The Primary Production Process

In the process of carbon reduction, there is a net accumulation of sugars, or their equivalents, in the organism's tissues. The costs of photosynthesis are extensively treated by plant physiologists and are out of the purview of this book. Other costs, related to movement of the photosynthates within the plant and allocation to symbiotic associates, are significant to the plant and to the ecosystem, and will be considered further on.

Gross primary production minus plant respiration yields net primary production. NPP is the resultant of two principal processes: (1) increases in biomass and (2) losses due to organic detritus production, which follows from or is dependent on the biomass production (Fogel, 1985). The detritus production includes leaves, branches, bark, inflorescences, seeds, and roots. Additional losses are traceable to exudation, volatilization, leaching, and herbivory (Cheng et al., 1993, 1996).

Measurement of aboveground components is at times tedious, but fairly complete in many studies (see reviews by Persson, 1980, and Swank and Crossley, 1988). In contrast, measurement of belowground production processes has been fraught with errors and many difficulties. However, the total allocation of NPP belowground is often 50% or greater (Coleman, 1976; Harris et al., 1977; Fogel, 1985, 1991; Kuzyakov and Domanski, 2000) (Table 2.1). A sizable portion of the total production is contributed by fine roots, which often have a high turnover rate of weeks to months (Table 2.2), which may be closely linked to nitrogen availability on a seasonal basis (Nadelhoffer et al., 1985, 1992; Pub-licover and Vogt, 1993). In addition to production of fibrous root tissues, there are accompanying inputs of soluble compounds, namely organic acids, sugars, and other compounds. All of these have a considerable impact on rhizosphere (the zone of soil immediately surrounding the root and comprised of root secretions, exfoliations, and the microbial

TABLE 2.1. Annual Production (Mg • ha 1) of Fine Roots (<2 mm) and Root Production as Percentage of Total NPP in Different Ecosystems


Age (years)

% Contribution


Coniferous forest

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