Soil food webs in agricultural ecosystems - Microbial Ecology in Sustainable Agroecosystems - page 65

4.2 Soil food web descriptions and models
A food web is a description of the trophic interactions within an ecosystem. By their nature,
they are incomplete, focusing on a subset of organisms and resources and averaging over
different spatial and temporal scales. Paine (1980) distinguished three types of food web
descriptions— connectedness , energy flux, and functional —that share a common underlying
structure but that utilize and convey different types of information about the system. These
three descriptions provide a convenient organizational and operational framework to
study and model agricultural practices and their impacts on ecosystem structure, function,
and stability. Within each description, different modeling approaches and formulations are
used (see Hunt et al., 1987; Moore and Hunt, 1988; Moore et al., 1993; de Ruiter et al., 1995).
The food web descriptions and models we use are based on those developed for soils
of the North American shortgrass steppe developed by Hunt et al. (1987). The native short-
grass steppe system supports grazing by cattle, while cultivated systems include con-
ventional tillage and no-tillage dryland winter wheat and irrigated corn and alfalfa. The
modeling approaches presented here have been adopted by several research groups that
have attempted to link the structure of soil food webs to the decomposition of organic
matter and the mineralization of nutrients (Andrén et al., 1990; Brussaard et al., 1988, 1997;
de Ruiter, Van Veen, et al., 1993; Hendrix et al., 1986; Hunt et al., 1987; Moore et al., 1988).

4.2.1 The connectedness food web

The connectedness food web defines the model's basic structure and foundation for the
energy flux and functional descriptions ( Figure 4.1 ) . For our purposes, the web depicts
the trophic interactions among functional groups of organisms and detritus within the

Phytophagous
Nematodes

Collembolans

Predaceous
Mites

Cryptostigmatic
Mites

Roots

Mycorrhizae

Nematode
Feeding Mites

Noncrypto-
stigmatic Mites Inorganic

N

Saprophytic
Fungi

*

Predaceous
Nematodes

Fungivorous
Nematodes

Resistant
Detritus

Omnivorous
Nematodes

Labile
Detritus

Flagellates

Bacteria

Amoebae

Bacteriophagous
Nematodes

Figure 4.1 The connectedness description of the shortgrass steppe soil food web from the Central
Plains Experimental Range (CPER), Nunn, Colorado. The solid vectors represent trophic interac-
tions in terms of carbon fluxes among resources, mutualists, and consumer (prey and predator)
functional groups. The dashed vectors represent nitrogen fluxes. The asterisk (*) indicates that all
living organisms contribute to the inorganic nitrogen pool as explained in the text and Figure 4.3.