Hydrochar and Arbuscular Mycorrhizal Fungi Improve Soil Properties and Plant Growth in Coal Gangue-Amended Sandy Soil

This study evaluated hydrochar and arbuscular mycorrhizal fungi (AMF) for improving coal gangue-amended sandy soil and plant performance. A ryegrass (Lolium perenne L.) pot experiment compared corncob, corn straw, and rice husk hydrochars applied at 2–8%. Selected 8% hydrochars were then evaluated in a four-month alfalfa (Medicago sativa L.) rhizobox experiment to examine soil and plant responses across a 30 cm soil profile and the additional response to AMF inoculation. Hydrochar significantly affected soil nutrient status and all seven ryegrass growth and root traits, with 8% rice husk hydrochar (RHH8) showing the strongest overall performance. After cultivation, soil organic matter was 32.85 g kg−1 under RHH8 versus 14.81 g kg−1 under the control (CK). In the rhizobox experiment, compared with 8% rice husk hydrochar alone (RHH), AMF addition to RHH was associated with higher soil nutrient levels, greater dry-sieved aggregation and water retention, and a greater maximum root length in alfalfa (33.46 vs. 25.68 cm). Total soil concentrations of all eight measured metals were below the GB 15618-2018 agricultural-land risk screening values at the measured endpoints. Overall, RHH performed consistently well, with additional improvements following AMF inoculation. These short-term findings support further evaluation for restoring coal gangue-amended sandy soil.