Hydrophobicity and surface free energy to assess spent coffee grounds as soil amendment. Relationships with soil quality

Highlights

• Raw spent coffee grounds (SCG) significantly increased the soil hydrophobicity (Hph).

• SCG effects on Hph are close related to soil type.

• A multiparametric analysis connects Hph and soil quality.

• Soil fungal activity increases with SCG and affects Hph.

• Water drop penetration time, contact angle and surface free energy assess the Hph.

Abstract

The aim of this work was to evaluate the effects of spent coffee grounds (SCG), a highly hydrophobic raw material, on the hydrophobicity of two Mediterranean agricultural soils. Physical, chemical, mineralogical and biological soil properties, most of them used to evaluate soil quality, were studied and related to the main hydrophobicity parameters. The in vitro assay was performed with two SCG doses (2.5 and 10%), two soils and two incubation times (30 and 60 days). Hydrophobicity was determined by the water drop penetration time test (WDPT), the contact angle (CA) with H₂O, formamide and diiodomethane, and the surface free energy components (SFE) calculated using the Van Oss model. The addition of SCG increased the WDPT, CA and SFE, being the latter which was related to a greater number of soil quality variables. Hydrophobicity was related to lower humus quality index (HQI), and a higher proportion of labile organic matter, as shown by Infrared and UV–vis spectroscopy. An increase in hydrophobicity was related to an improvement of soil physical quality: a high aggregate stability index, saturated hydraulic conductivity, porosity (total and macro), water retention, and a less bulk density. The most critical effect related to the increase in hydrophobicity was the significant decrease in the available water content. The SEM images showed a greater occlusion and stabilization mechanism of the SCG particles incorporated in Vega soil, probably due to its higher content of smectite and carbonates. The appearance of fungal biomineralizations of calcium carbonate is associated with SCG addition and could be considered as an interesting and little studied process of inorganic carbon fixation and secuestration. These results showed that hydrophobicity can afford relevant information that can help to asses soil quality status after an amendment with SCG.

Introduction

Given the alarming deterioration in the quality of agricultural soils in the Mediterranean area (Rodríguez-Entrena et al., 2014), one of the main objectives of conservation agriculture is the increase of soil organic matter, which also favours its physical, chemical and biological properties. Hence, it is mandatory to study in depth the use of biological residues, raw or transformed, of a very different nature (such as ground coffee, SCG) as organic amendments and their effects on the soil physical, chemical and biological soil properties and in general on its quality. The latter includes possible negative effects, such as changes or decreases in biological activity, or alterations in soil water dynamics, as some authors have described (Aranda et al., 2016, Hardgrove and Livesley, 2016). Water holding capacity and water retention characteristics, structural type and aggregate stability, hydraulic conductivity, bulk density, organic matter, CEC, pH and EC can be used to assess soil quality as key attributes (Arshad and Coen, 1992). Soil biological parameters, have been shown to be especially useful to detect small changes in soil conditions, thus providing information regarding subtle alterations of soil quality (García-Ruiz et al., 2008).

Raw bio-residues and inadequate composted by-products, present inconveniences like the more labile and less stable organic matter, although its most critical effect could be its water repellence or hydrophobicity (Comino et al., 2017). Soil hydrophobicity is generated by the accumulation of hydrophobic compounds, originated from vegetation (de Blas et al., 2010) or microorganisms (Schaumann et al., 2007) or produced by the decomposition of fresh organic matter with a low degree of alteration and/or humification (Comino et al., 2017). This hydrophobicity could have a negative impact on soil water regime, infiltration capacity (Mohawesh et al., 2014), hydraulic conductivity and water availability for the plant (Doerr and Thomas, 2000). Furthermore, some authors have reported that hydrophobicity reduces the accessibility of microorganisms to organic matter, decreasing their rate of decomposition, due to the lack of water that restricts their favorable life conditions (Goebel et al., 2005, Leelamanie, 2016). At the same time, hydrophobicity increases the stability of soil aggregates and consequently contributing to the occlusion of organic matter as a stabilization mechanism (Lützow et al., 2006). In this way, moderate hydrophobicity is considered common in soil, and its positive influence on soil structure and quality has begun to be recognized (Goebel et al., 2005, Mataix-Solera et al., 2011, Aranda et al., 2016). These aspects are essential in relation to the conservation of organic matter in the soil and the extension of the concept of soil as a carbon sink. More empirical data on these effects in the soil and their properties are necessary and it is important to properly evaluate hydrophobicity, especially in agricultural soils. Some authors have suggested that hydrophobicity should be included as indicator of soil physical quality due to its relationship with other soil properties and its easy determination (Jordán et al., 2013).

There are several ways to determine hydrophobicity, the most usual but with notable limitations being the water drop penetration time (WDPT) test (Chau et al., 2014). Another widely used measure is the contact angle (CA) (Goebel et al., 2005, Diehl and Schaumann, 2007), which shows some physical meaning and enables the study of water repellency (SWR) in a wide soil range from wettable to extremely repellent. However, these methods estimate only persistence and intensity, respectively between water drops and the soil surface, without providing a comprehensive thermodynamic characterization of the solid surface alone. Many phenomena that take place at the solid-liquid interface, including wettability, adsorption, and aggregation, depend largely on changes involved the surface free energy (SFE) (van Oss, 1994). Hence, a complete determination of SFE components according to the Van Oss et al. (1988) approach for acid-base and Lifshitz-Van der Waals interactions would be recommendable.

The SCG is a bio-residue generated in large amounts (15 million tons annually, according to Kamil et al., 2019), characterized by its high amounts of polysaccharides, lignin and protein (Ballesteros et al., 2014). Previous studies on the effect of SCG on agricultural soils, have concluded that their addition to Mediterranean soils has clear agronomic benefits, in terms of improving soil physical, chemical and biological soil properties (Cervera-Mata et al., 2018, Cervera-Mata et al., 2019a, Comino et al., 2020, Vela-Cano et al., 2019). The composition of SCG is also characterized by high amounts of lipids, 13–18% (Al-Hamamre et al., 2012, Petrik et al., 2014), which are retained in the SCG after the preparation of coffee beverage, which would justify the hydrophobic nature of this bio-residue (Gross et al., 1997). The positive effects of the SCG on the soil structural stability in the short term could be attributed to this hydrophobic character (Cervera-Mata et al., 2019a). However, there is still insufficient knowledge about their influence on soil hydrophobicity. In this field, new knowledge can be generated as a result of a more exhaustive investigation given that the soil hydrophobicity as a result of the organic amendments in agricultural soils is a problem that has hardly been addressed until now (Aranda et al., 2016, Jiménez-De-Santiago et al., 2019). Together with hydrophobicity, SCG have other limitations for their use as soil organic amendment, such as their phytotoxicity (Cervera-Mata et al., 2018, Cervera-Mata et al., 2019b). Therefore, the transformation and stabilization of SCG through composting or biocharization could be considered an alternative for the use of these wastes (Liu and Price, 2011, Vardon et al., 2013). However, the time, cost and other environmental consequences of these treatments make it advisable to investigate the possibilities of their reuse as raw material, although taking into account their limitations.

Considering this, we can hypothesize that the addition of untransformed SCG (raw material) as an organic amendment in intensive agriculture as greenhouse crops (very widespread in the southeast of Spain), will effect soil hydrophobicity, and in particular, surface free energy components, which are decisive in the processes of adsorption and absorption in the soil. The main aim of this work is to know these effects in two Mediterranean agricultural soils with contrasted mineralogical properties. Moreover, the relationships between hydrophobicity and other physical, chemical, compositional (organic and mineral) and biological properties, many of them essential to evaluate soil quality, will be measured. Therefore, the changes generated by SCG from the point of view of soil quality will be discussed, an issue that has been little studied in relation to hydrophobicity.