ABSTRACT

Toward the middle of the 1960s studies on biogeochemical cycles in terrestrial ecosystems began to appear in the literature. One of the best known studies at this time was the “ Hubbard Brook Ecosystem Study” [1,2]. In Europe in-depth studies were also made by the Duvigneaud school [3] mainly in Belgian forests. This approach to the study of the ecosystems considers the soil as a subsystem of the larger ecosystem, usually forests, because in these ecosystems, anthropogenic effects are usually minimal, and processes in soil are studied in relation to the larger ecosystem, i.e., taking into account the factors and processes in soil that influence the larger ecosystem [4].The former approach to the study of ecosystems subsequently became more widespread, and soil scientists, with the same object of study, i.e., the pedon (the smallest volume of soil that can be recognized as a soil individual), split in two general tendencies: the first group (usually biochemists) studies soil at a microsite level (e.g., in microcosms), whereas other soil scientists (usually biogeochemists) study soil as a subsystem of larger ecosystems. It is difficult to integrate the results, processes, and concepts derived from these very different approaches and methodologies.Numerous studies deal with biochemical processes and/or biogeochemical cycles in continental and oceanic temperate climates [5,6] and, more recently, in tropical forests [7-9]. However, there are fewer studies on ecosystems in Medi423

terranean climates. This lack of studies on Mediterranean ecosystems has resulted in the extrapolation of results from well-studied continental ecosystems to Medi­terranean ecosystems, usually with unequal results. In continental climates, rains fall mainly in summer (frequently, groundwater does not circulate in the soil profile during winter), whereas in Mediterranean climates, summer is dry, water leaching occurs in winter, and deep leaching is reduced or nonexistent. In fact, some authors consider Mediterranean ecosystems as “ closed,” especially with respect to deep leaching.The aim of this chapter is to describe the biogeochemical cycles in Mediter­ranean ecosysems, based on some specific and particular soil processes and flows, taking as reference a well-studied rainfall gradient existing at the Sierra de Gata mountains (Western Spain) to define the differential characteristics in relation to continental and oceanic ecosystems.