1. Introduction
Recently, the idea of urban gardening involving the individual cultivation of vegetables and other crops has gained huge popularity [
1]. Growing plants in urban areas transformed into community gardens is a way of spending free time, strengthening social ties and obtaining plants for consumption. Farms, green roofs and walls, and vertical gardens, as well as the production of plants in various types of containers, are becoming more and more popular in urbanized areas [
2]. The intensive development of urban gardening may be an impulse for a more comprehensive use of composts and compost-based substrates derived from waste biomass [
3,
4]. More extensive research is needed to assess the suitability of various biomaterials for the preparation of composts and natural potting media that are the basis for sustainable crop production [
5].
The development of waste management technologies aims primarily at preparing the waste for recovery, and particularly recycling. The utilization of waste and by-products by means of composting can be applied everywhere in the world and does not require an advanced infrastructure [
6,
7,
8]. The availability of waste materials and by-products from the paper, food, wood and agricultural industries is currently unlimited [
9,
10]. Converting waste rich in organic matter and elements into composts is the best method of its disposal, as macro and micronutrients return to the ecosystem [
11]. Research shows that the share of compost in the total production of substrates in the EU is only 7.9%. The most often used substrates include composted green waste, composted or aged bark and composted wood waste [
12].
Compost is an invaluable organic fertilizer in organic farming. It improves the fertility of substrates and soils, and their physical, chemical and biological properties [
13]. Composts are used in the sustainable production of potting media as a source of humus and nutrients [
7,
11,
14]. Their main advantage is their biostimulating effect on the growth and development of plants and increasing crop quality [
5]. Previous studies assessed different types of composts prepared from one, two, and seldom more types of nuisance waste materials [
15]. Obtaining a new material in the form of compost based on several types of waste materials may increase the potential benefits of the final product. Such a product is no longer classified as waste, as it features completely different properties that positively affect the natural environment [
7,
16,
17].
A potential raw material for the production of compost may be the waste from paper production—pulp and paper mill sludge [
18,
19,
20]. Waste from the pulp and paper industry is characterized by a diversified chemical composition and physical and chemical properties, which depend, for instance, on the technology of the pulp and paper production, the type of pulp, and the type of produced paper [
18,
21]. The estimated amount of waste generated by the European paper industry is around 11 million tons per year, 70% of which come from paper recycling [
22]. Several directives issued in the European Union significantly limited the possibility of landfilling this type of waste. Research shows that pulp and paper mill sludge can supplement the compost with organic matter and minerals and constitute a potential component of horticultural growing media [
10,
23].
Raw materials for the production of composts should also be sought in the food industry [
24]. By 2012, the EU countries produced about 88 million tonnes of food waste [
25]. Fruit and vegetable waste can be a rich and sometimes most valuable source of nitrogen in composts. A good example is apple (
Malus domestica Borkh.) pomace, containing significant amounts of dietary fiber and polyphenols, of which Poland is the greatest producer [
26]. During juice production, these substances usually do not make it into the final product but are retained in the pomace and are often irretrievably lost. The unique composition of by-products from fruit and vegetable processing makes them not a waste, but a highly valuable raw material [
27].
Spent mushroom production substrate can be potentially and successfully used in growing media [
28]. The production of this waste in Europe reaches over three million tonnes annually [
10,
29]. Poland, as the largest producer of mushrooms in the EU and third in the world, generates huge amounts of the spent substrate [
30]. Composting the spent mushroom substrates and adding them as a component to growing media can solve local ecological problems with the storage of post-production substrate. In Poland, the mushroom substrate is made of winter cereal straw, poultry manure, gypsum and water. Its analyses showed that it is rich in organic matter, nitrogen and other yield promoting elements [
31]. Unfortunately, the chemical composition of the substrate is diverse, which is related to the production technology. Research studies demonstrated that the mushroom substrate can be composted with other waste products, and the final material is a valuable organic fertilizer [
32].
In most European countries, high peat is the dominant component of soilless substrates. It offers unique physical and chemical properties and is safe in phytosanitary terms, but it is a non-renewable resource. Moreover, the exploitation of peatlands increases CO
2 emissions and negatively affects the environment [
33,
34]. The EU market consumes approximately 34,609 billion m
3 of substrates per year, of which peat accounts for 75% [
12]. Most of the current research on substrates is aimed at developing low-peat or peat-free compositions [
10,
12,
16]. Substrates not containing high peat are currently the most sought after and the most often developed for environmental and economic purposes [
35].
Our study investigated the use of several waste materials, such as pulp and paper mill sludge, fruit-vegetable waste, mushroom spent substrate and rye straw to produce a compost with high fertilizing value. Then, various growing media containing the compost and such components as low peat, high peat, wood fiber, or pine bark were assessed in terms of their suitability for container cultivation of vegetables. Our model plant was a cherry tomato (
Lycopersicon esculentum Mill.), as it is one of the most important vegetables and is widely grown in home gardens and balconies for its tasty, healthy and decorative fruit. Tomatoes are easy to grow and serve as a natural source of antioxidants that protect the body against many diseases [
36]. This study involved numerous physical and chemical analyses of the compost and the media. It also assessed tomato growth and yield, fruit color and their content of antioxidants and minerals. Our research hypothesis assumed that the yield and biological quality of tomato fruit grown in compost-enriched media are comparable or better than on traditional substrates, such as high peat.
2. Materials and Methods
2.1. Composts Preparation
The composts were prepared by Sobex Sp. z o.o. company (Trzebicz, Poland) between September 2018 and April 2019 from the following waste materials and by-products: pulp and paper mill sludge (Arctic Paper Kostrzyn S.A., Kostrzyn n/O, Poland), fruit-vegetable waste (Tymbark MWS Sp. z o.o., Tymbark, Poland), spent mushroom substrate (mushroom production farm, Rakoniewice, Poland), and rye straw chopped into 5–8 cm-long chaff (local farm). Physical and chemical analyses of the materials used for composting were performed at the Chemical Analysis Laboratory, Institute of Horticulture in Skierniewice (Skierniewice, Poland) and are summarized in
Table 1. The methods used for these analyses are presented in
Section 2.2.
The composition of the initial compost was calculated based on dry weight (DW) analysis, so that the C:N ratio was in the range of 20–30:1 [
37]. The compost mass contained 18% (DW) of pulp and paper mill sludge, 40% DW of fruit-vegetable waste, 32% DW of spent mushroom substrate, and 10% DW of rye straw. Each 100 kg of its fresh weight (FW) included 20 kg FW of pulp and paper mill sludge, 54 kg FW of fruit-vegetable waste, 22 kg FW of spent mushroom substrate, and 4 kg FW of rye straw. After mixing, the components were formed into trapezoidal heaps (1.3 m high, 2.4 m wide, 3 m long) and kept under a shelter protecting them from precipitation and sunlight (
Figure 1). Every month, the heaps were turned to aerate them. The relative moisture content was measured once a week to maintain a level of 55–65% [
38]. The temperature of the heap was measured every week with an agricultural thermometer with a 1.5 m-long probe (Dramiński, Poland). The composting lasted for 16 weeks. At its completion, biological tests required by law were carried out at the Polish Center for Testing and Certification SA (Piła, Poland).
2.2. Physical and Chemical Analysis of Components, Compost and Growing Media
The pH of the composting materials, final compost and growing media was determined in a suspension of the substrate and distilled water (
v:
v; 1:2) with the TESTER CP-505 m (Elmetron, Zabrze, Poland), and the electrolytic conductivity (EC) with the CCP-401 conductometer with an EC-60 sensor (Elmetron, Zabrze, Poland) [
35]. The moisture content, bulk density, air capacity, water capacity, shrinkage and total porosity were assessed according to European standards [
39,
40].
Compost samples for physical and chemical tests were collected every month. Each sample was prepared by mixing five collective samples taken from different five spots and spacing, packed in 3 L polyethylene bags and sent on the same day to the Chemical Analysis Laboratory (Institute of Horticulture—PIB, Skierniewice, Poland).
The final compost and the substrates were tested for their fractions of matter particles (% weight). Particle distribution was established using sieves with the following mesh size: >20 mm, 10–20 mm, 5–10 mm, 2–5 mm, 1–2 mm, <1 mm.
A Kjeldahl digestion kit and unit (Vapodest, Gerhardt GmbH, Konigswinter, Germany) were used to determine total nitrogen (N) content by titration. Organic carbon (C) was determined by Dumas’ method with the Carbon Sulfur Determinator CS-530 apparatus (Eltra, GmbH, Neuss, Germany). After wet mineralization of samples in a 65% HNO
3 and 75% HClO
4 mixture, the content of total forms of phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), iron (Fe) manganese (Mn), copper (Cu), boron (B), zinc (Zn) and molybdenum (Mo) was determined by plasma spectrometry (Inductively Coupled Plasma Optical Emission Spectrometry) with the Optima 2000 DV sequential spectrometer (Perkin-Elmer, Boston, MA, USA). The analyses covered the spectra typical of each element in nanometers (nm): P—213.617, K—766.490, Ca—317.933, Mg—285.213, S—181.975, Mn—257.610, Cu—327.393, Zn—206.200, B—249.772, Mo—281.616, sodium (Na)—589.592, iron (Fe)—238.204, nickel (Ni)—231.604, chrome (Cr)—267.716, lead (Pb)—220.353, cadmium (Cd)—228.802 and mercury (Hg)—253.616 [
41].
The content of absorbable macronutrients was determined with the universal method [
36,
42] used in Poland for the analysis of horticultural soils and growing media. The extraction solution included 0.03 N acetic acid (pH 3.2), and the medium ratio was 1:10 (
v:
v).
2.3. Experimental Design
The experiment was carried out at the Department of Horticulture, West Pomeranian University of Technology in Szczecin from March 24 to July 11, 2019. Thirty-day-old tomato seedlings (Lycopersicon esculentum ‘Maskotka’), produced from seeds in a greenhouse (22 °C/18 °C; day/night) in peat substrate with pH of 5.7 and an average seedling height of 21 cm, were planted on 25 April 2019 into plastic pots with a capacity of 3 dm3 using the following four growing media:
T0—control (100% peat substrate)
T1—25% compost, 30% sphagnum peat, 15% fen peat, 20% pine bark, 10% wood fiber;
T2—30% compost, 20% sphagnum peat, 30% fen peat, 10% pine bark, 10% wood fiber;
T3—30% compost, 0% sphagnum peat, 30% fen peat, 30% pine bark, 10% woody fiber.
In total, 24 plants were planted for each treatment, with 6 plants per replication. The media were mixed in volumetric proportions (
v:
v). A commercial 100% peat substrate mixed with the PG Mix fertilizer (2 g dm
−3) was used as a control. Sphagnum peat was sourced from Latvian peatlands, and fen peat and pine bark were provided by local Polish companies. Wood fibers were obtained by means of a thermal-mechanical method from Steico sp z o.o. (Czarna Woda, Poland). The chemical properties of the components are presented in
Table 2. Based on the chemical analysis of T1, T2 and T3 media, calcium nitrate (15.5% N; 26.5% CaO) was applied at 1.5 g per pot before planting the seedlings.
The seedlings were placed in a random sub-block system in a high, unheated tunnel covered with two layers of plastic, which was equipped with an automatic ventilation control system. The plants were placed on a nursery mat at 40 × 50 cm spacing (five plants per m2). The average temperature during the growing season ranged from 22 to 28 °C, and the relative humidity was 65–80%. No chemical protection or top dressing was applied, and sticky boards and biological protection (Koppert Biological Systems, Berkel en Rodenrijs, The Netherlands) were used for pest control. Eighty days after seeding, two of the largest leaves from the middle section of a plant from each treatment were harvested for the measurement of the relative chlorophyll content with the Chlorophyll Meter SPAD 502 (Konica, Minolta, Japan). The readings were taken from twelve plants from each treatment at four sites on the leaf blade and averaged. The fresh weight of the aboveground part of the plants was also determined from eight plants from each treatment.
2.4. Tomato Yield and Fruit Characteristics
The fruit yield was assessed once (102 days after seeding), when at least five tomato fruits had ripened per plant. All tomato fruits (marketable and green ones) were harvested separately from each plant, counted and weighed. The percentage of ripe tomato fruits was calculated.
To determine the fruit color, five ripe tomato fruits per treatment were randomly selected. After washing and drying the skins, the color index was determined using a Chroma Meter, model CR-400 (Minolta, Tokyo, Japan). The reading was taken three times for each fruit in its middle part, and then the mean value was calculated. The proportions of red color (red positive/green negative)—a*, yellow color (yellow positive/blue negative)—b* and saturation were analyzed [
43].
Biochemical analyses involved 300 g of ripe fruit from each treatment. The tomatoes were ground and then frozen at −20 °C for 12 hrs. Tomato samples were freeze-dried for 24 h in a Beta 2–8 LSC freeze dryer plus (Martin Christ Gefriertrocknungsanlagen GmbH, Osterode am Harz, Germany) and ground to a powder. Tomato methanolic extracts were prepared as described by Grzeszczuk et al. [
44].
The total content of polyphenols was determined spectrophotometrically with the Folin–Ciocalteu reagent as described by Łopusiewicz et al. [
45]. The absorbance was read with a spectrophotometer (UV-Vis Thermo Scientific Evolution 220) at 765 nm. Gallic acid was used for the preparation of the calibration curve and results were expressed as mg gallic acid equivalents per 100 g of the sample.
The content of flavonoids was determined according to the method of Łopusiewicz et al. [
45]. The absorbance was measured at 510 nm. Quercetin was used for the preparation of the calibration curve and the results were expressed as mg quercetin equivalent (QE) per 100 g of the sample.
To determine the content of ascorbic acid, the Tillmans titration method was used, consisting of the reduction of 2,6-dichlorophenolindophenol [
46]. This consisted of 2 mL of tomato methanolic extract being mixed with 2 mL of 2% oxalic acid and shaken vigorously. The solution was quickly titrated with 2,6-dichlorophenolindophenol until the pink color held for 30 s. The ascorbic acid content is expressed in milligrams per 100 g of FW.
The antioxidant activity was determined based on 1,1-diphenyl-2-picryl-hydrazyl (DPPH) and 2,2′-azobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) radical scavenging activity according to the protocol provided by Łopusiewicz et al. [
47]. In brief, DPPH radical scavenging activity was determined by mixing 1 mL of the methanolic tomato fruit extract with 1 mL of 0.01 mM methanolic solution of DPPH. Absorbance was measured at 517 nm after 30 min incubation in the dark at room temperature. For the ABTS assay, 3 mL of the ABTS solution was mixed with 50 μL of the methanolic tomato fruit extract, and after 6 min incubation in the dark at room temperature, the absorbance was read at 734 nm.
The macro and micronutrients contents in the fruits was determined. The fruits were rinsed three times with distilled water, drained and cut in half. Then, the samples were dried at 70 °C (skin downwards) in a forced air oven, and milled in a Cyclotec TM 1093 laboratory mill (FOSS, Hilleroed, Denmark). Subsequently, the samples were subjected to wet digestion in concentrated nitric acid (0.5 g of the plant material +5 mL of 65% HNO
3) in a closed microwave oven (Nova Wave SA, Microwave Tunnel Digestion System). After mineralization, the content of macro- and micronutrients in solutions was determined by the plasma emission method using the ICP-OES Optima 2000 DV inductively coupled plasma spectrometer. A Kjeldahl unit was used to determine the total N content [
35].
2.5. Statistical Analysis
The results were statistically analyzed in the Statistica 13.1 program (Tibco Software Inc., Poland) using one-way analysis of variance (ANOVA). Differences between means were assessed with Tukey’s test (p ≤ 0.05).