INTRODUCTION

A. J. Lotka (1925), in his classic overview of ecological function, considered the system-level features of carbon gain, or anabolism, and the system-level losses of carbon reduction, or catabolism. This chapter is concerned with the primary sources of organic carbon inputs to soils, or system anabolism. These inputs have a major impact on nutrient (nitrogen, phosphorus, and sulfur) dynamics and soil food web function, as will be shown in Chapters 5 and 6.

How can we best address the problems of measurement of primary production? Some ecological studies have declared that taking accurate measurements of belowground inputs to ecosystems is virtually impossible, and assumed that belowground production equals that of production aboveground for total net primary production (NPP) (Fogel, 1985). This rule-of-thumb is clearly inadequate and often very wrong (Vogt et al., 1986). Our objectives in this chapter include addressing the processes and principles underlying primary production, and indicating where the “state of the art” is now, and is likely to be, over the next several years. A wide range of new techniques is now available. We anticipate that information on and our understanding of belowground NPP will continue to increase.

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; Publicover 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 communities contained therein) (Hiltner, 1904; Curl and Truelove, 1986) processes. We cover these later in the chapter.

TABLE 2.2 Annual Losses (Due to Consumption and Decomposition) of Fine Root Biomass in Different Forests

From Fogel, 1985.

When comparing across ecosystems, one needs to be aware of marked differences in root morphology and distribution, i.e., root architecture (Fitter, 1985, 1991). Thus wheat roots in a Kansas field are not markedly different in size, with primary and secondary laterals arising from root initials. In contrast, coniferous tree roots are often comprised of long, supporting lateral roots and short roots, which do the primary job of water and nutrient absorption. Ecologists often use a rather simple, pragmatic classification approach: roots with a diameter of less than 2 millimeters (mm) are classified as fine roots, and roots with a diameter greater than 2 mm are classified as structural roots (Fogel, 1991).

METHODS OF SAMPLING

There are several methods for sampling roots, many of which have been reviewed by Böhm (1979). They may be generally classified into two principal approaches (Upchurch and Taylor, 1990): (1) destructive (sampling soil cores or monoliths), and (2) nondestructive, or observational, using rhizotrons or borescopes; termed minirhizotrons (Upchurch and Taylor, 1990; Cheng et al., 1990; Pregitzer et al., 2002).