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

Abstract

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

Spent Coffee Grounds Characterization and Reuse in Composting and Soil Amendment

“…The results showed that 10 kg/m of SCG enhanced weed control, reducing growth by 50%, whereas 5 kg/m of SCG improved the C and N contents [16]. Moreover, a recent study concluded that applying a pre-treatment on SCG, such as hydrothermal carbonization and alkali treatment, could also solve the toxicity effect of SCG components, as well as improve plant growth by increasing the rosette diameter, the number of new leaves, and the dry weight compared with untreated SCG [17]. Another study investigated the effect of SCG by-products in soil amendment, considering C and N values, such as biochar at 270 and 400 • C, hydrochar at 160 and 200 • C, defatted SCG, biochar from defatted SCG at 270 and 400 • C, and vermicompost [101].…”

“…Recent studies showed that SCGs have excellent potential to be used in agriculture because of their high amount of nutrients and low risk of heavy metal contamination [14,15]. Therefore, SCGs have already been reported to be a soil amendment or a top dressing to improve soil quality and control weeds [12,16,17], as well as composting with animal manure and other wastes to improve soil quality, enhance plant growth, and avoid intoxicating the soil [18,19]. Environmental pollution problems due to solid waste and new environmental regulations have forced the industry to pay more attention to waste management.…”

Unveiling the Potential of Coffee Grounds, Banana Peels and Egg Shells as an Eco‐Friendly and Sustainable Fertilizer

“…Furthermore, they play a crucial role in composting alongside animal manure and other organic waste materials, improving soil quality, stimulating plant growth, and ensuring soil remains free from toxicants. [21][22][23][24] In this context, the present study aimed to evaluate the effect of these waste materials as fertilizers for the cultivation of P. vulgaris (common bean), a plant of great economic and nutritional importance. Two types of fertilizers were prepared, a dry mixture and a wet mixture obtained by boiling the dry mixture in water for 15 min, and the germination and growth of the plants were monitored over a 4-week period.…”

“…Furthermore, they play a crucial role in composting alongside animal manure and other organic waste materials, improving soil quality, stimulating plant growth, and ensuring soil remains free from toxicants. [ 21–24 ]…”