Soil as a biological system.

Soil plays a fundamental and irreplaceable role in the biosphere because it governs plant productivity of terrestrial ecosystem, allows the completion of the biogeochemical cycles and microorganisms inhabiting soil degrade, sooner or later, all organic compounds including those more recalcitrant. The main characteristics of soil are the domination of the solid phase, the presence of aqueous and gaseous phase and its capacity of reactions by surface active particles. These characteristics influence the biological processes carried out by the organisms inhabiting soil. A peculiarity of soil as a biological system is that it is a structured, heterogeneous, discontinuous system with organisms living in discrete microhabitats called "hot spots", that represents a small proportion (generally lower than 5%) of the overall available space. The chemical, physical and biological characteristics of these microhabitats differ both in time and space. To explain the capacity of soil to degrade all organic compounds the concepts of "microbial consortia", acquisition of novel degradation pathways by soil microorganisms, "extracellular enzymes" and "enzymatic combustion" were introduced.