Evaluating the impact of manure-derived hydrochar soil amendments on greenhouse gas fluxes: A greenhouse experiment using soil mesocosms and corn plants.

Animal manure is an agricultural by-product that presents both nutrient value and environmental challenges, particularly through nutrient leaching and greenhouse gas (GHG) fluxes. Hydrothermal carbonization (HTC) offers a potential management strategy that converts wet manures into hydrochar, which is a more stable and nutrient-retentive amendment. This study evaluated how hydrochar derived from dairy manure, swine manure, and poultry litter, applied at 15 or 30 metric tons/ha, influences GHG fluxes and corn biomass production in a controlled greenhouse experiment using soil mesocosms with two fertilization histories (dairy-manure vs. inorganic/chemical-only). All treatments received an equal total nitrogen input through combinations of hydrochar-derived N and urea. Hydrochar treatments tended to reduce cumulative N2O fluxes, although not always significantly, compared to the urea-only control, with the largest reductions occurring after the second urea application. Across feedstocks, hydrochar treatments tended to show higher cumulative CO2 fluxes, although not statistically significant, relative to the control, with 15 tons/ha applications producing higher CO2 fluxes and more corn biomass compared to 30 tons/ha. Methane fluxes exhibited no consistent treatment-level trends. Corn biomass tended to be higher in the 15 tons/ha hydrochar treatments and lower in the 30 tons/ha treatments relative to the control. Overall, findings indicate that partially replacing urea fertilizer with hydrochar, can potentially increase biomass while reducing cumulative N2O fluxes. Generally, the increases in CO2 fluxes from hydrochar did not fully offset the reductions in N2O fluxes at the system level. The results suggest that hydrochar amendments can offer an agronomically efficient and environmentally beneficial nutrient-management strategy.