Field application of rice straw–sewage sludge compost in Mediterranean citrus orchards: effects on soil properties, nutrient status and fruit quality

Abstract. Intensive agricultural practices have degraded soil fertility and polluted natural resources in Spanish citrus orchards, highlighting the need for more sustainable management strategies. Composting rice straw (RS) and sewage sludge (SS)—two residues that are difficult to manage in Mediterranean regions—offers an environmentally sound alternative for residue valorisation and soil fertility restoration. This study assessed the agronomic performance of compost produced from RS and SS (RS/SS) at an industrial scale, in comparison with compost derived from pruning residues and sewage sludge (PR/SS), which is commonly produced in Mediterranean composting facilities. The effects of compost application at two rates over two consecutive years were evaluated through analyses of soil physical, chemical, and biological properties, as well as foliar nutrient concentrations, yield, and fruit quality. RS/SS compost contained higher nutrient levels, particularly P, suggesting its potential as a P-rich organic fertiliser. Compost application reduced soil pH and slightly increased electrical conductivity without exceeding critical thresholds. Active lime decreased, while organic matter, N, P, K, and Zn contents increased, accompanied by improvements in soil biological indicators. Effects on foliar nutrient status, yield, and fruit quality were limited. These results indicate that industrial-scale RS/SS compost application represents a sustainable strategy for residue valorisation and soil fertility improvement in Mediterranean citrus systems, enhancing soil properties and reducing dependence on mineral fertilisers.