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The Impact of the Farming System and Forecrops on Selected Soil Quality Indicators and Oat Yield

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30 September 2026

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02 October 2026

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Abstract
A field experiment was conducted in 2020–2022 at the Czesławice Experimental Farm (Lublin Voivodeship, Poland) to investigate the chemical properties and enzymatic activity of loess soil under oat cultivation, as well as crop productivity (grain yield and grain protein content). The first experimental factor was the farming system: (A) an organic system, without chemical plant protection products or NPK mineral fertilization (only the Humac Agro fertilizer, approved for organic farming, was applied); and (B) a conventional system, with chemical plant protection and NPK mineral fertilization at rates recommended for oat. The second factor comprised different forecrops for oat: potato, spring barley, narrow-leafed lupine, oat, and winter wheat. The organic system had a more fa-vorable effect on the soil contents of organic carbon (by 0.08 percentage points, pp), mag-nesium (by 10%), boron (by 13%), and zinc, manganese, and copper (by 5.5%, 18%, and 7%, respectively), and promoted higher soil enzymatic activity, with increases ranging from 9–14% (for alkaline phosphatase, urease, dehydrogenase) to 32–39% (for acid phos-phatase and protease). In contrast, the conventional system favored soil nitrogen, phosphorus, and potassium contents (increases of 0.02 pp, 12%, and 25%, respectively). The total sorption capacity of the soil was significantly higher under the organic system (by approximately 6%), particularly in interaction with potato as the forecrop (40.9 cmol(+) kg−1). The forecrops such as potato and narrow-leafed lupine had a more beneficial effect on soil quality and oat productivity than the cereal forecrops. Oat grain yield under the organic system was, on average, only 8% lower than under the conventional system. When favorable root-crops and legume forecrops were used, the productivity of organically grown oat matched that of oat grown conventionally after cereal forecrops. Organic oat cultivation resulted in a significantly higher grain protein content.
Keywords: 
conventional farming
;  
organic farming
;  
effect of forecrops
;  
soil quality
;  
oat productivity
Subject: 
Environmental and Earth Sciences  -   Soil Science

1. Introduction

This article analyzes the effect of various farming systems on selected soil quality indicators and oat grain yield. The choice of the farming system determines the biological, chemical, and physical environment of the soil, as well as its biochemical and enzymatic indicators [1,2].
The organic system is based on the rational and sustainable management of agricultural land in compliance with the principles of environmental protection and the conservation of natural resources. It ultimately ensures the production of healthy foods owing to its positive effect on water and soil quality [3,4]. Soil nutrient composition also depends on the quantity and quality of post-harvest residues of the crops grown in the rotation (both main crops and their forecrops) [5].
A comprehensive study by Futa et al. [6] demonstrated that key enzymatic indicators of soil quality reached considerably higher values in organic than in conventional systems: alkaline phosphatase activity increased by 67% on average, acid phosphatase by 29%, urease by 28%, and dehydrogenase by 25%, while the potential biochemical index of soil fertility improved overall by 39%.
Organic systems and sustainable practices that use animal manure, vermicompost, or other natural fertilizers increase soil total organic carbon (TOC) by approximately 18% and total nitrogen (Nt) by 15%, while also raising the level of available ammonium nitrogen (N-NH₄⁺) by 34%. In contrast, intensive conventional systems relying exclusively on synthetic mineral fertilizers often lead to a decline in soil organic matter (SOM) over time [7].
The farming system also modifies the physical properties of the soil. Organic or integrated systems (particularly under reduced or no-tillage conditions) improve wet aggregate stability and decrease soil bulk density, leading to better soil aeration and enhanced crop root development [8,9].
Soil quality under individual farming systems is also related to the forecrops. Oat has a well-developed, highly efficient root system that reaches up to 120 cm deep into the soil, allowing it to tolerate slightly acidic soils and to take up sparingly available nutrients more efficiently than other spring cereals [10]. However, the choice of forecrop directly affects the condition of the soil in which oat is subsequently grown [11].
Legumes (e.g., red clover, lupine, vetch) are highly beneficial forecrops for oat. Their cultivation generates residues enriched in nitrogen through symbiotic nitrogen fixation. This natural nitrogen input substantially improves the chemical profile of the soil, supporting subsequent oat growth while reducing the need for synthetic fertilizers [12]. Growing oat in monoculture or after other cereal forecrops may enhance the development of soil-borne pathogens (fungi and bacteria). In contrast, root-crop forecrops (potato, sugar beet) help to break the life cycles of soil and plant pests, protect soil structure, and maintain high microbial metabolic diversity [13].
Fess and Benedito [14] reported that crop yields under conventional farming were usually 15-20% higher than under organic management. This difference results from the direct, targeted, and synchronized supply of NPK to the soil and from effective chemical control of weeds and pests. Under organic management, yields are lower but show greater seasonal stability. The yield reduction in the organic system stems from slower mineralization of soil nutrients and greater weed pressure. Nevertheless, the long-term improvement in soil health under organic management may eventually narrow this yield gap [14].
This article also examines the consequences of the adopted farming systems for oat yields. While soil biological indicators generally change positively in organic and conservation systems, oat productivity is higher under conventional farming. To increase oat productivity in the organic system, efforts should focus on achieving a balanced soil nutrient supply and on improving soil biological and physical properties [15]. Natural and organic fertilizers suitable for organic farming have been shown to increase oat yields in some agroecosystems by approximately 15% compared with synthetic fertilization regimes in the conventional system. This increase is attributed to the restoration of efficient carbon use by microorganisms in the organic system and to better protection of soil structure by natural fertilizers [16].
It should be noted that although high-input conventional systems allow maximum yields to be achieved, organic and sustainable farming systems combined with crop rotation (favorable forecrops) and appropriate management of natural fertilization reduce yield variability, conferring substantially greater climatic resilience on the crop against the effects of drought (e.g., kernel abortion) and periodic water deficits [17,18].
The aim of this study was to determine selected chemical properties and the enzymatic activity of loess soil under oat, as well as oat productivity, depending on two farming systems (conventional and organic) and five forecrops (potato, spring barley, narrow-leafed lupine, oat, and winter wheat). The following hypothesis was formulated: rational management under the organic system, including the use of the organic-approved mineral fertilizer Humac Agro combined with favorable forecrops (potato, narrow-leafed lupine), would result in better soil quality parameters than the conventional system (entailing comprehensive chemical protection of oat and standard NPK mineral fertilization). It was also assumed that, under such organic conditions, oat productivity would not differ substantially from that achieved in the conventional system.

2. Materials and Methods

2.1. Experimental Design and Field Management

A strict field experiment with oat (Avena sativa L., cv. ‘Bingo’) was conducted at the Czesławice Experimental Farm (51°30′ N, 22°26′ E; Lublin Voivodeship, Poland) on a loess-derived Luvisol with a silt loam texture (PWsp), classified as a good wheat soil complex (soil quality class II) [19]. The study was carried out in 2020–2022. Oat was grown under organic and conventional systems in a split-plot design with three replicates. In both farming systems, the individual plot area was 4 m × 5 m (20 m2), giving a total of 30 experimental plots and a total experimental area of 600 m2.
Before the experiment was established (autumn 2019, 2020, and 2021), the soil had a medium content of available macronutrients (Table 1).
*The methodological assumption of this field experiment was that the initial soil nutrient composition at the organic and conventional plots should be similar in each study year. Accordingly, in each of the three study years, the experiment (sowing/planting of oat forecrops) was established under identical soil conditions. This allowed an objective assessment of the effects of the agricultural technology used in the individual farming systems (level and type of fertilization and pesticides) and of the forecrops on the evaluated soil quality parameters under oat. In addition, before the establishment of the strict field experiment and the sowing/planting of oat forecrops (2018–2020), white mustard was grown as a pre-forecrop on all conventional and organic plots to equalize field conditions.
The following experimental factors were adopted: I. Farming systems for oat: CS – conventional system, which included NPK mineral fertilization (ammonium nitrate – 34% N; granulated superphosphate – 40% P2O5; potassium salt – 60% K2O), seed dressing; herbicide, fungicide, and insecticide applications, and mechanical weed control (harrowing before oat emergence and at the 3-4-leaf stage); OS – organic system, with fertilization using the natural mineral fertilizer Humac Agro, approved for organic farming, and mechanical weed control (harrowing before oat emergence and at the 3-4-leaf stage), without seed dressing, herbicide, fungicide, or insecticide applications. II. Forecrops for oat: a. potato (Solanum tuberosum L.) cv. ‘Tajfun’; b. spring barley (Hordeum vulgare L.) cv. ‘Jowita’; c. narrow-leafed lupine (Lupinus angustifolius L.) cv. ‘Agat’; d. winter wheat (Triticum aestivum L.) cv. ‘Indiana’; and e. oat (Avena sativa L.) cv. ‘Pablo’.
Oat on the organic plots was cultivated in accordance with the principles of organic farming, with appropriate isolation distances maintained from the conventional system (150 m) and from transport routes (500 m). Oat fertilization in the conventional system during the study years (2020–2022) was as follows: N – 60 kg ha−1 (25 kg before sowing and 35 kg at stem elongation stage, BBCH 32–34); P – 45 kg ha−1 (before sowing); K – 80 kg ha−1 (before sowing). In the organic system, Humac Agro was applied to oat at a rate of 300 kg ha−1 (before sowing). The chemical composition of Humac Agro is as follows: humic acid content, min. 62% of dry matter; free humic acid content, min. 50% of dry matter; moisture content, 20%; macronutrient content on a dry weight basis: N = 10.3 g kg−1, P = 1.05 g kg−1, K = 1.18 g kg−1, Ca = 16.80 g kg−1, Na = 12.80 g kg−1; micronutrient content on a dry weight basis: Fe = 14.50 g kg−1, Zn = 64 mg kg−1, Br = 77 mg kg−1, Cu = 19 mg kg−1, Se = 6 mg kg−1.
In the organic system, crop management consisted solely of two passes with a tine harrow. In the conventional system, oat was likewise harrowed twice, and chemical plant protection products were applied (seed dressing, herbicide, fungicide, insecticide): seed dressing – Sarfun 025 FS (a.i. triticonazole, 25 g L−1); herbicide – Chwastox Turbo 340 SL (a.i. MCPA + dicamba, 300 g L−1 + 40 g L−1), 1.0 L ha−1; fungicide – Amistar 250 SC (a.i. azoxystrobin, 250 g L−1), 0.9 L ha−1; and insecticide – Karate Zeon 050 CS (a.i. lambda-cyhalothrin, 50 g L−1), 0.25 L ha−1.
Soil tillage for oat and its forecrops was based on a conventional plough system.
Throughout the study period, the sowing and harvest dates of oat were identical in the conventional and organic systems (sowing: 12–15 April; harvest: 25–28 August). The oat seeding rate was also identical in both systems, at 180 kg ha−1. The sowing and harvest dates of the oat forecrops in 2020–2022 were likewise the same in both farming systems, as follows: sowing/planting – potato, 2–4 May; spring barley, 16–20 April; narrow-leafed lupine, 28 March–2 April; winter wheat, 24–27 September; oat, 13–16 April; harvest – potato, 28–30 September; spring barley, 15–17 August; narrow-leafed lupine, 25–28 August; winter wheat, 12–15 August; and oat, 18–20 August.
Details of the fertilization of the oat forecrops in the analyzed farming systems (conventional or organic) are provided in Table 2 and Table 3.
The rates of NPK mineral fertilizers, farmyard manure, and the organic-approved fertilizer Humac Agro applied to the individual crops (oat and forecrops) were based on agronomic recommendations for each species and on the initial soil nutrient status.

2.2. Measurements and Analytical Methods

Oat grain yield from each experimental plot was determined after drying the grain to a uniform moisture content of 14% and expressed in t ha−1. Grain samples were then collected for protein determination. Nitrogen content of oat grain was first determined by the Kjeldahl method. Each grain sample was digested in sulfuric acid with CuSO4 and TiO2 as catalysts, followed by distillation and titration, during which nitrogen was converted to NH3. Crude protein content was then calculated using a conversion factor of N × 5.3 [20]. After the oat harvest, to determine selected soil chemical properties for each treatment combination (farming systems × forecrops), soil samples were collected from the 0–25 cm layer with a soil sampling probe from an area of 0.20 m² on each plot. Soil was sampled at four randomly selected points within each individual plot (20 m²). In total, 96 soil samples were collected from the entire experiment (600 m²) in each study year. The reported results represent the mean of the four samples collected from each plot.
The following laboratory analyses were performed on soil samples collected from the individual oat treatment combinations (farming systems × forecrops):
- organic C content was determined using a carbon analyzer (SDCHN435);
- total N content was determined by the Kjeldahl method;
- available P and K were determined by the Egner–Riehm method;
- Mg content was determined by atomic absorption spectrometry (AAS);
- micronutrient contents (B, Cu, Mn, Zn) were determined by flame photometry;
- the cation exchange capacity (CEC) of the soil (cmol(+) kg−1) was determined by the Kappen method.
The activity of selected soil enzymes was also assessed:
- dehydrogenase activity (expressed as mg TPF kg⁻¹ soil d.m. d⁻¹) was determined by the method of Thalmann [21]. Soil samples of 5 g were used, with 2,3,5-triphenyltetrazolium chloride as the substrate. Incubation was carried out for 48 h at 30 °C in Tris-HCl buffer (0.2 M, pH 7.4);
- acid and alkaline phosphatase activities (expressed as mg PNP kg⁻¹ soil d.m. h⁻¹) were determined by the method of Tabatabai and Bremner [22]. Determinations were performed on 1-g soil samples, with disodium p-nitrophenyl phosphate as the substrate. Incubation was carried out at 37 °C for 1 h in a universal buffer at pH 6.5 for acid phosphatase and at pH 11 for alkaline phosphatase;
- urease activity (expressed as mg N-NH₄ kg⁻¹ soil d.m. 18 h⁻¹) was determined by the method of Zantua and Bremner [23]. Urea solution was used as the substrate, with 10-g soil samples. Incubation was carried out at 37 °C for 18 h;
- protease activity (expressed as mg tyrosine kg⁻¹ soil d.m. h⁻¹) was determined by the method of Ladd and Butler [24]. Casein was used as the substrate, with 2-g soil samples. Incubation was carried out at 50 °C for 1 h in Tris-HCl buffer (0.2 M, pH 8.0).
2.3. Statistical Analysis
The results were compiled in tables and analyzed statistically by the analysis of variance (ANOVA), and differences between means were evaluated using Tukey’s test (to determine the honestly significant difference, HSD, at p ≤ 0.05). No significant differences between study years were found for the measured traits of the field experiment; therefore, the tables present means from the three study years. Standard deviation (SD) was also calculated for the data presented in Table 4, Table 5, Table 6 and Table 7.

3. Results

The total nitrogen content in soil under oat depended significantly on the farming system. Regardless of the forecrop, a higher total N content was found under the conventional system – on average 0.08%, i.e., 0.02 percentage points (pp) more than under the organic system. In both farming systems, a significantly higher total N content of the soil (by 0.05–0.09 pp) was determined after narrow-leafed lupine and potato forecrops than after cereal forecrops (spring barley, oat, winter wheat). Moreover, among the cereal forecrops in the conventional system, soil total N content was significantly higher after winter wheat than after oat. This highlights the importance of the forecrop in shaping this soil quality parameter. The significantly highest total N content (0.16%) was found for the conventional system × narrow-leafed lupine forecrop interaction (Table 4).
Soil phosphorus content also depended significantly on the farming system. Under the conventional system, it averaged 129.9 mg kg−1, i.e., approximately 12% more than under the organic system (115.3 mg kg−1). The forecrop species also significantly modified its content in the soil. In both farming systems, the highest phosphorus content was recorded after narrow-leafed lupine and potato; however, in the organic system, these values were not significantly higher than those after any of the three cereal forecrops. In contrast, in the conventional system, soil P content after potato and narrow-leafed lupine was significantly higher (by approximately 6–10%) than after spring barley and oat (Table 4).
Soil potassium content differed significantly between farming systems. Under the conventional system, it averaged 241.9 mg kg−1, being approximately 25% higher than under the organic system. The individual oat forecrops significantly affected its content in the soil in both farming systems. After potato and narrow-leafed lupine, soil K content was higher by approximately 9% (organic system) to 15% (conventional system) than after cereal forecrops (spring barley, oat, winter wheat) (Table 4).
A different correlation between farming systems was observed for soil magnesium content under oat. The organic system was significantly more favorable for soil Mg content, which averaged 66.3 mg kg−1, i.e., was over 10% higher than in the conventional system. Irrespective of the farming system, the significantly highest soil Mg content was determined after potato and narrow-leafed lupine compared with all three cereal forecrops (by approximately 9% in the organic system and approximately 20% in the conventional system) (Table 4).
The organic system also had a more favorable effect on organic C content in soil under oat, which averaged 0.87% and was significantly higher than in the conventional system (by approximately 0.08 percentage points). In both farming systems, potato and narrow-leafed lupine forecrops significantly increased soil organic C content compared with all cereal forecrops, by approximately 0.08 pp (organic system) and approximately 0.12 pp (conventional system) (Table 4).
The farming system significantly affected soil boron content under oat. Under the organic system, B content averaged 2.46 mg kg−1 and was approximately 13% higher than under the conventional system. The forecrop species also significantly affected its content in the soil: the highest B content was found after potato and narrow-leafed lupine compared with all cereal forecrops studied. In the organic system, the increase in B content after potato and narrow-leafed lupine was approximately 10%, whereas in the conventional system it was approximately 8%. Notably, in the organic system, differences in soil B content were also found among the cereal forecrops: the oat forecrop resulted in approximately 6% higher B content than winter wheat and spring barley (Table 5).
Soil copper content was significantly modified by both experimental factors. The highest Cu content was determined in the organic system (on average 6.85 mg kg−1) compared with the conventional system (6.39 mg kg−1), with a difference approximating 7%. Irrespective of the farming system, narrow-leafed lupine and, in particular, potato forecrops increased soil Cu content by approximately 5–7% compared to cereal forecrops (Table 5).
The farming system significantly modified also soil manganese content. In the organic system, soil Mn content averaged 197 mg kg−1 and was nearly 18% higher than under the conventional system (167 mg kg−1). In both farming systems, root-crop and legume forecrops significantly increased its content in the soil compared with cereal forecrops, by approximately 11–13% (organic system) and 7–8% (conventional system), respectively (Table 5).
The content of zinc in the soil under oat was significantly higher, by approximately 5.5%, in the organic system (on average 8.64 mg kg−1) than in the conventional system (on average 8.18 mg kg−1). The forecrop had no significant effect on Zn content in neither of the farming systems only a tendency toward its higher content was observed, mainly after potato and, to a lesser extent, after narrow-leafed lupine (Table 5).
The organic system resulted in a significantly higher sorption capacity of the soil (by approximately 6% on average) compared to the conventional system. With respect to the individual oat forecrops, it was significantly higher after potato and narrow-leafed lupine than after winter wheat, spring barley, and oat. Significantly the highest sorption capacity of the soil (40.9 cmol(+) kg−1) was found for the organic system × potato forecrop interaction (Table 5).
Soil enzymatic activity under oat depended significantly on both experimental factors (Table 6). All analyzed enzymes exhibited significantly higher activity under the organic system than under the conventional system, regardless of the forecrop. The increases in enzyme activity in the organic vs. conventional system were as follows: dehydrogenase, approximately 14%; acid phosphatase, approximately 32%; alkaline phosphatase, approximately 9%; urease, approximately 14%; and protease, 39%.
The effect of oat forecrops on soil enzymatic activity varied. In the case of dehydrogenase activity in both farming systems, the narrow-leafed lupine forecrop had significantly most favorable effect compared with the other forecrops (an increase of approximately 10–21%), followed by potato (an increase of approximately 12%) compared to the cereal forecrops (spring barley, oat, winter wheat). For acid phosphatase analyzed in the organic system, the potato, narrow-leafed lupine, and oat forecrops increased its activity by approximately 8–14% compared with spring barley and winter wheat. In the conventional system, the increase in its activity after potato and narrow-leafed lupine compared to all cereal forecrops was approximately 6%. In the case of alkaline phosphatase, the differences in activity between the potato or narrow-leafed lupine forecrops and the cereal forecrops amounted to approximately 8% (organic system) and 11% (conventional system). Soil urease activity was significantly enhanced after root-crop or legume forecrops than after cereal forecrops, by approximately 25–30% (organic system) and 28–34% (conventional system). In turn, protease activity was the highest after the potato forecrop, followed by narrow-leafed lupine, whereas cereal forecrops resulted in its significantly suppressed activity (by approximately 22–36% in the organic system and 23–27% in the conventional system) (Table 6).
Significantly highest dehydrogenase and urease activities were determined for the organic system × narrow-leafed lupine forecrop interaction, whereas significantly highest acid phosphatase and protease activities were recorded for the organic system × potato forecrop interaction (Table 6).
Regardless of the forecrop, the grain yield of oat grown under the organic system averaged 4.51 t ha−1 and was 0.4 t ha−1 (approximately 8%) lower than under the conventional system. This demonstrates the high tolerance of oat to organic cultivation, provided that fertilization and the forecrop are appropriately selected. Indeed, oat grown in the organic system after potato or narrow-leafed lupine produced higher yield than oat grown in the conventional system after cereal forecrops. In both farming systems, potato and narrow-leafed lupine forecrops had significantly the most favorable effect on oat yield compared with cereal forecrops, with mean yield increases of approximately 22% (organic system) and 17% (conventional system). Furthermore, among the cereal forecrops grown in the organic system, oat yield was significantly lower (by approximately 5–9%) after spring barley than after oat and winter wheat (Table 7).
The organic system had a significantly positive effect on oat grain protein content, which averaged 12.6%, i.e., was by 0.75 percentage points higher than under the conventional system. It was also significantly affected by oat forecrops. In the organic system, significantly higher grain protein content was found after potato and narrow-leafed lupine (13.21% and 13.12%, respectively) than after any of the cereal forecrops. Additionally, in the organic system, the oat forecrop resulted in a higher grain protein content than the other cereal forecrops (spring barley and winter wheat). In the conventional system, significantly the highest grain protein content was likewise ensured by narrow-leafed lupine and potato (13.01% and 12.91%, respectively) compared with all cereal forecrops. Significantly the lowest grain protein content was recorded for the conventional system × spring barley forecrop interaction (10.79%) (Table 7).

4. Discussion

4.1. Soil Chemical Composition as Affected by Farming System and Forecrops

The experimental results presented in this article show that growing oat on loess soil under the organic system, irrespective of the five forecrops studied, significantly improved many soil quality indicators compared with the conventional system. Statistical analysis confirmed a significant increase in soil parameters such as the contents of organic C, magnesium, boron, manganese, copper, and zinc, as well as in total sorption capacity of the soil under organic cultivation system. In contrast, the conventional system had a significantly positive effect on soil total N, phosphorus, and potassium contents. The high content of the aforementioned nutrients in soil under the organic system was also related to their high initial availability in the years preceding the establishment of the experiments: in 2019, 2020, and 2021, when the soil contents of organic C, B, Cu, Mn, and Zn were higher (although not significantly) under organic than under conventional system (Table 1). During the strict field experiment, fertilization of oat and its forecrops with Humac Agro in the organic system further enhanced the already beneficial soil contents of organic C, magnesium, boron, zinc, copper, and manganese. In contrast, standard NPK mineral fertilization applied in the conventional system resulted in the significantly highest soil contents of total nitrogen, phosphorus, and potassium.
These findings are consistent with those of other authors [25,26,27], who confirmed that the organic system is associated with faster mineralization of soil nutrients and a higher organic C content than the conventional system, which is related, among other factors, to the type and rates of fertilizers.
Other studies have shown higher total sorption capacity of the soil under organic than under conventional systems due to the higher organic matter content of such soils, associated with the use of natural fertilizers in organic farming. However, the extent of improvement in this soil quality parameter also depends on the crop species and its forecrops. The soil organic matter balance improves considerably after root-crop and legume forecrops. These plant species have also been reported to elicit a more favorable effect on soil ion exchange than cereal forecrops [28,29,30,31].
The organic system promotes nutrient accumulation in the soil. Fertilizers used in organic farming undergo more extensive mineralization and stimulate soil enzymatic activity. In turn, conventional farming relies on synthetic NPK fertilizers and chemical plant protection products, which directly affect crop growth rate and reduce pathogen infection, consequently leading to higher yields [25,32]. This relationship was confirmed in the present study, in which lower soil nitrogen, phosphorus, and potassium contents were recorded under the organic than the conventional system. Studies by other authors [33,34,35] have also demonstrated lower soil nitrogen, phosphorus, and potassium contents under organic cultivation. This is considered to occur particularly when organic cultivation is practiced over the long period without an adequate balance of N, P, and K in organic fertilizers [36,37]. In the present study, the mineral fertilizer Humac Agro was applied in the organic cultivation of oat and its forecrops, which prevented a “gap” in soil macro- and micro-nutrient contents from between the organic and conventional systems. Moreover, the present study demonstrated the important role of forecrops in shaping loess soil quality, irrespective of the farming system. Potato and narrow-leafed lupine forecrops had the most favorable effect on the analyzed soil quality parameters. This finding is consistent with observations made by other researchers [38,39] who have demonstrated the positive role of root crops and legumes in affecting soil quality, particularly with respect to the accumulation of phosphorus, potassium, and magnesium.
In the present study, the organic system had a significantly positive effect on soil magnesium and micronutrient contents, particularly upon the use of forecrops favourable for oat (potato, narrow-leafed lupine). Cereal forecrops reduced the soil contents of Zn, Cu, Mn, B. Studies by other authors have shown that, under the organic system, enhanced soil microbial activity and a substantial amount of organic matter lower the redox potential and increase the availability of micronutrient cations. In addition, adsorbed micronutrient fractions are actively transformed into plant-available forms [40,41,42].
The more favorable soil quality parameters in the organic system compared with the conventional system (enhanced microbial activity, increased organic C and micronutrient contents) are due to the integrated management of this farming system rather than from the single-directional approach characteristic of intensive conventional agriculture [43,44,45].

4.2. Soil Enzymatic Activity as Affected by Farming System and Forecrops

The present study showed a significant improvement in the enzymatic activity of soil under oat in the organic system compared with the conventional system. Root-crop (potato) and legume (narrow-leafed lupine) forecrops also exerted a statistically significant positive effect on soil enzyme activity, irrespective of the farming system. Fließbach et al. [46] have noted that the conventional system, associated with the intensification of agricultural production, generally has a negative effect on soil enzymatic activity. Other authors [47,48] likewise found that soils cultivated under the organic system exhibit 2- to 2.5-fold higher soil enzyme activity than those under the conventional system, which is consistent with our findings. A study by Bobuľská et al. [49] confirmed an explicit positive effect of the organic system on soil biological activity under the climatic conditions of Slovakia compared with the conventional system. Soils from organic farming showed enhanced enzymatic activity (by 36% on average), 65% higher soil respiration, and over 60% higher carbon content in soil organic matter.
Importantly, the high enzymatic activity under the organic system is largely determined by the influence of plant biomass in such crops, which – by definition – are free from chemical plant protection agents. The type and amount of plant biomass returned to the soil with post-harvest residues, and thus the forecrop species, are important as well. Biomass from root crops and legumes has a more favorable effect on soil enzyme activity than cereal biomass [1,6,50,51,52], which was also observed in the present study. Another thesis put forward by other authors and confirmed in our study is the strong dependence of soil enzyme activity (particularly that of dehydrogenase) on high contents of organic C and other organic matter in soil under organic farming, on the stratification and degree of decomposition of organic matter, as well as on favorable soil moisture and temperature conditions during the crop growing season [53,54]. Kobierski et al. [55] have noted that natural fertilization, typical of organic farming, introduces readily available organic matter into the soil, resulting in enhanced soil enzymatic activity. Moreover, in both the organic and conventional systems, these authors found a significant positive correlation between dehydrogenase and alkaline phosphatase activities and soil organic C content.
These observations are reflected in the present study, in which the highest soil enzyme activity was found in loess soil under the organic system when narrow-leafed lupine and potato were grown as oat forecrops. This was also directly related to the rational application of natural fertilization (Humac Agro) on these plots and to the highest organic C content under such conditions.
The present study revealed a strong relationship between the activity of the enzymes studied and the oat forecrop. Soil after root-crop and legume forecrops exhibited the highest enzyme activity, especially that of dehydrogenase, whereas the lowest soil enzyme activity was observed after cereal forecrops. This is because potato and narrow-leafed lupine supply the soil with organic matter whose properties are more favorable for soil enzyme activity than those of cereal post-harvest residues. Results reported by other authors for root-crops (sugar beet) and legumes (pea) have also confirmed that, irrespective of the farming system, these crops have the most favorable effect on soil enzymatic activity and on the overall balance of soil chemical constituents compared with cereals [29,38,46,56].
4.3. Oat Productivity as Affected by Farming System and Forecrops
The present study shows that oat grain yield was significantly lower in the organic than in the conventional system. However, the yield gap between these systems was offset by the choice of forecrop: when oat was grown after a root-crop or legume forecrop, its productivity reached the level of oat grown under the conventional system after a cereal forecrop. Studies by other authors have indicated higher productivity of most crop species under conventional than under organic management. However, some species (e.g., millet, oat, spelt wheat) tolerate the extensive conditions typical of organic cultivation better, and their yields do not differ substantially from those obtained under conventional systems [57,58,59,60].
The present results indicate that oat is a crop for which the yield gap between organic and conventional cultivation can be eliminated. This is supported by greater yield stability throughout years and stable soil biotic and abiotic properties, including enhanced soil enzymatic activity and higher sorption capacity. This finding is consistent with earlier studies [28,32]. Pużyńska et al. [61] have also noted that oat tolerates organic cultivation very well. Despite this tolerance, it produces higher yields when standard NPK mineral fertilization is applied at balanced rates (conventional system), improving photosynthetic efficiency, which in turn enhances productivity [62]. Alvarez [63] has conducted a meta-analysis of yield data for selected crops under organic and conventional farming and the associated intensity of land use. Based on peer-reviewed studies by other authors, he found that yields under organic management were on average 25% lower than under conventional farming and that, for cereals, the difference averaged 30%. Similar observations were reported by Viana et al. [64].
In the present study, the organic system resulted in higher protein content of the grain than the conventional system. This finding is consistent with reports by other authors [65,66,67]. In turn, other studies [68,69,70] have demonstrated a high oat grain protein content under organic conditions, with soil type and the selection of cultivars adapted to local agroclimatic conditions being of major importance. The oat cultivar ‘Bingo’ used in the present study was well adapted to the soil and climatic conditions of the Czesławice Experimental Farm (Poland).
In the present study, oat grain protein content was significantly influenced by the choice of forecrop, with narrow-leafed lupine having the most favorable effect. Studies by other authors have also indicated that the protein content of cereal grain is significantly determined by the forecrop type, mainly because of the specific effect of the forecrop on soil nitrogen availability and soil microbial activity. The use of legumes as forecrops contributes to a statistically significant increase in grain protein content compared with cultivation after other cereals or brassicas [71].

5. Conclusions

The results of the three-year strict field experiment conducted on loess soil indicate that the effects of farming systems and forecrops on soil quality and oat productivity vary. Conventional farming resulted in significantly higher soil contents of total nitrogen, phosphorus, and potassium. Organic farming, in turn, promoted a significantly higher sorption capacity of the and higher soil contents of organic C, magnesium, boron, copper, manganese, and zinc. From the perspective of soil health, the organic system, particularly in interaction with potato and narrow-leafed lupine, resulted in significantly higher activity of all analyzed soil enzymes (dehydrogenase, acid and alkaline phosphatase, urease, and protease).
The nutrient composition and enzymatic activity of the loess soil were also closely related to the oat forecrops, i.e., to the effect of their post-harvest residues on the soil. Potato, followed by narrow-leafed lupine, had the most favorable effect on soil chemical quality indicators and soil enzyme activity. Compared with cereal forecrops (winter wheat, spring barley, and oat), they substantially increased the contents of organic C, N, P, K, Mg, B, Cu, and Mn.
Oat grain yield, regardless of the forecrop, was approximately 8% higher under the conventional system than under the organic system. At the same time, growing oat in the organic system after an appropriately selected forecrop (potato or narrow-leafed lupine) produced grain yields comparable to those obtained in the conventional system after cereal forecrops. Under the organic system, oat grain protein content was significantly higher than under the conventional system, irrespective of the forecrop.
In conclusion, the study confirmed the hypothesis that appropriate management of oat cultivation in the organic system (forecrop selection, Humac Agro fertilization) makes it possible to maintain healthy soil and achieve satisfactory crop productivity with a high grain protein content.

Author Contributions

C.A.K. and E.H. conceptualization; E.H. and C.A.K. formal analysis; C.A.K. and E.H. investigation and methodology; E.H. and C.A.K. resources, C.A.K. and E.H. writing-original draft. All authors have read and agreed to the published version of the manuscript.

Funding

Research supported by the Ministry of Science and Higher Education of Poland as part of the statutory activities of the Department of Herbology and Plant Cultivation Techniques (SUBB.WRU.19.029 RiO), University of Life Sciences in Lublin.

Data Availability Statement

The data supporting the results of this study are included in the manuscript.

Acknowledgments

This research was supported by the Ministry of Agriculture and Rural Development in Poland.

Conflicts of Interest

The authors declare no conflict of interest.

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Table 1. Soil characteristics before the establishment of the field experiments with oat in 2019–2021 (prior to the effects of the experimental factors: farming system and forecrops)*.
Table 1. Soil characteristics before the establishment of the field experiments with oat in 2019–2021 (prior to the effects of the experimental factors: farming system and forecrops)*.
Farming system Organic C (%) Total N
(%) 		P
(mg kg−1) 		K
(mg kg−1) 		Mg
(mg kg−1) 		B
(mg kg−1) 		Cu
(mg kg−1) 		Mn
(mg kg−1) 		Zn
(mg kg−1)
Conventional 0.73 – 0.75 0.06 – 0.07 116 – 120 188 – 191 58.0 – 58.7 2.07 – 2.10 6.28 – 6.30 159 – 161 8.09 – 8.11
Organic 0.82 – 0.84 0.04 – 0.05 110 –112 175 – 177 60.2 – 61.5 2.25 – 2.28 6.77 – 6.79 186 – 190 8.56 –8.59
Table 2. Fertilization of oat forecrops used in the conventional system (2020–2022).
Table 2. Fertilization of oat forecrops used in the conventional system (2020–2022).
Fertilization Component Forecrops
Potato Spring barley Narrow-leafed lupine Winter wheat Oat
Mineral fertilization (kg ha−1) N
(before sowing/ planting)		 85 70 25 85 60
P
(before sowing/ planting)		 85 50 40 65 45
K
(before sowing/ planting)		 110 80 55 105 80
Manure fertilization
(t ha−1)		 (autumn; before planting) 20 - - - -
Table 3. Fertilization of oat forecrops used in the organic system (2020–2022).
Table 3. Fertilization of oat forecrops used in the organic system (2020–2022).
Mineral fertilization Forecrops
Potato Spring barley Narrow-leafed lupine Winter wheat Oat
Humac Agro;
before sowing/ planting
(kg ha−1)		 425 310 175 370 265
Originating from organic livestock production;
autumn; before sowing
(t ha−1)		 20 - - - -
Table 4. Contents of total nitrogen, phosphorus, potassium, magnesium, and organic C in soil under oat – mean of three study years.
Table 4. Contents of total nitrogen, phosphorus, potassium, magnesium, and organic C in soil under oat – mean of three study years.
Farming system Oat forecrops Total N (%) P
(mg kg−1) 		K
(mg kg−1 		Mg
(mg kg−1) 		C-
Organic (%)
Organic potato 0.10 ±0.009* 118.4
±3.2		 211.2
±4.2		 70.3
±0.9		 0.93 ±0.03
spring barley 0.06 ±0.006 113.6
±2.7		 168.9 ±2.9 63.1
±0.5		 0.81
±0.02
narrow-leafed lupine 0.11 ±0.009 117.1
±3.0		 199.5 ±3.2 69.4
±0.8		 0.91 ±0.03
oat 0.05 ±0.005 112.4
±2.5		 161.3 ±3.0 65.3
±0.6		 0.85
±0.02
winter wheat 0.06 ±0.004 115.2 ±2.6 172.4 ±3.8 63.7
±0.4		 0.83 ±0.02
Mean 0.06 115.3 182.6 66.3 0.87
Conventional potato 0.14 ±0.010 135.8
±3.5		 273.2 ±5.3 68.3
±0.8		 0.89 ±0.03
spring barley 0.08 ±0.007 127.4
±2.9		 223.4 ±5.0 54.7
±0.4		 0.76
±0.02
narrow-leafed lupine 0.16 ±0.011 134.7
±3.1		 266.8 ±5.1 66.5
±0.7		 0.85 ±0.03
oat 0.07 ±0.006 121.5
±2.6		 219.7 ±4.5 51.6
±0.3		 0.72 ±0.01
winter wheat 0.09 ±0.008 130.3
±2.8		 226.6 ±4.9 55.8
±0.5		 0.74 ±0.02
Mean 0.08 129.9 241.9 59.3 0.79
HSD (p ≥ 0.05) for farming system (A) 0.018 7.25 13.17 3.85 0.079
HSD (p ≥ 0.05) for oat forecrop (B) 0.016 7.09 13.35 4.24 0.081
HSD (p ≥ 0.05) for (A × B) interaction 0.019 7.08 n.s.** n.s. n.s.
*SD – standard deviation; **n.s. – not significant differences.
Table 5. Contents of boron, copper, manganese, and zinc in soil and total sorption capcity of the soil under oat – mean of three study years.
Table 5. Contents of boron, copper, manganese, and zinc in soil and total sorption capcity of the soil under oat – mean of three study years.
Farming system Oat forecrops B (mg kg−1) Cu (mg kg−1) Mn (mg kg−1) Zn (mg kg−1) Total sorption capacity of soil (cmol (+) kg−1)
Organic potato 2.61 ±0.08* 7.12 ±0.20 215 ±6.2 8.84 ±0.25 40.9 ±0.59
spring barley 2.30 ±0.06 6.71 ±0.18 188 ±4.7 8.56 ±0.21 36.4 ±0.56
narrow-leafed lupine 2.56 ±0.07 7.05 ±0.19 207 ±5.6 8.75 ±0.23 38.3 ±0.45
oat 2.49 ±0.06 6.64 ±0.17 182 ±4.8 8.60 ±0.22 35.9 ±0.46
winter wheat 2.33 ±0.05 6.74 ±0.18 193 ±4.9 8.47 ±0.21 36.7 ±0.42
Mean 2.46 6.85 197 8.64 37.6
Conventional potato 2.29 ±0.06 6.56 ±0.17 179 ±5.0 8.23 ±0.21 37.8 ±0.48
spring barley 2.06 ±0.04 6.31 ±0.16 165 ±3.8 8.16 ±0.18 34.4 ±0.46
narrow-leafed lupine 2.23 ±0.05 6.49 ±0.15 172 ±4.9 8.26 ±0.21 37.1 ±0.51
oat 2.12 ±0.03 6.28 ±0.13 163 ±3.9 8.09 ±0.17 33.9 ±0.43
winter wheat 2.09 ±0.03 6.34 ±0.16 157 ±3.8 8.17 ±0.18 34.2 ±0.44
Mean 2.16 6.39 167 8.18 35.4
HSD (p ≥ 0.05) for farming system (A) 0.204 0.391 14.8 0.432 2.19
HSD (p ≥ 0.05) for oat forecrop (B) 0.139 0.332 8.91 n.s. 2.24
HSD (p ≥ 0.05) for (A × B) interaction n.s.** n.s. n.s. n.s. 2.58
*SD – standard deviation; **n.s. – not significant differences.
Table 6. Soil enzymatic activity under oat – mean of three study years.
Table 6. Soil enzymatic activity under oat – mean of three study years.
Farming system Oat orecrops Dehydrogenase (mg TPFkg−1 d.m.) Acid phosphatase (mg PNP kg−1 d.m.) Alkaline phosphtase (mg PNP kg−1 d.m.) Urease (mg N-NH4kg−1 d.m.) Protease (mg tyrosine kg−1 d.m.)
Organic potato 2.92
±0.09* 		75.63
±1.83		 81.12
±1.98		 58.21
±1.34		 19.57
±0.79
spring barley 2.58
±0.08		 64.73
±1.64		 74.16
±1.86		 44.87
±1.26		 12.33
±0.53
narrow-leafed lupine 3.24
±0.12		 71.29
±1.76		 82.64
±1.89		 65.22
±1.39		 16.48
±0.72
oat 2.54
±0.07		 69.52
±1.72		 76.45
±1.85		 41.88
±1.25		 14.35
±0.66
winter wheat 2.59
±0.09		 64.59
±1.68		 74.02
±1.72		 45.02
±1.29		 12.19
±0.51
Mean 2.77 69.15 77.67 51.04 14.98
Conventional potato 2.61
±0.10		 59.84
±1.44		 73.87
±1.83		 54.34
±1.23		 11.09
±0.49
spring barley 2.19
±0.07		 55.63
±1.42		 67.17
±1.83		 36.25
±1.18		 8.12
±0.37
narrow-leafed lupine 2.78
±0.11		 59.36
±1.50		 79.09
±1.81		 56.14
±1.22		 10.43
±0.44
oat 2.25
±0.06		 56.49
±1.71		 68.65
±1.90		 35.18
±1.15		 8.21
±0.30
winter wheat 2.17
±0.05		 55.21
±1.59		 66.29
±1.84		 37.98
±1.17		 8.06
±0.28
Mean 2.40 47.30 71.01 43.97 9.18
HSD (p ≥ 0.05) for farming system (A) 0.289 4.021 4.381 6.014 2.312
HSD (p ≥ 0.05) for oat forecrop (B) 0.296 4.018 4.212 5.998 2.205
HSD (p ≥ 0.05) for (A × B) interaction 0.311 4.032 n.s.** 5.972 2.245
*SD – standard deviation; **n.s. – not significant differences.
Table 7. Oat grain yield and grain protein content – mean of three study years.
Table 7. Oat grain yield and grain protein content – mean of three study years.
Farming system Oat forecrops Oat grain yield (t ha-1) Oat grain protein content (%)
Organic potato 5.14 ±0.09* 13.21 ±0.11
spring barley 3.86 ±0.04 12.09 ±0.07
narrow-leafed lupine 5.22 ±0.10 13.12 ±0.10
oat 4.05 ±0.08 12.51 ±0.06
winter wheat 4.26 ±0.05 12.07 ±0.05
Mean 4.51 12.60
Conventional potato 5.41 ±0.07 12.91 ±0.08
spring barley 4.47 ±0.05 10.79 ±0.10
narrow-leafed lupine 5.49 ±0.08 13.01 ±0.08
oat 4.54 ±0.06 11.35 ±0.13
winter wheat 4.62 ± 0.05 11.23 ± 0.11
Mean 4.91 11.85
HSD (p ≥ 0.05) for farming system (A) 0.362 0.202
HSD (p ≥ 0.05) for oat forecrop (B) 0.256 0.165
HSD (p ≥ 0.05) for (A × B) interaction n.s.** 0.433
*SD – standard deviation; **n.s. – not significant differences.
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