Intensive tilled arable farming systems occur worldwide, but are most prominent in industrialized countries in temperate regions. Intensive tilled agriculture bears little resemblance to highly complex unmanaged ecosystems. Arable soils are extremely disturbed by cultivation, fertilizers, and pesticides, all of which alter their biophysical composition (Anderson 2000). Plant communities of these conventionally managed, tilled agro-ecosystems are unnaturally simple. Intensive tillage farming negates the activity of many soil organisms, such as earthworms (Marinissen & de Ruiter 1993), ants (Decaens et al. 2002), and mycorrhizal fungi (Helgason et al. 1998), but some soil organisms continue to play an important role in the decomposition of crop residuals or organic manure, and are involved in the processes of nitrification and pesticide degradation (Anderson 2000).
In intensive tilled agriculture, short rotations and the use of high-yielding crops can trigger development of soil-borne pathogens and viruses, nematode infestations, and soil-dwelling insects, causing major yield losses. Overcoming these outbreaks requires the breeding of resistant crop varieties, the frequent use of soil pesticides (Epstein & Bassein 2001), or the development of novel biological control practices (Whipps 2001). In contrast, in traditional small-holder (predominantly tropical) farms, the role of organisms is apparent and the activities of a few species of earthworms, termites, and other fauna dominate soil communities. After hand tillage (mostly on tropical farms) or abandonment of the site in shifting agriculture, soil earthworm communities usually recover rapidly (Decaens & Jimenez 2002). This is in contrast with the microbial recovery of soil communities after intensive tilled farmed systems in temperate zones (Siepel 1996; Korthals et al. 2001).
Intensively tilled arable soils are primarily controlled by abiotic factors. For example, erosion is a constraint to soil fertility in 10 to 20 percent of the world's regions (Bot et al. 2000). In parts of the tropics, seasonal rains can cause severe soil erosion, degrading soils and making agriculture less sustainable. The adoption of minimum tillage or organic farming in some cropping systems to improve soil organic matter, or the adoption of erosion control for improved soil moisture, results in a significant recovery of soil biodiversity (Mader et al. 2002). However, this recovery, including the return of earthworms, has not been consistently linked to improvement in crop yields (Brown et al. 1999; Mader et al. 2002).
Arable soils may sequester carbon, but this ecosystem service depends on a combination of climatic factors (temperature/moisture conditions), topography and soil properties (texture, clay content, mineralogy, acidity) (Robert 2001), deposition history (VandenBygaart et al. 2002), and management strategy (Brye et al. 2002).
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