Integrating Variables

In studies of soil systems, rates of litter breakdown have been used as integrating variables. That is, because litter breakdown rates are the result of the combined activities of the soil biota, breakdown rates may be used to evaluate effects of disturbance on the entire system. For example, conversion of agricultural systems into conservation tillage regimes will affect soil biology, notably by shifting the composition of microbial communities and increasing earthworm population densities, but with changes in other soil biota as well (Doran, 1980, Parmelee et al., 1990, Beare et al., 1992). We can evaluate the consequences of these

FIGURE 5.1. Changes in species diversity of soil microarthropods as a function of agricultural practice (from Crossley et al., 1992).

changes in soil biota by measuring litter breakdown rates (Fig. 5.1) (Crossley et al., 1992).

Other integrating variables include soil respiration, formation of soil structure, and nutrient dynamics. All of these variables are readily measured, and all are important for ecosystem function. Soil respiration estimates biological activity generally and is dominated by microbes, with an important contribution by roots (Cheng et al., 1993; Kuzyakov, 2002). Soil structure is the result of combined actions of biota and climate on mineral substrates. Nutrient dynamics are the most valuable of the integrating variables for predicting primary productivity.

Although microbes are responsible for the biochemical degradation of organic litter, fauna are important in conditioning the litter and aiding in microbial actions. The soil scientist Hans Jenny characterized soil fauna as mechanical blenders: "They break up1 plant material, expose organic surface areas to microbes, move fragments and bacteria-rich

1Although we are accustomed to the term litter breakdown, the term litter breakup appears to be exactly equivalent.

excrement around, up, and down, and function as homogenizers of soil strata" (Jenny, 1980). Breakdown rates for organic litter integrate the effects of these various activities into a single set of variables. The combination of microbial and faunal activities results in a set of positive interactions of the type termed "facilitation" by Bruno et al. (2003). Results of the interactions are likely to be more significant than that by either component—animal or microbial—acting alone.

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