Soil ecology and agroecosystem studies - Microbial Ecology in Sustainable Agroecosystems - page 9

Collem-
bolans

Shoots

Root
Feeding
Nematodes

Predaceous
Mites

Mites I

Roots

Mycorrhizae

Nematode
Feeding
Mites

Inorganic
N

Mites II

Saprophytic
Fungi

Predaceous
Nematodes

Fungivorous
Nematodes

Labile
Substrates

Omnivorous
Nematodes

Bacteria

Resistant
Substrates

Flagellates

Amoebae

Bacterio-
phagous
Nematodes

Figure 1.2 Flow diagram of aboveground and belowground detrital inputs in a shortgrass steppe
ecosystem (from Hunt, H.W., D.C. Coleman, E.R. Ingham, R.E. Ingham, E.T. Elliott, J.C. Moore, S.L.
Rose, C.P.P. Reid, and C.R. Morley. 1987. The detrital food web in a shortgrass prairie. Biology and
Fertility of Soils 3:57-68. With permission from Springer). Note the separation of flows between vari-
ous faunal groups, including the fungivorous Cryptostigmata, collembolans, and nematodes, and
the more generalized feeders, non-cryptostigmatic mites. A majority of the nitrogen mineralization
were calculated to come from the protists, feeding primarily on bacteria, and bacterial and fungal-
feeding nematodes (Ingham et al., 1985).

The complexity presented in Figure 1.2 c an be condensed into the dominant pathways
beginning with pools of detritus or SOM that differ in quality. These pools would serve as
the primary energy sources for a suite of bacteria and fungi, each of which is consumed by
a host of microbial consumers and predators. Metabolic wastes and by-products that cycle
back as energy sources and binding agents would be factored in much as C and N are in
the current generation of models. This approach preserves the basic premise of material
transformations that occur in the soil carbon models (Parton et al., 1987; Gijsman et al.,
2002) and material transfers that occur in food web models (Hunt et al., 1987; de Ruiter et
al., 1993) in a way that provides a common currency. A comparison ( Figure 1.3 ) of below-
ground food webs from a wide range of agroecosystems is instructive. The food webs
from Central Plains, Colorado, and Horseshoe Bend, Georgia, in the United States and
two European ones, from Kjettslinge, Sweden, and Lovinkhoeve, the Netherlands, show
varying degrees of aggregation of functional groups, depending in part on the expertise
of the investigators involved. For example, in some webs, flagellates were distinguished
from amoebae, and in others they were aggregated as one: protozoa. Earthworms were not
found at the conventional fields of Lovinkhoeve. With much greater emphasis on above-
ground and belowground interactions currently (see De Deyn and van der Putten, 2005), it
is noteworthy that herbage arthropods were considered only at the Swedish site. For ease
of depiction, material flows to the detrital pool, through the death rates and the excretion
of waste products, were not represented in the diagrams but were taken into account in the
material flow calculations and stability analyses (de Ruiter et al., 1998).
Moore et al. (2003) presented a first approximation of this approach by linking the
activities of organisms within the bacterial and fungal pathways to SOM dynamics and