Effects of contrasting organic amendments and agricultural management on soil fertility and crop productivity in the Mediterranean region

Soil organic matter depletion, climate change, and soil degradation represent critical challenges for Mediterranean agriculture, threatening productivity and sustainability. This thesis explored the effects of different organic amendments including manure, straw, biochar, sewage sludge, and cover crops on soil properties, plant growth, and nutrient dynamics under contrasting soil textures and cropping systems. Results showed that manure was the most effective amendment for rapid fertility restoration, particularly in sandy soils, increasing aboveground biomass by approximately 25-50% and enhancing soil nitrogen availability shortly after application due to its readily mineralizable nutrient pool. Biochar significantly improved soil physical structure and water retention (up to 15-30% increases in water holding capacity), while stimulating microbial activity and enhancing legume performance, although its effects were strongly dependent on soil texture and crop type. Straw application contributed to long-term stabilization of soil organic matter and aggregate formation, with increases in soil organic carbon of 10-20%, but short-term biomass production was occasionally constrained by nitrogen immobilization. Sewage sludge provided fertility levels comparable to manure, promoting plant growth and nutrient uptake, but induced moderate increases in soil electrical conductivity and trace element concentrations, raising concerns related to salinity and potential contaminants under repeated applications. Cover crops showed limited short-term effects on crop biomass but improved soil surface cover, magnesium availability, and overall soil protection, confirming their role as a long-term strategy for soil conservation in perennial systems such as orchards. Collectively, these findings demonstrate that no single amendment is universally optimal. Instead, integrating short-term nutrient-rich inputs with long-term soil health practices can enhance soil functioning, improve crop performance, and increase resilience to environmental stresses, offering a sustainable pathway for Mediterranean agroecosystems.