Composting performance, nitrogen retention, and microbial succession differ among urea-only and organic co-substrate-amended Camellia oleifera shell formulations

Camellia oleifera shell is a lignocellulosic by-product, and its inefficient decomposition limits waste reduction and nutrient recovery. To identify practical amendment strategies, we compared urea alone (CK) with urea combined with goat dung (GD), C. oleifera seed meal (CM), or rice bran (RB) in terms of nitrogen retention, compost maturation, and microbial succession during C. oleifera shell composting over 77 days. Organic co-amendments improved compost heating and prolonged thermophilic conditions relative to urea alone. RB achieved the highest nitrogen retention (91.64 ± 0.96%), whereas CK had the lowest measured inorganic nitrogen concentration. Co-amendments increased bacterial diversity and were associated with succession of fungal communities toward saprotrophy. Bacterial community patterns were related to the carbon/nitrogen (C/N) ratio, electrical conductivity (EC), and the extract optical index (E4/E6), while fungal succession was more strongly associated with composting time. Overall, supplementing urea with organic co-substrate was associated with nitrogen retention and several maturity indices relative to urea alone, although no single amendment performed best on every compost quality criterion. Although RB’s final total nutrient content was below the cited 4.0% criterion and final-product safety was not established, these findings provide a practical basis for converting C. oleifera shell into a more stable, nutrient-retaining compost and support the resource-efficient utilization of this agricultural by-product.