Short-term effects of spent coffee grounds on the physical properties of two Mediterranean agricultural soils

Abstract

We investigated the short-term effects of spent coffee grounds on the physical properties of two Mediterranean agricultural soils (Calcisol and Luvisol). The in vitro assay was performed with two spent coffee grounds doses (60 and 240 Mg ha-1), two incubation times (30 and 60 days) and two modalities: with and without lettuce seedlings (Lactuca sativa var. longifolia). Spent coffee grounds addition increased water retention at -33 and -1500 kPa, and decreased bulk density and plant-available water content. With spent coffee grounds, the percentage of macroaggregates increased, the percentage of meso- and microaggregates decreased and the structural stability of all types of aggregates increased. The stereomicroscopy images showed that: the structural aggregates were rounded, the porosity increased by 316%, the structure was ordered into smaller aggregates, the incorporation of spent coffee grounds particles could occur in intraped cracks and spent coffee grounds particles could act as a binding agent. Scanning electron microscopy also showed that spent coffee grounds particles interacted with mineral particles and integrated into soil structure; moreover, fungal hyphae also developed on many spent coffee grounds particles. If spent coffee grounds effects are compared with those described in the literature for other organic amendments, the same trend was observed in most of the soil physical properties although with a different intensity, principally with regard to structural stability.