Community composition of soil organisms under different wheat-farming systems - Microbial Ecology in Sustainable Agroecosystems - page 90

communities to different land-use or management strategies. For other invertebrate groups,
such as Carabidae or Lepidoptera, results were not convincing and failed to produce clear
correlations between diversity measures at different resolutions (Mandelik et al. 2007).
Bertrand et al. (2006) argued that the aggregation of data from species to higher
taxonomic levels is generally flawed by a misconception about Linnaean classification.
Allocations in current taxonomic classifications are based on a heterogeneous mix-
ture of historical and contemporary views, and even if all taxa would be monophyletic,
those pooled into the same rank would simply denote separate clades without further
equivalence. Bertrand et al. (2006) concluded that these ranks are no more comparable
than any other, nonnested taxa (“such as, for example, the genus Rattus and the phylum
Arthropoda”), and that taxonomic surrogacy lacks justification. Consequently, the concept
of taxonomic sufficiency has been repeatedly questioned in aquatic studies (Maurer 2000;
Terlizzi et al. 2003; Bevilacqua et al. 2009; Terlizzi et al. 2009) but rarely been challenged
in terrestrial ecology, partly because of the often-inconsistent taxonomy in soil organisms
(Wall et al. 2008). Swift et al. (2004) further stated that “the abstraction 'diversity' has often
not been separated from the specific attributes of a community of organisms that is under
study in any particular location or system.”
In light of that comment and the limited knowledge about the response of commu-
nity composition in soil organisms to farming systems, we study response patterns from
five soil-living/-dwelling taxa to four wheat-farming systems in an agricultural long-term
experiment. We use data from our case study to compare patterns based on multivari-
ate community data at different resolution levels (taxonomic and functional) and then
directly relate the observed patterns at different taxonomic and functional levels to data
on soil properties and associated ecosystem functions. The following section provides a
brief overview of the analyzed taxonomic groups, primarily focusing on studies from row
crops and methods that are directly comparable to results from our case study.
5.1.3 Effects on soil organisms
Soil organisms play an important role in agricultural systems as they directly affect
nutrient dynamics, soil structure, and plant growth (Barrios 2007; Power 2010). The ongo-
ing intensification of agricultural management significantly reduces the complexity of
soil communities and could result in a reduced biological capacity for self-regulation in
response to anthropogenic disturbances (Birkhofer, Bezemer, et al. 2008; Postma-Blaauw
et al. 2010). Soil properties change slowly under transition from conventional to organic
farming and may require several years or even decades to reach a new equilibrium (Clark
et al. 1998; Fließbach et al. 2007). Therefore, it is crucial to study the impact of organic
farming in agroecosystems with a long-term farming system history (Raupp et al. 2006).
In the following sections, we discuss published results that compare effects of different
farming systems on soil organisms. We limit the discussion to bacteria, fungi, nematodes,
enchytraeids, and generalist predators since these groups were also studied in our case
study from a long-term field experiment (for general textbooks, see Benckiser 1997; Collins
and Qualset 1999). This is followed by a case study relating community responses of soil
organisms from four wheat-farming systems to each other, to soil properties, and to func-
tional characteristics.
5.1.3.1 Bacteria and Fungi
Soil bacteria and fungi are the most important primary decomposers in agricultural
soils (up to 10 9 cells g −1 soil; Kennedy 1999) and contribute significantly to soil nutrient