organic and inorganic constituents. Most of the leaching occurs within a few days and is followed by a period of microbial colonization and growth, causing numerous changes in leaf condition. The next stage, fragmentation by mechanical means and invertebrate activity, usually follows some period of softening of tissue by microbial enzymes, and is complete when no large particles remain. Although the general model suggests sequential stages, leaf decomposition is a complex process and some of the events can occur simultaneously. For example fragmentation can occur during microbial colonization and not just at the end of the process, and invertebrate colonization may begin shortly after leaves enter the stream (Gessner et al. 1999, Hieber and Gessner 2002).

As much as 25% of the initial dry mass of freshly abscised leaves is lost due to leaching in the first 24 h. Constituents lost during leaching are primarily soluble carbohydrates and polyphenols (Suberkropp et al. 1976). Leaves of different plants show species-specific leaching rates: alder (Alnus rugosa) lost only about 4% of dry mass over several days whereas elm (Ulnus americana) lost 16% in an early study by Kaushik and Hynes (1971). Release of DOC

FIGURE 7.3 The processing or ''conditioning'' sequence for a medium-fast deciduous tree leaf in a temperate stream. Leached DOM is thought to be rapidly transferred into bioflms by microbial uptake.

by leaves of several plants in a stream in British Columbia, Canada, also revealed substantial differences in leaching rates (McArthur and Richardson 2002). During the first day, Western hemlock needles (Tsuga heterophylla) lost 14% of the total DOC released over a 7-day period, compared with 30% for western red cedar (Thuja plicata) and 74% for red alder (A. rubra) (Figure 7.4). By the end of the experiment, hemlock and cedar had released 40% and 20%, respectively of the DOC released by alder.

Differences in leaf chemistry and structure result in wide variation in breakdown rates (Webster and Benfield 1986). Leaves with a high initial nutrient concentration decompose more rapidly than leaves of lower nutrient content. For example, Kaushik and Hynes (1971) established a positive relationship between initial N concentration and rapidity of breakdown. Comparison of the decomposition rates of litter from red alder, western red cedar, and western hemlock also showed a positive correlation with initial N content and a negative correlation with C/N ratios (Richardson et al. 2004). Conversely, a high lignin content slows breakdown. A combination of initial N and lignin proved to be an effective predictor of breakdown rate of six

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