Co-application of hydrothermal carbonization aqueous phase and biogas slurry reduced ammonia volatilization in paddy

Application of biogas slurry (BS) can promote ammonia (NH₃) volatilization. Algae sludge and Quercus acutissima leaves are rich in resources and nutrients, and can be effectively converted into valuable products. Hydrothermal carbonization technology (HTC) is a sustainable method for the treatment of wet biomass. However, the large amount of hydrothermal carbonization aqueous products (HAP) contains harmful substances that require effective management. The combined application of HAP and BS can mitigate NH₃ emissions and facilitate resource recovery, presenting an eco-friendly approach to both nutrient recycling and pollution mitigation. This study explored the joint application of HAP and BS in paddy to decrease NH₃ volatilization and the factors influencing NH₃ volatilization. In this study, the HAP prepared from algae sludge and Quercus acutissima leaves at 180 °C and 220 °C was mixed with BS at a 1:1 total nitrogen content ratio, and the mixture was used instead of 25% or 50% urea. The experimental results indicated that the rice yield with the application of HAP and BS was equivalent to the control treatment only with urea (CK). Compared to the CK, HAP and BS treatments reduced soil NH₃ volatilization by 6.9%-55.5% and increased soil dissolved organic matter (DOM) by 2.7%-59.4%. The treatments using algae sludge and Quercus acutissima leaves prepared at 220 °C as substitutes for 50% of urea reduced NH₃ volatilization by 43.9% and 55.5%, respectively. Ammonium nitrogen, pH, total organic carbon, urease, and DOM were important factors influencing NH₃ volatilization. This study showed that substituting part of urea with HAP and BS for field application reduced NH₃ volatilization and increased soil organic matter content.