Effect of Catch Crops and Tillage Systems on the Content of Selected Nutrients in Spring Wheat Grain

Abstract

This paper presents the effects of catch crops (white mustard, lacy phacelia, and a mixture of legumes—faba bean + spring vetch) and two tillage systems (plough tillage and no-tillage) on some quality parameters of spring wheat grain. A field experiment in growing spring wheat in monoculture was conducted in the period 2016–2018 in Czesławice (central part of the Lublin region, Poland). An assumption was made that the nutritional composition of wheat grain could be influenced already at the stage of selection of agronomic practices by modifying the soil chemical and enzyme composition—being the “starting point” for grain quality. It was proven that all the catch crops tested in this study contributed to an improvement in the chemical composition of the soil used in the experiment (a significant increase in humus, P and Mg content). Both the catch crops and the conservation (no-tillage) system stimulated the activity of soil enzymes: dehydrogenase and urease. This resulted in more favorable soil conditions for spring wheat grown in monoculture. The cultivation of the catch crops (particularly white mustard) indirectly contributed to an increased content of dietary fiber and o-dihydroxyphenols in wheat grain. Moreover, the content of most of the amino acids determined and the essential amino acid index (EAAI) in wheat grain were found to be more favorable in the treatments with the catch crops. The highest content of all the macro- and micronutrients analyzed in wheat grain was found in the catch crop treatments (especially that with white mustard). It should be noted that spring wheat responded favorably to the reduced tillage system (no-tillage). Since the grain content of o-dihydroxyphenols, magnesium, calcium, copper, manganese, iron, selenium, and some amino acids (especially essential ones: Lys, Met, Trp) was found to be higher compared to plough tillage.

1. Introduction

Across the world, cereal grain is an essential raw material for the production of many foodstuffs. It is the main source of valuable dietary nutrients such as: hydrocarbons, protein, dietary fiber, vitamins, minerals, phenolic compounds, inulin, sterols, etc. [1,2,3,4]. Agricultural producers and consumers are more and more aware of the need to both produce and consume healthy food free of chemicals and to take care of the environment in which we live [5,6]. At the global scale, wheat (both winter and spring wheat) has an enormous importance in human nutrition. The possibility of influencing wheat grain quality through agronomic practices is still a current topic, especially in the context of the “fashion for ecology”, healthy lifestyle, and food safety [7].

Improvement in the soil environment (physical, chemical, and biological properties), particularly under monoculture, can be achieved through catch cropping. Ploughed-in catch crop biomass, or left in the field as mulch, provides, among others, favorable soil temperature and maintains soil moisture content at a higher level. Catch crops improve the soil availability of macro- and micronutrients available to plants and also stimulate soil enzymatic activity. This is of special importance for the quality of agricultural produce harvested from such crop stands [8,9]. Furthermore, catch crops are a factor that reduces soil nitrogen losses. They contribute to reduced occurrence of fungal pathogens in soil. Due to their allelopathic effects, they also reduce weed incidence. Such environmentally friendly farming exerts a lower pressure on the natural environment and climate change (catch crops increase the possibility of CO₂ sequestration by plant cover). Therefore, catch crops indirectly affect the quality of agricultural produce obtained [10,11,12,13].

On the other hand, apart from efforts to improve agricultural produce quality, agriculture seeks to reduce energy consumption of agricultural production by, among others, replacing the conventional plough tillage system by no-tillage (conservation) technology. Apart from the economic aspect, conservation tillage has been found to increase soil organic matter content and thus to enhance soil biological activity. Conservation tillage also decreases the risk of nutrient leaching beyond an agricultural ecosystem and reduces water and air erosion [14,15,16,17,18,19,20,21]. This study hypothesized that the quality of spring wheat grain yield could be influenced already at the stage of selection of agronomic practices (tillage system, catch cropping) by modifying soil conditions. Importantly, such an approach does not require special financial inputs from a wheat producer and is environmentally friendly. An assumption was made that the use of phytosanitary and allelopathic effects of catch crops on soil would indirectly contribute to improved quality (chemical) parameters of grain of spring wheat grown in monoculture. It was also expected that conservation tillage would promote the creation of a favorable soil environment rich in nutrients available to plants. Due to this, it would be possible to obtain good-quality grain yields.

The aim of the present study was to determine the effects of crop species grown as a catch crop and of tillage reductions (no-tillage relative to plough tillage) on the selected chemical components of spring wheat grain.

2. Materials and Methods

2.1. Experimental Site

A field experiment on monoculture cropping of spring wheat cv. ‘Monsun’ (Triticum aestivum L.) was established in 2015, while the study results included in this paper were collected over the period 2016–2018 (three-year monoculture). The experiment was conducted at the Czesławice Experimental Farm, belonging to the University of Life Sciences in Lublin (Poland), on loess soil with the grain size distribution of silt loam and classified as good wheat soil complex (soil class II, pH = 6.2). The experiment was set up as a split-plot design with 5 replicates in 27 m² (3 m × 9 m) plots for sowing and 16 m² (2 m × 8 m) harvesting plots.

2.2. Experimental Design

The design of the experiment comprised two factors: I. Type of catch crop in a spring wheat monoculture: A—control treatment (without catch crop); B—white mustard cv. ‘Borowska’ (Sinapis alba L.); C—lacy phacelia cv. ‘Stala’ (Phacelia tanacetifolia Benth.); D—faba bean cv. ‘Amulet’ (Vicia faba L. ssp. minor) + spring vetch cv. ‘Hanka’ (Vicia sativa L.). II. Tillage practices used after harvest of the spring wheat and before sowing of the catch crops, and subsequently, tillage practices after harvest of the catch crops and before sowing the cereal crop:

• Conventional plough tillage—after harvest of the spring wheat crop (first decade of August), wheat straw was removed from the field, subsoil ploughing and harrowing were carried out, and subsequently, the catch crops were sown (second decade of August); after the harvest of the catch crops (October), their aboveground biomass was shredded and incorporated into the soil during autumn ploughing (25 cm deep); in the spring, a tillage unit was used, mineral NPK fertilization was applied, and spring wheat was sown by seed drill (second decade of April).
• Conservation tillage—after harvest of the spring wheat crop (first decade of August), wheat straw was removed from the field, the field was tilled with a rigid tine cultivator (grubber), the catch crops were sown (second decade of August); after the harvest of the catch crops (October), their aboveground biomass was shredded and left in the field as mulch (until March 15); in the spring, the mulch was incorporated into the soil using a disk harrow (12 cm deep), the field was smoothed with a spike tooth harrow, mineral NPK fertilization was applied, and spring wheat was sown with a seed drill with disks (second decade of April). Detailed information on the agricultural practices used in the experiment is presented in

  Table 1. The design of agronomic operations related to the cultivation of catch crops and spring wheat.

  Date of Performance	Plough Tillage	Conservation Tillage
  First decade of August	Harvest of spring wheat (stubble left in the field; straw removed from the field)	Harvest of spring wheat (stubble left in the field; straw removed from the field)
  Second decade of August	Field preparation for sowing catch crops (subsoil ploughing, harrowing), sowing of catch crops	Field preparation for sowing catch crops (no-tillage)—rigid tine cultivator (grubber), harrowing
  Second decade of October	Catch crop biomass is cut and then shredded. Incorporation of biomass into the soil (ploughing-in of biomass)	Catch crop biomass is cut and then shredded. Biomass is left on the field surface (mulch)
  Third decade of October –first decade of April	Catch crop biomass mixed with the soil decomposes into organic matter	Catch crop biomass left in the field (mulch) slowly decomposes
  Second decade of March –second decade of April	Field preparation for sowing spring wheat (tillage practices as in plough tillage)	Field preparation for sowing spring wheat (tillage practices as in conservation tillage)

  .

The selection of catch crops included in the experiment resulted from the greatest reliability in yielding and the popularity of these species in agricultural practice in Poland (Lublin region).

Table 2. Catch crop dry biomass (t·ha⁻¹) after harvest.

Specification	Tillage System
	Plough Tillage			Conservation Tillage
	2016	2017	2018	2016	2017	2018
White mustard	4.22	4.07	4.01	4.15	4.01	3.98
Lacy phacelia	4.11	3.95	3.87	4.03	3.90	3.81
Faba bean + spring vetch	2.61	2.49	2.42	2.58	2.40	2.37

presents detailed data on the dry biomass yield of catch crops in particular years of the study.

In all treatments, mineral NPK fertilization was applied (adjusted to the requirements of the spring wheat and individual catch crop species). Based on the availability of the major macronutrients in the soil used in the experiment and taking into account “economical” crop protection to be used, the following rates of mineral fertilizers (kg ha⁻¹) were applied for the individual crops included in the field experiment: spring wheat (N–60, P₂O₅–50, K₂O–80), white mustard (N-40), lacy phacelia (N-40), faba bean + spring vetch (N-40).

During the experimental years, spring wheat was sown at the optimal agronomic time for the region (second decade of April at a rate of 200 kg ha⁻¹). The sowing of catch crops was carried out in the second 10 days of August in each year. The seeding rate was as follows, respectively: white mustard—20 kg ha⁻¹, lacy phacelia—5 kg ha⁻¹, faba bean + spring vetch—100 + 40 kg ha⁻¹.

Spring wheat seeds were dressed with the seed dressing Raxil 060 FS at a rate of 50 mL 100 kg⁻¹ of seeds. The other crop protection agents were used in the lower limits of the recommended rates (in line with the “economical” crop protection strategy followed in the experiment), and they were as follows: herbicide—Sekator 6.25 WG (amidosulfuron + iodosulfuron-methyl-sodium + mefenpyr-diethyl)—0.2 kg·ha⁻¹; fungicide—Alert 375 SC (flusilazole + carbendazim)—0.9 L ha⁻¹; growth retardant—Stabilan 750 SL (chlormequat chloride)—0.9 L·ha⁻¹.

2.3. Weather Conditions at the Study Site

The course of meteorological conditions during the experiment is presented in

Table 3. The sum and distribution of precipitation (mm) in 2016–2018.

Specification	Months												Annual Sum
	I	II	III	IV	V	VI	VII	VIII	IX	X	XI	XII
Sum monthly in 2016	41.9	147.1	11.6	29.0	116.2	58.4	84.8	53.3	137.5	11.1	54.6	32.5	778.0
Sum monthly in 2017	35.3	30.7	18.9	25.4	60.2	53.1	70.6	60.5	80.4	25.7	38.8	31.6	531.2
Sum monthly in 2018	38.1	29.2	16.5	34.8	60.9	59.2	65.0	74.2	60.3	20.2	44.7	50.3	553.4
Long-term average (1966–2006)	31.5	26.9	29.6	44.5	79.5	80.2	79.4	68.6	77.6	48.7	39.8	42.4	648.7

and

Table 4. Average air temperatures (°C) in 2016–2018.

Specification	Months												Annual Average
	I	II	III	IV	V	VI	VII	VIII	IX	X	XI	XII
Mean monthly in 2016	−8.3	−2.0	2.2	8.8	13.9	17.5	20.8	20/0	11.9	4.8	6.3	−5.4	7.5
Mean monthly in 2017	−7.9	−2.8	2.1	7.7	13.5	17.1	19.2	18.6	10.8	4.8	4.7	−5.7	6.8
Mean monthly in 2018	−4.1	−1.8	2.4	8.9	14.5	17.8	20.5	20.2	12.1	5.6	6.4	−4.6	8.1
Long-term average (1966–2006)	−3.2	−2.1	2.2	7.6	13.4	16.3	17.9	17.4	13.0	8.1	2.6	−1.0	7.7

. The annual rainfall in the Experimental Farm in Czesławice in 2016 was 778 mm and was 129.3 mm higher than the long-term average. Hence, 2016 should be considered wet. In contrast, 2017 should be considered dry, as the annual rainfall was 531.2 mm and was 117.3 mm lower than the long-term average. The year 2018, similarly to 2017, was a dry one. The annual rainfall total was 533.4 mm and was lower by 95.3 mm than the long-term average (Table 3).

The distribution of average temperatures throughout the growing season was subject to the expected fluctuations. All the months in which spring wheat developed were above the perennial average, 2017 turned out to be the coldest year. To sum up, the average annual air temperature in 2017 was lower than the long-term average by 0.9 °C, in 2016, it only differed slightly (by 0.2 °C) from the multi-year period, while in 2018, it exceeded the long-term average by 0.4 °C (Table 4).

To determine the temporal and spatial variation of meteorological factors and evaluate their impact on spring wheat growth, Selyaninov’s hydrothermal coefficient (K) was calculated [22] by dividing the monthly total rainfall by one tenth of the sum of average daily temperatures for a given month (

Table 5. Hydrothermal coefficient (K) during the growing seasons of spring wheat.

Month	Year
	2016	2017	2018
IV	1.35	1.07	1.21
V	1.17	1.82	1.74
VI	1.31	0.63	0.33
VII	0.80	0.29	0.58
VIII	0.95	0.76	0.81

Hydrothermal coefficient value: K ≤ 0.5 extremely dry; 0.51–0.69 very dry; 0.70–0.99 dry; K >1 no drought.

). The calculated values of the hydrothermal coefficient show that during the first months of the growing season, wheat plants were well supplied with water, whereas in May (in the years 2017–2018), we can even speak of an excess of water. However, in the summer months of 2017 and 2018 (VI–VIII), slightly dry or dry periods were found to occur, or even a severe dry period in July 2017 and in June 2018. In this respect, the 2016 growing season is an exception because it should be considered to be favorable for growing spring wheat in terms of water and thermal conditions.

2.4. Chemical Analyses of Soil

In order to determine the comprehensive effects of catch crops and tillage system on the crop stand (2015 and 2016–2018), soil samples were taken from the 0–20 cm layer and the selected soil chemical parameters were analyzed. Soil samples were taken using a soil sampling tube from an area of 0.20 m² in each plot in the spring period (before spring wheat was sown). Soil pH was determined electrometrically in water and 1 M KCl, a carbon analyzer (SDCHN435) was used to determine organic C content, while total nitrogen was analyzed by the Kjeldahl method [23]. The content of available forms of P and K was determined by the Egner–Riehm method, whereas the Mg content using 0.01 M CaCl₂ [24]. The soil humus content was determined by alkaline extraction using the following reagents: NaOH and Na₂O₃ at concentrations of 0.1 M to 0.5 M [25].

To determine enzymatic activity, fresh soil samples with natural moisture content were collected. Samples were taken with a cylinder (100 cm³) in triplicate in each plot. Dehydrogenase activity was determined using the method developed by Lenhard, which followed the Casida procedure, and it was expressed in μmol TPF kg⁻¹· h⁻¹ [26]. The determination of urease activity followed the Tabatabai and Bremner method [27], and this activity was expressed in mmol NH₄⁺ kg⁻¹·h⁻¹ [26].

2.5. Chemical Analyses of Spring Wheat Grain

Determination of N content was carried out by the Kjeldahl method (ISO 5983-1, Animal feeding stuffs, Determination of nitrogen content and calculation of crude protein content, Part 1: Kjeldahl method, 1997). Grain amino acid content was determined by HPLC (AAA 400, Ingos, Prague, Czech Republic). The amino acids were separated by ion exchange chromatography. A 0.37 × 45 cm column was filled with ion-exchange resin (Ostion LG ANB, Tessek, Prague, Czech Republic). Amino acid identification was done using a photometric detector at a wavelength of 570 nm for all amino acids, but for proline (Pro), at 440 nm [28]. Total dihydroxyphenol content was measured spectrophotometrically at a wavelength of λ = 725 nm (Shimadzu 1800 spectrophotometer, Shimadzu Corp. Kyoto, Japan) and expressed as caffeic acid equivalents. To make the measurement on the spectrophotometer, 50 µL–500 µL of the extract (depending on the expected value of absorption of the tested sample) was transferred into a volumetric flask. A total of 2.0 mL methanol, 10 mL H₂O, 2 mL Folin reagent, and 1.0 mL of a 10% solution of Na₂CO₃ were added. The samples were put aside for 0.5 h, and subsequently, they were made up with deionized water up to the mark and measured on the spectrophotometer at a wavelength of λ = 725 nm in relation to the reference sample [29]. The phenolic concentration in the extract was expressed as caffeic acid equivalents using the following formula:

$$\mathrm{Z}(\mathrm{mg} {\mathrm{mL}}^{-1})=\frac{\mathrm{c}}{\mathrm{V}}$$

(1)

• c—phenolic content (mg) in the sample read from the calibration curve.
• V—extract volume taken for analysis (mL).

The phenolic concentration in the material was expressed as caffeic acid equivalents using the following formula:

$$\mathrm{Z}(\mathrm{g}/100 \mathrm{g})=\frac{5\mathrm{c}}{\mathrm{Vm}}$$

(2)

• c—phenolic content (mg) in the sample read from the calibration curve.
• V—extract volume taken for analysis (mL).
• m—weighed portion of material (g).

Total dietary fiber content was determined by the enzymatic gravimetric method using a Fibertec 2010 system (FOSS, Hillerød, Denmark). The sample was subjected to digestion with the following enzymes: thermostable alpha-amylase, pepsin, and pancreatin; the weight of the undigested residue was determined, and the soluble dietary fiber supernatant was precipitated from the solution and its weight was determined. Mineral analysis of the isolate was performed by atomic absorption spectrometry using a Varian Spectra A 280 FS spectrophotometer (Varian, Inc., Palo Alto, CA, USA). The conditions of determination of the individual elements were as follows: potassium emission (without lamp), wavelength 766.5 nm, slit 0.2 nm, flame acetylene/air (stoichiometric ratio); calcium Varian lamp, current 4 mA, wavelength 422.7 nm, slit 0.5 nm, flame acetylene/air (stoichiometric ratio); magnesium Varian lamp, current 4 mA, wavelength 202.6 nm, slit 1.0 nm, flame acetylene/air (stoichiometric ratio); iron Varian lamp, current 5 mA, wavelength 248.3 nm, slit 0.2 nm, flame acetylene/air (stoichiometric ratio); manganese Varian lamp, current 5 mA, wavelength 279.5 nm, slit 0.2 nm, flame acetylene/air (stoichiometric ratio); zinc Varian lamp, current 5 mA, wavelength 213.9 nm, slit 1.0 nm, flame acetylene/air (stoichiometric ratio); copper Varian lamp, current 5 mA, wavelength 324.8 nm, slit 0.5 nm, flame acetylene/air (stoichiometric ratio); selenium Varian lamp, wavelength 196.0 nm or 204 nm.

The essential amino acid index (EAAI) was calculated as the geometric mean of all essential amino acids in relation to the content of these amino acids in the egg reference protein [30]. The EAAI was calculated according to the following formula:

$$\mathrm{EAAI}=\sqrt[n]{\left(\frac{a_1}{a_{1s }}\right)\times 100\times \dots \times \left(\frac{a_n}{a_{ns}}\right)\times 100}$$

(3)

• where:
• a_n—amino acid content in the protein tested, a_ns—amino acid content in the reference protein, n—the number of essential amino acids.

2.6. Statistical Analyses

The results were analyzed statistically by analysis of variance (ANOVA). HSD (Honest Significant Difference) values were determined by Tukey’s test at p _≥ 0.05. Moreover, to determine dependencies and relationships between the studied characteristics, correlation analysis was applied. An Excel 9.0 spread sheet and the statistical package Statistica v.10.0 software for Windows StatSoft. Inc. were used to collate and statistically analyze the results. The factors fixed in the studies were: farming system and catch crops. Years of research were a random factor. In statistical analysis, double and triple interactions between all factors were calculated: HSD (p ≥ 0.05) for interaction (tillage system × catch crop; HSD (p ≥ 0.05) for interaction (tillage system × catch crop × year). All calculated interactions were insignificant Therefore, only statistical significance for the main effects is given in the result tables. For the resulting data presented in in the result tables, the following were calculated: SD—Standard Deviation.

3. Results

3.1. Soil Conditions

Tillage system did not have a significant effect on the chemical components in the plough layer of the soil under spring wheat, which are shown in

Table 6. Results of some chemical determinations for the plough layer (0–20 cm) in the experiment—mean for 2016–2018.

Specification	Tillage System
	Conservation Tillage						Plough Tillage
	Total N ***%	P mg kg−1	K mg kg−1	Mg mg kg−1	C-org. %	% Humus	Total N %	P mg kg−1	K mg kg−1	Mg mg kg−1	C-org. %	% Humus
A **	0.05 a * (±0.01)	157 a (±1.6)	278 a (±2.4)	62 a (±0.4)	0.70 a (±0.08)	1.45 a (±0.05)	0.05 a (±0.01)	153 a (±1.2)	276 a (±2.1)	56 a (±0.3)	0.69 a (±0.07)	1.41 a (±0.05)
B	0.10 b (±0.02)	169 b (±1.9)	279 a (±2.1)	69 b (±0.7)	0.89 b (±0.07)	1.59 b (±0.07)	0.09 b (±0.02)	160 b (±1.4)	271 a (±2.0)	63 b (±0.4)	0.87 b (±0.06)	1.55 b (±0.06)
C	0.08 b (±0.01)	167 b (±2.0)	279 a (±2.0)	67 b (±0.4)	0.83 b (±0.06)	1.54 b (±0.08)	0.07 b (±0.01)	161 b (±1.3)	275 a (±1.9)	60 b (±0.2)	0.80 b (±0.04)	1.50 b (±0.07)
D	0.09 b (±0.01)	171 b (±1.8)	276 a (±2.5)	68 b (±0.3)	0.86 b (±0.07)	1.62 b (±0.09)	0.07 b (±0.01)	165 b (±1.3)	270 a (±1.9)	61 b (±0.3)	0.82 b (±0.05)	1.57 b (±0.08)
HSD (p ≥ 0.05)	0.025	9.4	n.s. ****	4.6	0.106	0.087	0.024	6.7	n.s.	3.9	0.105	0.085

HSD (p ≥ 0.05) for year—not significant differences. HSD (p ≥ 0.05) for interaction (tillage system × catch crop)—not significant differences. HSD (p ≥ 0.05) for interaction (tillage system × catch crop × year)—not significant differences. * Means in column with different letters (a–b) for catch crop and tillage system are significantly different (p _≥ 0.05). ** A—control, B—white mustard, C—lacy phacelia, D—faba bean + spring vetch. *** Total N–the sum of nitrate ammonium (NH₄N) and organic nitrogen (N_org). **** n.s.—not significant differences.

. Nonetheless, one should note a trend towards a more beneficial effect of conservation tillage on the soil quality characteristics (in particular the content of Mg, organic C, and humus). On the other hand, all the catch crops grown in the experiment (using both conventional and conservation tillage) contributed to a significantly higher soil content of nitrogen, phosphorus, magnesium, organic C, and humus compared to the control (without catch crop).

Soil enzymatic activity in the experiment was significantly dependent on both experimental factors and on year (

Table 7. Soil enzymatic activity in 0–20 cm layer.

Specification	Dehydrogenase Activity (μmol TPF kg−1·h−1)	Urease Activity (mmol NH4+ kg−1·h−1)
Tillage System
PT *	4.9 (±0.09) a ***	4.1 (±0.05) a
CT	6.1 (±0.11) b	5.5 (±0.08) b
HSD (p ≥ 0.05)	0.81	0.62
Catch Crop
A **	4.4 (±0.07) a	4.0 (±0.05) a
B	6.8 (±0.12) b	5.9 (±0.09) b
C	6.2 (±0.06) b	5.2 (±0.05) b
D	6.4 (±0.09) b	5.5 (±0.06) b
HSD (p ≥ 0.05)	0.90	0.83
Year
2016	6.0 (±0.09) a	5.1 (±0.07) a
2017	4.8 (±0.05) b	3.8 (±0.03) b
2018	5.1 (±0.06) b	4.1 (±0.04) b
HSD (p ≥ 0.05)	0.73	0.91

HSD (p ≥ 0.05) for interaction (tillage system × catch crop)—not significant differences. HSD (p ≥ 0.05) for interaction (tillage system × catch crop × year)—not significant differences. * PT—plough tillage, CT—conservation tillage. ** A—control, B—white mustard, C—lacy phacelia, D—faba bean + spring vetch. *** Means in column with different letters (a–b) for catch crop, tillage system, and year are significantly different (p ≥ 0.05).

). Under conservation tillage, dehydrogenase and urease activity was higher, respectively, by 19.7 and 25.0%, in comparison to conventional (plough) tillage. The catch crops, in particular white mustard, had an even more distinct effect on soil enzymatic activity. In the control plots (without catch crop), dehydrogenase activity was lower by 29.1−35.3%, while urease activity by 23.1−32.3%. The study results reveal that soil enzymatic activity was also affected by weather conditions during the study period. In the year 2016, which was characterized by the most favorable hydrothermal coefficient, dehydrogenase and urease activity was significantly higher than in the drier years of 2017–2018 (Table 5 and Table 7).

3.2. Total Dietary Fiber Content and O-Dihydroxyphenol Content

Total dietary fiber content in spring wheat grain was significantly dependent on catch cropping and weather conditions in the study years (

Table 8. Total dietary fiber (TDF) content and o-dihydroxyphenol content (expressed as caffeic acid equivalents) in spring wheat grain.

Specification	Total Dietary Fiber Content (%)	O-Dihydroxyphenol Content (g 100·g−1)
Tillage System
PT *	16.35 (±2.33) a ***	0.149 (±0.013) a
CT	16.69 (±2.55) a	0.161 (±0.039) b
HSD (p ≥ 0.05)	n.s. ****	0.011
Catch Crop
A **	15.58 (±2.11) a	0.137 (±0.009) a
B	17.38 (±2.89) b	0.169 (±0.042) c
C	17.04 (±2.45) b	0.157 (±0.030) b
D	17.19 (±2.53) b	0.152 (±0.025) b
HSD (p ≥ 0.05)	1.433	0.0124
Year
2016	17.43 (±2.97) a	0.164 (±0.040) a
2017	15.66 (±2.17) b	0.142 (±0.011) b
2018	15.51 (±2.09) b	0.136 (±0.008) b
HSD (p ≥ 0.05)	1.413	0.0152

HSD (p ≥ 0.05) for interaction (tillage system × catch crop)—not significant differences. HSD (p ≥ 0.05) for interaction (tillage system × catch crop × year)—not significant differences. * PT—plough tillage, CT—conservation tillage. ** A—control, B—white mustard, C—lacy phacelia, D—faba bean + spring vetch. *** Means in column with different letters (a–c) for catch crop, tillage system, and year are significantly different (p ≥ 0.05). **** n.s. —not significant differences.

). All the catch crops used in the experiment contributed to a statistically proven increase in grain TDF content in comparison with the control treatment, while white mustard (treatment B) had the most beneficial effect—an increase in grain TDF content by 1.8 p.p. (percentage point) relative to the control. The beneficial influence of the catch crops on this trait was independent of the tillage system (the method of management of catch crop biomass after harvest). The favorable hydrothermal conditions in 2016 during the growing season of spring wheat promoted significantly higher TDF accumulation in grain (by 1.77–1.92 p.p.) relative to the years 2017–2018. Despite that there is no statistically significant difference, one should note a tendency towards a higher TDF content in wheat grain by 0.34 p.p. under conservation tillage conditions in comparison with plough tillage.

The o-dihydroxyphenol content in spring wheat grain was significantly higher, by about 8%, in the treatment with conservation tillage in comparison to plough tillage. Regardless of the tillage system, ploughing in of catch crop biomass or leaving it in the field as mulch had a significant effect on increasing the o-dihydroxyphenol content in wheat grain, by about 10% (lacy phacelia, legume mixture) and 19% (white mustard), respectively. The effect of the white mustard catch crop (A) on the parameter in question was also significantly higher relative to the treatments with the other catch crops (C and D). When considering the differences in the o-dihydroxyphenol content in wheat grain over the study period, we note that it was significantly higher in the year 2016 (by 14–17% relative to the other years)—which was characterized by the most favorable hydrothermal coefficient during the growing season of this cereal crop (Table 8).

3.3. Amino Acid Content

Catch cropping in spring wheat monoculture positively affected changes in the amino acid composition of grain protein compared to the control without catch crop (

Table 9. Amino acid content in spring wheat grain (mg·g⁻¹).

Specification	Catch Crop					Tillage System			Year
	A **	B	C	D	HSD (p ≥ 0.05)	PT *	CT	HSD (p ≥ 0.05)	2016	2017	2018	HSD (p ≥ 0.05)
Asp (asparagine)	5.63 a (±0.05)	6.41 b (±0.06)	6.22 b (±0.04)	6.27 b (±0.04)	0.506	6.25 a (±0.03)	6.28 a (±0.04)	n.s.	6.39 a (±0.03)	6.24 a (±0.06)	6.19 a (±0.06)	n.s.
Thr (threonine)	3.26 a (±0.03)	3.59 b (±0.02)	3.55 b (±0.03)	3.51 b (±0.04)	0.234	3.61 a (±0.05)	3.49 a (±0.06)	n.s.	3.75 a (±0.03)	3.54 a (±0.05)	3.48 a (±0.06)	n.s.
Ser (serine)	5.45 a (±0.06)	6.39 b (±0.09)	6.24 b (±0.08)	6.19 b (±0.05)	0.586	6.11 a (±0.07)	6.15 a (±0.07)	n.s.	6.24 a (±0.08)	6.12 a (±0.07)	6.05 a (±0.09)	n.s.
Glu (glutamine)	38.1 a (±1.1)	41.3 b (±1.0)	39.3 a (±0.9)	40.8 b (±1.2)	1.28	37.1 a (±1.3)	39.9 a (±1.4)	n.s.	39.0 a (±0.8)	37.3 b (±1.2)	36.9 b (±1.0)	0.89
Pro (proline)	12.5 a (±0.6)	15.5 b (±0.7)	14.8 b (±0.5)	14,4 b (±0.6)	1.33	13.3 a (±0.7)	14.9 b (±0.6)	1.24	14.1 a (±0.5)	13.8 a (±0.8)	13.6 a (±0.8)	n.s.
Gly (glysine)	4.13 a (±0.07)	5.28 b (±0.08)	5.11 b (±0.09)	5.17 b (±0.08)	0.916	4.14 a (±0.06)	5.01 b (±0.08)	0.823	5.13 a (±0.04)	4.11 b (±0.07)	4.08 b (±0.08)	0.871
Ala (alanine)	4.09 a (±0.08)	5.31 b (±0.09)	5.12 b (±0.07)	5.19 b (±0.06)	1.043	5.02 a (±0.03)	5.09 a (±0.04)	n.s.	5.20 a (±0.05)	5.13 a (±0.06)	5.07 a (±0.07)	n.s.
Cys-A (cysteine-A)	1.45 a (±0.02)	3.59 c (±0.04)	2.43 b (±0.01)	2.38 b (±0.02)	0.923	2.16 a (±0.01)	3.22 b (±0.04)	0.936	3.17 a (±0.02)	2.11 b (±0.01)	2.18 b (±0.02)	0.928
Val (valine)	4.48 a (±0.06)	5.23 b (±0.05)	4.73 a (±0.06)	4.69 a (±0.04)	0.487	4.51 a (±0.05)	5.83 b (±0.06)	0.668	5.79 a (±0.07)	4.50 b (±0.06)	4.36 b (±0.05)	0.695
Met (methionine)	1.01 a (±0.01)	2.42 c (±0.03)	2.02 b (±0.02)	2.08 b (±0.02)	0.339	2.02 a (±0.02)	3.16 b (±0.03)	0.397	3.07 a (±0.04)	2.17 b (±0.02)	2.12 b (±0.02)	0.416
Ile (isoleucine)	3.45 a (±0.03)	3.63 a (±0.05)	3.58 a (±0.07)	3.52 a (±0.06)	n.s.***	3.63 a (±0.07)	3.81 a (±0.08)	n.s.	3.77 a (±0.06)	3.68 a (±0.05)	3.59 a (±0.07)	n.s.
Leu (leucine)	8.16 a (±0.9)	8.40 a (±0.7)	8,37 a (±0.6)	8.25 a (±0.8)	n.s.	8.53 a (±0.5)	8.64 a (±0.6)	n.s.	8.39 a (±0.5)	8.26 a (±0.7)	8.19 a (±0.7)	n.s.
Tyr (tyrosine)	2.16 a (±0.06)	2.88 b (±0.05)	2.65 b (±0.07)	2.61 b (±0.07)	0.415	2.74 a (±0.06)	2.49 a (±0.05)	n.s.	2.66 a (±0.08)	2.53 a (±0.06)	2.49 a (±0.05)	n.s.
Phe (phenylalanine)	4.86 a (±0.5)	5.87 b (±0.6)	5.73 b (±0.4)	5.65 b (±0.5)	0.712	5.61 a (±0.6)	5.54 a (±0.6)	n.s.	5.70 a (±0.5)	5.62 a (±0.6)	5.48 a (±0.5)	n.s.
His (histidine)	2.62 a (±0.03)	2.88 a (±0.04)	2.79 a (±0.05)	2.75 a (±0.04)	n.s.	2.69 a (±0.03)	2.74 a (±0.04)	n.s.	2.71 a (±0.04)	2.54 a (±0.03)	2.39 a (±0.03)	n.s.
Lys (lysine)	2.79 a (±0.07)	3.31 b (±0.06)	3.25 b (±0.05)	3.23 b (±0.06)	0.424	3.05 a (±0.04)	3.47 b (±0.06)	0416	3.35 a (±0.05)	2.96 b (±0.07)	2.88 b (±0.06)	0.384
Arg (arginine)	4.49 a (±0.08)	5.72 b (±0.09)	5.60 b (±0.09)	5.52 b (±0.07)	0.974	4.41 a (±0.05)	4.70 a (±0.07)	n.s.	4.68 a (±0.06)	4.44 a (±0.07)	4.37 a (±0.05)	n.s.
Trp (tryptophan)	3.49 a (±0.04)	8.45 c (±0.06)	5.24 b (±0.03)	6.12 b (±0.07)	1.187	4.12 a (±0.04)	6.75 b (±0.06)	1.034	6.61 a (±0.05)	4.34 b (±0.07)	4.25 b (±0.05)	1.098
EAAI ***	57.9 a	75.6 c	66.8 b	70.3 b	4.95	65.9 a	70.2 b	4.26	71.3 a	65.4 b	64.2 b	4.37

HSD (p ≥ 0.05) for interaction (tillage system × catch crop)—not significant differences. HSD (p ≥ 0.05) for interaction (tillage system × catch crop × year)—not significant differences. * PT—plough tillage, CT—conservation tillage. ** A—control, B—white mustard, C—lacy phacelia, D—faba bean + spring vetch. *** EAAI—essential amino acid index; means in rows with different letters (a–c) for catch crops, tillage system, and year are significantly different (p ≥ 0.05). *** n.s.—not significant differences.

). Except for the content of two amino acids (Ile, Leu), the study found a statistically proven increase in the content of the other amino acids determined as affected by the catch crops. It should be stated that white mustard had a particularly beneficial effect on the amino acid composition of wheat grain protein (especially in the case of the following amino acids: Trp (an increase in its content by 2.4 times), Met (an increase in its content by 2.3 times), Cys-A (an increase in its content by 60%), and Val (an increase in its content by 15%)), relative to the control treatment. As far as the above-mentioned amino acids are concerned, the positive influence of white mustard was also statistically significant compared to the effects of the catch crops in treatments C and D (lacy phacelia and the legume mixture). Tillage system differently affected the amino acid content in spring wheat grain (Table 9). Conservation tillage caused a significant increase in the content of the following essential amino acids: Trp, Met, and Lys (respectively, by 39, 36, and 12%), and in some non-essential amino acids: Val (by 23%), Cys-A (by 23%), Pro (by 11%), and Gly (by 7%), compared to plough tillage. The content of the other amino acids determined in spring wheat protein was similar for both tillage treatments (not statistically significant). The most favorable meteorological conditions that prevailed in 2016 significantly positively affected the content of essential amino acids in spring wheat grain (Lys, Met, Trp) and of some non-essential amino acids (Glu, Gly, Cys-A, Val) in comparison with the less favorable, in terms of weather, years 2017–2018, regardless of the experimental factors (Table 9).

The integrated essential amino acid index (EAAI) determined in the study for spring wheat protein indicates that where catch crops are grown (especially white mustard), protein is characterized by the highest biological value. This confirms the justification for growing catch crops as a factor positively contributing to the nutritional value of wheat grain since abandonment of catch cropping resulted in a significantly lower value of EAAI. Conservation (no-tillage) system also had a significant influence on the biological value of spring wheat protein as expressed by EAAI relative to plough tillage (an about 6% increase in EAAI). The study also revealed that regardless of catch cropping and tillage system, the value of EAAI was significantly positively correlated with the most favorable weather conditions during the 2016 growing season of spring wheat (Table 9).

3.4. Content of Some Macro- and Micronutrients

The catch crops, regardless of the tillage system (plough or conservation tillage), significantly positively affected accumulation of macro- and micronutrients in spring wheat grain. All the catch crop species included in the study caused an increase of these nutrients in the grain in comparison to the control treatment (without catch cropping). White mustard had a particularly large effect on the macro- and micronutrient content in spring wheat grain (especially a 3-time increase in grain selenium content relative to the control, 2.2-time increase in manganese content, and 1.8-time increase in copper content). Interestingly, as regards the Se, Mn, and Cu content in grain, the effect of the white mustard catch crop was significantly larger in comparison to the influence of the lacy phacelia and legume mixture catch crops. This proves the high suitability of this catch crop in monoculture cropping of spring wheat.

Plough tillage contributed to a significantly higher nitrogen and potassium content in spring wheat grain compared to the conservation tillage system (respectively by 16% and 17%). The grain zinc content was similar under both tillage systems. In the case of the other macro- and micronutrients evaluated (Mg, Ca, Cu, Mn, Fe, and Se), their content in spring wheat grain was significantly higher under conservation tillage conditions than for plough tillage. The conservation tillage system particularly promoted an increased content of manganese (by 41%) and selenium (by 32%) (

Table 10. Content of some macro- and micronutrients in spring wheat grain.

Specification	Catch Crop					Tillage System			Year
	A **	B	C	D	HSD (p ≥ 0.05)	PT *	CT	HSD (p ≥ 0.05)	2016	2017	2018	HSD (p ≥ 0.05)
Nitrogen (g kg−1)	20.1 a (±0.7)	22.4 b (±0.8)	21.7 b (±0.6)	22.8 b (±0.6)	2.07	21.4 a (±0.7)	18.0 b (±0.7)	2.12	21.1 a (±0.5)	20.7 a (±0.6)	20.5 a (±0.6)	n.s. ***
Potassium (g kg−1)	1.82 a (±0.03)	2.54 b (±0.04)	2.26 b (±0.05)	2.41 b (±0.05)	0.431	2.34 a (±0.03)	1.72 b (±0.04)	0.465	2.21 a (±0.05)	2.09 a (±0.03)	2.11 a (±0.04)	n.s.
Magnesium (mg kg−1)	653 a (±4.0)	791 b (±5.0)	724 b (±6.0)	731 b (±6.0)	68.3	695 a (±5.0)	919 b (±7.0)	75.4	865 a (±8.0)	734 b (±7.0)	725 b (±6.0)	74.3
Calcium (mg kg−1)	238 a (±2.0)	361 b (±3.0)	340 b (±2.0)	351 b (±3.0)	56.5	181 a (±0.09)	222 b (±2.0)	40.5	343 a (±4.0)	288 b (±3.0)	271 b (±2.0)	46.4
Copper (mg kg−1)	2.51 a (±0.07)	4.51 b (±0.09)	3.16 c (±0.05)	3.19 c (±0.06)	0.573	3.61 a (±0.08)	4.93 b (±0.09)	0.632	4.38 a (±0.09)	3.24 b (±0.08)	3.36 b (±0.06)	0.656
Manganese (mg kg−1)	15.9 a (±0.5)	35.2 b (±0.8)	24.0 c (±0.6)	24.6 c (±0.7)	7.25	15.6 a (±0.4)	26.4 b (±0.7)	7.06	30.5 a (±0.6)	22.5 b (±0.5)	23.7 b (±0.4)	7.11
Iron (mg kg−1)	28.5 a (±0.7)	40.8 b (±0.9)	37.3 b (±0.8)	38.3 b (±0.8)	6.98	36.4 a (±0.7)	45.0 b (±0.9)	6.79	39.7 a (±1.0)	30.4 b (±0.6)	29.2 b (±0.6)	6.62
Zinc (mg kg−1)	23.5 a (±0.6)	35.5 b (±0.7)	33.1 b (±0.8)	30.2 b (±0.7)	5.94	32.4 a (±0.6)	33.0 a (±0.6)	n.s.	32.3 a (±0.7)	29.1 a (±0.5)	31.1 a (±0.6)	n.s.
Selenium (mg kg−1)	16.5 a (±0.4)	49.2 b (±1.1)	29.8 c (±0,8)	33.5 c (±0.9)	9.42	20.3 a (±0.7)	34.7 b (±0.9)	8.86	37.2 a (±0.8)	28.7 b (±0.7)	29.2 b (±0.6)	7.44

HSD (p ≥ 0.05) for interaction (tillage system × catch crop)—not significant differences. HSD (p ≥ 0.05) for interaction (tillage system × catch crop × year)—not significant differences. * PT—plough tillage, CT—conservation tillage. ** A—control, B—white mustard, C—lacy phacelia, D—faba bean + spring vetch; means in rows with different letters (a–c) for catch crop, tillage system, and year are significantly different (p ≥ 0.05). *** n.s.—not significant differences.

).

Compared to the dry growing seasons in 2017–2018, the favorable (hydrothermal) weather conditions during the 2016 growing season had a significant effect on the content of most of the macro- and micronutrients evaluated (Mg, Ca, Cu, Mn, Fe, and Se) (Table 10).

The effects of the quality parameters of the loess soil on spring wheat grain quality were significantly manifested in combination with catch cropping under cereal monoculture. Tillage system was not a factor that caused significant differences in the correlations found. The significant correlation coefficients shown in

Table 11. Coefficient of correlation (r) for some soil chemical components and soil enzymatic activity in relation to selected quality characteristics of spring wheat grain (under catch cropping conditions, regardless of the tillage system).

Specification	Catch Crop	TDF	O-Dihydr.	Lys	Met	Trp	Se	Mn	Cu
Humus	control	0.13	0.11	0.23	0.19	0.21	0.19	0.23	0.24
	white mustard	0.65 *	0.61 *	0.87 *	0.85 *	0.89 *	0.81 *	0.71 *	0.78 *
	lacy phacelia	0.47	0.46	0.54 *	0.52 *	0.55 *	0.60 *	0.58 *	0.56 *
	faba bean + spring vetch	0.55 *	0.51 *	0.60 *	0.58 *	0.72 *	0.70 *	0.62 *	0.59 *
Organic C	control	0.10	0.13	0.26	0.22	0.25	0.06	0.15	0.07
	white mustard	0.58 *	0.62 *	0.85 *	0.82 *	0.91 *	0.57 *	0.54 *	0.55 *
	lacy phacelia	0.42	0.39	0.55 *	0.54 *	0.59 *	0.49	0.43	0.44
	faba bean + spring vetch	0.57 *	0.61 *	0.69 *	0.65 *	0.72 *	0.53 *	0.50 *	0.52 *
Total N	control	0.08	0.11	0.14	0.05	0.12	0.19	0.18	0.20
	white mustard	0.66 *	0.63 *	0.64 *	0.62 *	0.67 *	0.61 *	0.60 *	0.59 *
	lacy phacelia	0.55 *	0.57 *	0.52 *	0.51 *	0.55 *	0.50 *	0.48	0.46
	faba bean + spring vetch	0.51 *	0.54 *	0.60 *	0.59 *	0.63 *	0.59 *	0.52 *	0.53 *
Mg	control	0.26	0.22	0.24	0.21	0.19	0.08	0.08	0.10
	white mustard	0.61 *	0.63 *	0.69 *	0.73 *	0.75 *	0.56 *	0.51 *	0.53 *
	lacy phacelia	0.57 *	0.52 *	0.56 *	0.54 *	0.58 *	0.50 *	0.42	0.44
	faba bean + spring vetch	0.60 *	0.61 *	0.69 *	0.67 *	0.65 *	0.53 *	0.52 *	0.50 *
Dehydrogenase	control	0.20	0.18	0.17	0.15	0.16	0.32	0.26	0.31
	white mustard	0.92 *	0.88 *	0.78 *	0.75 *	0.80 *	0.78 *	0.65 *	0.67 *
	lacy phacelia	0.65 *	0.68 *	0.58 *	0.56 *	0.64 *	0.56 *	0.52 *	0.57 *
	faba bean + spring vetch	0.89 *	0.82 *	0.71 *	0.68 *	0.75 *	0.60 *	0.58 *	0.55 *
Urease	control	0.26	0.29	0.21	0.23	0.25	0.41	0.25	0.35
	white mustard	0.82 *	0.85 *	0.71 *	0.72 *	0.77 *	0.72 *	0.64 *	0.62 *
	lacy phacelia	0.63 *	0.66 *	0.54 *	0.51 *	0.57 *	0.58 *	0.54 *	0.50 *
	faba bean + spring vetch	0.73 *	0.75 *	0.66 *	0.63 *	0.69 *	0.63 *	0.59 *	0.53 *

* significant correlation coefficient (p ≥ 0.05).

demonstrate that the quality parameters of the soil under spring wheat crops had a measurable effect (in the case of catch cropping conditions) on the nutritional composition of wheat grain. Under monoculture without catch crop (control treatment), the correlation coefficients were not statistically significant in any case. The strongest correlation was found between soil humus and organic C content in relation to the content of essential amino acids (Lys, Met, Trp) in wheat grain and to Se, Mn, and Cu content when the white mustard catch crop was grown. In turn, soil enzymatic activity (Dh, Ur) exhibited the strongest relationship with the TDF and o-dihydroxyphenol content in spring wheat grain also in the treatments with the white mustard catch crop (the highest values of the correlation coefficient r). The soil enzymes (in particular dehydrogenase) also showed a high significant correlation with essential amino acid content—in the treatments with the white mustard and legume mixture catch crops. They also distinctly affected the correlation with grain Se, Mn and, Cu content, especially in the treatments with the white mustard catch crop. Total soil N and Mg content exhibited the strongest correlation with essential amino acid content (in the treatments with catch cropping, especially those with the white mustard and legume mixture catch crops). However, no significant correlations were found between soil P and K content and wheat grain quality, and hence, these results are omitted in this table (Table 11). For better readability of Table 11, it omits the correlations with the essential amino acids (which were positive, but at a statistically non-significant level) and also some macro- and micronutrients (in all cases, the correlations with the soil quality characteristics were positive in the catch crop treatments, but statistically not significant).

4. Discussion

The chemical composition of agricultural produce is dependent on many factors, both agronomic ones, which include fertilization, soil tillage, previous crop, and crop protection, and soil, genetic (varietal characters) and climate ones. The effects of these factors can have very different impacts that are dependent on the crop species, the component evaluated, and the region where research was conducted [31,32,33,34]. The dependence of the chemical (nutritional) composition of spring wheat grain on tillage system and catch crop is an issue that is seldom addressed in the literature of the subject. For this reason, the results obtained in the present experiment should be considered to be “niche” findings and it is not easy to present a discussion of the obtained results.

The correctness of the selection of catch crops for this study (see Table 1) is confirmed by the data contained in

Table 12. Average yield of catch crop dry matter [13,35,36,37].

Catch Crop	Dry Biomass (t·ha−1)
	Plants	Roots
White mustard	3.98–4.38	0.12–0.13
Tansy phacelia	3.62–4.35	0.08–0.10
Spring vetch + field pea	3.20–3.60	0.19–0.21
Yellow lupine + seradella	3.20–3.60	0.19–0.21
Oats + spring vetch + field pea	3.06–3.40	0.15–0.17
Seradella	3.05–3.20	0.07–0.09
Yellow lupine	2.72–2.90	0.25–0.30
Narrow-leafed lupine	2.39–2.89	0.24–0.29
Red clover	2.48–2.80	0.06–0.08
Westerwolds ryegrass	2.42–2.65	0.05–0.06

. It provides a chronological list of the most promising (most faithfully yielding) catch crops. Other studies conducted over the years 2011–2020 show that white mustard and tansy phacelia have the highest dry biomass yield. On the other hand, legume catch crops yield best in mixtures than in single sowing. A high yield of catch crops biomass is positively correlated with the quality features of wheat grain grown in such a stand. This was confirmed in our studies.

The present study proved that cultivation of catch crops to be ploughed in or left as mulch until spring contributed to an improvement in the chemical composition of loess soil and stimulated soil enzymatic activity. Due to this, nutrients contained in the soil were better taken up by spring wheat plants, which translated itself into the grain nutritional composition [20,38,39,40]. Furthermore, abandonment of plough tillage in favor of conservation tillage generally contributed to improved soil quality parameters, as expressed by the soil chemical composition and enzymatic activity—dehydrogenase and urease. This, in turn, resulted in an improvement in spring wheat grain quality. Woźniak and Gos [41] observed that no-tillage contributed to a higher content of soil organic C, total N, and available phosphorus in comparison to conventional tillage. Other scientific reports also show that no-tillage (particularly when combined with catch crops left as mulch in the field) can be an important factor determining agricultural produce quality [42,43,44,45,46]. To sum up, it can be said that fertile and nutrient-rich soil is the “starting point” for production of healthy and safe food [8,47,48,49].

In the present study, spring wheat grain was characterized by dietary fiber content at a level reported by other authors [50,51,52,53], but the highest content of this component was found in grain harvested from the plots with the white mustard catch crop. A study by Wu et al. [54] reveals that leaving crop residue on the field surface can be a source of many plant germination and growth inhibitors, among which phenolic compounds are characterized by high biological activity. An increased phenolic content in the soil after incorporation of white mustard, buckwheat, spring barley, or oat biomass into it was also found by Stokłosa et al. [55]. In this study, the catch crops (particularly white mustard) promoted the highest o-dihydroxyphenol content in spring wheat grain. Conservation tillage promoted greater accumulation of these compounds in wheat grain than plough tillage. In turn, Woźniak and Rachoń [56] as well as Buczek et al. [57] found plough tillage to have a more beneficial effect on wheat grain quality than no-tillage, while Djouadi et al. [58] obtained better wheat productivity and grain quality in the case of transition from conventional to conservation tillage (no-tillage). However, these authors note that better grain quality under no-tillage conditions is largely correlated with favorable hydrothermal conditions during the growing season. This observation was also confirmed in the present study.

Phenolic compounds are a large and important group of substances with strong oxidant properties. A significant amount of these compounds is also found in cereal grains, especially in whole grains and bran [59,60,61]. Research has shown that cereal species (particularly wheat) exhibit different richness in individual phenolic acids [62,63,64,65]. In this research, grains of spring wheat grown after the white mustard catch crop were richest in phenolic compounds. Żuchowski et al. [66] found that total phenolic acids in wheat grain are higher for organic technologies than for conventional ones. In these authors’ opinion, significant differences in the proportions of individual phenolic acids can be found both between tillage systems and wheat cultivars. Woźniak and Rachoń [56] as well as Gawęda and Haliniarz [67] observed that the chemical composition of spring wheat grain is more favorable when a good previous crop is grown (a legume crop) than under monoculture conditions, which is also confirmed by this study. Regardless of factors used in a field experiment, some authors [68] stressed the significant positive impact of environmental and meteorological conditions (nutrient-rich soil, an even distribution of rainfall, higher air temperatures during grain filling) on many quality traits of wheat grain, in particular phenolic compounds and related antioxidant properties. This is confirmed in this study.

From the point of view of human nutrition, the amino acid composition that determines the nutritional value of protein is extremely important. The following amino acids are the most important in nutrition: lysine, sulfur amino acids, threonine, tryptophan, valine, and isoleucine [69,70]. When comparing the amino acid composition of protein found in this study with the amino acid content in common wheat grain determined by other authors [11,71,72], substantial differences can be noticed. In a study by Matuz et al. [73], common wheat contained more aspartic acid, glutamic acid, tyrosine, valine, phenylalanine, isoleucine, and leucine, but less methionine, whereas the content of other amino acids in grains was at a similar level to that determined in our study.

The present study found no-tillage system to have a more beneficial effect on the content of essential amino acids (Lys, Met, Try) in spring wheat grain. Andruszczak [74], on the other hand, did not find the amino acid composition of wheat grain to significantly vary as affected by tillage system. Zhang et al. [75] noted that favorable water conditions during the wheat growing season significantly improved the essential amino acid index (EAAI) compared to dry conditions. The results of this study are a confirmation of this thesis because the calculated EAAI was significantly higher for the growing season that was most favorable in terms of hydrothermal conditions relative to the dry years. Reports of other authors [76,77,78] demonstrate that the biological value of grain protein of wheat and other cereals, as expressed by EAAI, is determined not only by species and varietal characteristics. It largely depends on environmental factors (agri-climate and soil type and quality), farming method as well as quality and quality of crop residue incorporated into soil. This study proved that incorporation of white mustard biomass into the soil resulted in the highest value of EAAI in spring wheat grain, which is of large importance for agricultural producers and consumers of cereal products.

A detailed evaluation of the mineral composition of agricultural produce is very important because, as demonstrated by Fan et al. [2], the content of zinc, iron, copper, and magnesium in cereal crops has decreased in the United States since 1960, which coincided with the introduction of semi-dwarf, high-yielding cultivars into cultivation. In Finland, Ekholm et al. [79] evaluated the macro- and micronutrient content of cereal food products, fruits, and vegetables and compared this content with the results obtained 30 years earlier. These authors showed significant changes in the content of elements in the evaluated products. In most cases, their amounts were found to be lower and only the amount of selenium had substantially increased due to the use of selenium-supplemented mineral fertilizers.

In the present experiment, in comparison with the results obtained by Suchowilska et al. [80], spring wheat had an almost half lower potassium and magnesium content, a more than 20% lower content of calcium and zinc, and a similar copper, manganese, and iron content. Zhao et al. [81] showed the grain Fe content in common bread wheats to range from 28.8 to 50.8 mg kg⁻¹ (on average 38.2 mg kg⁻¹), whereas the average amount of zinc in common wheat grain was 21.4 mg·kg⁻¹. The selenium content in common wheat grain greatly varied, ranging between 32.9 and 237.9 μg· kg⁻¹.

In this study, the cultivation of catch crops significantly modified the content of most of the macro- and micronutrients in grain. The catch crops (especially white mustard) caused an increase in the Cu and Se content in wheat grain in comparison to “pure monoculture” without catch crop. A study by Woźniak and Makarski [82] demonstrated that spring wheat grain harvested from treatments after previous legume crops was characterized by a higher content of P, Ca, Fe, and Zn compared to grain harvested from monoculture. In this study, the conservation tillage system positively modified the composition of most of the macro- and micronutrients determined in spring wheat grain (Mg, Ca, Cu, Mn, Fe, Zn, Se), whereas conventional tillage significantly increased the N and K content in grain. Kraska [83] also obtained similar results—conservation soil tillage with incorporation of the catch crops promoted higher content of zinc and iron in spring wheat grain; zinc content was higher by 2.4 to 2.8%, while iron content increased by 0.5 to 8.3% compared to plough and conservation tillage with autumn disking of the catch crops. Weber [84] reported that conservation tillage promotes greater availability of nutrients in the arable layer compared to plow tillage. At the same time, the introduction of a catch crop as a factor mitigating the negative effects of monoculture spring wheat cultivation increased the content of zinc and copper in the grain as compared to the control object without catch crop. In the study by Kraska [83], the content of copper and zinc in the grain of spring wheat cultivated after catch crops was higher, and the content of manganese was lower than in objects without catch crops. The highest content of zinc and copper was found in wheat grain cultivated after tansy phacelia. The results concerning the macro- and micronutrient content in cereal grain obtained by other authors [85,86,87,88,89,90] show large variations resulting from the crop growing technology used, the wheat cultivar grown, and weather conditions.

5. Conclusions

The positive effects of the catch crops were independent of the tillage system. Both ploughing in of catch crop biomass in autumn (conventional tillage) and leaving the biomass in the field until spring as mulch (conservation tillage) contributed to an improvement in the soil chemical parameters and soil enzymatic activity. In consequence, spring wheat grain harvested from the catch crop treatments (especially white mustard) was characterized by the best quality parameters: the content of TDF, o-dihydroxyphenols, and amino acids (in particular Lys, Met, Trp, Cys-A, and Arg), the best biological value of protein—the highest EAAI and the highest content of macro- and micronutrients. Lacy phacelia and legume mixture should also be considered to be important quality-enhancing factors for spring wheat grain, in spite of their slightly lower positive impact.

The conservation tillage system (no-tillage) can be recommended for growing spring quality wheat because it had a beneficial effect on the soil chemical and enzymatic parameters, which was translated into an improvement in most of the quality characteristics of wheat grain.

An even distribution of total rainfall determines obtaining a favorable nutritional composition of spring wheat grain and the best EAAI.

In order to confirm the positive effect of catch crops on the parameters of nutritional composition of spring wheat grain, analogous studies should be continued in regions with different soil and climatic conditions.