Case Study on 5^th Year Impact of Soil Tillage on Carbon/Nitrogen Agronomy Key Nexus in Winter Wheat—Soybean Rotation

Abstract

The scope of this research was to quantify the mid-term impact of different soil tillage on carbon/nitrogen agronomical key context under optimal growing conditions of the European moderate continental climate. A large-scale on-farm experiment was established in winter wheat/soybean two-crop long-term cultivation without fertilization on fertile Luvic Chernozem. Four treatments were conducted: (T1) ‘Deep Loosening’ with tillage depth of 50 cm, (T2) ‘Plowing’ to 30 cm, (T3) ‘Strip-Till’ with tillage depth of 20 cm, and (T4) ‘No-Till’; the tillage frequency at T1 and T2 was reduced and applied to soybean only, therefore, once per 2 years during the trial period 2020/21–2024/25. Unlike the crop yield, which decreased with tillage intensity decreasing (21.38 > 19.30 > 18.88 > 18.62 t/ha in dry matter cumulatively; T2 > T3 > T1 > T4), the carbon/nitrogen key agronomical parameters either increased (root nodules count/weight: thus confirmed convergent, occasionally reverse indicators; soil compaction: penetrometric resistance) or differed in varying patterns and extent (soil chemical indicators). In fertile Chernozem soils, tillage and indicators have different importance within the nexus studied; plowing still gives the most stable yields. To improve nitrogen fixing, farmers’ practices need to balance yield vs. soil health, including eliminating soil compaction.

1. Introduction

In the current understanding, win–win solutions are necessary for the agriculture’s main mission on food/feed sustainable production and its approaches concerning the recent environmental challenges [1,2,3]. Soil fertility, one of the basic prerequisites for the main mission and the challenges, is conditioned by its cultivation and is related to a number of technological properties of the soil, including parameters of the nitrogen nexus [4,5]. Reflecting a strong social and economic pressure to develop and apply as eco-smart as climate-smart soil cultivation, different soil tillage has been used worldwide for decades. However, the mid-term and long-term impact, and given the various agro-ecology preconditions, even the short-term effect, are the subject of various contradictions [6,7,8]. Despite the fact that some methods of soil tillage are usually referred to as soil-protective and environmentally friendly, the impacts on the indicative parameters could be contradictory [9,10,11,12].

Both carbon and nitrogen sit at the core of soil fertility, and concurrently at the heart of the eco-climate–agronomy connection. That nexus suggests the critical interlinkages between nitrogen/carbon use in agriculture, on the one side, and adaptive strategies on climate-smart solutions, on the other side [13,14]. When managed smartly, nitrogen enhances yields, helps to build soil carbon, and reduces emissions, making it essential for meeting some climate goals, carbon directives, and sustainable food systems [3,15,16]. Integrated fertility management, based on both nitrogen and carbon inputs, as a combination of organic matter (compost, manure) and mineral nitrogen, improves agronomic efficiency and reduces environmental losses [15,17]. Nitrogen, which came from nitrogen fixation by root nodules, is more efficiently utilized because it is preferentially used for the formation and growth of roots and only later for the formation of generative organs. Nitrogen from mineral components of the soil serves more for the formation of vegetative parts of the plant [14,15,16].

Tillage system has a marked impact on biological nitrogen fixation [5]. According to some studies, no-till practices contribute positively to the total nitrogen content of the topsoil in the long term [11,12,13,14,15,16]. Nitrogen emissions tend to be higher under no-till than conventional systems, according to some studies. These emissions can be mitigated by avoiding monoculture and incorporating legumes into crop rotations [18]. Although these carbon/nitrogen nexus advantages and disadvantages of different soil tillage are described differently, contradicting even the main issues of the topic. Probably due to contrasting conditional results obtained under various soil–climatic conditions, under which the research is carried out [3,8]. Classical plowing, for example, is associated with the formation of an unfavorable plow basin, a compacted layer of soil impermeable to water and plants at a depth where the plow body no longer interferes. However, reduced tillage involves the formation of a similar disk plate layer when tillage is reduced to disking. From the point of view of the risk of climatic damage to crops, the damage is higher the shallower the compacted layer is in the topsoil and the thicker the compacted layer is [5,19].

The first works that compare conventional soil tillage with no-till and reduced tillage in Slovakia are associated with the early 1970s, but their modern expansion is associated with the development of more technically advanced equipment and machines and dates back to the mid-1990s [13,14]. Similar to the majority of papers that came from other European countries, this research is based on small-plot experiments, thus a priori using the small-plot technique. Despite earlier papers, the current one is on-farm research, which is aimed at verifying the mid-term impact of soil tillage on yield development and the agronomic key nitrogen/carbon nexus under favorable soil–climate conditions. For this purpose, a technology fine-designed agronomy experiment was conducted with an emphasis on the use of large-scale contrasting tillage techniques at the on-farm semi-operational conditions and the sub-factorial approach concerning crop nutrition.

2. Materials and Methods

A large-scale experiment with four different treatments of soil tillage was established at a two-crop rotation cultivation system on fertile soil in the European moderate climate conditions on the East Slovakian Lowland in the autumn of 2020.

2.1. The Trial Site, Location, and Soil–Climate Conditions

The trial site is located in the Michalovce district, near the village of Žbince, at an altitude of 106 m (Figure 1). For the trial, it is a typical fertile Luvic Chernozem with favorable soil properties. The initial status of the values of the main soil chemical parameters is displayed in

Table 1. Main soil chemical properties, the initial status of topsoil (0–30 cm) at the trial beginning in September 2020.

N-NH4+ mg/kg	N-NO3− mg/kg	Nan. mg/kg	Nt %	P mg/kg	K mg/kg	Mg mg/kg	Ca mg/kg	pH/KCl	Cox. %	Humus %	C/N
11.7	7.0	18.7	0.169	141.1	567.4	366.7	3110	5.26	1.68	2.89	9.9
				high	very high	very high	good	acidic	middle	middle

N-NH₄⁺—ammoniacal nitrogen content, N-NO₃⁻—nitrate nitrogen content, N_an.—inorganic nitrogen content, P—available phosphorus content, K—available potassium content, Mg—available magnesium content, Ca—exchangeable calcium content, pH/KCl—soil reaction in KCl, C_ox.—soil organic carbon, humus—humus content, C/N—ratio of soil organic carbon to total nitrogen.

. Regarding the moderate climate, according to the 30-year normal (1961–1990), the site is characterized by an average yearly air temperature of 8.9 °C and a yearly precipitation amount of 550 mm, while 16.0 °C and 348 mm are the adequate values for the main vegetation period (April–September).

2.2. The Applied Agronomy and Experimental Layout

This large-scale experiment was conducted with the method of long plots, involving 4 trial members (the 4 tillage treatments,

Table 2. Soil tillage technologies for the treatment of crops.

Treatment	Soil Tillage	Crop	Tillage Depth	Tillage Frequency (During 2020/21–2024/25)	Main Tool (Tillage)	Main Tool (Drilling)
T1	‘Deep Loosening’	soybean	50 cm	1 per 2 years (November 2020, October 2022, October 2024)	Chisel plough (Maschio Gasparo, IT)	Mzuri Pro till (UK) (without strip till)
		winter wheat	(not applicable, without tillage)
T2	‘Plowing’	soybean	30 cm	1 per 2 years (November 2020, October 2022, October 2024)	Plough (Sukov, CZ)
		winter wheat	(not applicable, without tillage)
T3	‘Strip-Till’	soybean	20 cm	1 per year	Strip-tiller (Mzuri, UK)	Mzuri Pro till (UK) (with strip till)
		winter wheat
T4	‘No-Till’	soybean	(not applicable, without tillage)			Horsch Avatar (DE)
		winter wheat

) with a total area of 4 ha (1 ha per treatment). This experiment was conducted in the autumn of 2020, when the first soil tillage treatments were applied. Since then, a two-crop rotation was applied as follows: soybean was cultivated in 2021, 2023, and 2025; winter wheat was cultivated in 2022 and 2024. However, regarding field history, the same two crops were rotated from 2008 by conventional soil tillage with plowing.

Considering the field history, the soybean seed was not inoculated with nodule-forming bacteria due to having been cultivated regularly and long-term; therefore, nodule-forming bacteria are present in the soil. The soybean was sown (11 May 2021; 3 May 2023; and 1 May 2025) to a depth of 3.5 cm; only the cultivar of ‘Atacama’ (multiplier Matex, Veškovce, Slovakia) was used at a unified sowing rate of 0.55 million germinating seeds per hectare across the entire experiment. Winter wheat was sown (30 October 2021 and 11 October 2023) to a depth of 3.0 cm; only the cultivar of ‘Avenue’ (multiplier Matex, Veškovce, Slovakia) was used at a unified sowing rate of 3.50 million germinating seeds per hectare across this experiment.

Highly aimed intensive herbicide protection was applied in this experiment, across all tillage treatments (

Table 3. The applied pesticides.

Type	Year/Crop	Preemergent	Postemergent/On Leaf
Herbicides	2021/soybean	2 L/ha Stomp Aqua (pendimethalin 455 g/L) + 0.25 L/ha Command (clomazone 480 g/L)—11 June	0.7 L/ha Pulsar 40 (imazamox 40 g/L) + 1.25 L/ha Benta 480 (bentazone 480 g/L)—16 June
	2022/winter wheat	-	50 g/ha Orcane (halauxifen 100 g/kg, florasulam 100 g/kg, pyroxsulam 240 g/kg, cloquintocet-acid 212.5 g/kg) + 0.5 L/ha Shaman (alkylpfenol alkoxylate 990 g/L)—27 April
	2023/soybean	-	3 kg/ha Sharpen (saflufenacil 700 g/kg) + 0.25 L/ha Command (clomazone 480 g/L)—15 May, 0.7 L/ha Pulsar 40 (imazamox 40 g/L) + 1.25 L/ha Basagran 480 (bentazone 480 g/L)—14 June
	2024/winter wheat	-	0.6 L/ha Pegas (florasulam 6.25 g/L, 2 452.5 g/L,4-D) —16 June
	2025/soybean	-	0.6 L/ha Pulsar 40 (imazamox 40 g/L) + 2 L/ha Basagran 480 (bentazone 480 g/L)—21 June
Fungicides	2021/soybean	-	-
	2022/winter wheat	-	0.5 L/ha Agrozol (tebuconazole 250 g/L) + 0.5 L/ha Vuvuzela (prochloraz 450 g/L)—5 June
	2023/soybean	-	-
	2024/winter wheat	-	1.0 L/ha Agrozol (tebuconazole 250 g/L)—27 May
	2025/soybean	-	-

-: without application.

). Given the high nutrient content in the soil, no fertilizers were used (

Table 4. The nutritional doses during 2021–2025.

Crop	N	P	K	Amount of NPK
soybean	0	0	0	0
winter wheat	0	0	0	0

). Before the experiment was established, on October 26, 2020, finely crushed limestone (Ladmovce, Slovakia) was applied across the trial area uniformly at a dose of 3 t/ha CaCO₃, which was subsequently incorporated to a depth of 3 cm with cultivator discs (Sukov, Czech Republic). Crops were harvested using a large-scale New Holland technique. After harvest, the crop was weighed (scale: TCM 128/15-5309, Tenzona, Bratislava, Slovakia); then, the yield harvest moisture content was also determined (moisture meter: Minifra scan NIT analyser, ID 11217).

2.3. Followed Parameters

Within this research, the parameters used were as follows:

• Main soil chemical properties (total nitrogen content was determined by Kjeldahl method [20]; ammoniacal nitrogen was determined by the Nessler technique, and nitrate nitrogen was determined by the colorimetric method; available P, K, Mg, Ca were determined by Mehlich III method [21]; soil reaction in 1 mol dm⁻³ KCl solution was determined using potentiometric method (ISO 10390 2005) [22]; soil organic carbon was determined by Tjurin method (ISO 14235 1998) [23]. The averaged soil sample was analyzed per treatment (4) and soil layer (2), while the sample was averaged from 3 collection points per treatment (indirect repetitions);
• Root nodules. Soybean plant samples were collected on 3 July 2025, from two different places per treatment, with samples taken from a row with a total extent of 1.0 m per treatment. The root nodules were separated and counted, then dried at 105 °C to a constant weight and weighed gravimetrically (KERN, model AC 200-4M, Balingen, Germany);
• Soil penetrometric resistance (using soil penetrometer PEN 100, manufacturer: OROSZ, Budapest, Hungary). The in situ measurements were performed on 3 June 2025 and consist of the soil profile 0–60 cm, while the records were taken at 6 replications per treatment (4), and every 1 cm was measured;
• Dry matter yield of the crops. The yield data was taken at harvest by measuring especially on a scale (scale: TCM 128/15-5309, Tenzona, Slovakia) after harvesting the entire variant; subsequently, the absolute dry matter yield was calculated. The moisture content of the yield sample was determined in the NPPC laboratory (moisture meter: Minifra scan NIT analyzer, ID 11217, manufacturer INFRACONT, Budapest, Hungary), after drying of the samples (1 per each of treatments and year, 20 in total) at 105 °C to a constant weight;
• Weather parameters (average air temperature and sum of precipitation). The weather data was obtained from the nearest observation station of the Slovak Hydrometeorological Institute (SHMÚ Bratislava, Slovakia) installed in Michalovce (up to 10 km as the crow flies from the trial site in Žbince), ensuring the quality of authentic data.

2.4. Statistics

Within this paper, the main parameters of descriptive statistics were applied in general. The laboratory analyzed soil chemical properties, and samples were obtained in classical two replicates; the soil penetrometric measurements were performed in situ in 6 replications. If it is not specified, only the average data is displayed in this paper; if necessary, the standard error (5%) is added.

3. Results and Discussion

The results of the case study consist of several issues; these topics are presented hereinafter in causal logical order, leading to the nitrogen agro-environmental key nexus.

3.1. Weather Conditions, Air Temperature, and Precipitation

The course of weather conditions during the trial period is displayed in

Table 5. The course of weather conditions on the trial site, average air temperature, and sum of precipitation per year and during the main growing season (IV−IX, April till September) 2021–2024, and adequate values of the 30-year normal (1961−1990).

Parameter	Period	2021	2022	2023	2024	30-Year Normal
Temperature, °C	year	10.0	11.2	11.5	12.3	8.9
	IV−IX	17.1	18.2	17.8	19.5	16.0
Precipitation, mm	year	539	464	832	533	550
	IV−IX	307	295	461	378	348

;

Table 6. The course of weather conditions on the trial site of Žbince during the main growing season of 2025, and adequate values of the 30-year normal (1961−1990).

Parameter	Period	April	May	June	July	August	September
Temperature, °C	2025	11.6	12.9	21.3	21.4	20.7	-
	30-year normal	10.1	15.1	17.9	19.4	18.7	14.8
Precipitation, mm	2025	31	62	11	82	76	-
	30-year normal	41	57	70	74	62	44

-: The values are not applicable (manuscript of this paper prepared before 30 September 2025).

. Compared to the 30-year normal, years 2020–2024 were typical, with hot weather and a sufficient total amount of precipitation. However, the precipitations had a sporadic character and were not ideally distributed during the main growing season. Nevertheless, the years 2020–2024 can be considered as productive, fertile ones.

The main vegetation period of 2025 began with drought in April, continued with sufficient precipitation in May, and extreme drought in June.

3.2. DM Yields

The crops were harvested on 9 October 2021, 10 July 2022, 12 October 2023, 7 July 2024, and are planned for September/October 2025. The yields of the crops and the moisture content at the harvest are displayed in

Table 7. The crops yield in dry matter and moisture content at harvest, by the treatments.

Treatment	Moisture Content at Harvest, %				Dry Matter Yield, t/ha
	2021 Soybean	2022 Winter Wheat	2023 Soybean	2024 Winter Wheat	2021 * Soybean	2022 Winter Wheat	2023 Soybean	2024 Winter Wheat
T1	12.2	11.8	13.2	12.7	3.56	6.26	3.65	5.41
T2	12.0	9.7	12.9	12.5	4.03	7.04	4.09	6.21
T3	11.8	8.8	13.3	11.8	3.62	6.57	3.38	5.73
T4	11.9	8.3	13.8	11.5	3.44	6.33	3.19	5.66

* On 11 July 2021, the glacier caused 30% damage to the leaf area across this experiment. T1—‘Deep Loosening’, T2—‘Plowing’, T3—‘Strip-Till’, T4—‘No-Till’.

. The yield decreased with tillage intensity, decreasing 21.38 > 19.30 > 18.88 > 18.62 t/ha in dry matter cumulatively, T2 > T3 > T1 > T4; both crops followed this treatment order.

The soil type of current research is Luvic Chernozem, which belongs to the most fertile soil type across Central and Eastern Europe; therefore, various soil tillage systems are applied [24]. Tillage systems directly influence crop yields, which is a result of complex processes of soil structure, microbial activity, and nutrient cycling, particularly nitrogen, as one of the most important elements [11]. In Chernozem soils, naturally rich in organic matter, the formation of soil aggregates is especially important for the long-term stabilization of soil organic matter and many other technological parameters [25,26]. While the aggregate size distribution in Chernozems is partly determined by the soil physical properties, such as high clay and humus content, it can vary significantly under different tillage systems. Among these, tillage intensity has a great influence, often disrupting aggregates and reducing soil organic matter and stability of many other indicators over time, while according to an agronomical point of view, the key importance must belong to the crop yield stability [25].

According to the results of previous research with differentiated tillage methods, which we carried out in 1990–2025 in small-plot experiments in comparable soil and climatic conditions on Gleyic Fluvisol in the East Slovak Lowland, the yields of the main field crops (eight field crops, including winter wheat and soybean) decreased with a decrease in tillage intensity, and in terms of time, the reduction in crop yields was permanent [15,27,28]. According to various studies, the reduction in the intensity of tillage is accompanied by a decrease in yields, the so-called decline effect, which may subside in the medium to long term [1]. This is consistent with the results of Fecák et al. [29] on heavy soils.

It is improbable that plant production can be sustained if soil quality declines. Soil quality refers to the dynamic processes and properties of the soil that influence plant production risks, even over the long term [10,30]. Land use practices, including tillage, affect soil quality in both the short and long term. In Hungary, for example, a tillage method can be considered beneficial if it meets crop requirements without damaging the soil or if it enhances its physical and biological properties. This study emphasizes the importance of achieving and maintaining a balance between conserving soil quality and ensuring productive plant growth [31].

Several field studies under European continental climates (e.g., Czechia, Hungary, Serbia, Slovakia, Russia, Ukraine) assessed the effect of conventional tillage, reduced tillage, and no-till systems on crop yield, as well as nitrogen availability, soil microbiology, and crop nutrient profiles over multi-year periods (5–15 + years) [12,29]. In a paper on different tillage systems, Birkás et al. show that minimized tillage can reduce soil compaction compared to heavy tillage, promoting better water infiltration and higher crop yields [31]. The research of Fecák et al. [29] also confirms that the seed-filling stage was identified as the most sensitive to water stress (leading to a reduction in yield in this case).

3.3. Soil Nexus

3.3.1. Main Chemical Properties

The content of nutrients and the main soil chemical properties status of spring 2025 are displayed in

Table 8. The main chemical parameters of topsoil by treatments, and the following layers, as of spring 2025.

Treatment	Soil Layer	P mg/kg	K mg/kg	Mg mg/kg	Ca mg/kg	pH/KCl	Cox. %	Humus %	C/N
T1	0–15 cm	161.6	823.1	295.3	2525	5.51	2.01	3.46	10.6
	15–30 cm	168.3	617.6	279.2	2392	5.23	1.85	3.19	10.3
	0–30 cm	165.0	720.4	287.3	2459	5.37	1.93	3.33	10.5
	evaluation	high	very high	high	good	acidic	good	good	middle
T2	0–15 cm	159.7	865.5	360.8	2379	5.63	1.89	3.26	10.4
	15–30 cm	167.4	932.5	366.8	2415	5.62	1.89	3.26	9.8
	0–30 cm	163,6	899.0	363.8	2397	5.63	1.89	3.26	10.1
	evaluation	high	very high	very high	good	weakly acidic	good	good	middle
T3	0–15 cm	192.0	1049.4	392.7	2717	5.92	2.09	3.60	10.3
	15–30 cm	220.5	1092.3	408.9	2509	5.87	1.93	3.33	10.3
	0–30 cm	206.3	1070.9	400.8	2613	5.90	2.01	3.46	10.3
	evaluation	very high	very high	very high	good	weakly acidic	good	good	middle
T4	0–15 cm	245.6	911.0	261.6	1961	5.14	1.80	3.10	9.6
	15–30 cm	235.1	846.3	249.5	1969	5.12	1.87	3.22	10.6
	0–30 cm	240.4	878.7	255.6	1965	5.13	1.84	3.16	10.1
	evaluation	very high	very high	good	middle	acidic	good	good	middle

P—available phosphorus content, K—available potassium content, Mg—available magnesium content, Ca—exchangeable calcium content, pH/KCl—soil reaction in KCl, C_ox.—soil organic carbon, humus—humus content, C/N—ratio of soil organic carbon to total nitrogen; T1—‘Deep Loosening’, T2—‘Plowing’, T3—‘Strip-Till’, T4—‘No-Till’.

. After the experiments were carried out (spring 2025) in the depth 0–30 cm, the mean content of available phosphorus was in the range from 163.6 mg/kg to 240.4 mg/kg; the content of available potassium reached from 720.4 mg/kg to 1070.9 mg/kg; the content of available magnesium was from 255.6 mg/kg to 400.8 mg/kg, and the content of exchangeable calcium was from 1965 mg/kg to 2613 mg/kg. In terms of criteria for assessing the results of chemical analyses of arable soil (Slovak Republic, Regulation No. 151/2016, 2016) [32], middle-heavy Luvic Chernozem belongs to soils with a high to very high content of available phosphorus, a very high content of available potassium, a good to very high content of available magnesium, and middle-to-good content of exchangeable calcium.

Across this experiment, the average available phosphorus content was 193.8 mg/kg, whereby the highest content was on treatment T4 > T3 > T1 > T2 (240.4 > 206.3 > 165.0 > 163.6 mg/kg). Phosphorus is an essential nutrient for plant growth; it plays a key role in energy metabolism, root development, flowering, and fruit ripening. A significant portion of phosphorus is removed from the soil through the harvest of grain and biomass. ‘No-Till’ (T4) and ‘Strip-Till’ (T3) generally result in lower crop yields and, therefore, lower phosphorus offtake, leaving more phosphorus in the soil. In contrast, ‘Plowing’ (T2) and ‘Deep Loosening’ (T1) tend to produce higher yields, leading to greater phosphorus removal and a consequent reduction in its content in the soil.

No-Till means that the soil is virtually not disturbed, and sowing is carried out directly into the residues of the previous crop. Phosphorus is a poorly mobile element, and when the soil is not turned over, it accumulates in the surface layer—i.e., where fertilizers are applied or where organic matter mineralization takes place. Under ‘No-Till’, a higher content of available phosphorus was observed in the upper soil layer (0–15 cm) compared to the lower layer (15–30 cm). In contrast, with greater soil disturbance in other tillage treatments, higher concentrations of available phosphorus were found in the deeper soil layer.

‘Deep Loosening’ (T1) does not disturb the soil horizons like tillage and mainly opens the soil without mixing it. It does not transfer a significant amount of available potassium from the upper layer to the lower one, so the horizons remain relatively distinct. Potassium is relatively mobile, but not to the extent that it moves significantly deeper on its own. As a result, a lower content of available potassium was found in the lower soil layer. The same pattern was observed for available magnesium and exchangeable calcium. ‘Deep Loosening’ did not promote the mixing of nutrients between layers; it only improved the physical properties of the soil, such as permeability and aeration.

The soil reaction in KCl ranged from 5.13 to 5.90 at a depth of 0–30 cm under different tillage treatments. According to the assessment criteria (Slovak Republic, Regulation No. 151/2016, 2016) [32], this range is classified as acidic to slightly acidic. The values of exchangeable soil reaction depended on the tillage method. Higher average values were observed in different treatments in the following order: T3 > T2 > T1 > T4 (5.90 > 5.63 > 5.37 > 5.13).

Soil depth had an effect on soil reaction in KCl. As depth increased, soil reaction values decreased (Table 7). Higher soil reaction values were detected in the upper soil layer (0–15 cm), while lower values were observed at a depth of 15–30 cm. The difference between the first and second soil depths was only 0.01 in ‘Plowing’ (T2), 0.05 in ‘Strip-Till’ (T3), and 0.02 in ’No-Till’ (T4). A significantly greater difference (0.28) was observed in ‘Deep Loosening’ (T1).

In ‘No-Till’ (T4), a higher soil organic carbon content was observed at a depth of 15–30 cm (1.87%) compared to the 0–15 cm layer (1.80%). The lower soil organic carbon content in the upper layer is associated with faster organic matter decomposition due to higher temperatures, greater oxygen availability, and increased microbial activity. In contrast, in the lower layer (15–30 cm), decomposition is slower due to limited oxygen and reduced biological activity, resulting in a higher soil organic carbon content.

Under more intensive tillage systems (‘Deep Loosening’—T1, ‘Plowing’—T2, and ‘Strip-Till’—T3), a higher soil organic carbon content was found in the upper soil layer (0–15 cm) compared to the 15–30 cm depth. The top 0-3 cm soil layer, which is associated with higher carbon content especially under no-till in general [3,5,8,19], was not monitored separately.

The calculated ratio of organic carbon to total nitrogen is an indicator of humus quality. The ratio of organic carbon to total nitrogen varied from 10.1 to 10.5 at observed variants of tillage in total depth of 0–30 cm (Table 7). Soil organic matter decomposition was intensive, as determined by the ratio of organic carbon to total nitrogen. The ratio of organic carbon to total nitrogen was not significantly affected by the tillage.

Examining soil physical quality factors allows for the assessment of their interactions across different land use systems. One of the main objectives of land management is to minimize the damage caused by tillage methods and other technology-driven field operations. In practice, conventional land use systems often lead to a decline in soil quality, reducing both moisture management capacity. Furthermore, as soil quality deteriorates, its vulnerability to climatic extremes tends to increase workability [18].

3.3.2. Penetrometric Resistance and Key Precondition

Across this experiment, the total average soil penetrometric resistance was 26.7 kiloponds, whereby the treatment order was T2 < T3 < T1 < T4 (22.0 < 24.4 < 28.2 < 32.3 kp),

Table 9. The average values of soil penetrometric resistance (in kiloponds) by soil layers.

Treatment	Soil Layer
	0–15 cm	15–30 cm	0–30 cm	30–60 cm	0–60 cm
T1	24.4	23.7	24.0	32.4	28.2
T2	15.9	14.7	15.3	28.6	22.0
T3	21.2	21.2	21.2	27.5	24.4
T4	25.8	28.0	26.9	37.7	32.3

T1—‘Deep Loosening’, T2—‘Plowing’, T3—‘Strip-Till’, T4—‘No-Till’.

and Figure 2. According to Act No. 220/2004 Coll. [3], the threshold values of penetrometric resistance for compacted agricultural soils with a loamy texture range from 37 to 42 kp.

The lower threshold of 37 kp in the 0–60 cm soil layer was not exceeded at tillage treatments, evaluating them by the total average. However, in the ‘No-Till’ treatment, values exceeding the upper threshold of 42 kp were observed at depths between 40 and 44 cm (ranging from 42.9 to 43.6 kp), thus indicating potential soil compaction in this layer, under the criteria set by Act No. 220/2004 Coll [3].

Overall, the soil penetrometric resistance decreased with the increase in tillage intensity, while the treatment mutual differences were more marked, compared to the soil chemical parameters described earlier. This may indicate the predominant impact of the tillage on soil physical properties, through which the other parameters are preconditioned, including the overall favorable impact on the crop yield. This agrees with earlier findings and emphasizes that different soil tillage significantly influences the crop yield due to marked changes in physical characteristics of soil, especially bulk density and porosity, both of which affect soil water and nutrient dynamics [3,13,14,29,33,34,35,36]. This is also confirmed by Modiba et al. and Birkás et al. [34,37,38]. Higher penetration resistance and soil compaction significantly reduce yields, especially in crops with deep root systems. The use of minimal tillage systems and soil monitoring with a penetrometer can improve the physical properties of the soil and support higher yields.

Penetrometric measurements of Badalíková et al. [9] carried out as part of agronomic experiments with sugar beet confirmed that on Chernozem soils with favorable physical properties, the application of deep loosening is not necessary, as these soils can naturally regenerate their structure. Badalíková’s research [9] also emphasizes that preserving soil structure and limiting soil degradation supports rhizobia activity and increases the bioavailability of nitrogen for crops.

Long-term studies of Kotorová et al. [13] show that reduced tillage (RT) sometimes could result in lower bulk density and better macroporosity in the subsoil compared to conventional tillage (CT). However, CT typically has a higher total porosity at the surface layer due to mixing and loosening effects [3,5,8,13,19].

3.4. Carbon/Nitrogen Nexus

3.4.1. Soil Mineral Nitrogen and Total Nitrogen

The average soil mineral nitrogen content (Nan) was 39.5 mg/kg, occurring at 0–15 cm and 15–30 cm depths of the soil, ranging from 36.0 to 43.7 mg/kg. The average soil mineral nitrogen content was 38.7 mg/kg in the depth 0–15 cm and 40.2 mg/kg in the depth 15–30 cm (

Table 10. The content of mineral nitrogen forms in topsoil by treatments and the following layers, as of spring 2025.

Treatment	Soil Layer	N-NH4+, mg/kg	N-NO3−, mg/kg	Nan., mg/kg	Nt, %
T1	0–15 cm	18.5	19.0	37.5	0.190
	15–30 cm	22.8	17.2	40.0	0.179
	0–30 cm	20.7	18.1	38.8	0.185
T2	0–15 cm	17.4	18.6	36.0	0.182
	15–30 cm	18.2	25.5	43.7	0.193
	0–30 cm	17.8	22.1	39.9	0.188
T3	0–15 cm	17.8	20.0	37.8	0.204
	15–30 cm	20.3	19.6	39.9	0.188
	0–30 cm	19.1	19.8	38.9	0.196
T4	0–15 cm	20.8	22.5	43.3	0.188
	15–30 cm	17.9	19.2	37.1	0.176
	0–30 cm	19.4	20.9	40.2	0.182

N-NH₄⁺—ammoniacal nitrogen content, N-NO₃⁻—nitrate nitrogen content, N_an.—inorganic nitrogen content, N_t—total nitrogen; T1—‘Deep Loosening’, T2—‘Plowing’, T3—‘Strip-Till’, T4—‘No-Till’.

and Figure 3). The treatments order, by N_an average values decreasing, was as follows: T4 > T2 > T3 >T1 (40.2 > 39.9. > 38.9 > 38.8 mg/kg); however, according to the topsoil layers, the position of T4 was opposite to the order (T4 > T3 > T1 >T2 vs. T2 > T1 > T3 >T4; or 43.3 > 37.8 > 37.5 > 36.0 vs. 43.7 > 40.0 > 39.9 > 37.1 mg/kg, respectively, upper vs. lower layer).

The mineral nitrogen content at a depth of 0–30 cm in individual tillage variants was comparable. However, if we compare the inorganic nitrogen contents at two soil depths (0–15 cm, 15–30 cm), a lower inorganic nitrogen content was found at a depth of 0–15 cm in soil with higher disturbance ‘Deep Loosening’ (T1), ‘Plowing’ (T2), ‘Strip-Till’ (T3) compared to a depth of 15–30 cm. At a depth of 0–15 cm, a lower inorganic nitrogen content was found by 2.5 mg/kg with ‘Deep Loosening’, 7.7 mg/kg with ‘Plowing’, and 2.1 mg/kg with ‘Strip-Till’.

Differences in the distribution of inorganic nitrogen (N) between soil depths under various tillage treatments can be attributed to the extent of soil disturbance and the dynamics of organic matter decomposition. Lower concentrations of inorganic nitrogen in the 0–15 cm layer compared to the 15–30 cm layer, which may be explained by the mechanical disturbance and mixing of the soil, which facilitates the downward movement of nitrogen compounds and enhances microbial activity throughout the tilled profile. These processes can accelerate the mineralization of organic nitrogen and lead to partial leaching or redistribution of mineral forms of nitrogen (nitrate and ammonium) into deeper soil layers.

In contrast, under ‘No-Till’ conditions (T4), higher concentrations of inorganic nitrogen by 6.2 mg/kg were detected in the upper 0–15 cm soil layer. This is likely due to the accumulation of plant residues on the soil surface, which decompose slowly and release nitrogen directly into the topsoil. Additionally, the absence of soil disturbance in no-till systems preserves soil structure and microbial habitats in the surface layer, promoting localized mineralization and retention of nitrogen near the soil surface, while limiting its movement into deeper layers.

These findings highlight the significant influence of tillage intensity on the vertical distribution of mineral nitrogen in the soil profile, which may have further implications for nutrient availability, crop uptake, and potential nitrogen losses.

The average total nitrogen content in the 0–30 cm depth ranged from 0.182 to 0.196% (Table 9). A significant positive relationship (r = 0.90) was confirmed between soil organic carbon and total soil nitrogen. This means that the total nitrogen content in the 0–30 cm depth was higher under intensive tillage (from 0.18 to 0.196%) compared to ‘No-Till’ (0.182%), similar to that of soil organic carbon.

By comprehensive research of [39], several studies have reported no significant differences in nitrate nitrogen content within the top one-meter soil layer between deep soil loosening and no-till practices. Findings of [12] indicated that mineral nitrogen concentrations at 0–10 cm and 20–30 cm soil depths were lower under conventional ploughing than under no-till management. These results suggest that reducing or eliminating ploughing may contribute to improved nitrogen retention and overall nitrogen reserves in the soil.

Other conclusions are clearly in favor of a no-till system, according to which long-term implementation of reduced or no-till practices has led to higher concentrations of nitrate nitrogen in the topsoil compared to conventional tillage systems [4,40]. Compared to ploughing, no-till management stimulated microbial activity involved in nitrogen transformations by as much as seven times, especially within the upper 20 cm of soil [40,41]. No-till practice increased mineral nitrogen retention in the rhizosphere compared to conventional ploughing [7].

In the study of Kravchenko et al. [42], total nitrogen (N_t) exhibited seasonal variation, with levels decreasing during the growing season and recovering after harvest. Implementation of reduced tillage enhanced nitrogen accumulation in surface layers and led to more favorable C:N ratios, which also occurred in this work.

As opposed to conventional tillage approaches, Farhangi-Abriz et al. [36] observed that minimum tillage increased soil microbial biomass carbon, dehydrogenase activity, and nodule formation, leading to approximately 15% higher nitrogen uptake and around 6% improvements in soybean growth, chlorophyll content, ground cover, grain, protein, and oil yields. So, although it did not raise soil organic matter content, minimum tillage enhanced soybean productivity by stimulating microbial activity, nodulation, and nitrogen uptake.

3.4.2. Root Nodules, Convergent and Occasionally Reverse Key Indicator

The average root nodules count per soybean plant was 7.49, ranging from 6.70 to 9.18 within the treatments (

Table 11. The plant height and parameters of root nodules at soybean stand, status of 30 June 2025.

Treatment	Plant Height	Plant Count		Nodules Count	Nodules Dry Weight
	cm	No. per 1 m	No. per 1 m	No. per Plant	g per 1 m	g per Plant
T1	42.5	34	313	9.18	0.670	0.020
T2	44.1	33	203	6.85	0.295	0.009
T3	37.8	33	228	6.70	0.607	0.018
T4	40.2	35	256	7.23	0.912	0.026

T1—‘Deep Loosening’, T2—‘Plowing’, T3—‘Strip-Till’, T4—‘No-Till’.

, Figure 4). The highest count of 9.18 was found at ‘No-Till’ T4, and the lowest one of 6.70 at ‘Strip-Till’ T3. When ranking the tillage treatments by the nitrogen fixing root nodules dry weight per plant (0.009 < 0.018 < 0.020 < 0.026 g per plant in dry matter), the obtained order of classical ‘Plowing’ < ‘Strip-Till’ < ‘Deep Loosening’ < ‘No-Till’ was in reverse mode of the tillage intensity and crop yield as well.

The reverse order obtained on the root nodules to the crop yield is in agreement with our earlier findings on soybean different tillage and fertilization long-term studies based on small-plot trials carried out under similar soil–climatic conditions [14,15]. That reverse order can be explained by findings of Bengough et al. [43] on higher soil compaction, which limits oxygen diffusion into the soil, which can reduce symbiotic bacteria in the nodules and, thus, nitrogen fixation. This oxygen reduction creates anaerobic conditions, stressing symbiotic bacteria within root nodules, hindering their ability to convert atmospheric nitrogen into ammonia through biological nitrogen fixation, ultimately reducing crop yields. Also, by corresponding findings of Bordeleau and Prévost [44], the relationship between root growth and nitrogen uptake is limited by soil compaction. Although the number of nodules may not directly determine yield, a deep and healthy root system improves the plant’s ability to take up mineral nitrogen from the soil as well as from biological fixation. Concerning the nitrogen requirements at higher yields, Salvagiotti et al. [33] concluded that it was obvious that with a larger harvest, soybeans need more nitrogen than the nodules are able to supply, so a larger portion is taken directly from the soil.

When studying nutrition intensity, it was found that the soil inorganic nitrogen has a negative effect on root nodules forming mainly in cases when the soil N_an content exceeds 80 mg/kg, which could vary, especially according to soil fertility. Based on the earlier findings [14,15], soybeans can obtain 45–90% of their nitrogen through symbiotic fixation, with nitrogen being taken up from both the air and the soil. Integrating this information, the root nodules count and weight are sensitive but not entirely reliable indicators of fixation, as their size and metabolic activity also play a role under optimal growing conditions [14,15,38,45,46,47,48,49]. These findings align with this study, showing that nodule count as weight is a converging indicator that occasionally even reverses indicators.

Soil properties are frequently applied to model, forecast, or estimate various environmental processes, such as assessing environmental risks, predicting agricultural yields, estimating carbon stocks, and modeling climate change impacts [36,50,51]. In general, biological nitrogen fixation represents a sustainable strategy for enhancing soil fertility, as it contributes nitrogen to succeeding in crops and concurrently mitigates the environmental impacts associated with the use of synthetic fertilizers [8]. Medvedeva et al. [4] observed that tillage system strongly influences biological nitrogen fixation, obtaining higher yields under minimum and zero tillage compared to conventional moldboard ploughing. Additionally, soil compaction can hinder root development and negatively affect nodule formation.

Nitrogen fixation is a visible and sensitive indicator of the impact of soil cultivation, especially when assessing the sustainability and biological health of soil under different tillage systems. This influences mainly soil structure and aeration, moisture availability, organic matter content, and microbial activity (especially rhizobia). Although conventional tillage (CT) produced marginally higher yields (in tested rainfed wheat–soybean systems), it also led to increased nutrient losses and lower rates of biological nitrogen fixation [2].

Because nitrogen fixation depends on biological, chemical, and physical soil properties, it serves as a holistic indicator of how soil cultivation impacts the fertility of soil, microbial ecosystem health, and the sustainability of cropping systems. Increased nitrogen fixation under ‘No-Till’ or reduced tillage systems is attributed to improved conditions for microbial activity and reduced disruption of the soil environment [6]. Microbial communities within stable soil aggregates under no-till systems exhibit higher rates of organic matter cycling compared to those in conventional tillage systems, where aggregate disruption and compaction hinder microbial activity and reduce nitrogen fixation [46].

The value of nitrogen fixation lies in the increased soybean productivity resulting from improved nitrogen availability enabled by the symbiotic relationship between the bacterium Bradyrhizobium japonicum and soybean root hairs. Through the nitrogen fixation process by B. japonicum, atmospheric nitrogen can be assimilated by the plant. Certain soil characteristics and environmental conditions, as well as crop rotation, can influence the survival and activity of B. japonicum and subsequent soybean nodulation. Soil characteristics that influence the survival of B. japonicum include pH, temperature, texture, water content, and available nitrogen [45].

If soil pH/KCl is below 6.0, acidic soil reaction can limit nitrogen fixation [48]. As noted by Neupauer [48], the optimal soil pH/KCl for soybean cultivation ranges from 6.0 to 7.0, corresponding to a slightly acidic to neutral environment. Within this range, the availability of essential macro- and micronutrients required for healthy soybean growth is maximized. Additionally, nodule-forming symbiotic bacteria thrive best under these pH conditions, which facilitate effective rhizobial infection of soybean root hairs and promote successful nodule development.

The integration of no-till practices with crop residue retention promoted biological nitrogen fixation in soybeans, optimized nitrogen uptake by rotational wheat, and mitigated nitrogen leaching losses [49]. Impact of no-till (NT) mulching on crop yield is significantly influenced by precipitation levels, particularly in soybean–wheat systems [5]. Flooded soils often lead to anaerobic conditions, which are harmful to rhizobia. Additionally, soil compaction can hinder root development and negatively affect nodule formation. Cold soil temperatures, particularly below 25 °C, also slow down both nodule development and the process of biological nitrogen fixation [2,15,46]. For normal growth and development, soybeans need about 700 mm of precipitation per year. The moisture requirements of soybeans during the growing season correspond to the dynamics of dry matter formation. Soybeans are particularly sensitive to water shortages during flowering, pod formation, and seed setting [15]. If drought occurs, Rhizobia populations and nodulation may be reduced in dry soil [15].

Tedone et al. [8] conducted research on faba beans, in which the tillage system had a marked impact on biological nitrogen fixation: both the percentage of nitrogen derived from the atmosphere (% Ndfa) and the total amount of nitrogen fixed by faba bean were significantly higher under the no-till (NT) system compared to conventional tillage (CT). This suggests that no-till practices enhance the conditions for symbiotic nitrogen fixation, likely due to improved soil structure, moisture retention, and microbial activity. Higher organic matter content can also be positively influenced by soil condition, which also contributes to the activity of the soil microbial community [8]. Native soil nitrogen (N) remains relatively steady and continual, and its biological dynamics are closely mediated by interactions between soil microorganisms and plant roots. Tóth et al. [14] assessed the total number of microorganisms following the application of soil conditioners under different soil cultivation methods after soybean harvest. The highest average microbial counts were recorded under minimal tillage, followed by direct sowing, while the lowest values occurred with conventional tillage.

The largest difference was observed in the no-till farming system, where the C/N ratio in the top layer was 9.6, at which nitrogen in the form of NH₄⁺ may be released into the environment, from where it can be volatilized, nitrified, taken up by plants and microorganisms, and may participate in the formation of complexes with organic matter in the soil. C/N ratio controls how fast organic matter decomposes and whether nitrogen is tied up or released in the soil. In Slovak soils, such as Chernozem, the C/N ratio ranges from 6:1 to 12:1 [19].

4. Conclusions

This paper quantifies the mid-term impact of four different soil tillage systems on the agronomical key carbon/nitrogen nexus within an on-farm sub-factorial large-scale open-field technology experiment, which was carried out at optimal soil conditions of fertile Luvic Chernozem under European moderate continental climate.

Unlike the crop yield, which decreased with tillage intensity decreasing (21.38 > 19.30 > 18.88 > 18.62 t/ha in dry matter cumulatively; classical ‘Plowing’ > ‘Strip-Till’ > ‘Deep Loosening’ > ‘No-Till’, respectively), the key agronomical carbon/nitrogen parameters (main soil chemical indicators, root nodules count, soil penetrometric resistance were followed) differed in varying extent and manner. This study found that the count of soybean root nodules was the highest at the no-till treatment, where the soil tillage was the least intensive, and the yields were the lowest. However, the order of the treatments by the nitrogen fixing root nodules dry weight per plant (classical ‘Plowing’ < ‘Strip-Till’ < ‘Deep Loosening’ < ‘No-Till’; 0.009 < 0.018 < 0.020 < 0.026 g per plant in dry matter, respectively) was even in the reverse order of the tillage intensity and crop yield as well. This suggests the most efficient nitrogen fixing with root nodules at the most intensive soil tillage, where the yield was the highest, and, a priori, the highest nitrogen consumption occurred, but inorganic nitrogen content in soil was the highest at no-till since following another tillage order and carbon/nitrogen pattern.

It can be concluded that the number of nodules on soybean roots is related to the soil penetration resistance. Although it is not necessary for the nodules to be directly proportional to the crop yield, the crop yield clearly indicates the nitrogen uptake. Since the nitrogen consumption is/was greater at higher yield, the activity of nodules and the mobility of mineral nitrogen are/were, therefore, inhibited by soil compaction. The number of nodules may not be directly related to yield; as is logical, the ratio between fixed and mineral nitrogen from the soil is decisive, which is influenced by compaction, oxygen access, water, and mineral nitrogen availability. The number of nodules, therefore, only shows the potential for nitrogen fixation, but the actual yield is/was determined by the combined supply of nitrogen from the soil and from nodules. Therefore, the yield is/was directly related to nitrogen consumption, but not to the number of nodules.

Based on the obtained results, similar conclusions could be derived for the carbon/nitrogen nexus, further key indicators, and soil penetrometric resistance as well. Differences in the distribution of organic carbon and inorganic nitrogen between soil depths under various tillage treatments can be attributed to the extent of soil disturbance and the dynamics of organic matter decomposition. Under more intensive tillage systems (‘Deep Loosening’, ‘Plowing’, ‘Strip-Till’), a higher soil organic carbon content was found in the upper soil layer (0–15 cm), while in no-till, a higher soil organic carbon content was observed in the lower soil layer (15–30 cm). In general, this may indicate carbon losses came from the decomposition of crops post-harvest residues on the soil surface into the atmosphere, since the top 0-3 cm soil layer (where higher carbon content may have occurred especially under ‘No-Till‘) was included, but not monitored separately. In the case of inorganic nitrogen, the situation was the opposite. The lower inorganic nitrogen content was found in the upper soil layer with its higher disturbance (‘Deep Loosening’, ‘Plowing’, and ‘Strip-Till’) compared to ‘No-Till’, where lower inorganic nitrogen content was observed in the lower soil layer. The soil penetrometric resistance, which was measured within the soil profile of 0–60 cm, decreased with the tillage intensity, while the differences were more marked in comparison to other indicators. This confirms the predominant effect of the favorable physical soil conditions on yields and the following carbon/nitrogen nexus indicators, proving the mid-term marked impact of soil tillage.

All the findings were registered and can be concluded despite the fact that the tillage frequency at both classical ‘Plowing’ and/or ‘Deep Loosening’ was reduced, so that was applied to soybean only, once per 2 years during the trial period of 2020/21–2024/25, with wheat/soybean two-crop rotation. Since this research proves that tillage, as well as all the following indicators, have different importance and mid-term impact within the nexus studied, it is essential to maintain the soil in a certain optimal state so that its properties are harmonized with each other. Finally, it can be concluded directly that in fertile Chernozem soils, plowing still gives the most stable yields, and though no-till increases nodulation, farmers’ practices need to be oriented on yield vs. soil health balance. Therefore, further investigations on long-term impacts are required.