Organic Carbon and Ecosystem Services in Agricultural Soils of the Mediterranean Basin

Abstract

Soil organic carbon (SOC), the major component of soil organic matter (SOM), is extremely important in all soil processes. Organic material in the soil is essentially derived from plant and animal residues, synthesized by microbes and decomposed under the influence of temperature, moisture and soil conditions. The problem of soil organic carbon depletion is of particular concern in the Mediterranean basin, with mild or moderately cold humid winters and warm dry summers, since high temperatures and reduced soil moisture conditions accelerate decomposition processes. This depletion is often in combination with non-conservative agronomic practices such as deep tillage and the low inputs of organic matter to soils, as well as other soil degradation processes, e.g. soil erosion by water. Typically, soils developed in the Mediterranean basin exhibit a high spatial variability of soil properties, are prone to drought, have low water holding capacity, and are shallow particularly on slopes or stony on the soil surface. They are also relatively fragile, and vulnerable to different human activities arising from changes in land cover and land use such as deforestation, urban development and deep soil tillage, and as a result of unsustainable agricultural and forestry practices. In this situation many ecosystem services (ES) are severely threatened. Here we describe the main ecosystems services including provisional, regulating, aesthetic and supporting services, with a focus on the provision of services from soil carbon and crop sustainable management in the Mediterranean basin, including the threats derived from soil erosion and floods. We highlight the specific measures for a sustainable cropland management that can decrease soil organic carbon (SOC) losses, increase the external organic matter (OM) input, and how to efficiently combine both. We reviewed different measures adopting external organic input addition to soil, conservation agriculture by no-tillage, residues retention, cover crops, organic farming compared to conventional agriculture and sustainable crop management by irrigation. In arable cropping systems, we reported an increase in C sequestration rate ranging from 1.3 to 5.3 Mg C ha⁻¹ yr⁻¹ with the addition of organic external inputs, and equal to 0.27 Mg C ha⁻¹ yr⁻¹ with the adoption of cover crops. No tillage and reduced tillage can increase C sequestration rate by 0.44 and 0.32 Mg C ha⁻¹ yr⁻¹ respectively. The adoption of combined management practices, where organic matter inputs and conservation tillage practices are simultaneously applied, increase C sequestration rate by 1.11 Mg C ha⁻¹ yr⁻¹. Organic farming management increase C sequestration rate by 0.97 Mg C ha⁻¹ yr⁻¹ as average, ranging from 0.62 to 1.32 Mg C ha⁻¹ yr⁻¹ with compost application and manure combined with cover crops respectively. Organic farming is also effective in increasing soil organic carbon stocks by about 70% compared with conventional management, and depending on soil type in permanent crops such as olive groves. Regulated deficit irrigation in summer crops is able to decrease CO₂ emissions by about 10%, and consequently soil organic carbon losses without any negative effect on crop yields such as tomato. Soil erosion by water in permanent crops can be decreased by more than 70% with the use of cover crops, and by more than 40% with the adoption of temporary ditches on sloping soils in arable crops.