Site and experimental set-up

The research facility is located in the Central European Lowlands, the sub-area of the South Baltic Lagoon, in the Ilawa Lake District. The study area is characterized by a young glacial landscape within the range of the ice sheet of the Pomeranian glaciation of the Vistula. Winter rapeseed (Brassica napus L.) was grown in monoculture and in crop rotation in Balcyny (53°36’ N, 19°51’ E), Poland, in 2009-2013. The field experiment was set up on loess soil, class IIIa soil/arable soil of good quality Topsoil (Ap) was made up of heavy loamy sand, and the E-horizon consisted of clay underlain by light loam in the illuvial horizon (Bt). According to the World Reference Base for Soil Resources (^{WRB, 2014}), this corresponds to a Luvisol. Soil was slightly acidic (in KCl solution with pH 6.6), and its total N content was determined at 0.95 g kg^-1 and total organic C content at 10.05 g kg^-1. Soil concentrations of plant-available macronutrients (mg kg^-1) were 93.3 mg P kg^-1, 185.4 mg K kg^-1, 58.5 mg Mg kg^-1, and 550 mg Ca kg^-1. The concentrations of soil nutrients were according to the valid standards and standard methods applied in Poland. The contents of macronutrients were determined: Total N by the Kjeldahl method, P and available K by the Egner-Riehm method in calcium-lactate extract ((CH₃CHOHCOO)₂Ca) acidified with hydrochloric acid to pH 3.6, available Mg was assayed after the extraction of 0.01 mol CaCl₂ × 10^-3 m³ from soil, using the Atomic Absorption Spectrometry (AAS) and Ca by universal method of extraction with 0.003 N acetic. Soil pH was determined electrometrically in a solution of 1 M KCl and humus content by the Tiurin method.

Before the experiment, a mixture of spring cereals (oats, barley, wheat) was sown in all plots for green fodder, without fertilization. The results presented in this study were noted in the fifth year of the experiment in the rapeseed monoculture (2013) and in crop rotation: 2009 winter rapeseed, 2010 winter wheat (Triticum aestivum L.), 2011 field bean (Vicia faba L.), 2012 spring wheat (T. aestivum), and 2013 winter rapeseed. The open-pollinated rapeseed ‘Californium’ was grown, seeds (4.5 kg ha^-1) were sown in 20 August and dressed with insecticides imidacloprid 200 g and cypermethryna 50 g (Brasikol C 250 FS, Z.P.U.H. “Best-Pest” - Jaworzno, Poland) and fungicide tiuram 332 g and karbendazym 148 g (Funaben T 480 FS, Organika-Azot S.A. Jaworzno, Poland). Plants were harvested in the first half of July. Three levels of technologies were used: economically (low-input), moderately intensively (medium-input) and intensively (high-input), varied in amount of N and S fertilization as well as protection against pests. The applied fertilizer and pesticide treatments are given in Table 1. The experiment had a randomized block design with three replicates. The plot size was 12.0 m², the harvested plot area was 9.0 m².

Table 1: Treatments carried out in winter rapeseed (Brassica napus) plots experiment. 

*BBCH Monograph (2001).

Yield and content of macro and microelements

At the end of the experiment seeds were collected, dried and purified. Rapeseed seed were collected from the experimental field (9.0 m²) and its yield was calculated in tons per hectare at 15% humidity.

Seed samples (1 kg) were taken from the plot and subjected to chemical analysis for the content of macro- and micronutrients according to the methods used in agricultural chemistry. The seeds was mineralized in the acid mixture of HNO₃ and HClO₄ (4:1). The content of Cu, Zn, Mn and Fe was determined in the extract and mineralizate with the use of atomic absorption spectrometry (AAS) (Hitachi Z-8200 Polarized Zeeman Atomic Absorption Spectrophotometer, Hitachi, Tokyo, Japan). Total N was determined using the Kjeldahl method, P was determined with vanadium-molybdenum method, while K and Ca with atomic emission spectrometry (AES), and Mg with AAS in the material previously mineralized in H₂SO₄ with addition of H₂O₂ as an oxidizer.

Oil extraction and analysis

Fat content was determined with the use of near-infrared spectroscopy (NIR) (Infratec 1241 Grain Analyzer, Foss, Hillerod, Denmark), which takes measurements of transmission waves from the near-infrared region (570-1050 nm). Analysis of the fatty acids was done following the cold extraction of rape oil with chloroform/methanol (2:1 v/v). Fatty acid methyl esters (FAME) were prepared according to ^{Zadernowski and Sosulski (1978)} using a mixture of chloroform:methanol:sulphuric acid (100:100:1, v/v/v). Chromatographic separation was performed using a gas chromatograph (Agilent 7890A, Agilent Technologies Wilmington, Delaware, USA) with a flame-ionization detector (FID) and a 30 m 0.32 mm internal diameter capillary column. The liquid phase was Supelcowax 10 and the film thickness was 0.25 µm.

The conditions of separation were as follows: helium was used as a carrier gas; flow rate 1 mL min^-1; detector temperature 250 ºC; injector temperature 230 ºC; column temperature 195 ºC. The different acids were identified by comparing retention times with standards from Supelco (Bellefonte, Pennsylvania, USA). The fatty acid content is presented as the relative percentage (% total fatty acids) in rape oil.

Weather conditions

Poland’s climate can be described as a temperate climate, which is greatly influenced by oceanic air currents from the west, cold polar air from Scandinavia and Russia, as well as warmer, sub-tropical air from the south.

The mean monthly air temperatures (from winter rapeseed sowing till the end of November) were on a similar level as the analogous annual periods (Table 2). The drought recorded in August (with precipitation lower by 44.9 mm than in the annual periods) might have hindered seed germination, but the precipitation levels in the following months secured good plant growth before wintering. During the wintering period (December-March), when water resources should be accumulated for spring growth, precipitation was lower by 41.5 mm in comparison with the analogous periods in 1981-2010.

Table 2: Weather conditions in 2012-2013 and the multi-annual average of 1981-2010. 

Following melts, there were ground frosts in March, which presented a risk of potential plant damage due to thin snow cover. Weather conditions did also not favor plant development and growth at the stages from budding to silique formation - BBCH 53-79 (^{BBCH Monograph, 2001}). The recorded precipitation volumes between April and June were lower by 50.9 mm (lower by 30.8% as compared to the annual periods) and remained below the requirements of winter rapeseed.

Statistical analyses

The results were statistically processed in Statistica 10.0 (StatSoft, Tulsa, Oklahoma, USA) with the use of one-way ANOVA. Basic parameters and homogenous groups were determined by Tukey’s test at p = 0.05. The relationships between yield of seeds, content of fat, N, P, K, Mg, Ca, Cu, Fe, Zn, Mn and yield of fat: saturated fatty acids (SFA), monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA), were described by linear regression analysis.

Content of macro and microelements

The chemical analysis of winter rapeseeds demonstrated that, regardless of production technology, the average content of minerals in the fifth year of monoculture was as follows: 29.9 g N kg^-1, 0.595 g P kg^-1, 1.12 g K kg^-1, 0.298 g Mg kg^-1, 0.55 g Ca kg^-1, 3.18 mg Cu kg^-1, 115.6 mg Fe kg^-1, 42.8 mg Zn kg^-1, and 38.2 mg Mn kg^-1. In the fifth crop rotation year there was a year break in rapeseed: 29.2 g N kg^-1, 0.562 g P kg^-1, 1.12 g K kg^-1, 0.302 mg Mg kg^-1, 0.392 mg Ca kg^-1, 3.47 mg Cu kg^-1, 113.3 mg Fe kg^-1, 44.6 mg Zn kg^-1, and 42.4 mg Mn kg^-1 (Table 3). These results are comparable in their P and Cu contents with a higher amount of N, Mg, Fe, Mn and Zn, although they have a lower content of other elements compared to the data reported by ^{Fordonski et al. (2015)}.

Table 3: Macro and microelements content in seeds of winter rapeseed grown under different agricultural production systems. 

Averages two in rows followed by the same letter are nonsignificant (α < 0.05); ± standard deviation.

The content of N, P, K, and Mg in rapeseed did not differ significantly depending on its proportion in a crop rotation. The level of Ca was higher (by 17.0%) in rapeseed produced with a medium-input monoculture system compared to crop rotation technology. However, the content of this element was lower (by 34.9%) in the high-input crop rotation system than in monoculture. Considering the intensity of agricultural engineering procedures, only the highest level (high-input) generated a significant increase (10.0%) of N accumulation in the rapeseed compared to medium-input technology. Similarly, Ca content in a crop rotation was significantly higher with the high-input compared to both the low-input and medium-input technology. In the fifth monoculture year, a higher Ca content was recorded in the medium-input technology than the other levels (26.5% on average).

Depending on a crop rotation method, a generally higher content of microelements was measured in rapeseeds grown in the crop rotation system. However, a significantly higher content of Mn was found only for the low-input crop rotation technology and of Zn and Mn in the medium-input system. The rapeseed low-input crop rotation system was an exception, with a significantly lower Fe content (by 7.8%).

The highest fertilization level (high-input) generated a significantly higher content of Zn and Mn in rapeseeds in both crop rotation systems. A significantly higher Fe level was recorded in rapeseeds grown in the medium-input crop rotation technology and in the low-input monoculture system.

Yielding, fat content

Winter rapeseed cultivated for a number of years in the same field reacts with a substantial reduction in seed yield, but when seeded occasionally 2 yr in a row or in a short monoculture system it may generate yields at a similar level as after cereal plants (^{Rozylo and Palys, 2011}; ^{Jaskulska et al., 2014}). When cultivated in a monoculture and crop rotation system, winter rapeseed yielded high-level crops, from 4.04 to 6.25 t ha^-1 (Table 4).

Table 4: Seed and fat yield and fat content in seeds of winter rapeseed grown under different agricultural production systems. 

Averages in two rows followed by the same letter are nonsignificant (α < 0.05); ± standard deviation.

Regardless of the technology level, seed yield was higher by 18.6% with the crop rotation method than in the monoculture system. Significantly higher crops (by 47.3%) were obtained using low-input crop rotation technology compared to the low-input monoculture approach. The increase of agricultural engineering technologies contributed to diminishing differences in the yield between the crop rotation and monoculture systems. Increased intensity of agricultural technologies resulted in significantly higher seed crops only in the crop rotation system. ^{Jarecki et al. (2013)} found that higher level of agricultural engineering procedures, as compared with a lower input, generated a significant increase in seed yield by approximately 12%, which is a result of substantially higher number of siliques on the plant and thousand-seed weight. The level of oil in mature winter rapeseeds ranges between 45% and 50% on average (^{Liersch et al., 2013}). In personal studies, winter rapeseeds ‘Californium’ contained 47.2% fat on average (Table 4). The crop rotation method did not substantially modify the fat content in rapeseeds. Moreover, the intensity level of agricultural procedures did not impact the fat content in rapeseeds, as reported in the studies performed by ^{Jarecki et al. (2013)}.

The content of fat in seeds is mainly determined by genetics (^{Tanska et al., 2009}; ^{Wittkop et al., 2009}; ^{Ambrosewicz-Walacik et al., 2015}), although it may change being influenced by habitat conditions (^{Ozturk, 2010}; ^{Spychaj-Fabisiak et al., 2011}; ^{Faraji, 2012}; ^{Varényiová and Ducsay, 2016}). The fat yield was strongly correlated with seed yield (r = 0. 929) but was independent of fat content (Table 7). According to ^{Narits (2010}), N fertilization had a positive effect on seed yield and seed protein content. On the other hand, N fertilization, especially in higher rates, had a negative effect on oil content.

Fatty acid profile

The analysis of fatty acid composition demonstrated a high proportion of oleic (C18:1 c9), linoleic (C18:2) and linolenic (C18:3) acids (Table 5). There was no occurrence of the following fatty acids: erucic (C22:1n9), cis-13,16-docosadienoic (C22:2), lignoceric (C24:0), and nervonic (C24:1n9). In general, the proportion of winter rapeseed in the seeding structure had a varied effect on the fatty acid proportions. In rapeseeds from the monoculture, low-input and medium-input technologies resulted in a higher percentage contribution of oleic acid (C18:1 c9) while the high-input approach generated a reduction of its level. Low-input and high-input technologies contributed to an increase in the percentage proportion of C18:2 and C18:3 fatty acids in rapeseeds from the monoculture. The medium-input technology resulted in increased levels of C20:0, C18:1 c11, C18:2, C18:3, and C20:2 fatty acids in the crop rotation system rapeseeds.

Table 5: Fatty acid profile (%) in seeds of winter rapeseed grown under different agricultural production systems. 

Averages in two rows followed by the same letter are nonsignificant (α < 0.05); ± standard deviation.

^aC14:0 myristic acid; C15:0 pentadecanoic acid; C16:0 palmitic acid; C17:0 margaric acid; C18:0 stearic acid; C20:0 arachidic acid; C22:0 behenic acid; C16:1 palmitoleic acid; C17:1 margaric-oleic acid; C18:1 oleic acid c9; C18:1 octadecanoic acid c11; C20:1 eicosenic acid; C18:2 linoleic acid; C18:3 linolenic acid; C20:2 eicosadienoic acid.

^bM: Fifth year of monoculture (monoculture), Cr: fourth year break in rapeseed (crop rotation).

According to nutritional studies, a proper ratio of n-6 to n-3 polyunsaturated fatty acids in the daily ration should range from 6:1 to 4:1, although according to the experts of the International Society for the Study of Fatty Acids and Lipids (ISSFAL), the n-6 PUFA to n-3 PUFA ratio in the diet should not exceed 4 (^{Ntawubizi et al., 2010}). The percentage changes in the proportions of polyunsaturated fatty acids such as linoleic acid (C18:2) and linolenic acid (C18:3) did not exert any significant impact on the C18:2/C18:3 ratio in rapeseed from either crop rotation systems (Table 6). Greater differences in the C18:2/C18:3 acid ratio were reported with a varied level of agricultural engineering technology. An increased intensity of the technology significantly reduced the ratio of these acids both in the crop rotation and monoculture system. The recorded proportions of linoleic acid-to-linolenic acid approximated the levels reported by ^{Tanska et al. (2009)}.

Table 6: Saturated fatty acids (SFA), monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA) and C18:2/C18:3 in seeds of winter rapeseed grown under different agricultural production systems. 

Averages in two rows followed by the same letter are nonsignificant (α < 0.05); ± standard deviation

The average content of SFA in rapeseed oil was 7.41%, PUFA was approximately 28.2%, and MUFA was approximately 64.3%. Neither the level of technology nor the crop sequence impacted the content of SFA with C14, C15, C16, C17, C18, C20, and C22 atoms. The highest content of MUFA (66.1%) was recorded with the highest level of technology in the crop rotation system, and of PUFA (29.9%) with the low-input monoculture system. Rapeseed oil from the monoculture system contained a significantly higher amount of MUFA (medium-input) and PUFA (low- and high-input). Depending on the level of rapeseed saturation in crop rotation and technology, the MUFA:PUFA ratio ranged from 2.1:1 to 2.5:1 and was similar to typical rapeseed oil. According to ^{Liersch et al. (2013)}, oil with the monounsaturated-to-polyunsaturated fatty acid ratio of 2:1 perfectly fits into the nutritional recommendations.

A correlation analysis shows a negative relation between seed yield and content of P (r = -0.535) (Table 7). There was a positive relation between fat yield and seed yield (r = 0.929) and Mn level (r = 0.623) and a negative relation between P (r = -0.596) and Fe content (r = -0.669). The increase in SFA was closely correlated with K (r = 0.800) and Mg (r = 0.920) contents. There was a positive correlation between the MUFA content and the level of P (r = 0.604), and Ca (r = 0.876) with the content of Cu, Zn, and Mn (r = 0.487, r = 0.511 and r = 0.585, respectively). As the only lipid fraction, PUFAs were correlated with seed yield (r = -0.472). Together with the increase in Mg and Fe content, the amount of PUFA increased (r = 0.514 and r = 0.553, respectively) whereas the PUFA fractions decreased together with increasing Ca and Mn levels (r = -0.835 and r = -0.578, respectively).

Table 7: Correlations between seed yield, fat content, macro and micronutrients in seeds and fat yield, saturated fatty acids (SFA), monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA) in seeds of winter rapeseed grown under different agricultural production systems. 

ns: Nonsignificant differences.