The Effect of Organic Materials with Different Degrees of Decomposition on the Content of Nickel in the Lettuce Leaves Cultivated in Mineral Soil

Abstract

Studies were carried out in order to verify the research hypothesis that “the introduction of organic matter in the form of high peat, brown coal, and wheat straw to the mineral soil contaminated with nickel will reduce the content of this metal in the lettuce”, the main purpose of which was to determine the impact of various organic materials with varying degrees of decomposition on the content of nickel in lettuce leaves. The factors of the experiment were different types of organic material that were added to the mineral soil (loamy sand), such as high peat, brown coal, and wheat straw. The substrates in which lettuce was grown were contaminated with increasing doses of nickel: 0, 50, 75, and 100 mg Ni·dm⁻³ of the substrate. This research shows that the type of organic material introduced into the mineral soil as a source of soil organic meter has a significant impact on the content of nickel in lettuce leaves.

1. Introduction

Due to the high density of nickel, it is included in the heavy metal group, and since 1987, it has also been included among the essential elements for plant growth [1]. The plant requirement for nickel is small relative to other micronutrients, but its functions are very important [1]. Nickel ions, in plants, enter the active centers of many enzymes, and one of the most important is participation in the construction of the urease enzyme [1,2]. Due to the low demand of plants for this element, it is easy to exceed the limit of proper nutrition and lead to a toxic state [3,4]. This carries risks for human and animal health due to the accumulation of nickel in organisms, especially with the systematic consumption of foods containing the unfavorable metal [5,6]. As a result of thriving industry, the environment, and especially the soil, is becoming contaminated with heavy metals. Nickel enters the environment mainly from the metallurgical industry, sludge effluents, and agriculture (mainly with fertilizers and crop protection products). In view of the high threat posed by the high content of heavy metals in soils and substrates, the world is looking for ways to reduce their content and spread, as well as uptake by plants. Due to the high cost and lengthy process of cleaning the soil of heavy metals, ways are being sought to reduce the uptake of these elements by plants [7]. Many authors report that one way to reduce the availability of heavy metals to plants can be the use of organic matter and the properties of humus [8,9,10]. The effect of organic matter on the detoxification of cadmium and lead has been found by many researchers, while there is little research on the effect on the detoxification of nickel.

In view of the above, the main objective of the research conducted was to determine the effect of different organic materials (brown coal, high peat, and wheat straw) with varying degrees of decomposition (measured by the amount of organic carbon in the substrates) on the nickel content of edible parts of butter lettuce.

This research involved the addition of 30% of these organic materials to the mineral soil, based on preliminary research and research by Bosiacki and Tyksiński [11,12], in which a positive effect of Cd and Pb detoxification was obtained in selected vegetable species.

Organic materials most frequently used in horticultural vegetable crops were selected for research. When selecting organic materials, various degrees of decomposition were suggested. It takes about 1.4 years to completely mineralize straw, 4 to 13 years for high peat, and more than 13 years for brown coal [13].

In the process of humification of these materials, various qualities of humus are obtained (the content of humic, fulvic, and hymatomelan acids).

The content of these acids influences the achievement of more or less stable organic complexes, with functional groups such as -COOH, -OH, and C=O influencing the detoxification of heavy metals [14,15].

2. Materials and Methods

Vegetation experiments were carried out during the three years of this study (2018–2020) in an unheated greenhouse of the Department of Plant Physiology at the Experimental Station of the Horticultural Departments of the Faculty of Agriculture, Horticulture and Bioengineering—Marcelin during the spring–summer season. Lettuce, the edible part of which is leaves, namely butter lettuce (Lactuca sativa L.) of the cultivar ‘Zeralda’, was selected for this study. Seeds of butter lettuce of the ‘Zeralda’ variety came from the Vilmorin Garden Sp. z o.o. company (Poznan, Poland). The variety is intended for year-round cultivation. It is characterized by a fairly large, light yellow-green head, tolerates unfavorable growing conditions and high temperatures well, does not break out into a seed shoot, the ends of the leaves do not dry out, is very tasty, and tolerates transport well. It is also resistant to aphids and powdery mildew.

The factors of the experiment were different types of organic material added to the mineral soil—loamy sand (high peat, brown coal, wheat straw), increasing doses of nickel (50, 75, 100 mg Ni dm⁻³ of the substrate), and the years of testing (three years of testing)—and different degrees of decomposition of organic matter.

Organic matter was added to mineral soil only in the first year of this study in the amount of 30% by volume (share of mineral soil and organic matter 70/30% (v/v), preparing lettuce growing substrates for the three years of this study. Experimental vases with the addition of organic matter in which the research was conducted in the second and third years were stored in an unheated greenhouse.

Three vegetation experiments were conducted over the three years of this study. Each vegetation experiment consisted of 16 combinations, while each combination consisted of 5 replications. The replicates by species consisted of 4 lettuce plants growing in one 6 dm³ polyethylene container.

In the first week of April, in the first year of this study, the growing substrates for all years of the experiments were prepared. Experimental, polyethylene, drip-free vases were filled with mineral soil (which is the control) and mineral soil with the addition of particular types of organic matter. One liter of mineral soil weighed 1155 g, high peat 450 g, brown coal 745 g, and wheat straw 355 g. The basic component of the substrates was mineral soil—loamy sand. The granulometric composition of the mineral soil was determined using the Casagrande method with a modification by Prószynski [16], and it was 69% sand, 23% dust, and 8% clay parts. The C organic content was 1.14%.

Mineral soil was taken from the arable layer (0–20 cm) from Zlotniki near Poznań. Brown coal (2–4 mn fraction) came from the Konin Lignite Mine, Hartmann high peat (acidic pH 4.50, sphagnum, ground, fractionated), and wheat straw from the Experimental Plant in Swadzim.

The total nickel content of high peat, brown coal, and wheat straw was determined after mineralization of these organic materials using the “wet” technique in a mixture of HNO₃ and HClO₄ acids at a volume ratio of 3:1, and it was 0.82 in high peat, 0.39 in brown coal, and 0.82 mg kg⁻¹ dry weight in wheat straw.

The chemical composition of individual substrates before the introduction of nutrients and nickel is shown in

Table 1. Nutrient content, sodium content, and pH and EC of substrates before the establishment of the experiment.

mg dm−3	Mineral Soil (Loamy Sand)	ls + High Peat	ls + Brown Coal	ls + Wheat Straw
N-NH4	7	11	traces	2
N-NO3	14	4	5	9
P	16	34	28	33
K	52	82	78	89
Ca	6255	6188	6311	6198
Mg	98	52	178	87
Na	14	10	28	16
S-SO4	12	8	17	15
Cl	7	14	9	8
Fe	34.9	50.7	48.6	38.7
Mn	8.2	11.5	9.1	8.8
Cu	2.5	3.4	2.8	2.6
Zn	11.1	6.7	12.1	10.1
Ni	1.32	1.47	1.37	1.28
EC (mS cm−1)	0.193	0.190	0.211	0.197
pH (H2O)	7.15	7.03	7.20	7.09

and was determined using the universal method according to Nowosielski in 0.03 M CH₃COOH (macronutrients and sodium) [17] and by the Lindsay method (nickel, iron, manganese, copper, zinc) [18].

Based on the results of the chemical analysis of the substrates, nutrients were introduced into the substrates in the form of solutions, in the same amount each time each year, in the third week of April, before planting lettuce seedlings. The prepared chemical solutions were mixed with the tested substrates.

In the experiments, macronutrients were used by bringing to the following levels (in mg dm⁻³): 150 N—KNO₃, 150 P—KH₂PO₄, 250 K—KH₂PO₄, 130 Mg—Mg(NO₃)₂ 6H₂O. Magnesium was not added to mineral soil with brown coal, as this substrate contained a high amount of magnesium. Micronutrients were introduced into the substrates, bringing them to the following levels (in mg dm⁻³): 75 Fe—FeSO₄ 7H₂O, 15 Mn—MnSO₄ H₂O, 10 Cu—CuSO₄ 5H₂O, 20 Zn—ZnSO₄ 7H₂O.

Nickel was introduced into the tested substrates in increasing doses: 0, 50, 75, and 100 mg Ni dm⁻³ of substrate in the form of a solution prepared from nickel sulfate (NiSO₄ × 6 H₂O—262.86 g mol⁻¹), Chempur, and distilled water. The substrates prepared in this way were covered with foil until the start of cultivation in order to maintain constant humidity and prevent the growth of weeds.

Lettuce seedling was prepared in multi pots (8 × 13 pots) with 1 pot size of 3.5 × 3.5 × 4.5 cm. Klasmann peat substrate TS 1 medium basic was used to prepare the seedling. Lettuce seedlings were planted into growing containers when they reached 4–5 leaves proper. In 1 growing container, 4 plants were planted. During the growing season, an annual and one-time insecticide treatment was carried out against aphids (Aphididae sp.) and western thrips (Frankliniella occidantalis). Mospilan 20 SP was applied at a concentration of 0.04% using 10 L water/100 m² of crop. Harvesting was performed 10–11 weeks after sowing the seeds. The final yield of individual plants was weighed. The plant material was then crushed and dried at 105 °C.

2.1. Chemical Analyses of Substrates

Chemical analyses of the substrates were performed in the Department of Plant Physiology in the laboratory of the Plant Nutrition Laboratory. Substrates for chemical analysis were taken with a shortened Egner stick. Five substrate samples were taken from each combination (one average sample per container). Each substrate sample was thoroughly mixed and cleaned of root debris.

Substrate extraction for the content of N-NH₄, N-NO₃, P, K, Ca, Mg, Na, S-SO₄ (mg dm⁻³ of substrate) was performed using the universal method according to Nowosielski (extraction with 0.03 M CH₃COOH) [17]. Subsequently, ammonium and nitrate nitrogen were determined by the microdistillation method (according to Bremner with Starck’s modification); P was determined by the vanadium–molybdenum method; Mg, Ca, K, Na were determined by the FAAS atomic absorption method, (acetyl–air flame wavelength: 285.2 nm, burner—single-slit), with a Zeiss AAS-5 spectrophotometer; and Cl and S-SO₄ were determined by the nephelometric method.

Electric conductivity EC (mS cm⁻¹) was measured with a conductivity meter, while pH was measured with a pH meter (potentiometrically) [19].

Micronutrients Fe, Zn, Mn, Cu, and Ni were extracted from the substrate using the Lindsay method (mg dm⁻³) [18]. Nickel, iron, zinc, copper, and manganese were then determined by the FAAS atomic absorption method (acetylene–air flame, torch—single slit) with a Zeiss AAS-5 spectrophotometer. The Iszczerek method was used to determine organic carbon content, while the determination of sorption capacity was determined using the Kappen method [20].

2.2. Analysis of Nickel Content in Lettuce Leaves

Mineralization of plant material for the determination of nickel content in lettuce leaves was carried out using the “wet” technique in a mixture of HNO₃ and HClO₄ acids in a ratio of 3:1 [21]. The same method was used to test the reference material, certified by the IRMM (Institute for Reference Materials and Measurements) in Belgium, which was bran flour (Pseudevernia furfuracea), in each year of analysis for nickel content in the edible part of lettuce (

Table 2. Nickel content of the reference material after wet mineralization in a mixture of HNO₃ and HClO₄ acids in a ratio of 3:1.

Metal	Year of Research	Reference Material
		Certified Content	+/−	Content Obtained	Difference
Ni	I year	2.47	0.07	2.34	−0.13
	II year			2.21	−0.26
	III year			2.28	−0.19

).

Determination of Ni content (mg kg⁻¹ s.m.) was carried out by the FAAS atomic absorption method (acetylene–air flame, torch—single-slot) with a Zeiss AAS-5.

2.3. Nickel Toxicity Index

In the study conducted herein, an index of the toxic effects of nickel on the yield of lettuce leaves was used, called the tolerance index (Ti). The value of the tolerance index is the ratio of the amount of obtained yield of edible parts of lettuce growing in nickel-contaminated substrate to the amount of yield found in plants growing in nickel-uncontaminated substrate. The following values were adopted for the tolerance index (Ti):

-

T_i < 1—yield inhibition;

-

T_i = 1—no effect of increased nickel content in the substrate on yields;

-

T_i > 1—positive effect of nickel in the substrate on yielding.

2.4. Statistical Analysis

Statistical evaluation was performed using STAT BAT software (single-variate analysis of variance for factorial orthogonal experiments). Differences between mean values were determined using Duncan’s test at a significance level of α = 0.05. The statistical analysis of the test results concerned 48 combinations, and each combination consisted of 5 replications. The replicates by species consisted of 4 lettuce plants growing in one polyethylene container with a capacity of 6 dm³. The letters next to the result indicate the statistical significance of the combination, and those followed by the same letters do not differ significantly at α = 0.05.

Statistical evaluation involved three-factor analysis of variance for nickel content in lettuce leaves, as well as nickel content, C organic content, and sorption capacity in substrates after lettuce cultivation.

3. Results

3.1. Tolerance Index (Ti) of Lettuce to Increasing Doses of Nickel

After growing lettuce in mineral soil contaminated with increasing doses of nickel, the positive effect of this metal on yield was found in the third year of this study at an applied nickel dose of 50 mg dm⁻³ and in the first and second years of this study at a nickel dose of 100 mg dm⁻³ (Figure 1), obtaining a tolerance index value above one.

After growing lettuce in mineral soil to which high peat was introduced, contaminated with increasing doses of nickel, the positive effect of this metal on yield was found in the second and third years of this study at an applied dose of 50 mg Ni dm⁻³, 75 mg Ni dm⁻³, and in the second and third years of this study at a nickel dose of 100 mg dm⁻³ (Figure 2), obtaining a tolerance index value above one.

After growing lettuce in mineral soil to which brown coal was introduced, contaminated with increasing doses of nickel, the positive effect of this metal on yield was found in the first year of this study at applied doses of 50, 75, and 100 mg Ni dm⁻³, in the second year of this study at an applied dose of 100 mg Ni dm⁻³, and in the third year of this study at nickel doses of 75 and 100 mg Ni dm⁻³ (Figure 3), obtaining a tolerance index value above one.

Growing lettuce in mineral soil to which wheat straw was introduced, contaminated with increasing doses of nickel, the positive effect of this metal on yield was found in the first and third years of the study at the applied doses of 50 and 100 mg Ni dm⁻³, and in the first year of the study at a nickel dose of 75 mg dm⁻³ (Figure 4), obtaining a tolerance index value above one.

3.2. Nickel Content in Lettuce Leaves

Comparing the average nickel content in lettuce leaves growing in the tested substrates regardless of the nickel doses used and the years of this study, the lowest nickel content (10.41 mg kg⁻¹) was found in the leaves of plants growing in mineral soil with added brown coal, while the highest (12.04 mg kg⁻¹) nickel content was found in mineral soil with added straw, and it was 15.66% higher than the lowest result obtained (

Table 3. Nickel content (mg kg⁻¹) in lettuce leaves.

Substrate		Dose of Ni (mg dm−3)	Year of Research				Mean B·C		Mean B
			I		II	III
mineral soil (ls)		0	3.28 *a		5.69 b	5.15 b	4.71 a		11.04 b
		50	8.89 cd		13.18 jk	10.26 efg	10.78 c
		75	11.21 f–i		15.85 m	12.04 hij	13.03 d
		100	17.66 op		15.80 m	13.43 jk	15.63 g
Mean B·A			10.26 bc		12.63 f	10.22 b
ls + high peat		0	2.97 a		5.58 b	4.95 b	4.50 a		11.61 c
		50	9.90 def		13.24 jk	10.42 efg	11.19 c
		75	12.17 hij		16.65 mno	12.52 ij	13.78 e
		100	16.81 mno		18.28 pq	15.83 m	16.97 h
Mean B·A			10.46 bc		13.44 g	10.93 dc
ls + brown coal		0	2.80 a		5.68 b	4.93 b	4.47 a		10.41 a
		50	9.53 cde		10.52 efg	9.54 cde	9.86 b
		75	14.39 kl		13.03 jk	10.97 fgh	12.79 d
		100	15.73 m		15.48 lm	12.36 ij	14.52 f
Mean B·A			10.61 bcd		11.18 de	9.45 a
ls + wheat straw		0	2.70 a		5.50 b	4.82 b	4.34 a		12.04 d
		50	8.53 c		14.04 k	11.40 ghi	11.32 c
		75	13.93 k		17.60 nop	14.14 k	15.23 fg
		100	16.27 mn		19.24 q	16.31 mno	17.27 h
Mean B·A			10.36 bc		14.09 h	11.67 e
Mean A			10.42 a		12.83 b	10.56 a
Mean C		dose 0 Ni (I–III)		dose 50 Ni (I–III)		dose 75 Ni (I–III)		dose 100 Ni (I–III)
		4.50 a		10.79 b		13.71 c		16.62 d
Mean A·C	year	dose 0 Ni		dose 50 Ni		dose 75 Ni		dose 100 Ni
	I	2.94 a		9.21 d		12.93 f		16.62 i
	II	5.61 c		12.75 f		15.78 h		17.20 i
	III	4.96 b		10.40 e		12.42 f		14.48 g

A—year, B—substrate, C—dose; * followed by the same letters do not differ significantly at α = 0.05.

).

Increasing nickel doses used in this study significantly increased the nickel content of lettuce leaves.

Analyzing the average nickel content in lettuce leaves, obtained under the influence of the tested nickel doses, regardless of the substrates used and the individual years of the study (mean C), the lowest content (4.50 mg kg⁻¹) was found in leaves growing in substrates without nickel addition. A nickel dose of 50 mg dm⁻³ increased by 139.78% the nickel content of the leaves, while a dose of 75 mg Ni dm⁻³ and 100 mg Ni dm⁻³ successively increased by 204.67% and 269.34% the nickel content of lettuce relative to lettuce leaves obtained from plants growing in substrates without nickel addition.

The average nickel content in lettuce leaves harvested from plants grown in the different substrates without nickel addition did not differ significantly during the three years of this study.

Comparing the average nickel content during the three years of this study, which was obtained in lettuce leaves grown in the tested substrates contaminated with 50 mg Ni dm⁻³, the lowest content was found in the leaves of plants grown in mineral soil with added brown coal (9.86 mg kg⁻¹), while the highest (11.32 mg kg⁻¹) content was found in mineral soil with added straw. A similar result was obtained by contaminating the substrates with a dose of 75 mg Ni dm⁻³, finding the lowest content of this metal (12.79 mg kg⁻¹) in lettuce leaves growing in mineral soil with the addition of brown coal, and it did not differ significantly from the nickel content found in lettuce leaves growing in mineral soil alone contaminated with the same dose of nickel. The highest nickel content (15.23 mg kg⁻¹) was found in the leaves of lettuce growing in mineral soil with the addition of straw. An identical relationship was found in plants growing in the tested substrates contaminated with 100 mg Ni dm⁻³, where the lowest nickel content in leaves was obtained when lettuce was grown in mineral soil with added brown coal (14.52 mg kg⁻¹), while the highest (17.27 mg kg⁻¹) content was found in leaves of lettuce grown in mineral soil with added straw.

Comparing the effect of each year of the study and the nickel doses used, regardless of the effect of the substrates tested, a higher content of this metal was found within each tested nickel dose in lettuce leaves harvested in the second year of cultivation.

3.3. Nickel Content (mg dm⁻³) in Substrates After Lettuce Cultivation

After lettuce cultivation, regardless of the nickel doses used and the years of study (

Table 4. Nickel content (mg dm⁻³) in substrates after lettuce cultivation.

Substrate		Dose of Ni (mg dm−3)	Year of Research				Mean B·C		Mean B
			I		II	III
mineral soil (ls)		0	0.55 *a		0.63 a	0.61 a	0.60 a		8.23 a
		50	6.45 bc		6.42 bc	7.56 c	6.81 b
		75	10.16 de		10.77 de	11.34 ef	10.76 d
		100	14.84 hi		15.06 hi	14.36 h	14.75 e
Mean B·A			8.00 a		8.22 a	8.47 a
ls + high peat		0	0.56 a		0.89 a	0.52 a	0.65 a		12.80 b
		50	10.14 de		12.52 fg	9.36 d	10.67 d
		75	19.59 lm		18.43 kl	12.47 fg	16.83 f
		100	25.08 p		22.27 o	21.79 no	23.05 h
Mean B·A			13.84 cd		13.53 c	11.04 b
ls + brown coal		0	0.65 a		0.91 a	0.57 a	0.71 a		12.65 b
		50	9.39 d		9.47 d	6.04 b	8.30 c
		75	17.40 k		15.92 ij	11.09 e	14.80 e
		100	35.52 s		28.94 q	15.89 ij	26.78 j
Mean B·A			15.74 e		13.81 cd	8.40 a
ls + wheat straw		0	0.49 a		0.99 a	0.64 a	0.71 a		13.95 c
		50	6.91 bc		13.71 gh	12.80 g	11.14 d
		75	17.24 jk		20.69 mn	19.41 lm	19.11 g
		100	21.31 no		22.42 o	30.83 r	24.85 i
Mean B·A			11.49 b		14.45 d	15.92 e
Mean A			12.27 b		12.50 b	10.96 a
Mean C		dose 0 Ni (I–III)		dose 50 Ni (I–III)		dose 75 Ni (I–III)		dose 100 Ni (I–III)
		0.67 a		9.23 b		15.37 c		22.36 d
Mean A·C	year	dose 0 Ni		dose 50 Ni		dose 75 Ni		dose 100 Ni
	I	0.56 a		8.22 b		16.10 f		24.19 i
	II	0.85 a		10.53 d		16.45 f		22.17 h
	III	0.59 a		8.94 c		13.58 e		20.72 g

A—year, B—substrate, C—dose; * followed by the same letters do not differ significantly at α = 0.05.

), the lowest average nickel content of 8.23 mg dm⁻³ was found in mineral soil without the addition of organic matter. The introduction of organic matter in the form of tall peat, brown coal, and wheat straw significantly increased the amount of nickel compared with the content obtained in mineral soil without the addition of organic matter. The highest average content of the element in question (13.95 mg dm⁻³) was obtained in the substrate with the addition of wheat straw, and it was 69.50% higher than the content found in the mineral soil alone. Compared with the amount of nickel found in the mineral soil, the content of this metal in the substrate into which high peat was introduced and in the substrate with the addition of brown coal was 55.53% and 53.71% higher, respectively.

Regardless of the substrates used and the individual years of this study, it was found that with an increasing dose of nickel, the content of this element in the substrates increased. Analyzing the effect of the applied nickel dose over the years studied, the following relationships were found: In the substrates without nickel addition (0 mg Ni dm⁻³), the content of the tested metal over the three years of this study did not differ significantly from each other. In the substrates with the addition of 50 mg Ni dm⁻³, the lowest amount of nickel (8.22 mg dm⁻³) was obtained in the first year of cultivation, while the highest amount was obtained in the second year of cultivation. A 28.10% increase in nickel content was found in the second year of cultivation compared with the lowest content obtained in the first year of lettuce cultivation. In the substrates with an addition of 75 mg Ni dm⁻³, the lowest amount of nickel (13.58 mg dm⁻³) was obtained in the third year of cultivation, while the highest amount was obtained in the first (16.10 mg dm⁻³) and second year of cultivation (16.45 mg dm⁻³). In the first and second years of lettuce cultivation, a higher content of nickel was found in the substrates obtaining, respectively, an 18.55% and 21.13% increase in the tested element in relation to the content found in the substrates in the third year of this study. In the substrates to which nickel was applied at 100 mg dm⁻³, the lowest content (20.72 mg dm⁻³) was found in the third year of cultivation, while the highest nickel content (24.19 mg dm⁻³) was obtained in the first year of this study and was 16.75% higher than the content obtained in the third year of lettuce cultivation. Taking into account the effect of successive years of lettuce cultivation, regardless of the type of substrate and the doses of nickel used, the lowest content of this metal was found (10.96 mg Ni dm⁻³) in the substrates in the third year of this study.

3.4. Organic Carbon Content in Substrates After Lettuce Cultivation

The introduction of organic matter into the mineral soil significantly affected the organic carbon content of the tested substrates (

Table 5. Organic carbon content (%) in substrates after lettuce cultivation.

Substrate		Dose of Ni (mg dm−3)	Year of Research				Mean B·C		Mean B
			I		II	III
mineral soil (ls)		0	1.01 *ab		1.14 b	1.02 ab	1.06 a		1.01 a
		50	0.96 a		1.05 ab	0.95 a	0.99 a
		75	1.02 ab		1.07 ab	0.93 a	1.01 a
		100	1.02 ab		1.04 ab	0.93 a	1.00 a
Mean B·A			1.00 ab		1.07 b	0.96 a
ls + high peat		0	2.06 cde		2.15 de	2.06 cde	2.09 bc		2.07 c
		50	2.04 cde		2.04 cde	2.04 cde	2.04 bc
		75	2.07 cde		2.16 e	2.06 cde	2.10 c
		100	2.03 cde		2.15 de	2.00 cde	2.06 bc
Mean B·A			2.05 c		2.12 d	2.04 c
ls + brown coal		0	2.01 cde		2.04 cde	2.04 cde	2.03 bc		2.01 b
		50	2.01 cde		2.01 cde	2.04 cde	2.02 bc
		75	1.95 c		1.98 cd	2.06 cde	2.00 b
		100	2.03 cde		1.97 c	2.01 cde	2.00 bc
Mean B·A			2.00 c		2.00 c	2.04 c
ls + wheat straw		0	0.99 ab		1.08 ab	1.02 ab	1.03 a		1.03 a
		50	0.99 ab		1.05 ab	1.02 ab	1.02 a
		75	1.05 ab		1.05 ab	1.04 ab	1.05 a
		100	1.05 ab		1.08 ab	0.98 ab	1.04 a
Mean B·A			1.02 ab		1.07 b	1.01 ab
Mean A			1.52 a		1.56 b	1.51 a
Mean C		dose 0 Ni (I–III)		dose 50 Ni (I–III)		dose 75 Ni (I–III)		dose 100 Ni (I–III)
		1.55 a		1.52 a		1.54 a		1.52 a
Mean A·C	year	dose 0 Ni		dose 50 Ni		dose 75 Ni		dose 100 Ni
	I	1.52 ab		1.50 a		1.52 ab		1.53 ab
	II	1.60 b		1.54 ab		1.56 ab		1.56 ab
	III	1.53 ab		1.51 a		1.52 ab		1.48 a

A—year, B—substrate, C—dose; * followed by the same letters do not differ significantly at α = 0.05.

). The lowest organic carbon content (1.01%), regardless of the nickel doses used and the years of study, was found in the substrate that was a mineral soil without an addition of organic matter. This content was not significantly different from the organic carbon content obtained in the substrate with an addition of wheat straw. Compared with the organic content of mineral soil without an addition of organic matter, 104.95% more organic carbon was found in the substrate with an addition of high peat and 99.01% more in the substrate with an addition of brown coal. The mineral soil to which high peat was introduced had the highest organic carbon content of all the substrates tested, at 2.07% C_org.

In the mineral soil without an addition of organic matter, regardless of the nickel doses used, the highest organic carbon content (1.07%) was found in the second year of lettuce cultivation, while the lowest (0.96%) content was found in the third year of this study. In mineral soil to which high peat was introduced, regardless of nickel doses, the highest organic carbon content (2.12%) was found in the second year of lettuce cultivation, while it was significantly lower in the first (2.05%) and third (2.04%) years of this study. No significant differences in organic carbon content were found in the mixture of mineral soil and brown coal, regardless of the nickel doses applied. A similar relationship was found in the soil with an addition of wheat straw.

Analyzing the amount of organic carbon obtained in the substrates in successive years of this study, regardless of the nickel doses used and the type of substrate, the highest (1.56%) content was found in the second year of lettuce cultivation, while a significantly lower content was found in the first (1.52%) and third (1.51%) years of this study.

Analyzing the effect of nickel dose, regardless of the type of substrate and the years of this study, no significant effect was found on the organic carbon content of the substrates after lettuce cultivation.

3.5. Sorption Capacity (T) in Substrates After Lettuce Cultivation

The sorption capacity, denoted by the letter T, is the sum of all exchangeable cations that the soil can hold. The more colloids in a soil, primarily the clay fraction and humus, the higher the sorption capacity of that soil. The addition of high peat and brown coal, regardless of the nickel doses used and the years of this study, significantly affected the increase in the average sorption capacity of the substrates after lettuce cultivation (

Table 6. Sorption capacity T (cmol (+) kg⁻¹) in substrates after lettuce cultivation.

Substrate		Dose of Ni (mg dm−3)	Year of Research				Mean B·C		Mean B
			I		II	III
mineral soil (ls)		0	49.79 *b–i		49.71 a–g	50.00 d–j	49.83 ab		49.74 a
		50	49.75 b–h		49.46 ab	49.85 b–i	49.69 a
		75	49.75 b–h		49.64 a–e	49.90 c–i	49.76 a
		100	49.75 b–h		49.35 a	49.90 c–i	49.67 a
Mean B·A			49.76 bc		49.54 a	49.91 cd
ls + high peat		0	50.18 i–l		49.95 c–i	50.05 f–j	50.06 c		50.02 b
		50	50.08 g–k		49.99 d–j	49.90 c–i	49.99 bc
		75	50.35 j–m		49.65 a–f	50.14 h–k	50.05 bc
		100	50.03 e–j		49.98 c–j	49.98 c–j	49.99 bc
Mean B·A			50.16 e		49.89 cd	50.02 de
ls + brown coal		0	51.65 p		50.65 m	50.53 lm	50.94 e		50.83 c
		50	51.55 op		50.65 m	50.55 lm	50.92 e
		75	51.25 no		50.55 lm	50.45 klm	50.75 de
		100	51.18 n		50.58 m	50.35 j–m	50.70 d
Mean B·A			51.41 g		50.61 f	50.47 f
ls + wheat straw		0	49.94 c–i		49.76 b–h	49.91 c–i	49.87 abc		49.77 a
		50	49.85 b–i		49.58 abc	49.85 b–i	49.76 a
		75	49.83 b–i		49.60 a–d	49.75 b–h	49.73 a
		100	49.80 b–i		49.65 a–f	49.75 b–h	49.73 a
Mean B·A			49.85 cd		49.65 ab	49.82 bc
Mean A			50.29 c		49.92 a	50.05 b
Mean C		dose 0 Ni (I–III)		dose 50 Ni (I–III)		dose 75 Ni (I–III)		dose 100 Ni (I–III)
		50.18 b		50.09 ab		50.07 a		50.02 a
Mean A·C	year	dose 0 Ni		dose 50 Ni		dose 75 Ni		dose 100 Ni
	I	50.39 g		50.31 fg		50.29 efg		50.19 def
	II	50.02 a–d		49.92 ab		49.86 a		49.89 ab
	III	50.12 cde		50.04 a–d		50.06 bcd		49.99 abc

A—year, B—substrate, C—dose; * followed by the same letters do not differ significantly at α = 0.05.

). The highest substrate sorption capacity was found in mineral soil with an addition of brown coal.

In lettuce cultivation, the addition of high peat will significantly affect the sorption capacity compared with mineral soil without the addition of organic matter, while there were no significant differences in the sorption capacity found in the substrate with an addition of wheat straw compared with the sorption capacity in mineral soil (Table 6). Analyzing the sorption capacity that was obtained in the following years of this study in the substrates, regardless of the nickel doses used and the type of substrate, the lowest sorption capacity was found in the second year of lettuce cultivation. In contrast, the highest value of the studied parameter was obtained in the first year of lettuce cultivation.

Analyzing the effect of nickel dose, regardless of the type of substrate and years of testing, on the sorption capacity of the substrate, the highest sorption capacity was obtained in substrates without nickel addition (0 mg Ni dm⁻³).

4. Discussion

The availability of heavy metals to plants is influenced by granulometric composition, soil type, sorption properties, pH, oxidoreduction potential, and organic matter content, among other factors [22]. Krishnamurti and Naidu [15], on the basis of their studies, found that heavy metals in soil can be bound interchangeably, specifically bound to carbonates, to amorphous mineral colloids, in the metal–humic acid complex, in the metal–fulvic acid complex, in the bonds of easily reducible metal oxides, with crystalline Fe oxide, organically, and with aluminosilicates. There is a need in the world to carry out research to seek methods of detoxifying heavy metals in soils.

Soil organic matter consists of organic compounds with different chemical compositions and reactive properties. The immobilization of metals by organic matter in the soil is due to its ability to exchange cations and its ability to form chelating compounds and simple compounds with heavy metal ions [14]. The quality and quantity of organic matter in the soil depends on the material from which it is derived, such as plant residues, animal residues, and microbial residues, among other things, and also depends on the types of mechanical treatments performed [23].

Organic materials with different decomposition times in the form of high peat, brown coal, and wheat straw were used in this study. In the humification process, the tested organic materials create humus of various qualities in the soil, with different ratios of fulvic to humic acids, which influence the detoxification of heavy metals. Based on the results, it was found that the introduction of brown coal and high peat into mineral soil in lettuce cultivation increased the organic carbon content of the substrates and increased the total sorption capacity of these substrates.

To reduce the bioavailability and mobility of heavy metals in the soil, often-recommended organic materials include granulated or powdered brown coal, peat, the straw of various grains, and other materials, such as sawdust, bark, manure, or various types of composts. In conducting their research, many scientists have found the effect of various organic materials introduced into the soil to be a source of organic matter, detoxifying heavy metals such as cadmium, lead, zinc, and copper and reducing the content of these metals in the aboveground parts of plants. Confirmation of this is provided by the studies of [11,12,24,25,26,27]. However, research on the effect of organic matter on the mobility of heavy metals in the soil and their uptake by plants is inconclusive. Organic matter can immobilize heavy metals, but it can also stimulate their release [28].

This is evidenced by a study by Rupa et al. [29], who found that high concentrations of organic matter increased the uptake of heavy metals from the soil by wheat. As reported by Alashty et al. [30], the addition of municipal waste to the soil for a period of 3 years, despite increasing the amount of organic matter in the soil, increased the total content and the content of the bioavailable form of nickel as the amount of compost applied and subsequent years of application increased. This significantly increased the content of this heavy metal in lettuce and radish.

As reported by Murray et al. [8], the addition of compost to the soil increased the accumulation of lead and cadmium in vegetables. The increased metal content in plants can be explained by the high ratio of fulvic to humic acids in the applied compost. The authors claim that fulvic acid, unlike humic acid, can increase the availability of metals to plants [8]. According to Tejada et al. [31], humic acids have a higher adsorption capacity for nickel, unlike fulvic acids. Comparing the two acids, humic acid has proportionally fewer carboxyl groups [32,33].

Lobartini et al. [34] claim that humic acids, compared with fulvic acids, are more effective at complexing and chelating metals. They also claim that humic acids have much larger molecules and are more complex, which translates into a greater number and binding capacity, in contrast to the smaller and less complex fulvic acids.

Ongoing research into the effect of organic matter introduced into the soil on the nickel content of plants suggests that it may have an impact on reducing the content of this metal. According to Ociepa [35], fertilization with brown coal, as well as with a mixture of sewage sludge and brown coal in corn and Pennsylvania hogweed crops, lowered the bioavailability of nickel by 12–26% compared with the control. In the study, brown coal was applied at 25 t ha⁻¹, as well as the same amount of sewage sludge–brown coal mixture (at a ratio of 2.6:1).

Kuziemska et al. [36] found that the use of organic materials and lime in grass cultivation resulted in a decrease in the amount of nickel in the aboveground parts of the plants and a decrease in the content of the exchangeable fraction, as well as an increase in the proportion of nickel in the oxidizable and reducible fraction (addition of organic materials) and the residual fraction (liming).

In a study by Rehman et al. [37], organic additives in the form of biocarbon, cattle manure, and compost were used to evaluate the effect on nickel uptake by corn. The authors showed that the greatest reduction in nickel concentration in the shoots of the test plant was achieved under the influence of the addition of cattle manure (2% organic carbon), followed by the influence of the addition of biocarbon (2% organic carbon), obtaining, respectively, a reduction of 73.2% and 61.1% compared with the control. They also suggest that the effect of soil organic matter addition on Ni stabilization may be due to the high capacity of the for-cation exchange, an increase in soil pH, and an increase in micronutrient uptake by plants.

In a study by Naveed et al. [10], the application of fresh and composted manure reduced the stress and salinity associated with the presence of nickel in the soil, while composted manure reduced the uptake of nickel by canola.

In our own research, using various organic materials and studying their effects on nickel content in lettuce leaves, we found that among the organic materials tested, the addition of 30% brown coal to mineral soil reduced nickel content in lettuce leaves to the greatest extent. The authors explain the reason for this phenomenon as the formation of a larger amount of humic acids in the humification process of brown coal, which chelates with nickel more effectively and stably, limiting its availability for plant root systems. The addition of high peat and wheat straw to mineral soil increased the nickel content of lettuce leaves. This study concluded that the type of organic material introduced into the mineral soil as a source of organic matter for nickel detoxification is important.

Nickel is a nutrient needed for proper plant growth and development [1]. Researchers around the world are conducting studies to determine optimal levels of nickel for growing plants in soils, substrates, and soilless (hydroponic and aeroponic) conditions to achieve optimal qualitative and quantitative yields. In a study by Balaguer et al. [38], yield was found to decrease as the concentration of nickel increased from 0 to 20 mg Ni dm⁻³ in the nutrient solution. Also, in an experiment conducted by Kumar et al. [39], with an increase in the concentration of nickel applied (0–50 µM), a decrease in tomato fruit yield was shown. Palacios et al. [40], in a study on soil enrichment with sewage sludge, applied nickel to substrates at doses of 60, 120, and 240 mg kg⁻¹ of substrate. Only the 240 mg kg⁻¹ dose had an effect on yield, which reduced tomato fruit yield and quality. In contrast, in a study by Poulik [41], no negative effect of a nickel dose of 0 to 168 mg kg⁻¹ was observed on the growth, development, or yield of lettuce and tomato. In a study by Matraszek et al. [42], the addition of nickel at 10 mg kg⁻¹ to sand caused a decrease in lettuce yield.

The results obtained in the course of the present study on the tolerance index for the fresh weight of lettuce indicate a positive effect of applied nickel doses on yield in some experimental combinations. The results obtained indicate the need to continue research on the determination of optimal nickel content in substrates and soils for the cultivation of horticultural crops.

5. Conclusions

• The type of organic material introduced into the mineral soil as a source of soil organic matter is important for the nickel content of lettuce leaves.
• The introduction of organic matter into mineral soil can reduce, as well as increase, nickel content in lettuce leaves.
• Brown coal introduced at 30% by volume into mineral soil reduces the nickel content of lettuce leaves, increases the organic carbon content of the soil, and affects the higher sorption capacity of the substrates.
• High peat introduced at 30% by volume to mineral soil increases the nickel content of lettuce leaves, increases the organic carbon content of the substrate, and affects the higher sorption capacity of the substrates.
• Wheat straw introduced at 30% by volume into the mineral soil increases the nickel content of lettuce leaves and does not increase the organic carbon content or sorption capacity of the substrates.