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Castor (Ricinus communis L.) is an important oilseed crop worldwide whose inedible oil is widely used in the industrial, pharmaceutical, and agricultural sectors. Castor plants show high conversion potential for use as biorefining feedstocks. The present study was conducted to investigate the effects of two nitrogen fertilization levels (0 and 120 kg N ha−1) on seed and oil yield. From a biorefinery perspective, the residual biomass of seed processing was analyzed in terms of fiber composition and biomethane production carrying out a biological pretreatment using two white‐rot fungi (Pleurotus ostreatus and Irpex lacteus). Nitrogen fertilization resulted in an increase in seed and oil yields and a difference in capsule husk composition. Fungal pretreatment of capsule husks showed promising effects on anaerobic digestion, increasing the biomethane yield compared to untreated biomass. The highest lignin degradation and the lowest cellulose loss during pretreatment were obtained with I. lacteus, and this fungal pretreatment resulted in the highest biomethane yield (103.2 NmL g−1 volatile solids) for the fertilized biomass. [ABSTRACT FROM AUTHOR]