Divergent effects of food waste derived hydrochar on hydraulic properties and infiltration in a sandy soil

Hydrothermal carbonization of food waste (FW) and application of the solid product (hydrochar, HC) in agriculture, is expected to be an effective way to promote the circulation in food-energy-water nexus. However, little is known on the effects of applying FW-derived HC as a soil amendment on soil properties and functions. The current study investigated the effects of FW-derived HC on soil hydraulic properties and infiltration process. Two types of FW-derived HC were mixed into the top 0-20 cm soil layer at a rate of 1% and 2% (w/w) in a soil column experiment with grass growth. In-situ infiltration experiment was conducted within the soil column using a disc infiltrometer at different negative pressure (−20 cm, −6 cm, and −2 cm water head). Unsaturated hydraulic conductivity (K0) and sorptivity (S0) at each negative pressure head was estimated by fitting experimental cumulative infiltration data to a 4-terms infiltration model. Saturated hydraulic conductivity (Ks) and soil water retention curve (SWRC) were measured using undisturbed soil cores taken from the soil column. Soil pore size distribution and soil water constants were estimated from the SWRCs. The results indicate that application of FW-derived HC significantly (p < 0.05) affected soil structure by increasing meso (0.03−0.0002 mm) and macro (> 0.03 mm) pores, which can be attributed to the promoted soil aggregation. Saturated hydraulic conductivity was significantly increased and water retention characteristics were changed with significantly (p < 0.05) increased soil water holding capacity and plant available water content. Besides, FW-derived HCs were found hydrophobic and can render wettable sandy soil water repellent. The K0 and S0 were significantly (p < 0.05) decreased and cumulative infiltration was reduced when soil was wetted from an initial dry condition. Negative effect was found on grass growth with significantly (p < 0.05) decreased aboveground biomass after HCs application. Our study indicates that agriculture application of FW-derived HC as a soil amendment could effectively improve soil fertility by recycling nutrient elements and improving soil structure. However, attention should be paid to the hydrophobicity and phototoxicity of FW-derived HC to avoid the related detrimental influences on plant growth and environment.