Community composition of soil organisms under different wheat-farming systems - Microbial Ecology in Sustainable Agroecosystems - page 92

of nematodes may be particularly sensitive to agricultural intensification, and several indi-
ces based on nematode community composition have been developed to provide reliable
indicators of anthropogenic disturbance (e.g., Bongers 1990). Briar et al. (2007) reported
that bacterivorous nematodes were more abundant in organically managed fields at five of
nine sampling dates over a 4-year field study. However, the other trophic groups showed
a much weaker response to farming system. Ferris et al. (2004) also found consistently
higher numbers of bacterivorous nematodes, but only during the midseason in organically
grown tomatoes. No such difference was observed at the time of tomato planting or at the
end of the growing season. All other trophic groups did not show a consistent response
or varied too much to draw further conclusions. Sanchez-Moreno et al. (2009) also showed
that bacterial-feeding nematodes were significantly more abundant in organically man-
aged fields. The relative contribution of plant- and fungal-feeding nematodes to the whole
assemblage was greater in conventional fields.
In contrast to these studies, other publications only reported minor responses of
nematode communities to different farming systems. For example, van Diepeningen et
al. (2006) found no significant differences between organic and conventional farming sys-
tems for any of the nematode trophic groups. The proportion of bacterivorous nematodes
also did not respond to organic management but was positively related to nitrogen sta-
tus in pasture soils in New Zealand (Parfitt et al. 2005). Neher (1999) observed a greater
abundance of individuals from five families (Criconematidae, Heteroderidae, Plectidae,
Prismatolaimidae, and Tylencholaimidae) in organically compared to conventionally man-
aged soils. However, among all trophic groups and three different community indices,
only the plant parasitic nematode maturity index differed significantly between farming
systems. Yeates et al. (1997) did not find pronounced differences for bacterivorous nema-
todes, but fungivorous and plant-feeding nematodes were more abundant in organically
managed grassland soils.
5.1.3.3 Enchytraeidae
Enchytraeid worms belong to the soil mesofauna and are abundantly present (numbers
sometimes exceeding 30,000 individuals m −2 ; Didden et al. 1997) in various terrestrial eco-
systems. These worms with omnivorous feeding habits (Didden 1993) have been shown to
strongly regulate ecosystem processes of organic matter decomposition (Standen 1978) and
nutrient (C and N) mineralization (Briones et al. 1998; Cole et al. 2000; van Vliet et al. 2004)
through a positive effect of their grazing on microbial activity (Hedlund and Augustsson
1995). It has been shown that the activities of enchytraeid worms lead to increased plant
growth (Setälä 1995) and soil porosity (van Vliet et al. 1993), and that their role in the
decomposer food web can be proportionally greater than that of other groups (Laakso and
Setälä 1999). Enchytraeids depend on dead organic matter and therefore may benefit from
organic amendments, while the application of inorganic fertilizers may affect population
size and diversity negatively (Didden et al. 1997). Insecticide applications lead to negative,
neutral, or positive responses by enchytraeid populations (Didden 1991), with particularly
negative effects of fungicide application (Moser et al. 2004). Recent comparisons between
organically and conventionally managed systems have not documented abundance or
diversity differences for enchytraeids (Parfitt et al. 2005; Birkhofer, Bezemer, et al. 2008).
Interestingly, however, in a study by Bezemer et al. (2010) enchytraeid abundance and
richness responded characteristically to the presence of specific plant species, indicating
that enchytraeid worms have specialized plant preferences. The authors also observed
that enchytraeid biomass depended on the surrounding plant community. As organically
managed cereal fields generally have richer plant communities in the presence of arable