Impacts of magnesium sulfate and calcium superphosphate on nitrogen conservation, humification, and fertilizer effectiveness during corn starch residue-soybean meal-oyster shell co-composting

The rapid development of the aquaculture industry has led to substantial accumulation of oyster shell waste, creating an urgent need for effective treatment methods. Composting presents a promising solution. Due to their high alkalinity, direct composting of oyster shells is often inefficient. This study investigated the effects of magnesium sulfate (MS) and calcium superphosphate (CS) as mineral additives on the co-composting process of oyster shells with organic waste, focusing particularly on nitrogen retention, humification efficiency, and fertilizer value. The results demonstrated that CS facilitated conversion of humic precursors (reducing sugars, amino acids, polyphenols), increasing humification rate and extent. The CS treatment yielded the highest levels of humic substances (168.28 g/kg), humic acid (136.83 g/kg), and HA/FA ratio of 4.35 at the end of composting. In contrast, treatments incorporating MS showed lower maturity and higher electrical conductivity. X-ray diffraction analysis confirmed struvite (MgNH 4 PO 4 ·6H 2 O) formation in MS+CS, which mitigated NH 3 emissions and reduced nitrogen loss, thereby enhancing the fertilizing potential of the products. Although all matured composts promoted Chinese cabbage growth, the CS treatment exhibited the most pronounced effect. In conclusion, calcium superphosphate effectively optimizes the co-composting of oyster shells by significantly improving compost quality and fertilizer efficiency, whereas the addition of magnesium sulfate must be carefully regulated to avoid negative impacts. This research provides a practical technical pathway for sustainable oyster shell waste utilization and acidic soil amendment. • Calcium superphosphate enhances humification, producing up to 168.28 g/kg of humic. • Combined magnesium sulfate and calcium superphosphate cut NH₃ emissions by 24.58% • Rich in Ca and Mg compost improves soil quality and medium element content. • The compost boosted pak choi yield by 23.50%, confirming high fertilizing efficacy.