Livestock manure-derived hydrochar improved rice paddy soil nutrients as a cleaner soil conditioner in contrast to raw material

Hydrothermal carbonization (HTC) is a promising technology for resource disposal of livestock manure due to its advantages of high efficiency, low-energy consumption and no dehydration. Previous studies most focused on the characterization and applications in various fields of livestock manure-derived hydrochar (LMH). However, the effects of LMH as soil conditioner on agricultural soil nutrients, dissolved organic matter (DOM) changes and crop yields have not been clarified. The major factors affecting the rice yield of LMH after returning to the field are also seldom mentioned. In this study, a soil column experiment was conducted to explore the changes in soil basic characteristics, DOM and rice yield after returning cattle manure (CM) and cattle manure-derived hydrochar (CC180 and CC260) to the field on the premise of reducing inorganic fertilizer application. The results showed that CCs increased soil nutrient partly, including total phosphorus (by 6.8–18.9%), soil organic carbon (by 8.2%) and dissolved organic carbon (by 18.7%) compared to CM. Soil ammonium nitrogen and nitrate nitrogen concentrations of CCs decreased by 5.8–9.9% and 4.7–15.8% in comparison with CM. LMH increased the stability of phosphorus by increasing the content of calcium bound phosphorus in soil. Three-dimensional fluorescence spectra expressed that soil DOM contained humic-like acid substances (C1 and C2) and tyrosine-like substances (C4). The three substances contents in soil DOM under LMH treatment were higher than that of CM by 15.8–85%. In addition, infrared spectra and two-dimensional correlation analysis showed the structure and evolution of functional groups in soil DOM, which indicated that high temperature could increase the hydrophobic and saturated substances in soil DOM. Furthermore, CCs were more significant than CM in rice yield by 19.9–36.9%. The findings illustrated that LMH is more beneficial to agricultural soil as a soil conditioner to replace part of chemical fertilizer than CM.