Soil Amendments and Biostimulants from the Hydrothermal Processing of Spent Coffee Grounds

Purpose: Use of spent coffee grounds (SCG) in horticulture has deleterious effects on plant health and yields, suggesting that processing of SCG is necessary prior to field application. To this end, two products of an SCG based biorefinery were investigated: primary chars from the hydrothermal carbonisation (HTC) of SCG and enzymatic protein hydrolysates. Methods: Primary chars were produced under various HTC regimes from raw SCG, and alkaline pre-treated SCG. Primary chars were evaluated in germination toxicity tests and under soil stress conditions using Arabidopsis thaliana (Arabidopsis). Proteolytic enzymes were screened in production of SCG protein hydrolysates; biostimulant activity in the growth of Saccharomyces Cerevisiae, germination of Arabidopsis thaliana under normal and cold conditions and auxin-like activity was assessed. Results: Toxicity assays of primary chars with land cress (Barbarea verna L.) showed a maximum 35% increase in root length, relative to the control. In Arabidopsis growth trials, outstanding performances were recorded at 100 t/ha for primary chars produced from alkaline pre-treated SCG: rosette diameter and dry weight increased by 531 and 976%, respectively, relative to the control. SCG protein hydrolysate from Bacillus lichenformis proteolysis (glycine present at 47% of total) gave 140% increase in Arabidopsis seeds with expanded cotyledons, relative to the control. Auxin-like activity was also measured in the extracts. Conclusion: Cascade process design was used to valorise a major waste stream. Removal of phytotoxic components from SCG hydrochars enhanced plant growth, while biostimulant activity of SCG protein hydrolysates was observed. Graphical Abstract: [Figure not available: see fulltext.]