Conclusion

The recognition that earthworms are key operators of self-organized systems (SOS) in soils has important theoretical and practical implications. As regards soil ecology theory, we found a clear correspondence between the main characteristics of drilospheres and those of SOS as precisely defined by Perry (1995). This means that other characteristics of SOS that are more difficult to observe or assess also may be applicable to drilospheres and should be explored.

The shape and localization in soils of system spatial boundaries (for example, the limits among drilospheres and rhizospheres), the existence of discrete time boundaries at which different SOS interact (for example, critical stages during successional or invasive processes where earthworms trigger massive nutrient releases from organic reserves; Bernier and Ponge 1994, McLean and Parkinson 1997), the exact nature of the hierarchical organization of SOS in soils, and the place of drilospheres in them all are research topics that should be addressed in the future in order to better understand soil ecological function.

There is also a great need to depict and understand the nature of interactions among different SOS in soil. Interactions of soil invertebrates with plants and the vast domain of belowground-aboveground interactions are another research field that still is in its infancy and requires increased research efforts (Hooper et al. 2000, Blouin et al. 2006).

These new research questions should help address still-ignored mechanisms and patterns that affect soil function. We might thus find truly adapted concepts and theories for soils where current "aboveground" general theories often prove to be poorly applicable. Ecosystem engineering and self-organization are clearly levers that allow organisms to thrive in soils, and plants to grow better while having strong interactions with all soil organisms. Models based on purely trophic vertical (i.e., along food webs) or horizontal (i.e., among organisms with comparable ecological niches) interactions are unlikely to explain much of the soil function, except in soils where ecosystem engineers have been eliminated or never existed (Lavelle 2002).

On the other hand, the research field of plant-soil invertebrate interactions and roles played by microorganisms in them seems highly promising. As indicated by the SOS theory, these interactions that are critical in sustaining soil functions should take place at discrete scales of space and time that we need to discover while chemical and other mechanisms involved will be described.

The view of drilospheres as ecological "modules" that could be added when lacking, or replaced and/or repaired if damaged, has great practical consequences. To start, drilosphere restoration requires adequate environmental conditions and significant energy inputs (Senapati et al. 1999). In practical terms, there is a need to know basic soil and climate conditions that are required by species considered for reintroduction or enhancement (Barois et al. 1999, Fragoso et al. 1999). There is also a need to understand the way introduced individuals will interact with local microflora in order to establish efficient mutualist relationships (Gilot-Villenave 1994). The energy budget of the operation is also fundamental to determine how much organic matter should be brought, and in which form, to sustain drilospheric activities to a level that produces significant improvements in target ecosystem services (Lavelle et al. 2001).

Management of drilospheres, although highly promising, still requires important research and technological developments. Plant physiologists and soil fertility and soil ecology experts must coordinate their efforts to optimize the production of ecosystem services by a sustainable management of drilospheres and other soil self-organized systems. The best naturally selected or genetically modified plants will never achieve their potential for production in a degraded soil, nor will poorly productive traditional cultivars produce more in the most ecologically active soil. Conservation of soil biodiversity, water infiltration and storage, and C-sequestration also will have to be efficient in any of these systems to meet the rapidly growing demand for soil ecosystem services. In this necessary effort to optimize the provision of all soil ecosystem services at higher rates, all approaches need to be used in a comprehensive way.

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