Waste-based hydrothermal carbonization aqueous phase substitutes urea for rice paddy return: Improved soil fertility and grain yield

Hydrothermal carbonization is an emerging and efficient technology for utilizing wet waste. However, the utilization of hydrothermal carbonization aqueous phase (HAP), a carbon-rich liquid product containing massive quantities of nutrient elements, has been largely unexplored. In this study, HAP produced by hydrothermal carbonization of sewage sludge (SHAP) and pig manure (PHAP) at 180 °C and 260 °C was applied as a substitute for partial (9.1–55.9%) urea in rice paddy field. Results showed that HAP increased the abundance of humic acid-like substances and microbial metabolic by-products in soil dissolved organic matter (DOM) by 3.3–7.2 times and 42.1–101.6%, respectively, compared to the control treatment without HAP addition (CK). Meanwhile, HAP significantly increased soil fluorescence and humification index of soil DOM by 28.5–41.9% and 12.3–26.8%, respectively, in comparison with CK. Soil fertilized with SHAP260 had greater mineral nitrogen, total organic carbon and Olsen-P compared to CK. HAP increased the grain N uptake by 34.0–58.8% and rice yield by 6.3–34.1% with essential amino acids in grains improved by 8.6–20.9% compared to CK. This study stated that HAP as a substitute of partial urea for rice paddy return has positive effects on soil fertility and grain yields.