Effectiveness of composts from decentralised composting scenarios to promote degraded soil restoration and R. officinalis drought resilience

Soil degradation and water scarcity affect crop productivity and ecosystem resilience. Biotechnological strategies based on organic waste valorisation offer promising tools for restoring soil fertility and increasing stress tolerance. This study evaluated the effectiveness of four composts produced under different decentralized composting schemes: community (CA), decentralized urban (SO), small-scale agrocomposting from poultry manure (UP), and medium-scale agrocomposting from olive pomace and pig manure (TO), in improving a degraded soil and enhancing rosemary tolerance to drought. Soil microcosms were set up with a degraded soil and amended with different composts. Half of the microcosms were subjected to well-watered conditions and half to water-stress (75% and 20% of the soil water holding capacity, respectively). Composts promptly increased microbial abundance and dehydrogenase activity. With the TO compost plant grew better than other conditions, presumably due to its phosphorus and labile compound enrichment. Under drought stress, CA and SO composts mitigated biomass losses, demonstrating a protective role against drought-induced stress. Soil amended with pig and poultry-derived composts (TO and UP) introduced antibiotic resistance genes and, in the case of UP, the soil water extracts had significant ecotoxicological effects on both Daphnia magna and Lepidium sativum. In contrast, the CA compost - from organic fraction of municipal solid waste, yard trimmings and donkey manure - displayed the best overall performance, enhancing plant tolerance and soil microbial functionality without ecotoxicological effects. These findings highlight the biotechnological potential of community-scale composting as a circular and effective approach for degraded soil recovery and crop management under water-limited conditions.