Many studies have reported the effect of SCG on the growth, mineral content and bioactive compounds of several type of plants including edible plants (both for humans and for animals) such as beans, soybeans, broad beans, alfalfa, wheat, corn, clover, sorghum, sunflower, oats, rye, barley, buckwheat, lettuce, basil, ryegrass, tomato and Brassica [12,44,49,51,52,53,54,55,61,62,63,64,65,66,67,68], although inedible plants such as pine have also been studied [69]. In addition, SCG have been studied when fresh, that is, without any type of treatment [12,44,49,54,55,61,64,65,66], composted [44,62,65], combined with other types of waste [67,70], transformed into biochar or hydrochar [68,71,72,73] or supplemented with nitrogen fertilizers [51].

3.2.1. Effects on Plant Growth

The first reference on the effect of SCG on plant growth is in Kitou and Yoshida [53]. In a trial with pots, they studied the effect of this residue in a concentration of 1 and 2% on the growth of 12 edible plants. These authors found growth inhibition for most plants, attributing this effect to N immobilization, the multiplication of pathogenic fungi or the release of phytotoxins derived from fresh organic matter. Subsequently, many authors found the same detrimental effect of SCG on plant growth [12,49,52,55,61]. Hardgrove and Livesley [52] tested broccoli, leek, radish, sunflower and viola, Cervera-Mata et al. [51] and Cruz and Cordovil [66] tested lettuces, and Yamane et al. [49] tested alfalfa, guinea grass, crotalaria, sorghum, sunflower, oat, barley and rye. Yamane et al. [49] tried to corroborate the negative effect of SCG found in pots in a field trial. SCG displayed a negative effect on different plants, which was attributed to the presence of caffeine, tannins and polyphenols [74]. These authors suggested that SCG were better used for legume species to counteract the possible immobilization of N due to the addition of an untransformed residue. Cruz and Cordovil [66] also found growth inhibition in carrot, spinach and lettuce. This group studied the bioavailability of N and P when SCG was added, concluding that SCG immobilized these elements and could be the cause of the lack of growth exhibited by the crops. In this same line of research, Hardgrove and Livesley [52] and Cervera-Mata et al. [51] added fresh isolated SCG and fresh SCG with a nitrogen fertilizer to confirm the hypothesis of N immobilization. These authors reported limited growth in both cases, with and without the addition of the nitrogen fertilizer. In fact, the combined addition of high amounts of both SCG and N limited plant growth to a greater extent. Therefore, both groups concluded that plant growth inhibition must have been due to either SCG phytotoxic compounds or to an insufficient N dosage to overcome microbial immobilization.

SCG also limited seed germination for the generation of seedlings [63], which was attributed to unsuitable substrate conditions (lack of porosity when SCG were added). On the other hand, SCG reduced the stomatal conductance of plants, which is related to a strategy to adapt to stressful conditions. Conversely, Cruz’s group reported an increase in lettuce biomass using fresh SCG at concentrations of 2.5 and 10%, finding an inhibition with higher percentages [64]. In a later study [54], SCG were left fallow for 4 months before planting lettuces. In this case, as in the previous study, SCG increased lettuce biomass at a concentration of 10%, decreasing plant growth at amounts of 20 and 30%. In the next section, we will discuss how we can avoid SCG toxic effects by transforming them into other bioproducts such as vermicompost or biochar [62].

3.2.2. Effects on Mineral Content and Other Compounds

Different studies have recently been carried out on the reuse of SCG as an organic amendment to improve the mineral nutrition of edible plants (whether fresh, previously composted or directly composting on the ground) [12,54,55,57,61,62,63]. The addition of fresh SCG to cultivation substrates decreased the Mg, P, Ca, Na, Fe, Mn, Zn and Cu content in lettuce. This effect was attributed to mineral retention within the SCG matrix due to the presence of potential chelators or to the presence of caffeine [55]. Cervera-Mata et al. [61] found the opposite result: SCG increased the plant content of elements with nutritional importance, such as V, Fe, Co, Mn and Zn. They related it to the polyphenol, melanoidin or carbohydrate presence in SCG, which are molecules that have a chelating character and can mobilize these elements in the soil [62]. Other researchers also found an increase in some elements (Fe, Zn and Mn) in brown rice, after the application of SCG enriched with Fe and Zn [56]. Chrysargyris et al. [63] used fresh SCG as a cultivation substrate for seeds of the Brassica genus and observed that the levels of N, P and K increased, while those of Mg and Fe decreased. Caliskan et al. [69], who studied SCG and pine growth, reported that the addition of SCG increased the levels of N, K, Mg and P in a dose-dependent manner, whereas Ca and the C/N ratio decreased. Kasongo et al. [6] also investigated how the addition of different coffee residues (coffee husk and pulp) affected plant mineral nutrition. They found that these residues were able to favor Ca, Mg, K, N and P absorption, while decreasing the Cu, Zn, Mn and Fe concentration. However, most of the authors cited above did not use regular agricultural soils, but instead added SCG to very sandy, contaminated soils or to growing substrates, such as peat. This is an aspect that should be emphasized, since plants’ nutritional characteristics depend on the soil’s or growing medium’s chemical, physical and physicochemical properties [75].

SCG have been used not only to improve the mineral nutrition of crops but also to improve (in the case of lettuce) their content of carotenoids [64] and active compounds, as well as to improve their antioxidant capacity [55]. In this regard, SCG addition increased lutein and β-carotene by 90 and 72%, respectively, whereas chlorophylls increased by up to 61%. This increase in bioactive compounds occurred with SCG concentrations up to 10% [64]. The antioxidant capacity of lettuces increased linearly with the fresh SCG concentration, although the same did not happen with lettuces grown with composted SCG [65].

Sergio Pérez-burillo

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