Spent coffee grounds by-products and their influence on soil C-N dynamics

SCG are a bio-waste generated in great amount worldwide which are attractive as soil amendment for their high content of organic matter and nutritive elements. Nevertheless, several studies have shown that soil application of untreated SCG has detrimental agronomic and environmental effects due to their high degradability and content of noxious compounds (phenols, caffeine, and tannins). However, SCG can be valorised, in the frame of circular economy, by extraction of energy and valuable products (carbohydrates, proteins, bio-oil, bio-diesel) and generation of solid by products (biochar, hydrochar, compost) that can be utilized as soil fertilizers and amendments. Therefore, the aim of this work was the characterization of different solid SCG by-products (as second-generation products) and their assessment as effective organic amendments. The novelty of this study is that for the first time 8 different by-products derived from the same SCG were characterized and comparatively evaluated for their impact on the C and N cycles of soil. SCG was collected and treated to generate 8 different SCG by-products (biochars produced at 270 and 400 °C, hydrochars produced at 160 and 200 °C, vermicompost, defatted SCG and biochars produced from defatted SCG at 270 and 400 °C). SCG and derived by-products were characterized for SEM micromorphology, pH and EC values, and C, N, H, O, volatile matter, fixed C, LOI, carbonates, water soluble C and N, NO₃^- and NH₄⁺ content. SCG and SCG by-products assessment as organic amendments was performed with an incubation experiment. The residues were added (2.5%) to a moist Mediterranean agricultural soil and the amended soil samples were placed in mesocosms and incubated at 20 °C for 30 days. During incubation, CO₂ and N₂O emissions were measured every 6 h by means of a gas chromatography automated system for GHG sampling and measurement. The percentage of added C remaining (C_R) in the soil was calculated by fitting the cumulative respiration of amended soil to a two-pool model. After 2, 7 and 30 days of incubation, the control and amended soils were sampled and analyzed for their content of extractable organic C, N, NO₃^- and NH₄⁺ and microbial biomass C and N. Results showed that SCG by-products presented a great variability in their properties. SCG and hydrochars presented higher contents in volatile matter and water soluble C and N, and low content of fixed C, while biochars showed an opposite behaviour. SEM images confirmed the different characteristics of the SCG by-products: the biochar presented a porous structure, honeycomb-like form, due to the loss of the more soluble compounds, while the SCG and hydrochars' pores were filled with amorphous carbonaceous materials. Consequently, soil addition of SCG by-products showed a distinct impact on C and N cycle and microbial biomass content. Addition of SCG and hydrochars generated the highest cumulative CO₂-C emissions (2103-2300 μg g^-1), the lower amount of C_R (86.8-88.6%), increased the soil extractable organic C and microbial biomass C and N and caused N immobilization. On the other hand, the addition of biochars generated lower CO₂-C emissions (542-1060 μg g^-1), higher amounts of C_R (96. 3-99.9%) and lower amounts of extractable compounds and microbial biomass C and N, generating also N immobilization, but to a lesser extent. The addition of vermicompost generated 723 μg g^-1 of CO₂-C and 98% of C_R remaining. However, this by-product did not generate N immobilization being able to act as N fertilizer. None of the residues generated N₂O emissions. The different properties of the SCG by-products and their impact on C and N cycle indicated that they can be effectively applied to soil to exert different agronomical and environmental functions.