Combining Selenium Biofortification with Vermicompost Growing Media in Lamb’s Lettuce (Valerianella locusta L. Laterr)

Abstract

Leafy vegetables are a daily part of the human diet all over the world. At the same time, a worldwide problem of Se malnutrition is present in human populations, mostly due to low soil Se contents. As plants represent the main source of this element in the human diet, with Se being an essential trace element for humans and animals, plant foods containing Se can be used as an efficient means of increasing the Se in the human diet, as well as in animal feed (biofortification). At the same time, the production of growing media relies on limited peat reserves. The use of earthworms facilitates the production of composted organic masses mostly consisting of organic waste, called vermicompost. The aim of this study was to investigate the influence of three different growing media (commercial peat media, vermicompost, and a 1:1 mixture) on Se biofortification’s efficacy and yield in lamb’s lettuce. The Se biofortification was performed with sodium selenate (Na₂SeO₄). It was shown that biofortification increased the Se contents such that a mass of only 48.9 g of fresh leaves contained enough Se for the recommended daily intake in human nutrition (55 µg Se/day), which represents a significant potential for solving Se malnutrition. Furthermore, the use of a 1:1 vermicompost–commercial substrate mixture showed a similar performance to the peat growing medium, contributing to the preservation of peat reserves.

1. Introduction

Various anthropogenic activities, escalating urbanization, industrialization, and economic growth are leading to the production of huge quantities of solid waste around the globe. The management of this solid waste has now become an ecological and a technical problem for all [1]. Due to the rising costs and uncertain future availability of peat moss, there is a need for alternative components in commercial potting substrates. Nevertheless, peat-based commercial potting substrates have low ion exchange capacities, and there is a concern about the environmental impact of leachates containing high concentrations of chemical fertilizers [2]. The large-scale removal of peat from bogs is also destroying wildlife habitats, and the process of peat regeneration is extremely slow [3]. Combining these two insights with the fact of increasing garbage production and the increasing shortage of resources, one of the possible solutions to these problems might be vermicomposting. Vermicomposting is an efficient nutrient recycling process that involves harnessing earthworms as versatile natural bioreactors for organic matter decomposition. In other words, earthworms are capable of transforming garbage into “gold” [4]. Vermicomposting is a non-thermophilic biological oxidation process in which organic materials are converted into vermicompost, which is a peat-like material exhibiting high porosity, aeration, drainage and water-holding capacities, and rich microbial activities [4]. The process of vermicomposting is faster than composting because the material passes through the earthworm’s gut and presents an excellent soil additive, making it a high value product [5]. It is used as an organic fertilizer, soil amendment, and potting substrate component, with many characteristics matching those of conventional composts [2]. Therefore, many studies have proposed vermicompost as an alternative for maintaining economically viable crop production with minimal environmental pollution [6].

Plants recycle Se within the food chain. Thus, the biofortification of crops with Se, by means of adding Se along with fertilizers, is a useful technique to increase the consumption of Se by animals and humans [7]. In humans, Se absorption from products of plant origin is much easier compared to its absorption from products of animal origin [8]. Dietary Se deficiency has negative effects for human health, and more than 40 types of diseases have been associated with Se deficiency, such as Keshan disease, Kashin-Beck disease, cardiovascular diseases, liver diseases, some type of cancer and cataracts [8]. Similarly to Zn, Se supplementation to coronavirus disease 2019 (COVID-19) infected patients with low Se blood levels could be an option as a natural treatment against the virus [8]. Because Se deficiency in the diet is a common phenomenon in many countries worldwide, plants biofortified with Se are an excellent source of dietary Se that can help alleviate this problem [9]. According to the European Union regulations [10], the recommended daily dose of Se for adults is 55 µg. Se is a very important micronutrient for the proper functioning of humans, animals, and some microorganisms, as a structural component of selenoproteins [11], but its importance for plants is still the subject of research [12]. Some of the positive effects of Se found in plants are: promoting plant growth, alleviating UV-induced oxidative damage, improving the recovery of chlorophyll from light stress, and increasing the antioxidative capacity of senescing plants [12]. The protective role of Se in plants exposed to various environmental stresses, in most cases, has been attributed to the activation of the antioxidative defense system [12,13]. Se is a rare element, with an average concentration in igneous bedrock of only 0.05 mg kg⁻¹, less than any other nutrient element [8]. It exists in four different oxidation states: elemental Se (Se⁰), selenide (Se⁻²), selenite (Se⁺⁴) and selenate (Se⁺⁶), with other inorganic and organic matrices and soluble forms of selenite and selenate [14]. The plant-available Se in the soil, such as water-soluble and exchangeable Se, consists of mobile fractions that are readily taken up by plants [15].

Fresh-cut or minimally processed fruits and vegetables play an important role in the human diet, as opposed to highly caloric diets rich in lipids and sugars. Consequently, human nutrition has been orienting towards ready-to-eat foods such as pre-cooked and minimally processed vegetables or fruits [16]. With its relatively good storage quality, lamb’s lettuce is in increasing demand not only as a leafy salad but also as an ingredient in fresh-cut products and ready-to-eat salad mixtures [17]. Lamb’s lettuce has modest heat requirements, such that its production mostly occurs during the colder period of the year, with an optimal growth temperature between 5 and 10 °C [18]. Lamb’s lettuce has been reported to have a positive influence on certain diseases, such as diabetes, cardiovascular disorders and cancer [19].

The aim of this study was to investigate the efficiency of Se biofortification with sodium selenate (Na₂SeO₄) in three different growing media: A commercial substrate (CS), vermicompost, and a mixture of these two substrates in a 1:1 ratio.

2. Materials and Methods

2.1. Plant Growth Conditions and Growing Media Characteristics

Three different growing media were used for this experiment: commercial growing substrate (CS), vermicompost, and a CS–vermicompost mixture in a 1:1 ratio. The commercial growing substrate was characterized by the following properties: pH (H₂O) 6.3, electrical conductivity (EC) 0.42 dS/m, specific density 180 kg/m³, and a total porosity of 85% volume. CS is a mixture of frozen black sphagnum peat and very fine white sphagnum peat, with added water-soluble fertilizer and microelements. CS is a multipurpose substrate for production in containers and nutrients cubes of up to 6 cm for lettuce, cabbage, and celery, etc. The vermicompost used in the experiment was procured from the city municipal waste company UNIKOM d.o.o. Osijek (Osijek, Croatia), produced from municipal waste generated by the maintenance of public green areas (residues from cutting hedges, fallen leaves, grass clippings, and branch cuttings) and processed through the digestive tract of red wigglers (Eisenia fetida). The lamb’s lettuce cultivation was carried out in a plant growth room (Faculty of Agrobiotehnical Sciences Osijek, Osijek, Croatia) in a walk-in setup. During the experiment, the daytime temperature was set at 24 °C, and the nighttime at 20 °C; the relative humidity was 45%, and the light regime was 16 h day and 8 h night.

2.2. Experimental Design

The experiment was conducted with a completely randomized design. For the biofortification part of the experiment, the seeds of a lamb’s lettuce (Valerianella locusta L. Laterr), cv. Verte de Cambrai (Franchi sementi, Grassobbio, Italy), with a declared germination rate of 84% and a seed purity of 95% were used. The seeds were sown in styrofoam containers with 60 sowing holes (52 mL each) in a 10 × 6 arrangement. Four containers were prepared per substrate, representing four replicates for each of the growing media (CS, vermicompost, and mixture of both in a ratio of 1:1). Three seeds were sown per growcell. After three weeks, the seedlings were thinned to a single plant per growcell, and then fertilized with a pre-prepared solution of crystalline complex fertilizer N:P:K in a ratio of 20:20:20 + MgO (magnesium oxide) + ME (microelements) (Haifa Group, Haifa, Israel). The biofortification was carried out on the 6th week of the experiment, by adding 15 mL of 40 µM sodium selenate (Na₂SeO₄) solution, which was added per hole to half (30) of the growcells of each container. The plants were harvest 10 days after the Se application. The yield was expressed on a single-plant basis by weighing all of the plants from a single container treatment–substrate combination and dividing this by the number of plants.

2.3. Analysis of the Growing Media

Samples of the vermicompost, commercial substrate and mixtures of the vermicompost and commercial substrate in a 1:1 ratio were analyzed to measure for the following properties: pH, electrical conductivity (EC), organic matter (OM), ash content, the concentration of nitrogen, micronutrients (Zn, Cu, Mo and Ni), toxic elements (Cd, Pb, Cr, Hg and As), and the total Se concentration. The laboratory analyzes and element determination were conducted at the Faculty of Agrobiotehnical Sciences Osijek, Osijek, Croatia.

2.3.1. pH Determination

The electrometric measurement of the reaction (pH value) was performed with pH meters that measured the difference in electrical potential. The electrometric determination of the pH of the vermicompost was performed according to the European standard 13037:2011 [20] in a suspension of 60 mL fresh sample in 300 mL deionized water, i.e., in a volume ratio of 1:5 (sample:water), and after mixing with a shaker for 60 min.

2.3.2. EC

The electrical conductivity was measured according to the European standard EN 13038: 2009 [21] in a suspension of 60 mL fresh sample shaken on a shaker for 60 min in 300 mL deionized water, i.e., in a volume ratio of 1:5 (sample:water). Electrical conductivity is an indicator of the proportion of water-soluble electrolytes in the analyzed sample.

2.3.3. Determination of the Organic Matter and Ash Contents

The total organic matter and ash contents were determined by drying 5 g of the sample at 103 ± 2 °C for at least 4 h, and successively annealing the sample at 450 ± 10 °C for at least 6 h (EN, 2011.b). The sample was annealed in an annealing furnace, with the first weighing of the sample mass after 6 h, and then after each additional hour of annealing to a constant mass, i.e., when the difference between two consecutive weighings was <0.01 g.

2.3.4. Determination of the Organic C Content, Total N and C/N Ratio

The organic carbon content analysis was carried out by wet destruction, in which 50 mg of the dry sample was weighed into destruction cells, poured with 5 mL of 0.27 mol dm⁻³ K₂Cr₂O₇ and 7.5 mL of concentrated H₂SO₄, and destroyed for 30 min in a destruction block at 135 °C. After its destruction, the sample was quantitatively transferred to volumetric flasks, made up to 100 mL with deionized water, transferred to centrifuge tubes, centrifuged for 10 min at 2000 G, and filtered. In clear samples and a series of standard glucose solutions, the transmission values at 585 nm were measured with a spectrophotometer, and the organic carbon content (concentration) was expressed in %.

The determination of the nitrogen is based on the preparation of a stock solution of the sample by destroying the sample using mixtures of acids. An NaOH solution was added to the measured volume of the sample stock, and the N was distilled off as ammonia into a sample with an acid of known concentration. The amount of nitrogen in the analyzed sample was calculated from the acid consumption (EN, 2003). The C/N ratio was obtained using the data of the total carbon in the dry matter and the total nitrogen in the fresh matter according to the following formulae:

% C in fresh matter = (% C in dry matter × % dry matter) ÷ 100

(1)

C/N = % C in fresh matter ÷ % N in fresh matter

(2)

2.3.5. Determination of the Heavy Metal Concentrations

In order to determine the concentration of heavy metals (Zn, Cu, Ni, Mo, Cr, Cd, Hg, and Pb), a stock solution was prepared by destroying the dry sample by digestion with a mixture of concentrated nitric and hydrochloric acid in a ratio of 1:3 [22]. The heavy metal concentrations were determined by measurement in stock using a Perkin Elmer Optima 5300 DV Inductively Coupled Plasma Optic Emission Spectrometer (ICP-OES, Waltham, MA, USA). The concentrations of these elements are expressed in mg/kg dry matter of the sample.

2.4. Plant Material Analysis

Determination of the Total Se and Total Zn Concentrations in the Plant Tissue

The dry plant matter was ground in a special mill without heavy metal residues, and was destroyed by the wet process using the microwave technique. In total, 0.5 g of the dry sample was weighed into a Teflon dish and poured with 9 mL 65% HNO₃ and 2 mL 30% H₂O₂. After the digestion procedure under a controlled pressure and temperature in the microwave, the solution was filtered into metered vessels. Before determining the concentration, the reduction of the Se in the samples was performed. For the Se and Zn reduction procedure, 20 mL of the sample was transferred to clean 125 mL cuvettes with the gradual addition of 20 mL HCl. The solution was then transferred to a 50 mL polypropylene tube and made up to the mark with deionized water. The Se and Zn concentrations in the solutions were measured using the Perkin Elmer Optima 5300 DV Inductively Coupled Plasma Optic Emission Spectrometer (ICP-OES) technique.

2.5. Statistical Analysis

The statistical analysis was performed using the R programming language (4.0.4 version) and Microsoft Excel. A two-way type II analysis of variance (ANOVA) was carried out with the main-effects Se treatment and growing media, and assumed two-way interactions. The differences between the treatment means were considered significant at the p < 0.05 probability level in Fisher’s LSD test. Another one-way ANOVA was carried out for the decomposing of the significant interactions. The differences between the means were compared using Fisher’s least significant difference at a p = 0.05 probability level.

3. Results

3.1. Physico-Chemical Analysis of the Growing Media

The considerable differences were detected between the properties of the assessed growing media. It is shown in

Table 1. Basic properties of the growing media commercial substrate (CS), vermicompost and mixture 1:1.

Growing Media	% Organic Matter	% Ash	pHH2O	EC (mS/m)	Total N (g/kg)	C/N Ratio
CS	39.6	51.4	6.34	42.1	2.2	180:1
Vermicompost	12.6	87.4	9.23	67.9	5.4	23.3:1
Mixture 1:1	22.4	78.6	7.55	59.5	3.1	72.3:1

that the commercial substrate (CS) had the highest value of organic matter and the highest C:N ratio compared to the other two growing media. CS had the lowest EC, total nitrogen and ash content compared to the other two substrates. The vermicompost had the highest ash content, the highest pH value, EC and total nitrogen content, and compared to the other substrates it contained the lowest amount of organic matter and had the narrowest C:N ratio. The 1:1 mixture showed properties between CS and the vermicompost.

The concentrations of the trace elements and toxic heavy metals in the growing media are shown in

Table 2. Total concentrations (mg/kg) of the microelements (Zn, Cu, Se, Mo, Ni) and harmful heavy metals (Cd, Pb, Cr, Hg and As) in commercial substrate (CS), vermicompost and mixture 1:1.

Growing Media	Zn	Cu	Se	Cd	Pb	Mo	Ni	Cr	Hg	As
CS	60.2	16.9	0.55	0.64	20.1	0.098	7.1	4.6	<0.01	0.012
Vermicompost	106.0	34.0	0.21	0.49	18.9	0.720	23.0	30.0	0.063	5.570
Mixture 1:1	77.0	23.4	0.32	0.54	19.3	0.411	17.1	19.4	0.041	3.213

. CS had the highest determined concentrations of Cd, Se and Pb, and the lowest contents of Zn, Cu, Mo, Ni, Cr, Hg and As compared to the other two analyzed substrates. Vermicompost showed the highest levels of Zn, Cu, Mo, Ni, Cr, Hg and As, and on the other hand, the lowest concentrations of Cd, Se and Pb. The 1:1 mixture’s values were between the commercial substrate and the vermicompost.

3.2. Analysis of Plant Material

The main-effect growing media showed significant effects on the fresh and dry mass per plant and the Se content in the fresh plant material, while the main-effect Se treatment showed significant effects only on the Se content in both the fresh and dry mass (

Table 3. Analysis of variance for the fresh and dry weight per plant, and the Se concentrations. * represents significance at p < 0.05, *** represents significance at p < 0.001. In the case of a lack of significant effects, the p-values are given.

	Fresh Weight per Plant	Plant Dry Weight	Se mg/kg FW	Se mg/kg DW
Growing media	***	***	*	0.122
Se treatment	0.6735	0.129	***	***
Growing media: Se treatment	*	*	*	0.121

).

The yield of fresh mass per plant (g) varied significantly with respect to the treatment and growing media interaction. There were no significant differences detected between the yield in CS and the 1:1 mixture growing media, considering the control and treatment with Se. Significant differences between the Se treatment and the control in the vermicompost were found. A significantly higher yield was detected in the control than in the Se treatment (Figure 1).

Table 4. Mean values ± the standard deviations of the Se contents in the fresh weight (FW) and dry weight (DW) of the lamb’s lettuce tissue. The different letters represent the significance of the differences at the α = 0.05 level.

	Se FW (mg/kg)	Se DW (mg/kg)
Se Treatment
Control	0.0065 ± 0.0019 b	0.0691 ± 0.0131 b
Se	1.1236 ± 0.5914 a	12.2615 ± 4.3845 a
LSD 0.05	0.2794	2.629
Growing Media
CS	0.532 ± 0.584 ab	6.7207 ± 7.3576 a
Vermicompost	0.848 ± 1.01 a	7.4656 ± 9.0653 a
Mixure 1:1	0.314 ± 0.361 b	4.3096 ± 4.6585 a
LSD 0.05	0.342	3.221

shows the Se content in the fresh and dry weight of the lamb’s lettuce tissue. The Se treatment significantly affected the Se content increase in the dry and fresh plant mass over all three assessed growing media. In the fresh weight of the lamb’s lettuce, the Se content increased 172.86-fold compared to the control. The Se content in the dry mass increased by 177.45 times compared to the control. Significant differences were found between the concentration of Se in the fresh weight of the plant tissue with respect to the growing media over both the control and the Se treatment. The 1:1 mixture showed, significantly, the lowest Se contents compared to the other two growing media in the fresh-weight tissue, although no significant differences were found for the Se content in the dry mass of the lamb’s lettuce.

A significant effect of the Se addition to the growing media on the Zn uptake is shown in

Table 5. Mean values ± standard deviations of the Zn concentration in the fresh-weight (FW) and dry-weight (DW) lamb’s lettuce tissue. The different letters represent the significance of the differences at the α = 0.05 level.

Treatment	Growing Media	Zn FW (mg/kg)	Zn DW (mg/kg)
Control	CS	14.51 ± 0.910 a	170.85 ± 10.65 a
Se	CS	11.86 ± 0.064 b	149.95 ± 1.05 b
Control	Mixture 1:1	9.49 ± 2.462 cd	104.55 ± 0.15 c
Se	Mixture 1:1	6.83 ± 1.396 e	97.67 ± 5.72 cd
Control	Vermicompost	9.10 ± 0.635 d	88.245 ± 4.68 d
Se	Vermicompost	11.70 ± 0.379 bc	101.81 ± 6.39 c
LSD 0.05		2.226	10.52

. The fresh weight of the lamb’s lettuce in CS had a significantly lower Zn concentration in the treatment with Se. Vermicompost had, significantly, the highest concentration of Zn in the fresh weight with the addition of Se, and in the 1:1 mixture the Zn content was higher in the control compared to the Se treatment in the fresh plant tissue. In the dry mass of the lamb’s lettuce in CS, significant differences were detected between the control and treatment with Se. The control showed a higher Zn content compared to the Se treatment. The Se treatment significantly increased the Zn concentration versus the control in the dry weight of the lamb’s lettuce tissue in the vermicompost growing media, while no significant differences were found in the Zn concentration in the dry weight of lamb’s lettuce grown in 1:1 mixture.

4. Discussion

The aim of this study was to investigate the efficiency of Se biofortification in three different growing media: CS (commercial supstrate), vermicompost, and a mixture of the two in a 1:1 ratio. CS had the highest organic matter content, but also the widest C:N ratio (Table 1). The C:N ratio should be between 1:20 and 1:30, which is considered the most favorable C:N ratio because it does not lead to nitrogen depression, so the CS showed the least amount of N. Vermicompost had the optimal C:N ratio of 23.3:1 and the highest amount of N, although it showed the lowest content of organic matter. Electrical conductivity (EC) can serve as a measure of soluble nutrients—both cations and anions [23]—and due to that it would mean that vermicompost is the richest in nutrients, and CS the poorest. The lower EC could result in a lower cations content in the soil solution [23]. The 1:1 mixture had values between the vermicompost and the CS, and thus some properties of both were improved. The reason for the higher nutrient levels in the vermicompost might be the fact that earthworms enhance organic matter degradation [24]. Vermicompost and CS showed values for the trace elements and heavy metal concentrations in reverse order (Table 2). The maximum tolerable levels of the elements in unpolluted soils according to the WHO are: Cd 0.8 mg/kg, Zn 50 mg/kg, Cu 36 mg/kg, Cr 100 mg/kg, Pb 85 mg/kg, and Ni 35 mg/kg [25]. It was found that CS had the highest determined concentrations of Cd, Se and Pb, and the lowest amounts of Zn, Cu, Mo, Ni, Cr, Hg and As compared to the other two substrates. Vermicompost showed the highest concentrations for Zn, Cu, Mo, Ni, Cr, Hg and As, and on the other hand, the lowest concentrations of Cd, Se and Pb. Pb and Cd pollution in acidic soils yields a higher environmental risk, and suggests that efforts to increase the soil pH will effectively decrease both the Cd and Pb accumulation in plants grown in polluted acidic soils [26]. Salinity increases the heavy metal mobilization in soils. The extent of the mobilization depends on the type of heavy metal present, the total amount of heavy metal present, and the type of salt causing the salinization. This means that all of these factors must be explicitly taken into account when assessing the risk of salinization on heavy metal release from the soil [27]. Very low transfers of heavy metals to plant tissues occur at a high pH [28], which is good or bad for certain heavy metals. The term “heavy metals‟ refers to naturally occurring elements that have a high atomic weight, with a density greater than 4 g/cm³ [29].

The yield of the lamb’s lettuce differed according to the growing media (Figure 1). The highest yield was achieved in CS and the 1:1 mixture, without significant differences between the treatments with Se. With arising ecological issues linked to solid waste management [1] and the exploitation of limited peat reserves presenting a threat for natural habitats [3], this type of mixture might represent a good means of alleviation for these issues. Furthermore, peat substrates show poorer properties compared to vermicomposts [2], and vermicomposts might be per se even better optimized for growing other commercial horticultural plants. However, even for growing lamb’s lettuce, a 1:1 mixture can be used as an ecologically safe replacement for a commercial substrate with similar growing performance. Vermicompost showed a significantly lower yield compared to the other two growing media, and significant differences were also found in the treatments with and without Se in the vermicompost. Studies have shown that a high pH (7–10) or a high EC reduced rice [30] and strawberry [31] growth and biomass. On the other hand, the growth of geranium and calendula was better in all vermicompost-based growing media than compost-based growing media, despite the high pH of 7.3 [24]. A high solution pH increased the deficiency of nutrients, particularly Fe and P, and the associated chlorosis and loss of market appeal in leafy vegetables and yield [32]. In our study, high pH values were determined, and the Se treatment (Na₂SeO₄) probably increased the alkalinity even more, and caused a negative effect on the lamb’s lettuce yield. The Se biofortification was efficient, and the plants treated with Se had about 170 times higher Se concentrations compared to the control in the dry (177.45 times higher) and fresh (172.86 times higher) weight of the lamb’s lettuce (Table 5). A mass of 48.9 g of fresh biofortified lamb’s lettuce leaves from our experiment contained enough Se for the recommended daily intake in a human diet (55 µg Se/day). In total, 400 µg Se/day is considered a safe upper limit, and a high dose of Se supplementation showed that intakes up to 3200 µg Se/day gave no obvious Se-related serious toxicities in men [33]. Our results on the efficiency of Se biofortification are in accordance with other research on lettuce [7,9], radish [34], carrot [35] and lentils [36]. Lamb’s lettuce grown in vermicompost showed, significantly, the highest Se concentrations in its fresh weight, although there were no significant differences between the growing media and Se concentration in the dry weight of the lamb’s lettuce. The yield of the lamb’s lettuce was the lowest in vermicompost (Figure 1), such that the Se was more concentrated in the leaf with regard to its fresh weight (Table 5). Several studies have shown that an increase in the soil pH increases the plant’s Se uptake [37]. In our study, the Se biofortification was performed with sodium selenite; this could be the reason for the Se accumulation in the lamb’s lettuce, because selenate (SeO₄²⁻) is the predominant Se species in near-neutral pH environments under aerobic conditions, whereas selenite (SeO₃²⁻) predominates at lower pH and redox potentials [37]. Although there was no significant difference detected in the concentration of Se in the dry leaf mass of lamb’s lettuce, it can be seen that the different substrates produced different concentrations of Se in the leaf. It can be concluded that the rate of Se uptake depends on the concentration and chemical form of the Se in the soil solution, as well as the rhizosphere conditions, such as the pH and the presence of sulfate and phosphate, which alter Se form and compete with Se uptake [38]. In our research, an interesting phenomenon was noticed. In Table 5 it can be seen that Se biofortification had the influence on Zn uptake. CS and 1:1 mixture had a higher Zn content in the control, while contrarily, the Se biofortification decreased the Zn concentration in the fresh weight and dry weight of the lamb’s lettuce. The opposite was noted in the vermicompost, in which the addition of Se increased the Zn levels in the fresh and dry weight of the lamb’s lettuce leaves, while in the control, the Zn content was lower. It is known that Zn acts synergistically on Se uptake by the roots, and for accumulation in the leaves [39], which is in accordance with our results in vermicompost; therefore, Ei et al. [40] conducted a piece of research in which Se increased more than the Zn accumulation under the combined Se–Zn application. From our research, it can be concluded that, in CS and the 1:1 mixture, Zn and Se had an antagonistic relationship, and in the vermicompost they had synergistic relationship. Zn is an essential element in plants, and is a necessary co-factor of six classes of enzymes in plants, which include oxidoreductases, so these observations suggest that Zn might exert an Se detoxification effect at high Se doses [41]. Our results in CS and the 1:1 mixture are in line with the research form Mangueze et al. [42], which found that Se soil application had an antagonistic effect on Zn in rice grains. It is well-established that the metal transfer from soil-root-shoot/grains is controlled by element speciation, as well as various detoxification/tolerance mechanisms functioning in place inside plants [43].

5. Conclusions

The Se biofortification was successful in all three assessed growing media (CS, vermicompost, and a 1:1 mixture). CS and the 1:1 mixture showed no significant differences in the yield and Se concentration in the fresh and dry lamb’s lettuce. On the financial side, 100 kg commercial substrate costs approximately 36 euros. According to our results, the commercial substrate–vermicompost mixture in the 50:50 ratio exerted a similar performance to the commercial substrate in the cultivation of lamb’s lettuce. The savings in this scenario would be as much as 50%, or 18 euros per 100 kg, because in our case the vermicompost is produced from green waste from public areas. The use of Vermicompost per se resulted in reduced yields, but the lamb’s lettuce grown in the vermicompost showed the highest concentrations of Se, probably due to the high pH value and the use of Se in the form of sodium selenate. In our study, it was observed that Se enhanced the Zn uptake in vermicompost, although Zn was not amended, while in the other two growing media, the Se treatment reduced Zn uptake by lamb’s lettuce. Due to its positive performance, we propose vermicompost as an additive to commercial substrates in a 50:50 ratio, thus reducing the use of commercial substrates by 50% due to the increasing lack of peat, while maintaining the growing performance. Due to the extreme importance of Se for the human population, which generates more and more waste and seeks a solution, biofortification and the application of vermicompost are proposed as two potential solutions to address the two uprising issues.