Soil food webs in agricultural ecosystems - Microbial Ecology in Sustainable Agroecosystems - page 78

4.5.2 Effects on soil food webs
Several large research efforts have adopted the modeling approach presented to study
the impact of agricultural practices on biotic interactions and nutrient dynamics (Hendrix
et al., 1986; Hunt et al., 1987; Andrén et al., 1990; Moore and de Ruiter, 1991). The studies
adopted the practice of grouping biota into functional groups and in many cases used
the same parameter set presented in Table 4.3 to characterize the physiologies and life
histories of the groups, but with their own field biomass estimates ( Table 4.6 ) . The con-
nectedness, energy flux, and functional food web descriptions for the sites were similar to
one another. Similar functional groups were present at each site, distinct pyramids of bio-
mass and flux rates were present, and the asymmetry in the pairwise interaction strengths
apparent in Figure 4.5 a ppeared as well (de Ruiter et al., 1995). The compartmentalization
of interactions and fluxes into the root, bacterial, and fungal energy channels were present.
The models also captured the responses of soil biota and the mineralization of carbon and
nitrogen with surprising accuracy ( Figure 4.8 ), despite the fact that similar criteria were
used to formulate the descriptions and parameterizations.
Numerous studies have demonstrated that agricultural practices affect the densities
and activities of various taxa and the processes that they mediate, similar to the example
presented in Table 4.7 comparing no-till and conventional winter wheat management prac-
tices on prairie soils in Colorado ( Table 4.7 ) . The food web approach developed in the pre-
vious section provides a way to tie these responses together and in doing so has revealed
important patterns in the responses (de Ruiter et al., 1994; Schröter et al., 2003). For exam-
ple, the same type of information presented in Table 4.7 but from the Lovinkhoeve Farm
comparing integrated management to conventional management of winter wheat revealed
that the seasonal dynamics of the energy channels were affected in ways that affected
nitrogen dynamics and nitrogen availability ( Figure 4.9) .
Comparisons of the responses of soil food webs for systems under conventional tillage
compared to no-till or reduced tillage reveal that conventional tillage practices stimulate
microbial activity, increasing soil respiration and nitrogen mineralization rates (Elliott et
al., 1984; Hendrix et al., 1986; Andrén et al., 1990; Moore and de Ruiter, 1991; Wardle, 1995).

5000 500

4000 400

3000 300

2000 200

1000 100

C-mineralization
N-mineralization

1000 2000 3000 4000 5000

100 200 300 400 500

observed

Figure 4.8 Comparison between measured and calculated mineralization rates for the food webs
presented in Table 4.6. Values (kg ha −1 y −1 ) are for the 0- to 25-cm depth layer, except for the Horseshoe
Bend webs (0-15 cm). Open symbols, N-mineralization; closed symbols, C-mineralization.