This work investigates the effect of the application rate and type of sludge throughout the soil
carbon cycle in a semiarid Mediterranean agro-ecosystem. We study the two-year evolution of the various
pools of soil organic and inorganic carbon and their influence on soil respiration. We applied three rates (40, 80 and 160 Mg/ha) of two types of sludge –aerobically and anaerobically digested sewage sludge– in a calcareous Mediterranean soil. The study area is located in the southeast of Madrid (Spain) and is characterised by a Mediterranean climate with a marked seasonal and daily contrast. We analysed soil organic carbon, CO2 emissions, organic carbon fractions, soluble carbon, and inorganic carbon forms. Measurements were madeat three times over two years, and bimonthly for organic carbon and CO2.The results show that sludge type and rateof applicationexerta significant influence throughout the soil carbon cycle. Aerobic sludge has a greater effect over the short-term. Anaerobic sludge treatment appears to have less effect on the cycle at the beginning of the amendment, but is prolonged over time, as the differences with untreated soil persist even after two years. The application of organic amendments in calcareous Mediterranean soils also modifies the inorganic carbon pools and greatly increases the soil soluble hydrogen carbonates. All of these results are reflected in the rates of soil CO2 emissions, with the highest values recorded in soils amended with aerobic sludge. Our data points to the advisability of a review of the European Union’s recommendations regarding sludge and agriculture. We propose includinga sludge stabilization process and recommended application ratesaccording to the effects on soil biogeochemical cycles.