Soil ecology and agroecosystem studies - Microbial Ecology in Sustainable Agroecosystems - page 15

greater diversity of companion plants (weeds) in the agricultural plots. In a self-mulching
vertisol under an irrigated cotton-based cropping system in New South Wales, Australia,
Coleman et al. (2010) found that the diversity of bacteria, archaea, and eukarya (fungi,
micro- and mesofauna) was more influenced by previous cropping history (i.e., inclusion
of a legume crop in rotation) than short-term addition of carbon inputs (crop stubble).
Proteobacteria were the most abundant phylum and β-Proteobacteria were the largest
group. In contrast, Actinobacteria were the most abundant phylum in the Horseshoe Bend
agroecosystem plots in the United States (Upchurch et al., 2008) and in the cereal crop-
ping soils from the rain-fed region in South Australia (Gupta et al., 2010). Unlike bacterial
communities, soil protistan and nematode communities (e.g., fungi and protozoa) were
differentially influenced by cropping history and short-term manipulations. For example,
stubble addition and wetting-drying treatments that influence C availability and habitat
structure caused greater response in soil fungi and protozoan communities. Soil fauna
such as nematodes that require stable habitable pore structure were negatively influenced
by the effects of changing soil structure in response to wetting-drying events. In vertisols,
with smectitic clays, biota are regularly exposed to changing habitat structure; hence, their
response in terms of diversity and functionality may be strongly related to physicochemi-
cal characteristics compared to that in other soil types.
In more arid regions (e.g., large areas of the Sahel, Australia, and the arid regions of
North America), the keystone roles of termites in transporting and concentrating both
organic and inorganic nutrients are of similar magnitude to that of earthworms (Whitford,
2000). For more details on the roles of micro- and mesofauna in soil nutrient cycling, see
the work of Coleman et al. (2004) and Coleman (2008, 2011).
1.4.2 Changes in land use and impacts on detrital food webs
The impacts of changes in land-use practices are of increasing interest to ecologists world-
wide. Changes from agroecosystems to pasturelands and longer-term succession into
forested ecosystems lead to significant changes in the detrital food web. Thus, the more
frequently disturbed tillage agricultural system tends to be more bacterial dominated ver-
sus the fungal-dominated no tillage or pastureland (Wardle et al., 2004). Soil communities
usually respond more slowly to changes occurring with land abandonment than in the
communities aboveground (Korthals et al., 2001). Slow development rates of the below-
ground community may be an important factor controlling ecosystem services and the
outputs (goods) provided by these restored ecosystems (Wardle et al., 2004).
Further examples of aboveground influences (e.g., herbivory) on belowground pro-
cesses are becoming more evident as the result of recent experimental studies. Thus, grass-
hopper ( Romalea spp.) feeding on corn leaves caused a greater exudation of labile carbon
from the roots (Holland et al., 1996), which was further transmitted along the detrital food
web to increased carbon uptake by microbivorous nematodes in no-tillage plots in the
Horseshoe Bend agroecosystem study in the Georgia Piedmont (Fu et al., 2001; Coleman
et al., 2006, 2009).
1.5 Conclusions
The three domains of life (archaea, bacteria, and eukarya) are well represented and active
in soil systems. Much of the metabolic activity in soils is microbially mediated and faunally
influenced. Because agroecosystems are the most amenable to experimental manipulation
of any terrestrial ecosystems, many experiments on them have provided intriguing results.