The Inﬂuence of Digestate on the Static Strength of Spring Rapeseeds ( Brassica napus var. arvensis )

: Biogas production occurs during methane fermentation from organic substrates and the mass remaining after fermentation, containing organic matter and valuable minerals having regard to plant nutrition, forms the digestate, which could be useful for fertilizing purposes and very beneﬁcial in the case of the fertilization of rapeseeds. This paper focuses on the use of two forms of fertilization of rapeseeds—digestate and mineral fertilizers—in order to reduce the compressive strength of rapeseeds. The object presents results of compressive strength tests of three rape varieties (Bios, Feliks, Markus). The uniaxial compression tests between two parallel planes were made using a Zwick/Roell Z005 testing machine. Comparative analyses for the analyzed variables were carried out applying parametric and non-parametric statistical tests. On the basis of the conducted research, it was found that the distribution of the increase in the force crushing Bios and Feliks rapeseed varieties in both forms of cultivation was proportional to the increase in their mass. However, with a relatively comparable mass of Bios cv. seeds, in the case of the digestate use, a stronger correlation was found between the seed pressing force and its mass than for the multi-component fertilizer, understanding the need to apply more force to crush the seeds for this form of cultivation. In the conducted tests, the average size of rapeseed diameters of all varieties and forms of cultivation ranged from 1.81–1.95 mm, which indicates their good suitability for industrial purposes. difference in mean seed strength was observed between the highest and lowest mean strength values, which was 4.91 N, while the smallest was recorded for the control combination (0.84 N). Digestate fertilization for cultivation of the Bios and Feliks varieties contributed to an increase in the static strength of their seeds in relation to the control combination and statistical analysis pointed to NPK-digestate homogeneous groups. It was found that the distribution of the increase in the crushing force for Bios and Feliks rapeseed varieties in both forms of cultivation was proportional to the increase in their mass. However, with a relatively comparable mass of seeds of Bios cv., a stronger correlation was observed with the use of the digestate than with a multi-component fertilizer. This shows that seeds from this form of cultivation need a larger force to crush. In the conducted tests, the average rapeseed diameters of all varieties and forms of cultivation ranged from 1.81 to 1.95 mm, which indicates their good suitability for industrial purposes. Of all the varieties of rapeseed, the highest average weight was obtained in cultivation using the digestate (4.76 mg), while in traditional cultivation, it was 4.69 mg and in control combinations it was 4.61 mg.


Introduction
Methane fermentation from organic substrates, which may be natural fertilizers, plant biomass, or products of the agri-food industry, contributes to the production of biogas. During this process conducted under controlled conditions in an agricultural biogas plant, the organic matter decomposes with the production of biogas, the main component of which is methane [1]. In Poland, the annual efficiency of installations for the production of agricultural biogas is 391,328,847.8 m 3 /year, giving a total electric power of 101.093 MW. However, in the Lublin province, 36,100,000 m 3 /year is acquired [2]. The mass remaining after fermentation, containing organic matter and valuable minerals from the point of view of plant nutrition, forms the named digestate. The basic direction of digestate management after taking into account its physicochemical properties should be fertilizer use. A biogas plant located also observed that after some time, seeds with a certain moisture content at a constant temperature and under constant load begin to change their strength to dynamic loads [21]. It was also noted on the basis of the measurement of airflow resistance in seeds that during storage, rapeseeds are permanently deformed, especially at higher moisture content (11%) [35]. Although Figiel indicated that the storage of rapeseeds in a silo for 16 weeks in industrial conditions does not have a direct impact on their strength properties, the observation was made using quasi-static speeds [36]. In addition, it was also found that the increase in the seed moisture contributed to the reduction in the strength of individual seeds and an increase in the elastic modulus of the seeds.
The aim of this study was to undertake research determining the static strength of rapeseeds fertilized in a traditional manner (with the use of mineral fertilizers) compared with the use of digestate.

Experimental Material
Three varieties of spring rapeseed (Bios, Feliks and Markus) from the Plant Cultivation Station Strzelce Sp. zoo. were subjected to tests. The seeds were sown on experimental plots in south-eastern Poland in Lublin province in 2015-2016. The aforementioned varieties of spring rape were subjected to plot growing in three variants. The first one used natural fertilizer (digestate) originating from a biogas plant (Biogas-Tech, Sp. zoo., Lublin, Poland) in Piaski, Lublin province. Digestate was spilled onto the plots in an amount of 97 L per 27 m 2 (36,000 L ha −1 ). The pH value of the digestate used for rapeseed cultivation was 8.73. In the second variant, a multi-component mineral fertilizer Yara NPK 5-14-28 (5% N, 14% P, 28% K, 12.5% SO 3, and 3% CaO) in the amount of 0.405 kg/plot (150 kg·ha −1 ) was used. Doses of each fertilizer were selected based on the manufacturer's recommendations. In the third case, a control plot was used, in which no form of fertilization was applied. After harvesting, the seeds were cleaned and then left for one month under laboratory conditions at 20 • C, under conditions of approx. 70% humidity to equilibrate the moisture content ( Figure 1). storage, rapeseeds are permanently deformed, especially at higher moisture content (11%) [35]. Although Figiel indicated that the storage of rapeseeds in a silo for 16 weeks in industrial conditions does not have a direct impact on their strength properties, the observation was made using quasistatic speeds [36]. In addition, it was also found that the increase in the seed moisture contributed to the reduction in the strength of individual seeds and an increase in the elastic modulus of the seeds. The aim of this study was to undertake research determining the static strength of rapeseeds fertilized in a traditional manner (with the use of mineral fertilizers) compared with the use of digestate.

Experimental Material
Three varieties of spring rapeseed (Bios, Feliks and Markus) from the Plant Cultivation Station Strzelce Sp. zoo. were subjected to tests. The seeds were sown on experimental plots in south-eastern Poland in Lublin province in 2015-2016. The aforementioned varieties of spring rape were subjected to plot growing in three variants. The first one used natural fertilizer (digestate) originating from a biogas plant (Biogas-Tech, Sp. zoo., Lublin, Poland) in Piaski, Lublin province. Digestate was spilled onto the plots in an amount of 97 L per 27 m 2 (36,000 L ha −1 ). The pH value of the digestate used for rapeseed cultivation was 8.73. In the second variant, a multi-component mineral fertilizer Yara NPK 5-14-28 (5% N, 14% P, 28% K, 12.5% SO3, and 3% CaO) in the amount of 0.405 kg/plot (150 kg•ha −1 ) was used. Doses of each fertilizer were selected based on the manufacturer's recommendations. In the third case, a control plot was used, in which no form of fertilization was applied. After harvesting, the seeds were cleaned and then left for one month under laboratory conditions at 20 °C, under conditions of approx. 70% humidity to equilibrate the moisture content ( Figure 1). The moisture content of the tested seeds was 7%. Depending on the type of cultivation, 100 seeds were randomly selected from each variety and weighed with an accuracy of 0.1 mg using the scales (Max 50/1/WH). After preparing the raw material, the rapeseeds were subjected to tests to determine the compression force.

Determining the Compressive Force
The uniaxial compression tests between two parallel planes were made using a Zwick/Roell Z005 testing machine equipped with a compression head with a maximum force of 50 N. During testing, the speed of the moving plate was constant and amounted to 3 mm/min. Measurements were carried out until the seeds ruptured, recording the changes in a loading force as a function of the measuring head displacement.
The cracking point corresponding to a clear decrease in the pressure force on the loaddeformation characteristics was determined automatically using Zwick's TestXpertII.V3.5 software. The force (CF) at which the seeds cracked was accepted as the granule breaking force. The moisture content of the tested seeds was 7%. Depending on the type of cultivation, 100 seeds were randomly selected from each variety and weighed with an accuracy of 0.1 mg using the scales (Max 50/1/WH). After preparing the raw material, the rapeseeds were subjected to tests to determine the compression force.

Determining the Compressive Force
The uniaxial compression tests between two parallel planes were made using a Zwick/Roell Z005 testing machine equipped with a compression head with a maximum force of 50 N. During testing, the speed of the moving plate was constant and amounted to 3 mm/min. Measurements were carried out until the seeds ruptured, recording the changes in a loading force as a function of the measuring head displacement.
The cracking point corresponding to a clear decrease in the pressure force on the load-deformation characteristics was determined automatically using Zwick's TestXpertII.V3.5 software. The force (CF) at which the seeds cracked was accepted as the granule breaking force.

Statistical Analysis
While processing the obtained results, the following software was used: MS Office 2007 package, and Statistica version 13.1 by StatSoft. Results are presented using basic elements of descriptive Sustainability 2019, 11, 2133 4 of 10 statistics: mean value, median, standard deviation, minimum and maximum values. On the other hand, the compliance of the normal distribution was verified by means of the Shapiro-Wilk test and the homogeneity of the variance by the Brown-Forsyth test. Comparative analysis for the analyzed variables was carried out applying non-parametric by the Kruskal-Wallis tests. Correlation tests were used to assess simple relationships between single parameters. For continuous variables with normal distributions, the Spearman's rank correlation non-parametric test was used. The observed dependencies were considered statistically significant at α < 0.05.

Results and Discussion
Weather conditions that prevailed during the research period are presented in Table 1. They concerned the period from March to September in 2015 and 2016 and were recorded by IMGW-PIB from Lublin Radawiec Station. The macronutrient and heavy metal contents in the digestate used were also tested ( Table 2). Laboratory tests were carried out at the District Chemical-Agricultural Station in Lublin in accordance with KQ/PB-17-76-77: 2012 [37]. Maize silage, whey, and green waste matter were used as a feedstock in the biogas plant for the production of the biogas and digestate. Table 2. Content of macronutrients and selected heavy metals in the digestate used for the cultivation of spring rape.

Element
Unit Content Table 3 presents the results of the research on the content of the macroelements in the soil before the application of selected fertilizers and after the harvest of plants, which were carried out at the District Chemical-Agricultural Station in Lublin. Soil samples were tested for the content of bioavailable components regarding phosphorus and potassium in accordance with the applicable standards for the above-mentioned components PN-R-04024: 1997, and magnesium in mg per 100 g soil according to PN-ISO 10390: 1997 [38,39]. Soil acidity, pH in KCl, and possible liming needs were taken into account. Application of the fertilization method affected the change in the weight of seeds. The highest significant mean weight was found for the Feliks cv. (4.91 mg) in the cultivation using digestate, while the lowest was found for Markus cv. (4.44 mg) in the control crop. Of all the rapeseed varieties, the highest average weight was obtained by seeds when using the digestate (4.76 mg), whereas in traditional cultivation it was 4.69 mg and on control plots (4.61 mg). Considering the rapeseed type, the highest average weight was obtained for seeds of the Feliks variety (4.79 mg), although Bios cv. had a very similar value of 4.78 mg. Markus cv. Were the lightest seeds (4.49 mg). The total average weight of all tested seeds was 4.69 mg ( Figure 2). However, statistical analysis using the Kruskal-Wallis test showed no effect of fertilization on the mass of rapeseeds. www.mdpi.com/journal/sustainability bioavailable components regarding phosphorus and potassium in accordance with the applicable standards for the above-mentioned components PN-R-04024: 1997, and magnesium in mg per 100 g soil according to PN-ISO 10390: 1997 [38,39]. Soil acidity, pH in KCl, and possible liming needs were taken into account. Application of the fertilization method affected the change in the weight of seeds. The highest significant mean weight was found for the Feliks cv. (4.91 mg) in the cultivation using digestate, while the lowest was found for Markus cv. (4.44 mg) in the control crop. Of all the rapeseed varieties, the highest average weight was obtained by seeds when using the digestate (4.76 mg), whereas in traditional cultivation it was 4.69 mg and on control plots (4.61 mg). Considering the rapeseed type, the highest average weight was obtained for seeds of the Feliks variety (4.79 mg), although Bios cv. had a very similar value of 4.78 mg. Markus cv. Were the lightest seeds (4.49 mg). The total average weight of all tested seeds was 4.69 mg ( Figure 2). However, statistical analysis using the Kruskal-Wallis test showed no effect of fertilization on the mass of rapeseeds.  In the conducted research, three forms of fertilization for individual rape varieties were used. Results of spring rapeseed strength tests and values of the variability coefficients are presented in Table 4.
Based on Table 4, it can be observed that the use of digestate in the cultivation of rape for both Bios and Feliks varieties contributed to an increase in the static strength of their seeds in relation to the control combination. The highest compressive strength value was recorded for Bios cv. (29.81 N) and the smallest value was 5.63 N (for Feliks cv.). When cultivating using the digestate, the range of compressive strength values for seeds also had the smallest differentiation in the Markus variety ranging from 16.53 N to the largest 23.67 N in Bios cv. In the same cultivation, the largest difference in Sustainability 2019, 11, 2133 6 of 10 average seed strength, occurring between the highest and the lowest average value of the force, was also observed, which amounted to 4.91 N. In the case of crops using the mineral fertilizer, the lowest value of the force required to crush the rapeseeds was 3.86 N for the Feliks variety and the highest was 26.7 N for Bios cv. In this cultivation, the compressive strength range was from 20.36 N for Markus cv. to 22.54 N for Feliks. On the other hand, the difference in mean seed strength occurring between the highest and lowest average strength value was 2.98 N, which was smaller than for the variant with the digestate.
Considering the case with the absence of any form of fertilization, it was observed that in all varieties of rape, the average compressive strength was the lowest. The minimum compressive strength was recorded for the Markus variety, which reached a value of 3.19 N and it was the lowest of all seeds tested. However, the highest strength was achieved by Bios cv. at the level of 25.49 N. In the control combinations, the range of compression strength for rapeseeds was also not very different and ranged from 18.31 N for Feliks variety to 21.41 N for Bios cv. However, it was more than half the value when compared to the digestate cultivation variant. In turn, the difference in average seed strength occurring between the highest and the lowest average strength value amounted to 0.84 N, which was the smallest of all forms of rape cultivation.
When assessing the variability coefficient for the compressive force of the rapeseeds, the lowest values were reached with cultivations using the digestate, 29.92% on average. This indicates greater uniformity in the size of rapeseeds compared to other forms of cultivation. The highest value of this coefficient was obtained for cultivation using a multi-component fertilizer (45.67%-Markus variety). Elevated values in all cases indicate high variability in the size of the rapeseeds in the samples tested.
Considering the size of the seeds, Lamb and Jahnson showed that seeds with a diameter below 1.8 mm have lower agricultural usefulness as well as lower technological and nutritional value [40]. It is a consequence of relatively small kernels in smaller seeds with a small amount of reserve compounds [41]. In the tests carried out, the range of medium-sized rapeseed diameters ranged from 1.81 to 1.95 mm, which indicates their good suitability for the food industry. Seeds above 2 mm in diameter (with larger weight) are the source of the best oil in terms of quality. Unfortunately, oil is produced from the smallest seeds, which causes its deterioration through higher oxidation and hydrolysis processes [29]. In addition, Tańska et al. showed that the strength properties of seeds are dependent on the size of the seeds and correlated with the equivalent diameter of seeds [26]. All measured strength indices showed that seeds with larger diameters were significantly resistant. This was confirmed by the study of Szwed and Tys, which suggest that in bulk mass, small seeds are the main cause of seed spoilage [42].
The statistical analysis using the Kruskal-Wallis test showed with a probability p = 0.638 that the fertilization variants used did not significantly affect the compressive force. Only for the Bios and Feliks cv. was the static strength of the seeds more varied. In the Bios variant, there was a probability p = 0.101 with stronger dependence observed in the homogeneous group NPK-digestate (p = 0.767). However, for the Feliks cv., a significant effect of the fertilization variants on the compressive force was found with the probability p = 0.001, with one homogeneous group NPK-digestate p = 0.308 (Table 5). To assess the simple relationships between seed compression force and mass, Spearman's rank correlation tests were used at the significance level α = 0.05 (Table 6) for variables with a normal distribution. In the case of Bios variety, there was a relation between the mass and size of the rapeseeds vs. the compressive force. It was noticed that when the seed mass increased, the force needed to crush it was greater. This phenomenon was observed in the case of seeds that were fertilized with both digestate and mineral fertilizer. It is also worth noting that when using the digestate, this correlation was stronger (Rs = 0.621) than for the multi-component fertilizer (Rs = 0.448). This proves that with a relatively comparable mass of seeds in both types of cultivation, greater seed crushing force should be used in the case of cultivation with the digestate.
Considering Feliks cv., a similar relationship can be seen. It applies to all three cases where without any form of fertilization, increased compressive forces were needed with increasing seed mass. Looking at the correlation coefficients from Table 5, it can be seen that at the significance level of p < 0.05, their values were very close to each other (0.370, 0.377, 0.379). Thus, in this case again, with the increasing mass of seeds, it was necessary to use a greater crushing force.
Taking into consideration Markus cv., no significant correlations were found between the weight of seeds and the compressive force for each type of crop. This may be due to the individual properties of seeds of this rape variety.
Comparing the results of the present research to those by Tańska and Konopka in which the digestate was not used, the value of the rapeseed strength in view of their size was in the range from 10 to 18 N, with large seeds (of larger weight) showing much less elastic deformation than medium seeds [26]. Referring to the conducted research, values obtained with the use of a multi-component fertilizer were similar and were in the range of 7-21 N in three of the considered cases. Rapeseeds maintained their compressive properties in a similar range in a treated crop, the values of which were between 7 and 24 N. It can, therefore, be concluded that the compression strength ranges are similar to each other in the considered cultivations. This was confirmed by statistical analyses in two cases for the Bios and Feliks varieties, which showed that the strength of seeds increases with increasing weight.
Considering that in the current literature the research on the strength of rapeseeds does not take into account cultivations using the digestate, it is necessary to undertake further study in this field.

Conclusions
In cultivations using the digestate, the largest difference in mean seed strength was observed between the highest and lowest mean strength values, which was 4.91 N, while the smallest was recorded for the control combination (0.84 N). Digestate fertilization for cultivation of the Bios and Feliks varieties contributed to an increase in the static strength of their seeds in relation to the control combination and statistical analysis pointed to NPK-digestate homogeneous groups. It was found that the distribution of the increase in the crushing force for Bios and Feliks rapeseed varieties in both forms of cultivation was proportional to the increase in their mass. However, with a relatively comparable mass of seeds of Bios cv., a stronger correlation was observed with the use of the digestate than with a multi-component fertilizer. This shows that seeds from this form of cultivation need a larger force to crush. In the conducted tests, the average rapeseed diameters of all varieties and forms of cultivation ranged from 1.81 to 1.95 mm, which indicates their good suitability for industrial purposes. Of all the varieties of rapeseed, the highest average weight was obtained in cultivation using the digestate (4.76 mg), while in traditional cultivation, it was 4.69 mg and in control combinations it was 4.61 mg.