SUMMARY CHARACTERIZATION AND USE OF COMPOSTED TWO-PHASE OLIVE MILL WASTE (‘ALPERUJO’) AS ORGANIC SOIL IMPROVER. AGRONOMIC AND ENVIRONMENTAL ASSESSMENT The overall objective of this PhD Thesis is to characterize and assess the use as organic soil improvers of 2 composts prepared from two-phase olive mill waste (‘alperujo’) –one irrigated with well water during the composting process (compost A) and the other one with a liquid, animal fatty-proteinaceous waste from the pharmaceutical industry (compost A+H)–, developing several agronomic and environmental studies with the aim of accomplishing this objective. Outstanding differences in physical, physico-chemical and biological characteristics, in composition, and in maturity and stability parameters were found between the 2 composts, A being more stable and mature than A+H. These two composts improved soil physical properties and increased soil fertility in a 1-year experiment, the effect of compost rate (12 to 96 Mg total organic matter/ha) being in general more pronounced than compost source (A vs. A+H). Bioassays performed with two different textured soils (one sandy loam from Pisa–Italy and one clay loam from Valencia–Spain) disclosed that A+H application immobilized mineral N, and that both microbial respiration and enzyme activities as well as mineral N content in soil were higher in the Italian soil in comparison with the Spanish one. Both composts differed remarkably in their effects on the first crop grown immediately after they were applied to the soil (radish, 1st year), with values significantly higher for A; these differences disappeared in the following 2 crops of the rotation, lettuce and green bean (in the 1st year). Compost application rate of 24 Mg/ha and the standard mineral fertilization programme (F1) gave better results than compost application rate of 12 Mg/ha and the reduced fertilization (for both nitrogen and potassium, F2), respectively. These effects decreased with time, being in the 1st year (direct effect) higher than in the 2nd one (residual effect). In addition, A and A+H enhanced soil fertility –especially in potassium– in comparison with unamended controls after the 1st year of the experiment, this effect being maintained during the 2nd year but to a lesser extent. Investigations carried out on the soluble organic matter (MOS) extracted with alkali from the composts revealed that, from the 3 factors studied –MOS Source (compost A vs. A+H), MOS application Rate, and mineral Fertilization programme–, MOS application rate was the one which most consistently and significantly affected the parameters measured in the crop (lucerne) and in the soil (calcareous), the best results being recorded with the lowest MOS application rates (from 200 to 3200 mg total organic carbon/kg soil). Experiments developed about compost suppressiveness towards phytopathogens allowed to isolate 49 actinomycete strains from A, A+H and the soil amended with these 2 composts, which were co-cultured with 5 phytopathogenic fungi and 1 bacteria in in vitro assays, 12 isolates showing a remarkable antagonistic activity; 11 strains belonged to Streptomyces genus, and 1 –with chitinolytic activity– to Lechevalieria genus. In vivo suppressive assays carried out with the phytopathosystem melon ‘Rochet’–Fusarium oxysporum f. sp. melonis by using compost and peat mixes as container growing media revealed that substrates containing compost A showed greater antagonistic effects than those prepared with A+H. The study of the effects of compost A on heavy metal and arsenic bioavailability, and on growth of Brassica plants in 2 polluted soils (S1, pH=7.9 and S2, pH=3.8) demonstrated that compost impaired both growth and shoot accumulation of trace elements in plants growing in S1, B. carinata and B. juncea being the most suitable species for phytoextraction purposes, whereas compost application to S2 increased soil pH, thus allowing plant growth, B. carinata and B. oleracea being the most appropriate species in these conditions. Finally, 8 sulphur-oxidizing bacteria strains were isolated from compost A amended with elemental sulphur (S0) aiming to lower compost pH, thus providing a suitable material for containerised plant growth. Seven of these isolates were identified as Paracoccus thiocyanatus, and 1 as Halothiobacillus neapolitanus. Indigenous compost strains showed high acidification efficiency (higher than 68%) when A was amended with S0, and no additional effect was found with the inoculation of reference strains.