Digestate from an Agricultural Biogas Plant as a Factor Shaping Soil Properties

Abstract

In the context of a circular economy, special attention should be paid to the rational management of biodegradable waste. Currently, a potentially valuable waste material, rich in ingredients available to plants, is digestate, obtained as a by-product in agricultural biogas plants. The presented study aimed to determine the impact of digestate (DIG) from an agricultural biogas plant on soil reaction (pH), electrical conductivity (EC), sorption properties (SBC, HAC, CEC, BS), and chemical composition of soil. The research was based on a pot experiment in which increasing doses of liquid (LD) and solid (SD) forms of DIG were used in corn cultivation, balanced in terms of the amount of N introduced into the soil. The composition of DIG varied and depended on the LD or SD form. The LD was characterized by a lower pH value and higher EC compared to the SD form. The LD contained much less TC, fewer macroelements, and fewer trace elements. The application of LD significantly increased in the soil the content of TC, N_tot, available K, P, Fe, and Mn, and exchangeable cations K⁺. The SD significantly increased the content of available P, Mg, and Mn and exchangeable cations Ca²⁺ and Mg²⁺ in the soil. Both forms of digestate increased the total content of heavy metals (Cu, Cr, Pb, and Ni) in the soil. However, they did not pose a threat to the environment concerning their legally permissible levels.

1. Introduction

Energy obtained from the combustion of conventional sources, including hard coal, brown coal, natural gas, and crude oil, contributes to environmental pollution and degradation [1], deepening climate change [2]. The main factors causing environmental degradation include the global increase in CO₂ emissions, related to the increase in demand for energy consumption [3]. To meet this trend and minimize the effects of environmental pollution, alternative renewable energy sources (RESs) are being sought. Currently, RESs meet only 12.6% of global energy demand, with 83% from fossil fuels and 6.3% from nuclear energy [4]. RESs include energy from water, geothermal sources, wind, sun, and biomass [5]. By 2030, under EU directives, Poland is obliged to achieve a 32% share of RESs in gross final energy consumption [6]. For this purpose, energy production from biogas is being developed [7], which enables rational management of biodegradable waste [8].

In the process of transformation of organic matter in agricultural biogas plants (ABPs), two products are produced: biogas used for energy purposes and a by-product in the form of digestate (DIG) [9]. Organic materials are usually the following: liquid manure, farmyard manure, bird droppings, dewatered sewage sludge, rotten and expired fruit remains, waste from the agricultural and food industry (beet pomace, molasses), waste from the wood industry (cereal straw, hay), and various types of silage [10]. Due to the nature of the input material used, the obtained DIG is characterized by a very high content of easily available nutrients for plants, mainly nitrogen (N), phosphorus (P), potassium (K), sulfur (S), and essential microelements [11,12,13,14,15,16,17]. The beneficial properties of DIG mean that it can be used in the environment, mainly in agriculture as a substitute for mineral fertilizers or as a substance that improves soil properties. The use of DIG for fertilizer purposes allows this waste to be managed following the assumptions of a circular economy and sustainable circulation of all agricultural production [18,19]. In Poland, legal regulations have been developed specifying the method and principles of using DIG from ABPs [20,21,22]. For this method to be completely safe, ABP owners must follow the selection of designated input material [22], and farmers applying this material to the soil—the recommended doses, dates, and method of application [20,21].

The high degree of mineralization caused by microorganisms makes DIG a much better source of macronutrients compared to, for example, raw slurry. The use of DIG in agriculture also reduces greenhouse gas and odorous active volatile compounds emissions. It is worth emphasizing that DIG is also free from harmful pathogens because they die during the methane fermentation process [23]. Moreover, the minerals in DIG occur in forms easily available to plants. This mainly concerns the N content, in which the ammonium form (N-NH₄⁺) accounts for up to approximately 80%, while in manure, this share is only 10 to 15% [16,24].

However, due to the high concentration of minerals, before introducing DIG into the soil, a detailed analysis of the chemical composition should be carried out, mainly in terms of the content of N, P, and heavy metals, in order not to exceed their permissible concentrations and prevent eutrophication of waters [25]. The final chemical composition of DIG may vary significantly, depending on the raw material origin. These differences directly affect the fertilization potential of post-fermentation products. An important aspect is determining the appropriate doses of DIG introduced into the soil, corresponding to the doses introduced with traditional mineral fertilizers [26].

DIG can be managed in two ways, i.e., by application directly to the soil as an unseparated product—liquid form, or after separating the liquid fraction as a solid fertilizer. Currently, the main trend is the use of unseparated DIG, which is supported by the economic aspect of reducing the costs of drying, storage, and transport to the field. The environmental aspect is also important here, related to the limitation of the losses of nutrients through the leaching and oxidation of the ingredients contained in this raw material [27]. As a result of the separation of the digestate, the distribution of nutrients is usually uneven. The liquid fraction (LD) is rich in N and K compounds, while the solid fraction (SD) is characterized by a higher content of P and organic matter [28].

As scientific research indicates [29], there is a positive effect of DIG not only on the soil quality and fertility but also on several physicochemical properties such as hydrolytic acidity (HAC), moisture retention capacity, and the content of humic compounds. The positive reported effect of DIG on soil properties usually concerns the effect of the simultaneous use of DIG and mineral fertilizers [30]. However, not much research has been devoted to issues related to the impact of DIG application itself on soil properties and the content of available forms of macronutrients and heavy metals, including microelements valuable for plants. Due to the increasing practice of DIG dewatering, it seems urgent to investigate the impact of both forms of DIG on the above-mentioned soil features, so that in the future we will be able to more consciously select the doses of DIG concerning the intended effects while maintaining ecological safety. The above-mentioned arguments became the basis for researching the impact of the use of liquid digestate (LD) and solid digestate (SD) forms of DIG on the properties of the soil and the content of valuable macro- and microelements and toxic trace elements.

2. Materials and Methods

2.1. Pot Experiment

The pot experiment carried out in a vegetation hall of the University of Warmia and Mazury in Olsztyn considered the influence of digestate (DIG) in liquid (LD) and solid (SD) form on selected soil properties and the content of available macronutrients and heavy metals (HM). The test plant was medium-early corn variety LG 3250 (Figure 1). Two series were considered in the study: the first one included four increasing doses of LD (objects 2–5) whereas the second one included four increasing doses of SD (objects 6–9). To compare the impact of LD and SD on selected features, the study also included a control object—without DIG addition (treatment No. 1). Each experimental object was considered in 3 replicates (pots).

Increasing doses of DIG, both LD and SD, were balanced with the amount of nitrogen (N) introduced into the soil. The assumed doses of N introduced into the soil from DIG, regardless of the form of DIG, were, respectively, as follows: 28, 56, 84, and 112 mg N kg⁻¹ soil. Concerning the N doses that can be introduced into the soil under field conditions, only the first two doses were within the guidelines for rational fertilization with N. The first dose was 50% and the second dose was 100% of the amount of N (170 kg N ha⁻¹ for agricultural lands) that can be introduced into the soil with natural fertilizers under the Water Law Act [31]. However, the third dose constituted 100% of the demand for N of maize grown for green fodder, which according to the OSN action program amounts to 255 kg N ha⁻¹ [32]. In turn, the fourth dose was used to increase the effect due to the experimental conditions (pot experiment) and the short period of plant growth.

The experiment used polyethylene (PE) pots with a top diameter equal to 22 cm and with a depth equal to 25 cm. The pots were filled with 9 kg of soil, and soil moisture was maintained at 60% of full water holding capacity during the experiment. Throughout the entire experiment, no additional fertilizers were applied to the soil. The experiment lasted 69 days, and the maize was harvested at the BBCH-53 development phase (panicle development, visible top) (Figure 1). After harvesting the above-ground corn, plant mass and soil samples were taken from each pot and dried in the open air.

2.2. Physicochemical Properties of the Soil Used in the Experiment

The soil for the presented pot experiment was collected from the arable layer (0–25 cm) of the agricultural field. The soil was characterized by low electrolytic conductivity (EC), acidic reaction, average content of the total carbon (TC), and a low degree of saturation of the sorption complex with base cations (BS) (

Table 1. Basic soil parameters (in dry mass).

Parameter	Unit	Value
Sum of base cations (SBC)	mmol kg−1	15.33
Hydrolitic acidity (HAC)	mmol kg−1	23.00
Cation exchange capacity (CEC)	mmol kg−1	38.33
Base saturation (BS)	%	39.86
Soil reaction (pHKCl)	−log10(H+)	4.60
Electrical conductivity (EC)	μS cm−1	108.2
Total carbon (TC)	%	0.408
Total nitrogen (Ntot)	%	0.061
C/N	ratio	6.73
Exchangeable cations:
Magnesium (Mg2+)	mg kg−1	0.643
Potassium (K+)	mg kg−1	110.9
Calcium (Ca2+)	mg kg−1	457.8
Sodium (Na+)	mg kg−1	17.27
Bioavailable macronutrients:
Phosphorus (Pav)	mg kg−1	85.00
Potassium (Kav)	mg kg−1	87.00
Magnesium (Mgav)	mg kg−1	30.00

). Among the exchangeable base cations Ca²⁺ and K⁺ predominated, and among the available macronutrients, K_av and P_av. The lowest content of exchangeable cations and available elements was recorded in the case of Mg.

Among the trace elements determined (

Table 2. Content of trace elements in the initial soil.

Element	Total Forms (TF)	Available Forms (AF)	The Share of AF in TF
	mg kg−1 (in Dry Mass)		%
Fe	6938.3	800.2	11.54
Mn	244.1	80.69	33.06
Zn	20.52	11.11	54.14
Cu	6.333	1.051	16.60
Pb	49.04	5.954	12.14
Cd	0.280	0.038	13.57
Ni	8.933	0.160	1.79
Cr	62.14	3.967	6.38
Co	4.340	0.193	4.45

), the definitely highest content—total forms in the soil—was recorded in the case of Fe (6938.3 mg) and Mn (244.1 mg kg⁻¹ soil). The lowest content was found in the case of Ni, Cu, Co, and Cd, which amounted to 8.933, 6.333, 4.340, and 0.280 mg kg⁻¹ of soil, respectively. The share of available forms of these elements in their total content ranged from approximately 2 to 55%. The highest share was recorded for Zn (54.14%) and Mn (33.06%), and the lowest for Co (4.45%) and Ni (1.79%).

2.3. Characteristics of Digestate from the Agricultural Biogas Plant in Boleszyn

DIG was obtained from the Agricultural Biogas Plant in Boleszyn (Figure 2a). The biogas plant is located in Boleszyn in the Grodziczno commune, Nowe Miasto County (Poland). The main purpose of a biogas plant is the production of electricity. The production cycle produces thermal energy and post-fermentation mass (DIG). The thermal energy is partly used for their own needs, i.e., heating of fermentation tanks, substrates, and the entire infrastructure of the company, as well as external heating of two neighboring towns: Boleszyn and Mroczno, including single-family houses, apartment blocks, and public buildings (school, shops, community centers). The DIG is used to fertilize fields belonging to the company and external recipients.

The biogas plant is divided into two parts, the older part with a capacity of 2 MW and the newer one with a capacity of 1.6 MW, the total capacity of both installations is 3.6 MW. The biogas plant is based on agricultural raw products. The main solid input is corn silage obtained from the fields of the biogas plant, additional solid substrates are straw and chicken manure. Liquid substrates are pig slurry and corn-based distiller’s grain; additionally, cattle slurry is occasionally dosed in smaller quantities. Daily processing is approximately 100 Mg of solid material and 120–150 m³ of liquids. Approximately 200–300 m³ of digestate (DIG) is released from the tanks daily, stored in lagoons, and then transported in barrels to be applied to agricultural fields. Hydraulic retention time (HRT), depending on the selection of the feedstock used in the biogas plant, ranges from 15 to 55 days. The methane potential of DIG based on typical agricultural components ranges from 250 to 300 m³ of methane per Mg of input. In a biogas plant, the substrate is kept in the liquid phase and the dry matter content does not exceed 12–15%.

The DIG used in the research is a by-product from the methane fermentation process derived from typical agricultural waste, mainly pig slurry and corn silage. DIG was used in the experiment in two forms, as a liquid (LD) raw material (Figure 2b) and a solid (SD) processed material (dewatered, dried, and granulated in the biogas plant) (Figure 2c). Both forms of DIG have been subjected to analytical tests for the content of dry matter, total carbon (TC), macronutrients (N_tot, P, K, Mg, Ca, Ca), trace elements (Fe, Mn, Zn, Cu, Pb, Cd, Cr, Co, Ni), EC, and pH. The LD and SD properties are presented in

Table 3. Chemical characterization of DIG.

Elements	Liquid Digestate (LD)		Solid Digestate (SD)
Electrical conductivity (EC in mS cm−1)	21.70		3.40
Soil reaction (pH in H2O)	7.90		8.25
Content of dry matter (%)	3.75		91.91
Macronutrient Content
Elements	in Dry Mass	in Fresh Mass	in Dry Mass	in Fresh Mass
TC (%)	25.21	0.95	33.04	30.12
Ntot (%)	12.13	0.46	2.50	2.28
C:N (ratio)	2.08	2.07	13.22	13.21
P (g kg−1)	9.60	0.36	10.69	9.75
K (g kg−1)	104.8	3.93	12.98	11.83
Mg (g kg−1)	3.12	0.12	4.56	4.16
Ca (g kg−1)	14.37	0.54	17.90	16.31
Na (g kg−1)	41.36	1.55	5.72	5.22
Trace Element Content—Total Forms (mg kg−1, in Dry Mass)
Fe	2195.0	82.31	5145.8	4690.8
Mn	153.8	5.77	223.4	203.7
Zn	222.4	8.34	229.4	209.1
Cu	38.63	1.45	32.96	30.0
Pb	97.50	3.66	87.50	79.76
Cd	2.417	0.09	2.58	2.36
Co	11.25	0.42	11.25	10.26
Ni	14.50	0.54	15.17	13.83
Cr	1.93	0.07	3.80	3.46

.

2.4. Methods of Laboratory Analysis

LD and SD samples were collected at the Agricultural Biogas Plant in Boleszyn, then placed and securely closed in tight containers and transported to the Department of Agricultural and Environmental Chemistry, UWM in Olsztyn. The total content of macronutrients (N_tot, P, K, Mg, Ca, and Na) was determined in the fresh mass of both LD and SD forms. Part of the material was dried in a dryer at 60 °C until constant weight was obtained. After determining the dry matter content, the material was crushed in a mortar to obtain a homogeneous structure. In the LD and SD samples prepared in this way, the following contents were determined: TC and close to the total content of trace elements (Fe, Mn, Zn, Cu, Ni, Pb, Cd, Cr, and Co). Moreover, dried SD was used to determine pH and EC. These indicators (pH and EC) in LD were determined in fresh mass.

After the experiment, samples of soil were taken separately from each pot. All samples, both the initial soil samples and the soil taken from the pots, were dried in the open air, sifted through a sieve with a mesh diameter of 2 mm and additionally crushed in a mortar. Chemical analyses of soils included reaction, salinity, sorption properties, content of exchangeable cations (K⁺, Mg²⁺, Ca²⁺, Na⁺), as well as the content of TC, N_tot, and macronutrients (available forms: P_av, K_av, and Mg_av), and the content close to the total and available trace elements (Fe, Mn, Zn, Cu, Ni, Pb, Cd, Cr, and Co).

Chemical analyses were performed using the following methods:

–

pH—potentiometric method in a KCl solution with a concentration of 1 mol dm⁻³, using a pH 538 laboratory pH meter and a WTW electrode [33];

–

EC—conductometric method using the HANNA HI8733 conductivity meter;

–

HAC and SBC—Kappen method [33];

–

exchangeable basic cations were determined on an atomic absorption spectrophotometer—AA240FS Fast Sequential Atomic Absorption Spectrometer—after previous extraction of the soil with a 1M ammonium acetate solution [33];

–

TC on the TOC–L (Total Organic Carbon Analyzer) device from SHIMADZU using the SSM-5000A (Solid Sample Module) adapter [34];

–

N_tot—Kjeldahl distillation method. For this purpose, soil and DIG samples were mineralized in concentrated sulfuric acid (VI) with the addition of hydrogen peroxide as a catalyst. Distillation was carried out on a MODEL K-355 steam distiller [35];

–

macronutrients (general forms): P—spectrophotometrically using the vanadium–molybdenum method [36], K, Mg, Ca, and Na—using an atomic absorption spectrophotometer—AA240FS Fast Sequential Atomic Absorption Spectrometer [37]. These determinations were made after prior mineralization of the samples in concentrated sulfuric acid (VI) with the addition of hydrogen peroxide;

–

macronutrients (available forms)—P_av and K_av using the Egner-Riehm method [38], and Mg_av by the Schachtschabel method [36];

–

trace elements (forms close to total) using an atomic absorption spectrophotometer—AA240FS Fast Sequential Atomic Absorption Spectrometer [37], using MERCK standards. Before determination, the analytical material (soil, LD, SD) was mineralized in a MARS 6 microwave oven (CEM Corporation, USA) in MARSXpress Teflon vessels according to the US–EPA3051 methodology, using acids, 65% HNO₃ and 38% HCl, in a ratio of 3:1 [37,39]. To control the accuracy of the obtained results, the reference material CRM0120-50G (Trace Metals—Sandy Loam 2) was analyzed in parallel;

–

trace elements (available forms) using an atomic absorption spectrophotometer—AA240FS Fast Sequential Atomic Absorption Spectrometer [37]. Soil samples for determination were extracted in 1 M HCl [33].

CEC and BS were calculated based on the HAC and SBC values in the soil [40,41]. In turn, based on the determination of the content of total forms and digestible trace elements, the share of digestible forms in the total content was calculated.

2.5. Statistical Analysis

The statistical analysis of the obtained results included the Pearson’s simple correlation coefficient (r) and the LSD test. The Pearson’s coefficient was used to determine the relationship between the examined features and also allowed to determine the direction of the influence of increasing doses of DIG on the tested feature. The significance of the obtained r values was determined based on statistical tables [42], considering the following: *—significant at p ≤ 0.05; **—highly significant p = 0.01; ^n.s.—not significant. LSD analysis (two-way ANOVA) was used to assess the significance of the impact of increasing doses of DIG on the tested features. Significance and homogeneous groups were determined by Duncan’s test, at p ≤ 0.05. Various lowercase letters next to the given values indicate the influence of the interaction of the DIG form and doses on the examined feature; capital letters indicate the influence of the DIG dose or form. Microsoft Excel, v. 2206, was used for statistical calculations [43] alongside Statistica 13.3 PL [44].

3. Results

3.1. Sorption Properties of Soil and the Content of Exchangeable Cations

Examination of the soil material from the experiment in terms of soil sorption properties allowed for the assessment of the impact of the addition of liquid digestate (LD) and solid digestate (SD) on the size of the sorption complex (CEC) and the degree of its saturation with basic cations (BS). The average values of individual parameters were hydrolytic acidity (HAC)—24.05, sum of exchangeable cations (SBC)—21.60 mmol kg⁻¹ soil, CEC—45.65 mmol kg⁻¹ soil, and BS—47.24% (

Table 4. Sorption properties of soil.

Doses of DIG in mg N kg−1 of Soil	Treatments		Mean for Doses	Treatments		Mean for Doses	Treatments		Mean for Doses	Treatments		Mean for Doses
	LD	SD		LD	SD		LD	SD		LD	SD
	HAC (mmol kg−1 of Soil)			SBC (mmol kg−1 of Soil)			CEC (mmol kg−1 of Soil)			BS (%)
0	25.00 b	25.00 b	25.00 C	22.00 a b	22.00 a b	22.00 A	47.00 ab	47.00 ab	47.00 B	46.82 abc	46.82 abc	46.82 A
28	23.50 ac	24.00 ab	23.75 AB	22.00 ab	19.33 a	20.67 A	45.50 abc	43.33 c	44.42 B	48.36 abc	44.51 a	46.44 A
56	23.50 ac	24.00 ab	23.75 AB	19.33 a	23.33 b	21.33 A	42.83 c	47.33 ab	45.08 B	45.05 ab	49.22 bc	47.13 A
84	24.50 ab	24.50 ab	24.50 BC	23.33 b	22.00 ab	22.67 A	47.83 b	46.50 ab	47.17 B	48.71 abc	47.32 abc	48.02 A
112	22.50 c	24.00 ab	23.25 A	22.00 ab	20.67 ab	21.33 A	44.50 ac	44.67 ac	44.58 B	49.44 c	46.17 abc	47.80 A
Mean for Series	23.80 A	24.30 A	24.05	21.73 A	21.47 A	21.60	45.53 A	45.77 A	45.65	47.68 A	46.81 A	47.24

DIG—digestate; LD—liquid form of DIG; SD—solid form of DIG; HAC—hydrolytic acidity; SBC—sum of base cations; CEC—cation exchange capacity; BS—base saturation. Based on LSD p ≤ 0.05 analysis, various lowercase letters (a, b, c) next to the given values indicate the significant influence of the interaction of the DIG form and doses on the examined feature; capital letters (A, B, C) indicate the significant influence of the DIG dose or form; the same letters indicate the lack of significance of the factors considered.

). In the treatments fertilized with digestate (DIG), both the LD and SD soil HAC values, as well as the CEC value, were lower than those shown in the control sample (without the addition of DIG). LSD analysis showed a significant effect of the 1st, 2nd, and 4th dose of LD on the HAC and CEC values. In turn, SBC determined in the soil after DIG application, as well as BS, remained at a similar level compared to the control sample. LSD analysis showed that the type of DIG and the dose had no significant effect on the obtained SBC and BS values.

3.2. The Content of Exchangeable Cations

The average content of exchangeable cations in the soil after the experiment was similar to their amount in the initial soil in the case of Ca²⁺ and Na⁺, higher in the case of K⁺, and lower in the case of Mg²⁺ (Table 1 and Figure 3). The content in the soil after corn harvest was, respectively, 454.8 mg, 17.75 mg, 54.17 mg, and 0.547 mg kg⁻¹ (Figure 3). However, when comparing the content of exchangeable cations in the soil in the control object and those with the addition of DIG, one should note the significant and positive impact of SD application on the amount of Ca²⁺ and Mg²⁺ cations, and of LD on the content of K⁺ cations. As a result of the application of the highest dose of SD, the content of Mg²⁺ cations increased by 16% and Ca²⁺ by 9%. However, the addition of the highest dose of LD resulted in an increase in the content of K⁺ cations by almost 19% compared to the control. The LSD analysis showed that the highest DIG dose had a significant impact on the obtained results. In the case of the content of Na⁺ cations in the soil, the type of DIG, not its dose, turned out to be important (Figure 3). A significantly higher average content of Na⁺ cations was recorded in the LD series and was 31% higher than their average content in the SD series. In this series, in all objects with the addition of SD, a higher content of Na⁺ cations was observed than in the control, but these changes were not statistically significant. However, in the series with the addition of SD, the first three doses resulted in a decrease in the content of exchangeable Na⁺ cations, and the fourth dose resulted in a significant increase.

3.3. pH and EC Value as well as TC, N_tot, and C:N Ratio Content

After the application of LD and SD, the soil pH was in the range of 4.52 to 4.67 (Figure 4). The lowest pH value (pH = 4.52) was recorded in the control sample, without the addition of DIG. Against the background of increasing doses of DIG, the pH value increased significantly. However, there were no significant differences in the soil pH value between objects treated with different forms of DIG. The most beneficial effect of LD on the pH value was observed at the fourth dose, and SD at the third dose, at which the pH value was 4.67 and 4.62, respectively.

In the presented work, an increase in EC values was observed after DIG application (Figure 4). According to LSD analysis, in the case of SD, the lowest and highest doses significantly influenced the increase in EC. In turn, in the case of LD, a significant increase in EC values was observed only after the highest dose was administered. Moreover, it should be noted that the soil with SD added was characterized by higher salinity than with LD added. The exception was the highest dose of these additives, at which, regardless of the DIG form, the EC value remained at a similar level (36.53–36.93 µS cm⁻¹).

The use of individual doses of LD and SD influenced the TC content differently (Figure 4). Despite the lack of a linear relationship, the use of LD generally resulted in a significantly higher TC content in the soil compared to the control. Moreover, the percentage of TC was significantly higher in the LD soil than in the SD soil. SD application did not have a significant impact on the TC content in the soil. The content of N_tot in the soil ranged from 0.057% (control object) to 0.063% (object with the 2nd and 3rd dose of LD) (Figure 4). In both cases, this content increased successively after two initial doses of DIG (both LD and SD). Both forms of DIG had a positive effect on the N_tot content in the soil, as the results obtained were higher in the treatments with the addition of DIG than in the control sample. LSD analysis highlights the statistically significant impact of the second dose of LD on the N_tot content in the soil. It should also be noted that a significantly higher content of N_tot was observed in the soil with the addition of LD than SD.

In the conducted research, the average value of the C:N ratio in the soil was 7.593 (Figure 4). LD used in the first two doses had a positive impact on the C:N ratio. However, higher doses of LD resulted in a narrowing of the C:N ratio. In turn, SD had a negative impact on the C:N relationship, narrowing its value. However, this effect was small and statistically insignificant.

3.4. Content of Digestible Macronutrients (K_av, P_av, Mg_av)

Application of both forms of DIG to the soil resulted in enriching the soil with available K_av, P_av, and Mg_av (Figure 5). The average content of available K_av in the soil without DIG was 0.32 g kg⁻¹. Application of the 2nd, 3rd, and 4th doses of both forms of DIG increased the K_av content in the soil, but only the 3rd and 4th doses had a statistically significant effect. The highest K_av content, equal to 0.062 g kg⁻¹ (series with LD) and 0.040 g kg⁻¹ of soil (series with SD), was recorded at the 4th dose of DIG. In objects with SD, despite the observed positive changes, only the highest dose had a statistically significant effect. The average P_av content in the soil from objects treated with LD was 0.076 g, while with SD it was 0.079 g kg⁻¹ of soil. LSD analysis proved a significant effect of the 4th dose of LD and the 2nd, 3rd, and 4th doses of SD. Similarly, DIG application influenced the Mg_av content in the soil. The average Mg_av content in the soil was 0.029 g in the LD objects and 0.033 g kg⁻¹ of soil in the SD objects. The soil with the addition of SD was characterized by a significantly higher Mg_av content, as indicated by LSD analysis. LD had a non-significant effect on this trait.

3.5. Content of Trace Elements

The content of total metal forms (Fe_tot, Mn_tot, Zn_tot, Cu_tot, Pb_tot, Cd_tot, Co_tot, Cr_tot, Ni_tot) varied and depended on the DIG form used and the dose introduced (Figure 6). The average content of the tested metals in the soil, regardless of the DIG form, was as follows, in descending order: Fe_tot (7163.4) > Mn_tot (263.9) > Zn_tot (21.84) > Cr_tot (57.92) > Pb_tot (53.46) > Ni_tot (8.461) > Cu_tot (5.047) > Co_tot (4.601) > Cd_tot (0.687 mg kg⁻¹ soil). In the LD series, a significantly higher average content of Fe_tot, Mn_tot, and Cd_tot in the soil was recorded than in the SD series. In turn, SD contributed to a significant increase in the average content of Cu_tot, Pb_tot, Co_tot, Cr_tot, and Ni_tot in the soil. However, the Zn_tot content in the soil was similar in both series of experiments. LSD analysis proved that higher DIG doses usually had a significant impact on the change in the content of heavy metals in the soil compared to the control object. In the case of Zn_tot, Pb_tot, Cd_tot, and Co_tot, increasing SD doses did not show any significant impact.

Due to the necessity of certain trace elements for plants, including Fe, Mn, Cu, and Cu, as well as undesirable content of potentially toxic substances, including Pb and Cd, it is important to know their available content (Figure 7).

The average content of available forms of metals in the tested soil, regardless of the DIG type, was as follows, in descending order: Fe_av (804.7) > Mn_av (80.21) > Zn_av (8.864) > Pb_av (5.012) > Cr_av (4.066) > Cu_av (1.103) > Co_av (0.228) > Ni_av (0.190) > Cd_av (0.036 mg kg⁻¹ soil). LSD analysis showed that in the LD series, there was a significantly higher average content of Fe_av, Mn_av, Zn_av, Co_av, and Cr_av in the soil than in the SD series. In turn, the application of SD contributed to a significant increase in the average content of Pb_av and Cd_av in the soil.

Apart from the type of DIG, the dose of DIG was an important issue in shaping the content of available forms of metals in the soil. LD application increased the content of available Fe_av, Mn_av, Cu_av, and Pb_av in the soil, which was observed especially at higher LD doses. At the highest LD dose, the recorded increase in the content of Fe_av, Mn_av, Cu_av, and Pb_av compared to the control object was 6.9%, 12.4%, 21.9%, and 20%, respectively. LD had a different impact on the content of Cr_av in the soil, which decreased with the increase in the dose of this material. The greatest decrease in the content of Cr_av was recorded at the third dose of LD, which amounted to 20.2% compared to the control. However, the second dose of LD had a significant effect on the decrease in the content of Cr_av. Concerning the other metals (Zn, Cd, Co, and Ni), there was no impact of increasing LD doses on the content of their available forms in the soil.

Application of SD to the soil significantly influenced the content of Pb_av, Fe_av, and Cr_av. In the case of Pb_av, the effect was positive, while in the case of Fe_av and Cr_av, it was negative. A significant increase in the content of Pb_av and a decrease in the content of Cr_av in the soil was recorded after the first dose of SD. In turn, only the highest dose of SD resulted in a significant reduction in the content of Fe_av. The recorded difference in the content of these elements between the control and the object with the highest SD dose was 4.6% for Fe_av, 20.6% for Pb_av, and 15.1% for Cr_av.

3.6. The Share of Available Forms of Trace Elements in Their Total Content

In the soil treated with DIG, the highest share of available forms of heavy metals in the total content was found in the case of Zn, Mn, and Cu, and the lowest for Cd, Ni, and Co (Figure 8 and Figure 9). This phenomenon was observed in both types of DIG. In the LD series, this share was on average 40.80% for Zn, 30.92% for Mn, and 24.76% for Cu (Figure 8). In this series, the lowest average contribution was observed for Cd (4.60%) and Ni (3.51%). In turn, in the series with SD (Figure 9), the average share of Zn and Mn was 40.57% and 30.12%, respectively. A slightly lower share was recorded for Cu (21.06%), and the lowest for Ni (2.55%).

Application of increasing doses of LD to the soil (Figure 8) usually significantly and positively influenced the share of available forms of Fe (r = 0.902 **) and Mn (r = 0.839 **), and in the case of Co—significantly (r = 0.666 *). However, a negative and highly significant relationship with increasing LD doses occurred in the case of the share of available forms of Ni (r = −0.900 **), Cr (r = −0.833 **), and Cu (r = −0.752 **). In the case of the remaining elements (Zn, Pb, and Cd), no clear trend was observed. In turn, the application of increasing doses of SD had a significant and positive effect only on the increase in the share of available forms of Mn (r = 0.673 *) (Figure 9). Moreover, with an increase in the SD dose, a highly significant decrease in the content of available forms of Cu (r = −0.741 *), Ni (r = −0.678 **), and Cr (r = −0.723 **) in the soil was observed. A decrease in the share of these metals was observed already after the introduction of the first dose of SD. A linear relationship was not demonstrated for Fe, Zn, Pb, Cd, and Co.

3.7. Correlations between the Content of Available Forms of Trace Elements and Soil Properties

The observed changes in the content of available heavy metals in the soil obtained after the experiment were often related to the soil properties. Several relationships and associations were observed in the LD series (

Table 5. Correlation between the content of available metals and selected properties of soil treated with LD.

Selected Soil Properties	Content of Available Metals
	Feav	Mnav	Znav	Cuav	Pbav	Cdav	Niav	Crav	Coav
pH	0.685 **	0.579 n.s.	0.540 n.s.	0.618 *	0.384 n.s.	–0.281 n.s.	0.103 n.s.	–0.472 n.s.	0.413 n.s.
EC	0.612 *	0.631 *	0.552 n.s.	0.471 n.s.	0.489 n.s.	0.139 n.s.	–0.218 n.s.	–0.402 n.s.	0.177 n.s.
HAC	–0.751 **	–0.380 n.s.	–0.225 n.s.	–0.372 n.s.	–0.246 n.s.	0.378 n.s.	0.013 n.s.	0.326 n.s.	–0.306 n.s.
SBC	0.088 n.s.	0.247 n.s.	0.068 n.s.	–0.016 n.s.	0.145 n.s.	0.417 n.s.	–0.395 n.s.	–0.049 n.s.	–0.193 n.s.
CEC	–0.319 n.s.	0.014 n.s.	–0.060 n.s.	–0.210 n.s.	–0.004 n.s.	0.560 *	–0.335 n.s.	0.130 n.s.	–0.328 n.s.
BS	0.450 n.s.	0.370 n.s.	0.160 n.s.	0.151 n.s.	0.235 n.s.	0.198 n.s.	–0.382 n.s.	–0.173 n.s.	–0.033 n.s.
Ntot	0.356 n.s.	0.677 *	0.263 n.s.	0.621 *	0.598 *	–0.080 n.s.	0.447 n.s.	–0.794 **	0.496 n.s.
TC	0.322 n.s.	0.314 n.s.	0.346 n.s.	0.442 n.s.	0.367 n.s.	–0.449 n.s.	0.491 n.s.	–0.255 n.s.	0.374 n.s.

EC—electrical conductivity; HAC—hydrolytic acidity; SBC—sum of base cations; CEC—cation exchange capacity; BS—base saturation; N_tot—total nitrogen; TC—total carbon; *—correlation coefficient r significant for p ≤ 0.05; **—correlation coefficient r significant for p ≤ 0.01; ^n.s.—not significant.

). Soil reaction had a highly significant positive impact on the content of available Fe (r = 0.685 *) and significantly on the Cu content (r = 0.618 *). EC significantly correlated with the content of Fe (r = 0.612 *) and Mn (r = 0.631 *). Fe content and HAC were highly significantly correlated but with a negative significance. CEC was positively correlated with the content of available Cd (r = 0.560 *). The most significant correlations with the content of available metals were found in the content of N_tot, mainly with Mg, Cu, and Pb (0.598 * ≤ r ≤ 0.677 *). These relationships were generally positive. A negative relationship was noted in the N_tot–available Cr system (r = −0.794 **).

In the case of the series with SD, pH correlated positively and highly significantly with the Pb content (r = 0.783 **), significantly with the Cd content (r = 0.588 *), and highly significantly and negatively with the Cr content (r = −0.739 **) (

Table 6. Correlation between the content of available metals and selected properties of soil treated with SD.

Selected Soil Properties	Content of Available Metals
	Feav	Mnav	Znav	Cuav	Pbav	Cdav	Niav	Crav	Coav
pH	–0.405 n.s.	–0.005 n.s.	–0.276 n.s.	0.517 n.s.	0.783 **	0.588 *	0.421 n.s.	–0.739 **	0.435 n.s.
EC	–0.260 n.s.	–0.004 n.s.	–0.384 n.s.	0.513 n.s.	0.527 n.s.	0.124 n.s.	0.092 n.s.	–0.648 *	0.449 n.s.
HAC	–0.030 n.s.	–0.184 n.s.	0.090 n.s.	–0.492 n.s.	–0.349 n.s.	0.000 n.s.	–0.088 n.s.	0.455 n.s.	–0.554 *
SBC	0.035 n.s.	0.366 n.s.	–0.048 n.s.	–0.394 n.s.	0.248 n.s.	0.367 n.s.	0.689 **	0.051 n.s.	0.334 n.s.
CEC	0.024 n.s.	0.287 n.s.	–0.020 n.s.	–0.501 n.s.	0.132 n.s.	0.340 n.s.	0.612 *	0.174 n.s.	0.155 n.s.
BS	0.052 n.s.	0.384 n.s.	–0.080 n.s.	–0.294 n.s.	0.321 n.s.	0.377 n.s.	0.705 **	–0.043 n.s.	0.465 n.s.
Ntot	0.196 n.s.	0.367 n.s.	–0.073 n.s.	0.389 n.s.	0.457 n.s.	0.250 n.s.	0.289 n.s.	–0.497 n.s.	0.568 *
TC	–0.216 n.s.	–0.014 n.s.	–0.249 n.s.	–0.405 n.s.	–0.082 n.s.	–0.030 n.s.	0.067 n.s.	0.373 n.s.	–0.404 n.s.

EC—electrical conductivity; HAC—hydrolytic acidity; SBC—sum of base cations; CEC—cation exchange capacity; BS—base saturation; N_tot—total nitrogen; TC—total carbon; *—correlation coefficient r significant for p ≤ 0.05; **—correlation coefficient r significant for p ≤ 0.01; ^n.s.—not significant.

). Numerous connections in the SD series were also found between other soil properties and the content of available metals. EC significantly negatively correlated with Cr (r = −0.648 *). Co content and HAC were significantly correlated, but negatively (r = −0.554 *). However, the content of available Ni forms was correlated with the sorption complex (SBC, CEC, and BS), and these were positive relationships (0.612 ** ≤ r ≤ 0.705 **). The addition of SD also resulted in a significant correlation between the content of N_tot and Co (r = 0.568 *).

4. Discussion

The use of organic fertilizers for fertilizing purposes is currently crucial for the development of agricultural systems. DIG obtained from agricultural biogas plants can be a rich source of nutrients for plants [28]. The use of DIG improves the chemical properties of soils, including an increase in the content of many valuable ingredients, which was proven in the presented work and the available literature [45,46], and the degree of these changes often depends on the form of DIG used [47]. According to Panuccio et al. [48], liquid and solid forms of digestate differ significantly, both in terms of chemical composition and biological properties.

The use of DIG in agriculture is also desirable because the soils of many European countries, including Poland, are characterized by a low content of organic matter, and as it is known, the abundance of humus increases the sorption capacity and improves buffering properties [49]. Sorption properties are one of the key features that allow determining the method of soil use and the degradation processes occurring in it. This parameter is responsible for regulating the processes related to the leaching of nutrients from the soil, determining its fertility [50]. The soil sorption complex has the ability to immobilize or retain pollutants in the soil for a long time. The soil sorption properties depend on HAC, SBC, CEC, and BS [51]. The presented study did not demonstrate significant changes in SBC and BS as a result of the use of DIG, regardless of its type (Table 4). However, significantly lower HAC values were observed in objects fertilized with DIG, which was influenced by the particularly liquid form of DIG. Obtaining such results was undoubtedly influenced by the soil used in the research, which was characterized by a low BS value (39.86%) (Table 1) and the lack of additional mineral fertilization. Similar results were also obtained by other researchers [51,52]. In the study by Stańczyk-Mazanek et al. [51], there was no significant difference in the change in the sorption complex with increasing doses, regardless of the form of DIG (LD and SD). It should be noted that SD application, in our research, had a positive effect on the amount of Ca²⁺ and Mg²⁺ cations (Figure 3). However, the LD application had a positive effect on the content of K⁺ cations. A statistically significant effect was observed after the highest doses of DIG. In the case of the content of Na⁺ cations in the soil, the type of DIG, not its dose, turned out to be important. The soil saturation with Na⁺ cations was influenced more significantly by the addition of LD than SD. This observation is important, due to the fact that Mg²⁺, K⁺, Ca²⁺, and Na⁺ cations, apart from H⁺ and NH₄⁺, are the main exchangeable cations. Among them, Ca²⁺, Mg²⁺, and H⁺ cations are most important in the saturation of the sorption complex, and Na+ in saline soils. According to Widłak [53], K⁺ affects the structure of the soil and the root system, and Ca⁺ is responsible for regulating the water–air relations of the soil. Both elements make the soil alkaline and shape its good granular structure. In turn, Na⁺ in larger amounts is not desirable because it disturbs the soil structure and water management.

The next important parameter is the soil reaction, which affects its fertility and additionally shapes its physical, chemical, and biological properties [51]. Soil reaction is also related to the solubility of minerals and the presence of soil microorganisms [54]. The soil used in the experiment had a low pH of 4.60 and was classified as acidic. Our results indicate that DIG application may increase soil pH (Figure 4). The most beneficial effect of LD on the pH value was observed at the fourth dose, and SD at the third dose. Similar results were obtained also by other researchers [30,54], who showed a reduction in soil acidification after the application of DIG. The pH value increases when DIG is used alone or together with mineral fertilizers. This is particularly beneficial in the case of acidic soils [30]. The nature of the DIGs with their alkaline reactions (LD − pH = 7.90; SD − pH = 8.25) contributed to the increase in pH values recorded in the currently presented research (Table 3). An increase in soil reaction after the use of digestate was also noted by Koszel and Lorencowicz [55] and Fuchs and Schleiss [56]. In turn, Odlare et al. [57] did not observe significant differences in soil reaction after fertilization with digestate. However, in the research by Makádi et al. [58], despite the alkaline reaction of the digestate used, a slight decrease in soil pH was found. The authors therefore suggest the need to constantly monitor the chemical properties of soils subjected to such treatments, especially those with low buffering capacity.

The structure and yield quality of the soil is related to the content of total carbon (TC) and total nitrogen (N_tot). The amount of TC in each soil formation is different and depends on the type of use. The share of TC in the soil increases by introducing aboveground and underground biomass from plants and organic residues contained in natural and organic fertilizers. Among organic fertilizers, DIG is also mentioned as a source of TC [30]. In the conditions of field experiments [59], after the application of DIG at doses of 170 kg N ha⁻¹, an increase in the organic carbon content in the soil was demonstrated. This increase was 3.7% after the use of liquid digestate and 15% after the use of solid digestate. In our research, the use of DIG influenced the TC content in the soil in various ways. LD application generally resulted in significantly higher TC content in the soil compared to the control. Moreover, the TC content was significantly higher in the LD soil than in the SD soil. This significant but small increase in the TC content in the soil as a result of the use of LD and the lack of significant changes in the series with SD in our research could be the reason for the lack of application of additional fertilizer ingredients.

The use of DIG for fertilizing or improving soil structure can be considered as a kind of recovery of nutrients from the raw mass used for gas production purposes. DIG application increases the total N content in the soil, but part of the supplied N is not completely mineralized in the soil during one cycle. This incomplete mineralization of N contained in DIG may reduce N losses during leaching, which additionally protects other elements of the environment [30,47]. DIG is a substance rich in N, and N content is closely related to the type of substrate used for biogas production. In the study by Slepetiene et al. [29], fertilization with solid digestate at a dose of 170 kg N ha⁻¹ resulted in a significant (five-fold) increase in the content of mineral nitrogen in the soil layer at a depth of 0–40 cm. In the present study, both forms of DIG had a positive effect on the N_tot content in the soil. It should also be noted that a significantly higher N_tot content was observed in the soil with the addition of LD than SD. The use of LD shows faster utilization of ingredients by plants than in the case of SD due to the availability of easily soluble forms of N. In the light of the available literature [30], DIGs can be effectively used as nitrogen fertilizers in combination with small doses of mineral fertilizers. N plays many key roles; it is responsible for stimulating the growth of above-ground parts of plants, the development of the root system, and regulating the consumption of other elements. N shows a synergistic interaction with P and K [53]. However, attention should be paid to the potentially negative impact of excess ammonia in the soil, the source of which may be DIG. Excess ammonium in the soil may adversely affect earthworms, leafhoppers, and nematodes living in the surface layer of soil [28].

Taking into account the content of C and N in the soil, the C:N ratio is very important, which is a key criterion when assessing the agronomic management of organic fertilizers [30]. The narrow C:N ratio in DIG makes it necessary to use it with materials such as bark, straw, and sawdust, characterized by a wide range of C to N, so that the fertilizer has a more effective effect. A narrow C:N ratio affects the rate of mineralization taking place in the soil; the lower it is, the faster the process. Pilarska et al. [52] indicate a higher C:N ratio in liquid fertilizers than in solid fertilizers. Also, in our research, a higher C:N ratio was found in LD (13.21) than in SD (2.07) (Table 3). LD used in the first two doses had a positive impact on the C:N ratio (Figure 4). However, higher doses of LD resulted in a narrowing of the C:N ratio. However, the observed changes were not significant concerning the control treatment and could be caused by an increase in the N_tot content in the soil and decreasing TC content in these treatments. In turn, SD had a negative impact on the C:N relationship, narrowing its value. However, this effect was small and statistically insignificant. This was probably the result of the lack of a positive effect on the TC content in the soil and, at the same time, a slight increase in the N_tot content. The low C:N ratio obtained was also due to the lack of support for corn growth with mineral fertilizers and low parameters of the initial soil (Table 1).

Macronutrients such as K, P, and Mg are important elements, apart from N, necessary for plant growth. Their availability in the soil affects its fertility [54]. As in the case of N, the amount of available elements in DIG depends on the feedstock used in the biogas plant. DIG contains significant amounts of P, but most of the P in DIG is in the inorganic form [30]. In the DIG used for research, regardless of its form, the K content predominated (Table 3). A significant P content was also recorded in SD. The definitely lowest content in both forms of DIG was recorded in the case of Mg. Regardless of the nature of DIG, the presented work demonstrated that the application of both forms (LD and SD) to the soil increased the content of available forms of K_av, P_av, and Mg_av. In turn, Koszel and Lorencowicz [55], after using DIG, noted an increase in the content of P, K, and Mg in the soil, which was 19, 123, and 3%, respectively. In the experiment of Tan et al. [60], the content of total forms of P and K and available forms of N, P, and K in the soil after the application of digestate also increased compared to the control. In a study by Slepetiene et al. [29], an increase in P and K content was recorded using only 170 kg ha⁻¹ of nitrogen in the form of SD and LD. The results of Makádi et al. [58] also indicate an increase in the content of P and K in the soil due to the use of digestate. The literature in [30] also reported an increase in the P_av content in the soil as a result of the use of FM, which the authors explain by the action of microorganisms accumulating P from inorganic forms supplied from FM. Moreover, the authors suggest that the use of high doses of DIG with a low N:P ratio may lead to the accumulation of P in the soil. Therefore, to make better use of N by plants and reduce the accumulation of P in the soil, joint fertilization with DIG and mineral fertilizers containing N should be skillfully established.

A problem in modern plant cultivation is excessive soil salinity, which limits the uptake of nutrients and water, and this contributes to reduced soil fertility. Moreover, excessive salinity may cause the disappearance of the lumpy structure of the soil and reduce its permeability. The salinization process is defined as the natural or anthropogenic accumulation of soluble salts in the soil, caused by the storage of chlorides, sulfates, and carbonates of potassium, calcium, and sodium [53]. In the pot experiment, an increase in EC was observed after DIG application (Figure 4). The soil with SD added was characterized by a higher EC value than with LD added. The exception was the highest dose of these additives, at which, regardless of the DIG form, the EC value remained at a similar level (36.53–36.93 µS cm⁻¹). The obtained results may indicate that the ingredients contained in LD are mostly present in easily soluble forms, unlike SD. It should be noted that LD was characterized by an EC value that was six-fold higher than SD (Table 3). These results may indicate that the solution components influencing the EC value are separated during digestate dewatering.

When using DIG, the content of trace elements should also be taken into account, which may be a “harmful” ballast in DIG. Excessive levels of heavy metals pose a threat to human health. This threat results from their ability to accumulate and migrate in the soil profile. They enter circulation through the root system of plants, thanks to which the flora plays a key role in the food chain [53]. According to Urbanowska et al. [61], each post-fermentation mass will contain trace elements regardless of the substrate used, and their amount will depend on the degree of contamination of the input. Therefore, it is also necessary to determine the content of heavy metals not only in the feedstock material for biogas plants and the obtained post-fermentation mass but also in the soil from the area where the use of this mass is planned for fertilization or soil quality improvement purposes. A regulation of the Ministry of Agriculture and Rural Development on June 18, 2008 [21] on the implementation of certain provisions of the Act on fertilizers and fertilization indicates the permissible values of contamination with heavy metals in organic fertilizers supporting the cultivation of plants. In the case of Cr, the maximum value is 100 mg, Cd—5 mg, Ni—60 mg, Pb—140 mg, and Hg—2 mg per kg of dry matter of the fertilizer or plant-supporting agent. The DIG used in the tests did not exceed these permissible levels (Table 3). Colombo et al. [62] showed that as the soil pH and organic matter content increase, the availability of microelements gradually decreases. The authors explain this phenomenon by the fact that, probably in the presence of organic material, heavy metals transform into complete forms that are more stable and therefore less bioavailable to plants.

Joka et al. [63] state that large amounts of heavy metals contribute to reducing the effectiveness of the fermentation process to a similar extent as pesticides and detergents. Skwaryło-Bednarz et al. [64] define the determination of the total content of heavy metals as a good indicator of chemical soil degradation. The content of the tested metals in the soil after the experiment varied greatly and depended on the DIG form used (LD or SD) and the introduced doses (Figure 6 and Figure 7). The use of increasing doses of LD had a positive effect on the content of total forms of Cu, Ni, and Cr, and a negative effect on Fe, Mn, and Co. In turn, the application of increasing doses of SD implied an increase in the content of Cu, Cr, and Ni in the soil and a decrease in the content of Fe and Mn.

Bioavailable metals are easily taken up by plants, both those necessary for their functioning and those toxic for it. Their amount taken is related to other parameters. DIG used in the presented research influenced the content of available metals in the soil (Figure 7). LD implied, to a greater extent than SD, an increase in the content of Fe, Mn, Zn, Co, and Cr. However, SD resulted in an increase in Pb and Cd content in the soil. Furthermore, the observed results were noted at higher doses of DIG.

It is important that in the composition of organic fertilizers such as compost or digestate, the content of available metals is only a small part of the total content. In the soil treated with DIG, the highest share of available forms in the total content was found for Zn, Mn, and Cu, and the lowest for Cd, Ni, and Co (Figure 8 and Figure 9). This was independent of the DIG type. The observed changes in the content of available heavy metals in the soil obtained after the experiment were often related to the soil properties. Typically, soil pH is related to the uptake and transport of heavy metals, and as the pH increases, the toxicity of these elements decreases. The higher the soil pH, the easier it is to retain heavy metals. The presented results indicate that the increase in soil pH due to the application of LD was positively correlated with the content of Fe_av and Cu_av (Table 5), and SD was positively correlated with the content of Pb_av and Cd_av, and negatively with the content of Cr_av (Table 6). Other soil properties that change as a result of DIG application are also important. The presented research also proves that changes in the EC value, the properties of the sorption complex, and the content of TC and N_tot in the soil as a result of DIG application may vary the content of available forms of heavy metals in the soil (Table 5 and Table 6). In the study by Barłóg et al. [65], the highest accumulation of N, K, S, Na, Zn, and Fe in spring barley was obtained in the combination containing only digestate. In turn, for comparison, winter wheat treated with NPK mineral fertilization alone accumulated the smallest amounts of P, K, Mg, Na, Mn, and Zn.

The results obtained in the presented research were undoubtedly influenced by numerous factors, such as the acidic reaction of the initial soil, the lack of additional support for mineral fertilization, the cultivation of corn with high soil requirements, and the short cultivation period, which could have contributed to the incomplete decomposition and utilization of the ingredients from SD. However, many positive effects observed at work can be the basis for using DIG in both liquid and solid form. The use of DIG causes an increase in the content of TC, N_tot, P_av, K_av, and Mg_av, which was proven in the presented work and as indicated by research by other authors [45,46]. Moreover, the effects after applying DIG are more visible than after applying mineral fertilizers, so DIG can be not only a valuable organic fertilizer [46] but also constitute a substitute for mineral fertilizers [28,46]. The separation of DIG into LD and SD fractions should be considered not only from the environmental point of view [28] but also from the economic side [11]. It is very important to take into account the costs of transporting DIG from the biogas plant to the place where it is to be used. If this distance is too large, the profitability of using LD in agriculture may be negligible [11]. However, SD, as a DIG type with a concentrated and stabilized composition [45], may be more economically profitable, not only due to transport but also easier storage. According to Czekała [66], unprocessed digestate contained relatively little dry matter, which ranged from 3.65 to 10.23%. To further stabilize the DIG, some researchers [9,45] draw attention to the possibility of additional DIG composting. Composting prevents the potential phytotoxicity of DIG [9]. Like other researchers [47,67], we believe that DIG in agriculture has great potential, provided that its raw materials are not polluted with harmful substances and that they will be properly fermented in a biogas plant. Alburquerque et al. [17] believe that digestate is characterized by a high fertilizing potential, mainly related to the NH₄-N content; however, its use in agriculture may be limited due to the content of some elements (Cu and Zn), salinity, biodegradability, or phytotoxicity. Rational use is also important, with the selection of appropriate doses, terms, and methods of application.

5. Conclusions

The DIG used in the presented research differed in its chemical composition and had a different impact on the analyzed soil properties and the content of not only macroelements but also trace elements. The observed changes were correlated with the form (LD or SD) and the DIG dose used. LD was characterized by lower pH, higher EC, and significantly higher moisture content than SD. Moreover, LD contained much less TC, macroelements (N, P, K, Mg, Ca, Na), and trace elements (Fe, Mn, Zn, Cu, Pb, Cd, Co, Ni, and Cr). Both forms of DIG showed a negligible effect on the soil sorption complex, apart from the reduction in HAC as a result of the application of LD. However, they significantly increased the pH and EC values of the soil, but in the case of SD, the increase in EC values was more pronounced than in the case of LD. Moreover, LD had a significant effect on the increase in the content of TC and N_tot in the soil, which was additionally reflected in a slight increase in the C:N ratio, which was not observed after the application of SD. The application of increasing doses of LD significantly increased the content of K_av, P_av, Fe_av, and Mn_av and exchangeable cations K⁺ in the soil, and SD significantly influenced the content of available P_av, Mg_av, and Mn_av and exchangeable cations Ca²⁺ and Mg²⁺. Moreover, both forms of DIG significantly increased the content of total forms of Cu_tot, Cr_tot, Pb_tot, and Ni_tot but significantly reduced the share of available forms of Cu_av in the soil. Despite the recorded significant impact of DIG on the content of heavy metals in the soil, their amount does not pose an environmental threat. However, it provides indications for in-depth chemical analyses in the event of above-standard content of these elements in the feedstock material for biogas plants.