Co-application of biogas slurry and hydrothermal carbonization aqueous phase substitutes urea as the nitrogen fertilizer and mitigates ammonia volatilization from paddy soil.

Biogas slurry (BS) and bio-waste hydrothermal carbonization aqueous phase (HP) are nutrient-rich wastewater. To prevent environment contamination, transforming BS and HP into synthetic fertilizers in the agricultural field can potentially realize resource utilization. We hypothesized that acidic HP could neutralize alkaline BS, adjusting floodwater pH from 6.88 to 8.00 and mitigating ammonia (NH3) volatilization from the paddy soil. In this soil column study, the mixture of BS and HP was applied to paddy soil to substitute 50%, 75%, and 100% to urea. With a low (L) or high (H) ratio of HP, treatments were labeled as BCL50, BCL75, BCL100, BCH50, BCH75, and BCH100. Results showed that microbial byproduct- and fulvic acid-like substance were the main components in BS and HP using 3D-EEM analysis, respectively. Co-application of BS and HP mitigated the NH3 volatilization by 4.2%-65.5% compared with CKU. BCL100 and BCH100 treatments significantly (P < 0.05) mitigated NH3 volatilization by 65.5% and 56.8%, which also significantly (P < 0.05) mitigated the yield-scale NH3 volatilization by 49.6% and 42.3%, compared with CKU. The low NH4+-N concentration and pH value in floodwater were the main reason explained the NH3 mitigation. Therefore, this study demonstrated that BS and HP co-application can substitute the urea as a valuable N fertilizer in a rational rate and meanwhile mitigate the NH3 volatilization. This study will provide new ideas for the utilization of BS and HP resources in the context of ammonia mitigation.