Management practices to preserve soil organic matter in semiarid mediterranean environment

Organic matter (SOM) is a key constituent of soil as it is a “revolving nutrient fund” and improves soil structure, maintains crop production and minimizes erosion. In semiarid environments, the major problem for sustainable farming systems is the continuous decline of SOM towards levels too low for agricultural purposes. Furthermore, SOM is per se a dynamic entity. Its quantity and quality depend on numerous factors including climate, vegetation type, nutrient availability, disturbance, land use and management practices. In particular, soil organic carbon (SOC) stocks in Mediterranean semiarid agrosystems are constrained by 1) limited C inputs because of low precipitation and high evapotranspiration rates, 2) secular agriculture under intensive tillage systems combined with long bare fallows, and 3) the removal of crop residues for animal feed. Enhancing SOC content may be achieved by avoiding those techniques that speed up mineralization process or by increasing residue inputs, or both. A reduction in tillage intensity has been widely recognized as a successful strategy to reduce SOC losses. Conventional tillage (CT) systems is supposed to accelerate SOM mineralisation and consequently increase CO2 flux from soil to the atmosphere. Ploughing favours residue mixing throughout soil, thus improving not only physical contact between soil microorganisms and crop residues but also soil microclimatic conditions for crop residue decomposition (e.g., higher soil moisture content, oxygenation and temperature). In contrast, no tillage systems (NT) reduce microbial activity and, therefore, SOM decomposition. The higher soil bulk density expected under NT, associated with reductions in soil porosity, may lead to a more limited O2 supply for heterotrophic decomposition. However, although many studies suggest that NT increases SOC within the soil profile compared to CT, other studies indicate no net change in SOC. The latter studies suggest that NT only stratifies the SOC, as a near-surface increase in SOC was offset by a concomitant decrease in the subsurface. Organic manuring and inorganic fertilization are the most common practices applied in agricultural management to improve soil quality and crop productivity, respectively. Organic amendments and inorganic fertilizers, above all when coupled together, may indirectly influence soil C inputs through the returned crop residues and rhizodepositions, while directly controlling C outputs via soil microbial activity. In particular green manuring, by introducing into the soil fresh organic matter with a low C/N ratio, enhance SOM content and quality, thus sustaining a high potential microbial activity and biomass. Under Mediterranean climate, both high compost inputs and reduced tillage may have beneficial effects on soil microflora activities and nutrient availability. Reversing CT to sustainable agriculture usually decreases soil bulk density, enhances SOM as favours the immobilisation of C and N and increases most soil microbial quality indicators. Despite all the benefits listed above, soil C sequestration, through conversion to a restorative land use and adoption of recommended management practices, is more intense in cooler and wetter than warmer and drier climates. In this chapter we review in detail 1) the effects of the most widespread agricultural management practices on SOM dynamics, either quantitatively and qualitatively, and 2) their potential role and reliability to preserve SOM in semiarid Mediterranean environment.