Urine-derived solids as nutrient sources to enhance microbial wood degradation in a composting environment

Human urine and woody biomass are two abundant biological resources that can be co-valorized through biological wood oxidation (BWO), a meso-thermophilic composting process that generates low-temperature heat and nutrient-rich soil amendment. Here, we investigated the potential of three urine-derived solid formulations as nutrient supplements to enhance microbial wood degradation at 40 °C: phosphoric acid-dehydrated urine (PDU), oxalic acid-dehydrated urine (ODU), and struvite. Among these, PDU achieved the highest wood dry mass loss (27.5 % after 42 days) and oxygen consumption, with its effectiveness attributed to greater phosphorus availability, biochemically stabilized urea that gradually hydrolyzed in situ, and a sustained mildly acidic pH, conditions which collectively supported fungal colonization and carbohydrate degradation in wood. In contrast, struvite exhibited low solubility, high alkalinity, and an unfavorable N/P ratio (0.4) that limited nutrient availability and suppressed microbial activity, resulting in minor wood degradation. Complementary experiments with synthetic nutrient solutions showed that N supplied as urea was more effective than its hydrolyzed products (ammonium bicarbonate and ammonium hydroxide) and ammonium sulfate, likely because it released ammonium gradually and buffered pH shifts during composting. These findings highlight the potential of integrating acid urine dehydration with BWO as a means to advance nutrient circularity and energy sustainability. • Urine-derived solids were used as N and P sources for wood biodegradation at 40 °C. • Acid-dehydrated urine provides nutrients and acidity for efficient wood degradation. • Struvite inhibited fungal growth and wood degradation due to high pH and low N/P. • In-situ urea hydrolysis maintained a favorable and stable pH condition in BWO.