A Mixture of Green Waste Compost and Biomass Combustion Ash for Recycled Nutrient Delivery to Soil

agronomy

Article A Mixture of Green Waste Compost and Biomass Combustion Ash for Recycled Nutrient Delivery to Soil

Kristina Buneviciene 1 , Donata Drapanauskaite 1, Romas Mazeika 1 and Jonas Baltrusaitis 2,*

1 Lithuanian Research Centre for Agriculture and Forestry, Instituto al. 1, Akademija, LT-58344 Kedainiai,˙ Lithuania; [email protected] (K.B.); [email protected] (D.D.); [email protected] (R.M.) 2 Department of Chemical and Biomolecular Engineering, Lehigh University, B336 Iacocca Hall, 111 Research Drive, Bethlehem, PA 18015, USA * Correspondence: [email protected]; Tel.: +1-610-758-6836

Abstract: The use of major nutrient-containing solid residuals, such as recycled solid waste materials, has a strong potential in closing the broken nutrient cycles. In this work, biofuel ash (BA) combined with green waste compost (GWC) was used as a nutrient source to improve soil properties and enhance wheat and triticale yields. The main goal was to obtain the nutrient and heavy metal release dynamics and ascertain whether GWC together with BA can potentially be used for concurrent bioremediation to mitigate any negative solid waste effects on the environment. Both BA and GWC were applied in the first year of study. No fertilization was performed in the second year of the study. The results obtained in this work showed the highest spring wheat yield when the GWC (20 t ha−1) and BA (4.5 t ha−1) mixture was used. After the first harvest, the increase in the mobile forms of all measured nutrients was detected in the soil with complex composted materials (GWC + BA). The content of heavy metals (Cd, Zn, and Cr) in the soil increased significantly with BA and all GWC + BA mixtures. In both experiment years, the application of BA together with GWC resulted in fewer Citation: Buneviciene, K.; heavy metals transferred to the crops than with BA alone. Drapanauskaite, D.; Mazeika, R.; Baltrusaitis, J. A Mixture of Green Keywords: biofuel ash; green waste compost; nutrients; heavy metals Waste Compost and Biomass Combustion Ash for Recycled Nutrient Delivery to Soil. Agronomy 2021, 11, 641. https://doi.org/ 1. Introduction 10.3390/agronomy11040641 The use of secondary, e.g., recycled or reused, materials is very important for sustainable development. One such secondary material is biofuel ash. In developing countries, Received: 25 February 2021 about 60% of all municipal solid waste is incinerated [1]. Of this, only 4–13% [2] is returned Accepted: 25 March 2021 to the environment in the form of ash, as biofuel combustion ash is disposed of in landfills Published: 26 March 2021 and only a part is further processed and used in agricultural soils. Recycling biofuel ash in agricultural soils (a) reduces the need for landfilling; (b) leads to the return of valuable nu- Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in trients to the ecosystem; and (c) decreases soil acidification. As a result, energy production published maps and institutional affil- from wood-burning becomes more sustainable [3,4]. iations. One way to use biofuel ash is to mix it with green waste compost. Different guidelines for mixing compost with ash apply in different countries. In Austria, ≤2% biofuel ash can be added to the compost mix and no limits apply in Germany [5,6]. Other works used 12% ash in a mixture with compost and found no adverse effects to the plants [7]. Other researchers have also shown that up to 15% use of biofuel ash had no negative impact [6]. Copyright: © 2021 by the authors. Both biofuel ash and green waste compost contain various macro- and microelements Licensee MDPI, Basel, Switzerland. This article is an open access article required for soil and plants [8]. Biofuel ash also contains heavy metals [9]. The concentra- distributed under the terms and tion of these heavy metals and their release properties can potentially be mitigated when conditions of the Creative Commons mixing the ash with organic and compostable waste since compost can act as a biological Attribution (CC BY) license (https:// mediator to mitigate any toxic effects of biofuel ash. Researchers have found that the creativecommons.org/licenses/by/ addition of biofuel ash to green waste compost has effectively increased the macro- and 4.0/). microelements of the final product, including P, K, Mg, Ca, as well as Cu and Zn [10], while

Agronomy 2021, 11, 641. https://doi.org/10.3390/agronomy11040641 https://www.mdpi.com/journal/agronomy Agronomy 2021, 11, 641 2 of 15

other researchers have found improved microorganism activity [11]. Other reports have found that ash and green waste compost (a) increased the amount of organic matter in the soil; (b) improved the physical, chemical, and biological properties of the soil; (c) improved the soil texture (air and water permeability); and (d) improved the plant yield [12]. This study aims to investigate if biofuel combustion bottoms ash (BA) in combination with green waste compost (GWC) could present any sizeable environmental risk or, instead, offer new opportunities for sustainable solid waste recycling and nutrient management in the environment. We studied the response of soil properties as well as spring wheat and spring triticale yield improvement to green waste compost with ash application in a 2-year ﬁeld experiment. The ultimate goal was to perform a comprehensive assessment of the complex composted materials to not only reduce the consumption of mineral fertilizers but also the amount of biofuel ash in landﬁlls.

2. Materials and Methods 2.1. Materials and Physicochemical Characterization Biofuel ash and green waste compost were obtained from biomass processing centers in Lithuania. The GWC used in this study was obtained from JSC Alytaus regioninis atlieku˛ tvarkymo centras (Alytus, Lithuania) and BA obtained from JSC Alytaus šilumos tinklai (Alytus, Lithuania). The BA was formed by burning wood pellets. All reagents for chemical analysis were reagent grade from Fisher Scientific (Leicestershire, UK) and used as received. Double distilled water was used in all experiments [13]. A chemical analysis of the recycled materials used in the field experiments was carried out. The samples were ground, sieved, and extracted with aqua regia. Atomic Absorption Spectroscopy (AAS) (Thermo Fisher Scientific, Waltham, MA, USA) was used to measure the concentration of total calcium Ca and total magnesium Mg; a flame photometer (FP) (Sherwood, Cambridge, UK) for total potassium K; a UV—VIS spectrophotometer (Shimadzu, Duisburg, Germany) for total phosphorus P; and inductively coupled plasma mass spectrometry (ICP-MS) (Thermo Fisher Scientific, Waltham, MA, USA) for the heavy metals (Cu, Cd, Cr, Ni, Pb, and Zn) [14]. Total nitrogen (Ntot) was determined using the Kjeldahl nitrogen distiller method while organic carbon (Corg) was determined using a dry combustion method with a total carbon analyzer Liqui TOC II [15].

2.2. BA and GWC Mixtures for Composting The BA and GWC used, with their corresponding nomenclature, is shown in Table1. Based on previous experiments with an ash content of up to 15% [6] and 16% [16] in the mixture, mixtures of 7%, 13%, and 18% of BA by weight were used. The application rates were 20 t ha−1 GWC + 1.5 t ha−1 BA, 20 t ha−1 GWC + 3.0 t ha−1, and 20 t ha−1 GWC + 4.5 t ha−1, respectively. The GWC rate of 20 t ha−1 corresponded to 60 kg GWC per ﬁeld (30 m2) while that of BA of 3 t ha−1—to 9 kg. In mixtures with 7%, 13%, and 18% ash, the amounts applied were as follows: GWC 60 kg + BA 4.5 kg, GWC 60 kg + BA 9 kg, and GWC 60 kg + BA 13.5 kg. The rate of GWC was calculated based on the maximum permitted N rate of 170 kg ha−1, as indicated in the EC Directive 91/676/EEC [17]. Differently from the current literature data [18], the BA and GWC were not composted together but rather mixed manually before ﬁeld application.

Table 1. Green waste compost and biofuel ash samples used in the experiments.

Sample Abbreviation Control (without any fertilizer material) data Control Biofuel ash (3.0 t h−1) BA Green waste compost (20.0 t h−1) GWC Green waste compost (20.0 t h−1) mixed with biofuel ash (1.5 t h−1) GWC + BA1.5 Green waste compost (20.0 t h−1) mixed with biofuel ash (3.0 t h−1) GWC + BA3.0 Green waste compost (20.0 t h−1) mixed with biofuel ash (4.5 t h−1) GWC + BA4.5 Agronomy 2021, 11, 641 3 of 15

These were used in 2-year (total of 16 months) field experiments to assess the availability and transfer potential of nutrients from BA and GWC to the soil, the resulting changes in soil chemical composition, and yield improvement of spring barley as well as spring triticale.

2.3. Field Experiment Description and Experimental Design At the beginning of the field experiment, soil samples were randomly collected from 8 different spots at a depth of 0–20 cm. The samples were thoroughly mixed to form a composite and analyzed for initial soil properties. Agrochemical properties measured from the upper −1 −1 soil layer were as follows: mobile P2O5 130 ± 10 mg kg , mobile K2O 150 ± 10 mg kg , mobile Ca 4000 ± 100 mg kg−1, mobile Mg 830 ± 10 mg kg−1, Cd 0.079 ± 0.005 mg kg−1, Zn 31.5 ± 0.4 mg kg−1, Cr 15.5 ± 0.2 mg kg−1, Ni 12.2 ± 0.3 mg kg−1, Pb 8.56 ± 0.11 mg kg−1, and Cu 9.49 ± 0.12 mg kg−1. Next, soil samples were collected after the first-year harvest (after 4 months), before the second-year sowing (after 12 months), and after the second- year harvest (after 16 months) from the start of the experiment. The field experiments were conducted from 2018 to 2019 at the Lithuanian Research Center for Agriculture and Forestry, Institute of Agriculture (55◦2304900 N 23◦5104000 E) in Akademija, Lithuania. The soil at the experimental area is loamy (Endocalcaric Epigleyic Cambisol) [19]. The experiment consisted of 6 different variants, which were arranged in the field in three replications in randomization blocks. The field size was 30 m2 (3 m × 10 m). For each application, three replicates were tested. Additionally, three plots without any fertilizer material were used as control. No mineral fertilizers were added. The crop seedbed was prepared with a soil finisher/field cultivator shortly before seeding. GWC and BA mixtures were spread manually on the soil surface and incorporated into the soil (0–15 cm) before the sowing of spring wheat. The spring wheat cultivar “Collada” (Einbeck, Germany) and spring triticale cultivar “Milkaro” (Koscian, Poland) was chosen. The sowing rate was 270 kg ha−1 (spring wheat) and 250 kg ha−1 (spring triticale). Seeds were sown on 19 April 2018 and on 16 April 2019. The field was fertilized only in the first year of the two-year experiment. This was done to monitor the availability of the nutrients in the second year as the recycled materials were hypothesized to have slow-release properties. Each plot of each experiment was harvested with a small combine harvester. Crop yield was determined by weighing. Spring wheat and spring triticale grain and straw from each experimental plot were collected and weighed. Soil mobile potassium (K2O), mobile phosphorus (P2O5), mobile calcium (Ca), and mobile magnesium (Mg) were determined using an ammonium lactate–acetic acid extraction, as described by Egner, Riehm and Domingo [20]. Inductively coupled plasma mass spectrometry (ICP-MS) (Thermo Fisher Scientific, Waltham, MA, USA) was used to find the content of the heavy metals in soil, too. Mineral nitrogen (Nmin) was determined using laboratory prepared spectrometric method LVP-D-05:2017. ICP-MS was used to determine the total P, K, Ca, Mg, and heavy metals in the spring wheat and spring triticale grain and straw. The Kjeldahl method was used to determine the total nitrogen (Ntot) in spring wheat and spring triticale grain and straw. All chemical analyses were performed in triplicate (n = 3).

2.4. Meteorological Conditions At the experimental site, the sum of the active temperatures was 2100–2200◦, the annual precipitation was 500–600 mm, and the mean annual temperature was 6.5 ◦C. Me- teorological conditions during the experimental period were assessed according to the data from the Lithuanian Hydrometeorological Service under the Ministry of Environment (Vilnius, Lithuania). In 2018 and 2019, the temperature of the growing season was higher than the average perennial. Both years were equally humid in the growing season. Precipi- tation was 247 mm in 2018 and 245 mm in 2019. The average monthly temperature and the cumulative precipitation are shown in Figure1. Agronomy 2021, 11, 641 4 of 15

2.4. Meteorological Conditions At the experimental site, the sum of the active temperatures was 2100–2200°, the annual precipitation was 500–600 mm, and the mean annual temperature was 6.5 °C. Mete- orological conditions during the experimental period were assessed according to the data from the Lithuanian Hydrometeorological Service under the Ministry of Environment (Vilnius, Lithuania). In 2018 and 2019, the temperature of the growing season was higher Agronomy 2021, 11, 641than the average perennial. Both years were equally humid in the growing season. Pre- 4 of 15 cipitation was 247 mm in 2018 and 245 mm in 2019. The average monthly temperature and the cumulative precipitation are shown in Figure 1.

Figure 1. RainfallFigure sum1. Rainfall (mm) andsum the(mm) average and the temperature average temperature (◦C) for the (°C) testing for the period testing of 2018–2019,period of 2018–2019, as well as the long- as well as the long-term average (1928–2018). The shaded area represents the time when the spring term average (1928–2018). The shaded area represents the time when the spring wheat (2018) and spring triticale (2019) wheat (2018) and spring triticale (2019) were grown. were grown.

2.5. Statistical Analysis2.5. Statistical Analysis A one-way analysisA one-way (ANOVA) analysis of variance (ANOVA) was of varianceused to compare was used the to comparesoil characteris- the soil characteristics tics before andbefore after soil and fertilizing, after soil fertilizing, the spring the wheat spring and wheat spring and triticale spring triticaleyield and yield prop- and properties of erties of the BA,the GWC, BA, GWC, and GWC and GWC+ BA mixtures + BA mixtures,, and the and spring the spring wheat wheat and spring and spring triticale triticale grain and grain and straw.straw. Means Means were were compared compared using using Duncan’s Duncan’s test test at p at ≤ p0.01≤ 0.01 and and p ≤ p0.05.≤ 0.05. The The statistical statistical softwaresoftware package package SAS (version SAS (version 9.3, SAS 9.3, Institute, SAS Institute, Cary, Cary,NC, U.S.) NC, [21] USA) was [21 used] was used for all for all analyses.analyses.

3. Results and3. Discussion Results and Discussion 3.1. Nutrient Composition3.1. Nutrient of the Composition BA, GWC, of and the BA,GWC GWC, + BA and Mixtures GWC + BA Mixtures Table2 shows the chemical composition of the measured nutrients (P ,K , Ca , Table 2 shows the chemical composition of the measured nutrients (Ptot, Ktot, Catot, tot tot tot 2+ and Mgtot) in the as-received BA and GWC. Both were chieﬂy comprised of Ca ion- and Mgtot) in the as-received BA and GWC. Both were chiefly comprised of Ca2+ ion-containing chemicalcontaining compounds. chemical Notable compounds. was the total Notable P content was in the the total GWC P content and BA in of the ~3.5– GWC and BA of ~3.5–8.2% by weight for all the major nutrients measured. It is the main nutrient needed 8.2% by weight for all the major nutrients measured. It is the main nutrient needed by by plants for the synthesis of organic matter and metabolism [22]. The total P content in plants for the synthesis of organic matter and metabolism [22]. The total P content in the the present work in GWC was 0.52% (Table2). Other authors found also found it rather present work in GWC was 0.52% (Table 2). Other authors found also found it rather low, low, 0.16–0.42% [23], while some reports found the total P concentration in various types 0.16–0.42% [23], while some reports found the total P concentration in various types of of compost ranged from 0.8 to 1.8% [24,25]. The K concentration in the BA was ~16.9%. compost ranged from 0.8 to 1.8% [24,25]. The K concentration in the BA was ~16.9%. Mean- Meanwhile, in the compost, it was 25 times less (~0.69%) and can be considered as low [23]. while, in the compost, it was 25 times less (~0.69%) and can be considered as low [23]. Similar to our results, the total K concentration in green waste compost has been reported Similar to our results, the total K concentration in green waste compost has been reported to range from 0.5 to 0.7% [26]. to range from 0.5 to 0.7% [26]. The total Ca content in the GWC was ~3.08% and in the BA ~20.9%. The total Mg content was ~0.51% and ~3.15% for the GWC and BA, respectively. Collectively, it can be seen that GWC does not serve as a significant source of Ptot,Ktot, Catot, and Mgtot to the plants. The total (Ntot) and mineral (Nmin) nitrogen and organic carbon (Corg) were also evaluated, as Ntot is one of the most important nutrients. The literature indicates that Ntot is very low at <0.5% and very high at >2.0% in GWC [27]. The Ntot measured was >0.5% in GWC, i.e., 0.77 ± 0.01%. The Nmin in GWC was found to be 0.13 ± 0.01%. The Corg content of the GWC was found to be 9.27 ± 0.80%. Corg and Ntot were not detected in BA due to the oxidation of C during combustion and the evolution of N as a gas [28,29]. Hence, the addition of GWC is critical to BA to provide the nitrogen needed by plants [30]. If the ratio of organic carbon to total nitrogen (Corg:Ntot) is less than 10, the microorganisms performing the compost mineralization consume more nitrogen, which is absorbed from the GWC. As a result, the amount of it available to the plants is reduced. A ratio between Agronomy 2021, 11, 641 5 of 15

16 and 20 is considered optimal, and >25 is very good [27]. The Corg:Ntot ratio in the GWC found in the current experiment was 12, i.e., sufﬁcient for both microorganisms and plants. However, to increase the low Corg:Ntot ratio, other carbon-containing substrates can be added (e.g., biochar, granular activated carbon, and digestate from the anaerobic digestion process) [31].

Table 2. Chemical composition of the major nutrients (P, K, Ca, and Mg) in BA and complex composted materials (%).

BA GWC GWC + BA1.5 GWC + BA3.0 GWC + BA4.5 Nutrients % Total P 3.64 d 0.52 a 0.74 a,b 0.93 b,c 1.08 c Total K 16.9 d 0.69 a 1.82 b 2.80 c 3.61 c Total Ca 20.9 d 3.08 a 4.33 a,b 5.40 b,c 6.29 c Total Mg 3.15 e 0.51 a 0.70 b 0.85 c 0.99 d a,b,c,d,e The difference between the values with the same letter (a,b,c,d,e) was not statistically signiﬁcant between the different materials when p < 0.05. BA—biofuel ash; GWC—green waste compost; GWC + BA1.5—green waste compost (20 t ha−1) + biofuel ash (1.5 t ha−1); GWC+BA3.0—green waste compost (20 t ha−1) + biofuel ash (3.0 t ha−1); GWC + BA4.5—green waste compost (20 t ha−1) + biofuel ash (4.5 t ha−1).

The corresponding concentrations of heavy metals measured in the BA, GWC, and BA/GWC mixtures are shown in Table3. In this experiment, there were six heavy metals (Cd, Pb, Ni, Cu, Zn, and Cr) detected. Notably, Zn and Cu also can serve as microelements in low concentrations in soil [32]. The BA contained an almost 50 times higher concentration of microelements and heavy metals when compared to GWC, with ~9028 mg kg−1 of Zn, ~216 mg kg−1 Cu, ~240 mg kg−1 Cr, ~11.3 mg kg−1 Ni, ~216 mg kg−1 Pb, and ~75.8 mg kg−1 Cd. In general, the amount of elements in the ash depends on the type of wood used and their amount can vary over a very wide range. Studies conducted studies with ash found Zn concentration of 4920 mg kg−1 [33]. The content of heavy metals increased linearly with the increasing amount of BA in the mixtures. Hence, GWC + BA1.5 had the lowest concentration of microelements and heavy metals delivered to the soil primarily due to the higher content of the green mass. BA mixed with GWC complies with the maximum allowable limits, as shown in Table3[34].

Table 3. Chemical composition of heavy metals (Ni, Cr, Cd, Pb, Cu, and Zn) in the BA and complex composted materials (mg kg−1).

Heavy BA GWC GWC + BA1.5 GWC + BA3.0 GWC + BA4.5 Max Allowable Limits Metals mg kg−1 Ni 11.3 c 8.44 a 8.64 a,b 8.81 b 8.95 b ≤50 Cr 43.7 c 15.3 a 31.1 a,b 44.6 a,b 55.8 b ≤70 Cd 75.8 d 0.60 a 5.86 a,b 10.4 b,c 14.1 c ≤2 d b c c Pb 216 15.9 a 29.9 41.9 51.9 ≤120 Cu 240 d 21.5 a 35.1 b 46.8 b,c 56.5 c ≤300 Zn 9028 b 128 a 751 a 1285 a 1730 a ≤800 a,b,c,d,e The difference between the values with the same letter (a,b,c,d,e) was not statistically signiﬁcant between the different materials when p < 0.05. Max allowable limits in composts (mg kg−1) in Lithuania [34] are shown in the last column. BA—biofuel ash; GWC—green waste compost; GWC + BA1.5—green waste compost (20 t ha−1) + biofuel ash (1.5 t ha−1); GWC+BA3.0—green waste compost (20 t ha−1) + biofuel ash (3.0 t ha−1); GWC+BA4.5—green waste compost (20 t ha−1) + biofuel ash (4.5 t ha−1).

The cumulative amount of heavy metals introduced into the soil is shown in Table4. GWC and GWC + BA1.5 had the lowest contents of heavy metals delivered to the soil. The content of heavy metals in the complex composted materials did not exceed the max allowable levels permitted by EPA [35]. Agronomy 2021, 11, 641 6 of 15

Table 4. The amount of heavy metals (Ni, Cr, Cd, Pb, Cu and Zn) introduced into the soil (kg ha−1).

Heavy BA GWC GWC + BA1.5 GWC + BA3.0 GWC + BA4.5 Max Allowable Limits Metals kg ha−1 Ni 0.03 0.17 0.19 0.20 0.22 36 Cr 0.72 0.31 0.67 1.03 1.39 - Cd 0.23 0.01 0.13 0.24 0.35 4 Pb 0.65 0.32 0.64 0.97 1.29 100 Cu 0.65 0.43 0.75 1.08 1.40 - Zn 27.1 2.56 16.1 29.6 43.2 370 Max allowable limits in kg ha−1 according to EPA [35] are shown in the last column. BA—biofuel ash; GWC—green waste compost; GWC + BA1.5—green waste compost (20 t ha−1) + biofuel ash (1.5 t ha−1); GWC + BA3.0—green waste compost (20 t ha−1) + biofuel ash (3.0 t ha−1); GWC + BA4.5—green waste compost (20 t ha−1) + biofuel ash (4.5 t ha−1).

3.2. Nutrient and Heavy Metal Release from GWC + BA Mixtures as Measured via Chemical Composition of the Soil Important nutrient release patterns into the soil from the BA, GWC, and GWC + BA mixtures were measured during a 2-year field experiment and changes in mobile K2O, P2O5, Mg, and Ca in soil were recorded each year (Figure2). Crops were harvested 4 months after the start of the experiment. After the spring wheat harvest, the amounts of mobile forms of all nutrients (K2O, P2O5, Ca, and Mg) differed significantly from the corresponding control soil. The mobile K2O amount increased linearly in treatments with BA, GWC + BA3.0, and GWC + BA4.5 as a function of BA content. This was due to the high amount of total K in the BA, suggesting the solubilization and release of normally insoluble forms of K. Differently, the mobile Ca measured in the soil increased by about 1000 mg kg−1 in the GWC and GWC + BA1.5 treatments after 4 months. The mobile Mg increased by about 80 mg kg−1 in the same treatments. Thus, the higher availability of these nutrients in the soil can also be proposed to contribute to higher nutrient uptake and an increased yield of spring wheat (vide infra). About two-fifths of the annual rainfall fell in cool weather when the soil was not covered with plants, as shown in Figure1. As a result, the leaching of mobile Ca and Mg cations intensified, resulting in the peak measured. There was a marked decrease in soil nutrients after winter (Figure2). The mobile Ca content in the soil closely follows the GWC content instead of BA, suggesting that, while both contain an effectively equal concentration of Ca, the latter is responsible for its facile release. However, there was still a positive nutrient balance in Year 2 of the experiment when compared to the control. An important conclusion from Figure2 is that that mobile K 2O and P2O5 are BA originated while the mobile Ca and Mg mostly originated from the GWC, providing a synergistic nutrient delivery. Diffusion of heavy metals down the soil profile is normally difficult due to their low solubility and they are absorbed in the topsoil [21]. The most dangerous for the human body are Pb and Cd [36]. These elements are not considered to be essential for living organisms, so even small amounts of them can be detrimental [37]. Many chemical processes are involved in the transformation of these heavy metals in soil, affecting availability to plants, but precipitation–dissolution and adsorption–desorption are the most important controlling their bioavailability and mobility [13]. In the present experiments, Cd was statistically significantly (p ≤ 0.01) detected when fertilized with BA and all mixtures after the first harvest (Figure3). After 4, 12, and 16 months, the Cd content was significantly different only when BA and GWC + BA4.5 were applied, i.e., treatments with the most BA content, as shown in Table3. However, these did not exceed the allowable limits, as during the experiment, the Cd concentration increased to 0.25–0.30 mg kg−1. The concentration of Cd permitted in the soil is 1–3 mg kg−1 [38]. Furthermore, the Cd content at the end of the experiment in soil with the GWC + BA1.5 and GWC + BA3.0 treatments decreased. This was likely influenced by the Cd uptake by the spring wheat grain. The soil that is saturated with Ca ions is more resistant to any heavy metal release and resulting mobility because Ca (as well as Mg) reduces the availability of heavy metals to the plants [39]. This, in turn, Agronomy 2021, 11, 641 7 of 15

There was a marked decrease in soil nutrients after winter (Figure 2). The mobile Ca con- Agronomy 2021, 11tent, 641 in the soil closely follows the GWC content instead of BA, suggesting that, while both 7 of 15 contain an effectively equal concentration of Ca, the latter is responsible for its facile release. However, there was still a positive nutrient balance in Year 2 of the experiment when compared to the control. An important conclusion from Figure 2 is that that mobile suggests that heavy metals normally toxic to plants are more difﬁcult to absorb using GWC K2O and P2O5 are BA originated while the mobile Ca and Mg mostly originated from the GWC, providing+ BAa synergistic mixtures containingnutrient delivery. high levels of Ca and Mg.

Figure 2. Change in the chemical composition of soil mobile K2O, P2O5, Ca, and Mg after GWC, BA, Figure 2. Change in the chemical composition of soil mobile K2O, P2O5, Ca, and Mg after GWC, BA, and GWC + BA application.and The GWC control + BA represents application. no additives. The control The represents shaded area no representsadditives. theThe chemical shaded area soil compositionrepresents the after the crop chemical soil composition after the crop was harvested. * = significance at p ≤ 0.05 and ** = signifi- was harvested. * = significance at p ≤ 0.05 and ** = significance at p < 0.01. BA—biofuel ash; GWC—green waste compost; cance at p < 0.01. BA—biofuel ash; GWC—green waste compost; GWC + BA1.5—green waste com- GWC + BA1.5—green waste compost (20 t ha−1) + biofuel ash (1.5 t ha−1); GWC+BA3.0—green waste compost (20 t ha−1) + post (20 t ha−1) + biofuel ash (1.5 t ha−1); GWC+BA3.0—green waste compost (20 t ha−1) + biofuel ash −1 −1 −1 biofuel ash(3.0 (3.0 t tha ha−1); GWC+BA4.5—green); GWC+BA4.5—green waste waste compost compost (20 (20 t ha t ha−1) + )biofuel + biofuel ash ash (4.5 (4.5 t ha t−1 ha). ). In addition to Cd, an increase in the Zn and Cr concentrations in soil was also mea- Diffusion of heavy metals down the soil profile is normally difficult due to their low sured. The Zn concentration was found to be significantly different from the control in all solubility and they are absorbed in the topsoil [21]. The most dangerous for the human treatments except GWC. Higher soil Zn concentrations in the BA and GWC + BA mixtures body are Pb and Cd [36]. These elements are not considered to be essential for living or- treatments compared to the GWC treatment can be explained by its large concentration in ganisms, so even small amounts of them can be detrimental [37]. Many chemical processes BA (Table3). However, at the end of the experiment (after 16 months), the Zn concentration are involved in the transformation of these heavy metals in soil, affecting availability to was not significantly different from the control in either treatment. A portion of it was also plants, but precipitation–dissolution and adsorption–desorption are the most important absorbed by spring wheat and triticale. Despite increased Zn levels, the application of BA controlling their bioavailability and mobility [13]. In the present experiments, Cd was sta- in quantities tested in the present investigation should not create a risk for the environment. tistically significantlyThis is because(p ≤ 0.01) the detected permitted when amount fertilized of Zn with in the BA soil and is all 150–300 mixtures mg kgafter−1 [38]. All of the the first harvestapplications (Figure 3). After used within4, 12, and the 16 2-year months, experiment the Cd resultedcontent was in 30–60 significantly mg kg−1 (Figure3). An different only whenincrease BA and in Cr GWC was observed+ BA4.5 were after applied, 4 months, i.e., 12 treatments months, andwith at the the most end BA of the experiment. content, as shownThe in maximum Table 3. However, allowed concentration these did not of exceed Cr in the the soil allowable is 100 mg limits, kg−1 as[38 dur-]. At the end of the −1 ing the experiment,experiment, the Cd theconcentration content was increased about 15–20 to 0.25–0.30 m kg−1, i.e.,mg 5–7kg times. The lessconcentra- than allowed. The Cu −1 tion of Cd permitteddetected in the in thesoil soilis 1–3 after mg 4 kg and [38]. 12 months Furthermore, differed the significantly Cd content onlyat the in end the BA treatment. of the experimentThe in maximum soil with the allowed GWC concentration + BA1.5 and ofGWC Cu is+ BA3.0 50–140 treatments mg kg−1. Thedecreased. experiment resulted in This was likelyabout influenced 10 mg by kg the−1. Finally,Cd uptake another by the heavy spring metal wheat measured grain. The in the soil soil, that Pb, is did not exhibit saturated with Caany ions significant is more variationsresistant to over any time heavy or metal with the release different and resulting applied fertilization mobility treatment, as shown in Figure3. Similarly, no Ni increase was measured in the soil beyond the control concentrations.

Agronomy 2021, 11, 641 8 of 15

Agronomy 2021, 11, 641 because Ca (as well as Mg) reduces the availability of heavy metals to the plants [39]. This,8 of 15 in turn, suggests that heavy metals normally toxic to plants are more difficult to absorb using GWC + BA mixtures containing high levels of Ca and Mg.

FigureFigure 3. 3.Heavy Heavy metals metals measuredmeasured inin thethe soilsoil withinwithin 1616 months.months. ** == significancesignificance at pp ≤≤ 0.050.05 and and ** ** = = significancesignificance at atp p< <0.01. 0.01. BA—biofuelBA—biofuel ash;ash; GWC—greenGWC—green wastewaste compost; GWC+BA1.5—green waste waste −1 −1 −1 compostcompost (20(20 tt ha−)1 )+ +biofuel biofuel ash ash (1.5 (1.5 t ha t ha); −GWC+BA3.0—green1); GWC+BA3.0—green waste waste compost compost (20 t ha (20) + t biofuel ha−1) + ash (3.0 t ha−1); GWC+BA4.5—green waste compost (20 t ha−1) + biofuel ash (4.5 t ha−1). biofuel ash (3.0 t ha−1); GWC+BA4.5—green waste compost (20 t ha−1) + biofuel ash (4.5 t ha−1). In addition to Cd, an increase in the Zn and Cr concentrations in soil was also meas- Nmin is a very important nutrient for plants and it consists of ammonia and nitrate ured. The Zn concentration was found to be significantly different from the control in all and nitrite nitrogen [40]. Nmin was measured in the soil before and after the experiment. treatments except GWC. Higher soil Zn concentrations in the BA and GWC + BA mixtures The results obtained are shown in Figure4a. The N min before the experiment is shown with treatments compared to the GWC treatment can be explained by its large concentration in a solid line. The concentration of Nmin in the soil depends on the mineralization of organic matter,BA (Table the 3). amount However, of precipitation, at the end of asthe well expe asriment the amount (after 16 of months), it absorbed the Zn by plantsconcentra- [41]. tion was not significantly different from the control in either treatment.− A1 portion of it was The Nmin content determined before the experiment was 16.8 mg kg .Nmin decreased andalso ranged absorbed from by 11.0 spring to 11.8 wheat mg and kg− triticale.1 after the Despite experiment. increased Zn levels, the application of BA in quantities tested in the present investigation should not create a risk for the en- Ntot was measured in the grain and straw of spring wheat and spring triticale. The −1 resultsvironment. are shown This is in because Figure4 theb. Itpermitted was found amount that the of heavyZn in the metals soil is in 150–300 the mixtures mg kg did [38]. not All of the applications used within the 2-year experiment resulted in 30–60 mg kg−1 (Figure significantly affect the Ntot uptake in the plants. 3). An increase in Cr was observed after 4 months, 12 months, and at the end of the experiment. The maximum allowed concentration of Cr in the soil is 100 mg kg−1 [38]. At the end of the experiment, the content was about 15–20 m kg−1, i.e., 5–7 times less than allowed. The Cu detected in the soil after 4 and 12 months differed significantly only in the BA treatment. The maximum allowed concentration of Cu is 50–140 mg kg−1. The experiment resulted in about 10 mg kg−1. Finally, another heavy metal measured in the soil, Pb, did not exhibit any significant variations over time or with the different applied fertilization treatment, as shown in Figure 3. Similarly, no Ni increase was measured in the soil beyond the control concentrations. Nmin is a very important nutrient for plants and it consists of ammonia and nitrate and nitrite nitrogen [40]. Nmin was measured in the soil before and after the experiment.

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The results obtained are shown in Figure 4a. The Nmin before the experiment is shown with a solid line. The concentration of Nmin in the soil depends on the mineralization of organic Agronomy 2021, 11,matter, 641 the amount of precipitation, as well as the amount of it absorbed by plants [41]. 9 of 15 The Nmin content determined before the experiment was 16.8 mg kg−1. Nmin decreased and ranged from 11.0 to 11.8 mg kg−1 after the experiment.

(a)

(b)

min min Figure 4. (aFigure)Nmin 4.in ( thea) N soil in before the soil and before after the and experiment. after the experiment. Nmin before N the before experiment the experiment is shown withis shown a solid line. (b) with a solid line. (b) Ntot concentration in spring wheat (2018) and spring triticale (2019) grain and Ntot concentration in spring wheat (2018) and spring triticale (2019) grain and straw. * = significance at p ≤ 0.05 and ** = straw. * = significance at p ≤ 0.05 and ** = significance at p < 0.01. BA—biofuel ash; GWC—green significance at p < 0.01. BA—biofuel ash; GWC—green waste compost; GWC + BA1.5—green waste compost (20 t ha−1) + waste compost; GWC + BA1.5—green waste compost (20 t ha−1) + biofuel ash (1.5 t ha−1); GWC + biofuel ash (1.5 t ha−1); GWC + BA3.0—green waste compost (20 t ha−1) + biofuel ash (3.0 t ha−1); GWC + BA4.5—green BA3.0—green waste compost (20 t ha−1) + biofuel ash (3.0 t ha−1); GWC + BA4.5—green waste com- −1 −1 waste compostpost (20(20 tt haha−1)) + + biofuel biofuel ash ash (4.5 (4.5 t tha ha−1). ). 3.3. Chemical Composition of Major Nutrients and Heavy Metals in Spring Wheat and Spring Ntot was measuredTriticale Grain in the and grain Straw and straw of spring wheat and spring triticale. The results are shown in Figure 4b. It was found that the heavy metals in the mixtures did not The study sought to determine whether increased levels of heavy metals in materials significantly affect the Ntot uptake in the plants. affect and interfere with plant nutrient uptake. It was also important to find out how much heavy metals are absorbed by plants. The comparison of the nutrient concentrations in 3.3. Chemical Composition of Major Nutrients and Heavy Metals in Spring Wheat and Spring grains and straw among the different treatments shows that total P significantly (p ≤ 0.05) Triticale Grain and Straw increased in spring wheat grain treatments with BA, GWC + BA3.0, and GWC + BA4.5. The study Itsought was determined to determine that whether the amount increased of total levels P inof springheavy wheatmetals strawin materials was ~10 times lower affect and interferethan with in grain, plant asnutrient shown uptake in Figure. It 5wasa. Total also important K content into find straw out was how statistically much significant heavy metals are(p ≤absorbed0.01) compared by plants. to The control comparison with BA of and the GWC nutrient + BA concentrations mixtures in 2018. in Specifically, grains and strawit among increased the ~2 different times comparedtreatments toshows the control. that total Interestingly, P significantly in ( thep ≤ 0.05) second year of the experiment, with no added nutrients, the straw still contained a statistically significant higher content of K. However, the total K uptake of spring triticale straw in 2019 was − − ~5 g kg 1 when compared to ~20 g kg 1 in 2018. Hence, while there is a significant immediate nutrient release effect, a slow-release pattern can be suggested to last into the second year of experiments. The total amount of K in the cereals was not statistically Agronomy 2021, 11, 641 10 of 15

increased in spring wheat grain treatments with BA, GWC + BA3.0, and GWC + BA4.5. It was determined that the amount of total P in spring wheat straw was ~10 times lower than in grain, as shown in Figure 5a. Total K content in straw was statistically significant (p ≤ 0.01) compared to control with BA and GWC + BA mixtures in 2018. Specifically, it increased ~2 times compared to the control. Interestingly, in the second year of the experiment, with no added nutrients, the straw still contained a statistically significant higher −1 Agronomy 2021, 11, 641 content of K. However, the total K uptake of spring triticale straw in 2019 was ~5 10g ofkg 15 when compared to ~20 g kg−1 in 2018. Hence, while there is a significant immediate nutrient release effect, a slow-release pattern can be suggested to last into the second year of experiments. The total amount of K in the cereals was not statistically significant com- significantpared to the compared control in to both the controlstudy years. in both The study amount years. of total The amountCa in straw of total during Ca inthe straw first duringyear of thethe firstexperiment year of thewas experiment twice as high was as twice in grain as high (Figure as in grain5a). However, (Figure5a). only However, GWC + onlyBA3.0 GWC and +GWC BA3.0 + andBA4.5 GWC differed + BA4.5 significantly differed significantly (p ≤ 0.01) from (p ≤ the0.01) control. from theIn the control. second In theyear second of the yearexperiment, of the experiment, the total Ca the content total Caof the content same oftreatments the same was treatments also statistically was also statisticallysignificant ( significantp ≤ 0.01) in ( pthe≤ spring0.01) in triticale the spring grain. triticale Analysis grain. of Analysistotal Mg ofin totalspring Mg wheat in spring and wheattriticale and grain triticale and straw grain showed and straw that showed the total that Mg the content total Mg of straw content in ofthe straw first year in the of first the yearstudy of was the statistically study was statistically significant ( significantp ≤ 0.01) with (p ≤ GWC0.01) + with BA mixtures. GWC + BA Spring mixtures. wheat Spring straw wheatabsorbed straw ~3 absorbedtimes more ~3 Mg times than more spring Mg thantriticale. spring The triticale. Mg content The Mg of the content grain of was the about grain −1 was1.2 g about kg−1 in 1.2 both g kg experimentalin both experimental years. As expected, years. As all expected, nutrients all (total nutrients P, K, (total Ca, and P, K, Mg) Ca, andtested Mg) were tested lower were in lowerthe spring in the wheat spring and wheat triticale and triticalegrains and grains straw and in straw the second in the secondyear of yearthe study. of the study.

(a)

Figure 5. Cont.

Agronomy 2021, 11, 641 11 of 15 Agronomy 2021, 11, 641 11 of 15

(b)

Figure 5. ((aa)) Nutrient Nutrient and and ( (bb)) heavy heavy metals metals concentration concentration change change in in spring spring wheat wheat (2018) (2018) and and spring spring triticale (2019) grain and straw after fertilization with BA, GWC, and the GWC + BA mixture. * = triticale (2019) grain and straw after fertilization with BA, GWC, and the GWC + BA mixture. significance at p ≤ 0.05 and ** = significance at p < 0.01. BA—biofuel ash; GWC—green waste com- * = significance at p ≤ 0.05 and ** = significance at p < 0.01. BA—biofuel ash; GWC—green waste post; GWC + BA1.5—green waste compost (20 t ha−1) + biofuel ash (1.5 t ha−1); GWC + BA3.0—green −1 −1 wastecompost; compost GWC (20 + BA1.5—green t ha−1) + biofuel waste ash compost(3.0 t ha− (201); GWC t ha +) BA4.5—green + biofuel ash (1.5waste t ha compost); GWC (20 + t BA3.0—ha−1) + −1 −1 biofuelgreen waste ash (4.5 compost t ha−1). (20 t ha ) + biofuel ash (3.0 t ha ); GWC + BA4.5—green waste compost (20 t ha−1) + biofuel ash (4.5 t ha−1). Zn is an essential micronutrient and phytotoxic only at enough high concentrations in plantsZn is of an about essential 100–500 micronutrient mg kg−1, whereas and phytotoxic Cd becomes only attoxic enough at much high lower concentrations concentra- in −1 plantstions of of 10–20 about mg 100–500 kg−1 [42]. mg The kg measured, whereas grain Cd becomes Zn concen toxictrations at much in this lower experiment concentrations were −1 ~17–20of 10–20 mg mg kg kg−1 in both[42]. experiment The measured years grain while Zn the concentrations straw Zn concentrations in this experiment were about were 2 −1 ~17–20to 3 times mg lower kg infrom both 2018 experiment to 2019 (Figure years while 5b). The the strawCd mobility Zn concentrations is higher compared were about to other2 to 3 heavy times lowermetals from (Cd > 2018 Ni > to Zn 2019 > Cu (Figure > Pb =5 b).Cr) The[43] Cdand mobility thus it has is highera high comparedpotential for to bioaccumulationother heavy metals in food. (Cd > According Ni > Zn > to Cu European > Pb = Cr) legislation [43] and [44], thus the it has permitted a high potentialCd con- centrationfor bioaccumulation in grains is in 0.2 food. mg kg According−1. The average to European grain Cd legislation concentration [44], themeasured permitted in this Cd −1 workconcentration was <0.05 in mg grains kg−1. is Straw 0.2 mg Cd kg concentrations. The average showed grain Cdsimilar concentration trends to those measured of the in −1 grainthis work Cd. However, was <0.05 Cd mg exhibited kg . Straw a statisticall Cd concentrationsy significant showed difference similar between trends the to grains those withof the the grain BA Cd.and However,GWC + BA Cd mixtures. exhibited The a statisticallyamount of Cd significant in straw was difference statistically between differ- the entgrains from with the the control BA and when GWC BA + was BA mixtures.used. Alth Theough amount the concentration of Cd in straw of wasCu in statistically the soils different from the control when BA was used. Although the concentration of Cu in the soils varied only in the BA treatment (Figure 3), it varied significantly in the spring wheat straw varied only in the BA treatment (Figure3), it varied significantly in the spring wheat straw with the BA and all GWC + BA mixtures (Figure 5b). The concentration of Cu in spring with the BA and all GWC + BA mixtures (Figure5b). The concentration of Cu in spring wheat grain also increased with BA, GWC + BA3.0 and GWC + BA4.5. Cu from GWC + wheat grain also increased with BA, GWC + BA3.0 and GWC + BA4.5. Cu from GWC + BA was absorbed by the grain and the straw and part of it leached into the groundwater. BA was absorbed by the grain and the straw and part of it leached into the groundwater. Spring wheat and spring triticale grain and straw data indicate that many application Spring wheat and spring triticale grain and straw data indicate that many application rates rates of BA can be applied on this type of soil as it does not appear to accumulate poten- of BA can be applied on this type of soil as it does not appear to accumulate potentially tially toxic trace elements. toxic trace elements.

Agronomy 2021, 11, 641 12 of 15 Agronomy 2021, 11, 641 12 of 15

3.4.3.4. QuantifyingQuantifying Spring Wheat andand Spring Triticale GrainGrain Yields GrainGrain yieldyield improvementimprovement of the selected cropscrops used in the rotation, when fertilized usingusing BA,BA, GWC,GWC, andand GWCGWC ++ BA mixtures, is shown in Figure 66.. InIn thethe ﬁrstfirst yearyear ofof thethe experiment,experiment, therethere waswas aa statisticallystatistically signiﬁcantsignificant (p < 0.01) increase in spring wheat grain yieldyield inin allall variantsvariants comparedcompared toto thethe control.control. The highest increase in spring wheat grain yieldyield waswas achievedachieved byby fertilizationfertilization withwith thethe GWC + BA4.5 mixture, i.e., spring wheat wheat yield yield −1 waswas obtainedobtained atat 4.614.61 tt haha−1 (Figure 66).). TheThe lowestlowest increaseincrease waswas observedobserved inin fertilizationfertilization withwith purepure greengreen wastewaste compostcompost andand waswas 6.8%6.8% higherhigher comparedcompared toto the control.

FigureFigure 6.6.Two-year Two-year (2018–2019) (2018–2019) data data of of the the BA, BA, GWC, GWC, and and GWC GWC + BA + mixtures’BA mixtures’ effect effect on spring on spring wheat andwheat spring and triticalespring triticale yields. Theyields. difference The difference between between the values the withvalues the with same the letter same (a,b,c,d,e) letter (a,b,c,d,e) was not statisticallywas not statistically significant significant between the between different the materials different when materialsp < 0.01. when BA—biofuel p < 0.01. ash;BA—biofuel GWC—green ash; GWC—green waste compost; GWC + BA1.5—green waste compost (20 t ha−1) + biofuel ash (1.5 t waste compost; GWC + BA1.5—green waste compost (20 t ha−1) + biofuel ash (1.5 t ha−1); GWC ha−1); GWC + BA3.0—green waste compost (20 t ha−1) + biofuel ash (3.0 t ha−1); GWC + BA4.5—green + BA3.0—green waste compost (20 t ha−1) + biofuel ash (3.0 t ha−1); GWC + BA4.5—green waste waste compost (20 t ha−1) + biofuel ash (4.5 t ha−1). compost (20 t ha−1) + biofuel ash (4.5 t ha−1). In the second year of the experiment, when fertilization was not conducted, the BA, In the second year of the experiment, when fertilization was not conducted, the BA, GWC, and GWC + BA still increased the spring triticale grain yield, but the increase was GWC, and GWC + BA still increased the spring triticale grain yield, but the increase was not not statistically significant, as shown in Figure 6. This was likely due to the reduced nu- statistically significant, as shown in Figure6. This was likely due to the reduced nutrient trient content in the soil after 12 months, as shown in Figure 2, as significant rainfall took content in the soil after 12 months, as shown in Figure2, as significant rainfall took place place in the winter after the first year of the experiment. The amount of precipitation in in the winter after the first year of the experiment. The amount of precipitation in the the 2018–2019 winter was 1.6 times higher than the long-term (1928–2018) average, as 2018–2019 winter was 1.6 times higher than the long-term (1928–2018) average, as shown shown in Figure 1. In contrast to the first year, spring triticale grain yield was greatest in Figure1. In contrast to the first year, spring triticale grain yield was greatest when when amended with GWC + BA1.5 and GWC. Interestingly, for the BA, GWC + BA3.0, amended with GWC + BA1.5 and GWC. Interestingly, for the BA, GWC + BA3.0, and GWC and GWC + BA4.5, the yield improvement was the lowest. BA contains high basicity (>12 + BA4.5, the yield improvement was the lowest. BA contains high basicity (>12 pH units), potentiallypH units), potentially slowing down slowing spring down triticale spring growth. triticale A notablegrowth. second-yearA notable second-year yield increase yield in GWCincrease and in GWC GWC +and BA1.5 GWC can + BA1.5 be associated can be associated with the unique with the properties unique properties of GWC itself.of GWC In particular,itself. In particular, GWC is known GWC tois (a)known slowly to decompose(a) slowly decompose and release and plant release absorbable plant carbonabsorbable [45]; (b)carbon decrease [45]; large(b) decrease cations large and micronutrient cations and micronutrient release into the release soil [ 46into]; and the (c)soil bind [46]; (chelate) and (c) heavybind (chelate) metals, thusheavy making metals, them thus not making readily them available not readily for the available crops [47 for]. Thisthe crops capacity [47]. is likelyThis capacity limited, asis likely suggested limited, by muchas suggested lower 2-year by much yields lower with 2-year GWC +yields BA3.0 with and GWC GWC + + BA4.5.BA3.0 and Furthermore, GWC + BA4.5. the nutrients Furthermore, from BAthe arenutrients likely atfrom a sufficient BA are likely content at anda sufficient release ratecontent to sustain and release plant growthrate to insustain the first plant year grow of experimentsth in the first but year the of presence experiments of GWC but likely the extendspresence their of GWC availability, likely extends as shown their in availability, Figure6. as shown in Figure 6. TheThe secondarysecondary useuse ofof raw raw materials materials (BA (BA and and GWC) GWC) is is linked linked to to a sustainablea sustainable environ- envi- ment.ronment. From From the the overall overall results, results, it canit can be be seen seen that that ash ash enriched enrichedthe the compostcompost with the P P andand KK neededneeded byby thethe plants.plants. BA + GWC mixtures increased increased the the grain grain yield yield by by up up to to 20% 20% comparedcompared toto thethe controlcontrol (in(in thethe firstfirst yearyear of the experiment). The results also showed that that higher rates of BA can be used in mixtures. Spring wheat and spring triticale appear to

Agronomy 2021, 11, 641 13 of 15

higher rates of BA can be used in mixtures. Spring wheat and spring triticale appear to accumulate few potentially toxic elements (Pb, Ni, Cr) when using 1.5, 3.0, and 4.5 t ha−1 BA rates in mixtures. An experiment with BA + GWC mixtures is being carried out in acidic and depleted soils (pH 5.5). The results obtained from this and future experiments will help to evaluate the uptake of nutrients and heavy metals in plants in soils with a different pH.

4. Conclusions In this work, BA combined with GWC was used as nutrient-containing materials to test their synergistic properties towards wheat and triticale growth. In particular, BA and GWC were applied thoroughly mixed. The resulting mixtures were analyzed for their chemistry and tested in a field experiment (2018–2019) towards their propensity to release nutrients. The experimental design focused on (1) the control; (2) BA; (3) GWC; (4) GWC + BA1.5; (5) GWC + BA3.0; and (6) GWC + BA4.5. The results obtained in this work showed that the highest increase in spring wheat grain yield was achieved by fertilization with the GWC + BA4.5 mixture, i.e., spring wheat yield was obtained at 4.61 t ha−1. The lowest increase was observed in fertilization with pure green waste compost and was 6.8% higher compared to the control. In the second year of the experiment, when fertilization was not conducted, BA, GWC, and GWC + BA still increased the spring triticale grain yield, but the increase was not statistically significant. After the first harvest, an increase in the mobile forms of all the measured nutrients (K2O, P2O5, Ca, and Mg) was observed in the soil. No fertilization was performed in the second year of the study. Consecutively, a marked decrease in all soil nutrients was observed after 12 months. The content of heavy metals (Cd, Zn, and Cr) in the soil increased significantly compared to the control. Total P increased in grain and K, Ca, and Mg in straw. Only three heavy metals (Cd, Zn, and Cu) were detected in grain and straw. The results obtained show that the application of BA together with GWC results in less heavy metal transfer to the crops.

Author Contributions: Conceptualization, K.B. and R.M.; methodology, K.B. and R.M.; software, K.B.; validation, K.B.; formal analysis, K.B. and D.D.; investigation, K.B. and D.D.; data curation, K.B. and J.B.; writing—original draft preparation, K.B. and J.B.; writing—review and editing, K.B. and J.B.; visualization, K.B.; supervision, J.B. All authors have read and agreed to the published version of the manuscript. Funding: This work is supported by Engineering for Agricultural Production Systems program grant no. 2020-67022-31144 from the USDA National Institute of Food and Agriculture (Washington, DC, USA). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is contained within the article. Acknowledgments: Many thanks to the researchers from the Agrobiology Laboratory and Depart- ment of Plant Nutrition and Agroecology of the Institute of Agriculture, Lithuanian Research Centre for Agriculture and Forestry. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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