Temporal dynamics of hydrochar-soil interaction: evidence from a pot experiment

Hydrochar could be used as a strategy to recover soil health while managing organic wastes. However, before large-scale applications, pilot experiments are needed to rule out potential adverse effects on the soil biotic community. A greenhouse experiment was conducted on an agricultural 50-years unused soil by using pots across five treatments: control (C) and soil + hydrochar obtained from sewage sludge (HS) and thistle residues (HC), at two different doses (3.4% w/w and 6.8% w/w, denoted as HS3, HS6 and HC3, HC6). Several soil properties were analysed at four different times (18, 92, 146 and 517 days). To evaluate the temporal dynamics of the variables, the percentage change for each property relative to the control was calculated. Furthermore, an additive soil health index (SHI) was also calculated to assess the overall effects of the different hydrochar treatments. Results demonstrated increases in pH, soil respiration, and organic carbon under hydrochar application. In particular, in HS6 and HC6, organic carbon persisted for up to 517 days, which, together with a significant reduction in the mineralisation quotient, may suggest a potential role in soil carbon storage. Furthermore, the soil health index highlighted an overall hydrochar-stimulating effect up to 146 days. However, hydrochar's transient effects highlight the need for further research to extend its benefits and optimise its role in soil restoration.