Recycling C and N from biogas slurry and wastewater of hydrothermal carbonization to rice-paddy systems: Enhanced soil dissolved C and N retention

Abstract

This study explores a sustainable agricultural approach by mixing biogas slurry (BS) with hydrothermal carbonization aqueous phase (HCAP) to recycle carbon and nitrogen to rice-paddy system. Over two years, the effects of combining swine-derived BS with vegetable-derived HCAP as an alternative for synthesized nitrogen fertilizer in a rice-paddy system were evaluated. Four nitrogen substitution rates were tested: 0 % (control group, CKU), 50 % (low nitrogen substitution rate, BSVL), 75 % (medium nitrogen substitution rate, BSVM), and 100 % (high nitrogen substitution rate, BSVH). Results demonstrated combined application of BS and HCAP significantly improved soil dissolved organic matter (DOM) and total nitrogen content by 28.5–82.5 % and 5.8–7.2 %, respectively, with positive correlations to the increase in nitrogen substitution rate (P < 0.05). The DOM components revealed substantial increases in microbial by-product-like and fulvic acid-like substances in the soil, by 0.6–2.0 folds and 2.8–10.3 folds, respectively. Ammonia volatilization was significantly reduced by 15.6–46.3 % and 2.2–12.6 % across two years, correlating with pH and ammonium levels in floodwater (P < 0.05). Additionally, substituting chemical nitrogen fertilizer with BS and HCAP maintained grain nitrogen content without compromising rice nitrogen uptake. The results of structural equation model indicate that substituting nitrogen with BS and HCAP enhanced the recycling of carbon and nitrogen in paddy soil by improving soil DOM and total nitrogen accumulation. Overall, this study presents a viable strategy for recycling carbon and nitrogen from BS and HCAP into paddy soil, thereby substituting chemical fertilizers and enhancing soil fertility.