Catálogo de publicaciones - libros

Soil organic carbon (SOC) represents a significant reservoir of carbon within the global carbon cycle that has been estimated to account for 1,200–1,550 Pg C to a depth of 1 m and for 2370–2450 Pg C to a depth of 2 m (Eswaran et al. 1995; Lal 2004a). Comparative estimates of organic C contained in living biomass (560 Pg) and atmospheric CO_2-C (760 Pg) (Lal 2004a) indicate that variations in the size of the SOC store could significantly alter atmospheric CO_2-C concentrations. A 5% shift in the amount of SOC stored in the 0–2 m soil profile has the potential to alter atmospheric CO_2-C by up to 16%.

The terrestrial nitrogen (N) cycle comprises soil, plant and animal pools that contain relatively small quantities of biologically active N, in comparison to the large pools of relatively inert N in the lithosphere and atmosphere, but that nevertheless exert a substantial influence on the dynamics of the global biogeochemical N cycle. After carbon (ca. 400 g kg^−1) and oxygen (ca. 450 g kg^−1), N is the next most abundant element in plant dry matter, typically 10–30 g kg^−1. It is a key component of plant amino and nucleic acids, and chlorophyll, and is usually acquired by plants in greater quantity from the soil than any other element. Plant N provides the basis for the dietary N (protein) of all animals, including humans.

Phosphorus (P) and sulphur (S) are essential elements for all living cells. Among the biomolecules that contain P are nucleic acids (DNA and RNA), phospholipids, sugar phosphates (e.g. glucose-6-phosphate) and molecules with an energy-rich pyrophosphate bond (e.g. ATP), whereas S is contained in two amino acids (cysteine and methionine) and various coenzymes, vitamins and sulpholipids. The forms, amounts, transformation processes and cycling rates of the two elements in terrestrial ecosystems are usually studied either from an agronomic point of view, i.e. from the perspective of imminent deficiencies, since both elements are major plant nutrients and therefore essential to achieve sufficient crop yields, or from an environmental point of view, where a surplus of these elements in ecosystems may lead to eutrophication or even direct toxicity effects in the case of S.

Plants require eight micronutrients for normal growth and development: Fe, Mn, Zn, B, Cu, Mo, Ni and Cl. Through their involvement in various enzymes and other physiologically active molecules, micronutrients are important for gene expression; biosynthesis of proteins, nucleic acids, growth substances, chlorophyll and secondary metabolites; metabolism of carbohydrates and lipids; stress tolerance, etc. Micronutrients are also involved in structural and functional integrity of membranes and other cellular components (Rengel 2003).

In addition to their role as organs for anchorage in soil, soil exploitation, and uptake of water and nutrients, plant roots can modify the physico-chemical conditions in the surrounding soil via alterations of root activity. The soil volume that is directly or indirectly influenced by the activity of plant roots is called the rhizosphere (Hinsinger 1998). As early as the beginning of the last century, the German phytopathologist Lorenz Hiltner (1904) recognised that the rhizosphere, as the interface between the soil matrix, plant roots and soil microorganisms, plays a critical role in nutrient cycling in ecosystems. Root-induced physico-chemical changes in the rhizosphere are major determinants of the plant availability of nutrients and toxic elements in soils. Organic compounds released from plant roots as rhizodeposits can have a direct impact on the solubility of mineral elements or can indirectly influence turnover and availability of nutrients by interaction with soil micro-organisms. Thus, rhizodeposition is a key factor determining fluxes and pool sizes of mineral nutrients in ecosystems.

Microbial communities carry out fundamental processes that contribute to nutrient cycling, plant growth and root health. Microorganisms play a key role in nutrient cycling because they (1) decompose organic material (plant residues and soil organic matter) and release inorganic nutrients that can then be taken up by plants; (2) affect nutrient availability by solubilisation, chelation, oxidation and reduction; (3) store nutrients in, and release nutrients from, the microbial biomass; and (4) affect plant growth by release of stimulating or inhibiting substances. Microorganisms in the rhizosphere - the soil surrounding the root — are of particular importance for plant nutrient uptake and growth because of their vicinity to the roots. Via their effect on plants, rhizosphere microorganisms influence the composition and amount of residues returned to the soil.

Soil fertility is a measure of the ability of soil to sustain satisfactory crop growth in the long-term, and can be determined by physical, chemical and biological processes intrinsically linked to soil organic matter content and quality (Fig. 7.1). Given that a decrease in soil fertility is a major constraint to productivity, investing in practices leading to soil fertility enhancement is likely to generate large returns (Syers 1997). In recent years, increased concerns for healthy food production and environmental quality, and increased emphasis on sustaining the productive capacity of soils, have raised interest in the maintenance and improvement of soil organic matter through appropriate land use and management practices (Loveland and Webb 2003; Puget and Lal 2005; Whitbread et al. 2003).

About 53 million km^2, or 40% of the Earth’s land surface is grassland, containing about one-third of the global stock of terrestrial C. Grasslands are ecosystems where the dominant vegetation component is comprised of herbaceous species, with less than 10% tree cover (Jones and Donnelly 2004). Grasslands are either natural vegetation (e.g. the steppes of central Asia and prairies of North America) or anthropogenic in origin (e.g. north-western and central Europe, New Zealand, parts of North and South America and Australia). Grasslands are heavily relied upon for food and forage production, and about one-third of world milk and beef production occurs on grassland managed solely for these purposes (Conant et al. 2001).

The climate in the area surrounding the Mediterranean is characterised by the alternation of a rainy season in the cold months with a dry season in the warm months. Optimal moisture and temperature optima often do not coincide, although there are numerous spatial and temporal variations. The topography, soils and vegetation of the Mediterranean areas are also variable. Most of the area is covered by hills and mountains, resulting in a very rugged and highly dissected landscape. Soils are very variable, but the majority have been derived from calcareous formations. The soils are relatively shallow, slightly acidic (mean pH 5.6), with low concentrations of carbon (14 mg kg^−1), nitrogen (14 mg kg^−1), and phosphorus (17 mg kg^−1).

Drylands occupy approximately 40% of the Earth’s land surface and have low inputs of mean annual precipitation (P) relative to mean annual potential evapotranspirational (ET) losses (Millennium Ecosystem Assessment 2005). The United Nations Educational, Scientific and Cultural Organization (UNESCO 1979) proposed the following classification scheme for drylands: hyper-arid zone (P/ET <0.03), arid zone (P/ET 0.03–0.20), semi-arid zone (P/ET 0.20–0.05) and subhumid zone (P/ET 050–0.75). The majority of studies summarised in this chapter were conducted in arid and semi-arid zones with mean annual precipitation ≤300 mm.