Effect of Nitrogen on the Properties of Flax (Linum usitatissimum L.) Plants and Fibres

This paper presents a three-year study of the influence of different amounts of nitrogen on the properties of flax plants and fibres. At the same time, the acclimatization ability of five different cultivars of fibre flax was estimated through the valorisation of their morphological (technical stem length, stem thickness) and physical-mechanical properties of the fibres (length, fineness, tenacity). Cultivar trials with fibre flax were set up across three years (2008–2010) at the following locations: the experimental fields of the Faculty of Agriculture in Zagreb on anthropogenized Eutric Cambisol and the College of Agriculture at Križevci on pseudogley on level terrain. The selected cultivars were fertilized without and with different nitrogen rates (0, 30, 60 and 90 kg/ha) in different time. The trials were carried out according to the RCBD in four replications. According to the results of the three-year study of flax and fibres, significant differences were established among the cultivars and among the added nitrogen rates under study. Based on the results of the morphological and textile-technological properties of flax, the cultivars Viola and Agatha achieved higher values at the location of Križevci, where it was not necessary to add more than 30 kg N/ha.


Introduction
Flax (Linum usitatissimum L.) was one of the earliest ancient plants to be domesticated by humans and grown for seed, oil and the strong fibres produced in its stems [1].
Flax fibres are used as textile raw material, at first for composing cords and weaving yarn, and then for fashionable garments or high-quality fabric upholstery. Over the past decades, its numerous existing applications and the constant development towards more innovative materials make flax a plant of growing industrial interest, especially for the use of flax fibres as composite reinforcement. However, much fewer studies focus on flax as a plant, investigating the relationships between the properties of flax fibres and the development of the plants from which they are obtained. It is necessary to take advantage of the fact that flax is a crop with great potential due to its multiple uses and its high adaptability [2][3][4].
For more than 20 years, there has been cooperation between agronomists and textileoriented specialists with the aim of revitalizing the sustainable and ecological production of flax fibres in Croatia. Since Croatia does not have its own cultivars [5,6], it is necessary to introduce cultivars from Europe, which are grown in climatic conditions similar to Croatian cultivars [7][8][9][10][11][12]. Therefore, it may be assumed that the newly formed climate conditions would cause the reduction in the valuable properties of flax plant and fibres. Sowing a suitable cultivar is an important factor to enhance growth, yield and the components and quality parameters of flax depending on which area it is planted in with regard to climatic conditions. The applying of appropriate fertilizers at a suitable time and in the required quantities needed for the plant is one of the most essential elements to ensure that the plant provides a high output. Nitrogen fertilization plays an important role in increasing the productivity and quality of flax.
According to our previous two-year study [8,13,14] of the agronomic and morphological properties of fibre flax in two locations, significant differences were established between the cultivars, without and with topdressing by nitrogen. Higher values of the studied properties were obtained with a nitrogen topdressing with 27 kg/ha [14]. According to the obtained results, all investigated varieties showed satisfactory resistance to lodging, regardless of nitrogen fertilization. A large amount of nitrogen increases vegetative flax mass, which causes lodging and increases the stem thickness, and the amount of wooden core decreases the yield and quality of fibres, i.e., further fibre processing and the suitability of flax fibres as a textile raw material for yarn and fabric manufacturing are prevented [8,[15][16][17][18]. Phosphorus positively affects the development of the root and fibre strength and takes part in seed formation, while potassium stimulates fibre and seed formation. It creates a stronger bond among fibre bundles, and the stem is technically easier to process [3,8].
Many researchers from Egypt concluded that using nitrogen fertilizer at a suitable and required level could improve the yield, components and quality of flax [19][20][21][22]. This fertilizer is recommended for the highest yield of straw and seed with the best-quality new cultivars, Giza and Sakha, respectively, under the climatic conditions of Egypt. A group of authors from Ethiopia studied the potential development of flax fibre and oil quality (Linum usitatissimum L.) under the influence of different rates of nitrogen application. For example, they determined that increasing the nitrogen fertilizer rate increased the flax fibre length. A fibre length of about 58 cm was obtained from a 138 kg N/ha fertilizer rate and the lowest rate was recorded in the control treatment [23].
Bast fibres, including flax fibres, are characteristic of a high level of non-uniform morphology, which can be neither substantially affected nor removed by production technology, but noticeably affects fibre properties. Furthermore, the properties of flax fibres are also the result of the effect of different factors influencing the growth and development of flax plants such as cultivars and agroecological cultivation conditions, as well as fibre extraction from the stem of the flax plant and flax processing into fibres. Closely related parameters, which have an impact on the quality of flax fibres, are ergonomic, morphological and phenological plant properties such as root shape, length, thickness, stem colour, branching, leaf shape and retting procedure [24,25]. It can also be asserted that each step from plant cultivation to fibre production has its own specific impact on fibre quality [3]. Fibre fineness, length and strength are the most important processing properties, determining the quality and suitability of flax fibres as a textile raw material for yarn and fabric manufacturing [2].
There is the assumption that, under different climate conditions, a reduction in the agronomic properties of imported cultivars occurs, in turn causing a reduction in textiletechnological fibre properties. Agronomists and textile-oriented specialists from Croatia investigated acclimatization ability of foreign flax cultivars by determining their morphological and phenological flax properties and textile-technological fibre properties [9]. They concluded that the cultivar Viola has a better acclimatization ability and is recommended for cultivation in Croatia. The fibre flax cultivar, Viola, produces fibres of higher quality, which are longer, finer, and have a lower degree of resolution. Therefore, these authors are expanding their research to include a larger number of different cultivars [26,27]. Based on the results of the morphological and textile-technological properties of flax, the cultivars, Agatha, Electra and Viola, achieved higher values, and their fibres satisfy the current market demands for their use. The cultivars fertilized with nitrogen produced taller plants, longer technical stem and fibre lengths, thicker stems, and slightly coarser and stronger fibres. These plants entered the stage of flowering and yellow maturity later than the plants without nitrogen. According to the results obtained for the studied properties of fibre flax fertilization, it is not necessary to add more than 30 kg of nitrogen/ha. Considering the obtained results, the investigation continued, and the influence of nitrogen fertilization on the agronomic properties (dry stem yield, dry stem yield after retting, total fibre yield, share of total fibre, long fibre yield and share of long fibre) of fibre flax cultivars was investigated [11]. Based on the analysis of the results, it was concluded that the cultivars, Agatha, Viola and Electra, recorded the highest values for the investigated properties (dry stem yield, dry stem yield after retting, total fibre yield, long fibre yield) over three years and the highest share of total fibres and long fibres over one or two years. Additionally, the optimal nitrogen rate for fibre flax, according to the obtained results, was 30 kg N/ha. This paper presents a continuation of the research on the impact of different amounts of nitrogen on the properties of flax plants and fibres. At the same time, the acclimatization ability of five different cultivars of fibre flax was estimated through the valorisation of their morphological (technical stem length and stem thickness) and physical-mechanical properties (length, fineness and tenacity).

Flax Fibre Growing and Processing
Cultivar trials with foreign fibre flax were carried out in the experimental field of the . According to type affiliation, the soil in Zagreb is anthropogenized Eutric Cambisol, while in Križevci is it pseudoglay on level terrain. Both soils have silty loamy textures. Because of their high powder content, they are prone to creating a crust. The content of the nutrients in the soil and pH are given in Table 1. Fertilization with 100 kg ha −1 P (as superphosphate) and 150 kg ha −1 K (potassium salt) was carried out within basic tillage. Flax was fertilized with different N rates (0, 30, 60 and 90 kg ha −1 ) at different times (urea (46%) and with calcium ammonium nitrate (27% N)). No nitrogen was added in the first trial treatment. In the second fertilization treatment, all nitrogen was added before sowing (30 kg N/ha). In the third fertilization treatment, 30 kg N/ha was added before sowing, and 30 kg in a single fertilizer application was added at the average plant height of 10 cm; in the fourth fertilization treatment, 30 kg was added before sowing, and 30 kg was added at the average plant height of 10 and 20 cm each.
The trials were laid out according to the randomized complete block design (RCBD) with four replications. The main trial plot size was 10 m 2 (10 rows × 0.1 m row spacing × 10 m length). Sowing was carried out using a plot seeder (Wintersteiger, Ried, Austria.

Plant Properties
The investigated morphological properties were technical stem length and stem thickness. Pulling by hand was carried out at the yellow-green ripening stage. Plants were pulled from an area of 1 m 2 . Technical stem length was measured from the cotyledon node to the first branch. Stem thickness was determined in the middle of the technical stem length using an electronic micrometre. Flax stems were then placed in tank of water at 30 • C for 4 days under controlled conditions. Afterwards, retting stems were removed from the tank, dried at 60 • C for 30 h and weighed. A scutching machine was used to separate straw (woody matter) from the fibres.

Fibre Properties
Flax fibres are characterized by the following textile-technological properties: length, fineness and tenacity. Fibre length was determined by the measurement of technical fibres method according to HRN ISO 6989 2003. Fineness (HR EN ISO 1973 2021) and tenacity (HR EN ISO 5079 2020) of flax fibres were determined on tensile testers, Vibroscop and Vibrodyn 400 (Lenzing Instruments, Gampern, Austria). Used standards and regulations are adapted for testing technical flax fibres. For this purpose, cogged steal clamps were placed on the standard clamps, and the selected testing speed was 3 mm/min with a load cell capacity of 1500 mg. The selected gauge length of the sample was 5 mm to ensure that all elementary fibres were included in the tested sample during fineness and tenacity testing.
Used standards and regulations were adapted for testing technical flax fibres. Due to the non-homogeneity of flax fibres, the number of measurements was increased from 50 to 100. The measurements of the tested properties of fibres were performed on conditioned samples.

Statistical Analysis
Data of all the properties studied in each location and year were statistically processed by an analysis of variance. Differences between mean values were analysed using Duncan's multiple range test (DMRT) [28].  Values with the same letter are not significant at a level of 5%; as the significance value decreases, the letters become lower since they are in alphabetical order.
In addition, statistically significant differences were recorded among different nitrogen rates for technical stem length in  (Tables 4 and 5). No significant interaction was recorded for any properties or any locations; therefore, interactions were not included in the factors shown here and were not discussed any further. Accordingly, the factors affected the studied properties independently.
The technical stem length should not be shorter than 60 cm, and it is desirable at a length of about 100 cm. The obtained values (Tables 2 and 3 Slightly lower values than the average, as determined by a group of authors in their previous research [29][30][31], are the result of unfavourable weather conditions during the growth and development of flax (after planting-low temperature with the appearance of frost; excessively high temperatures in May-premature flowering; excessive rainfall in June-unevenly ripening). In addition, the production of fibre flax in sandy soil at Zagreb is unfavourable in the years with unevenly distribution of rainfall. Stem thickness affects Polymers 2022, 14, 558 6 of 10 fibre quantity and quality, i.e., the fibres fineness depends on the thickness of the stem. A very thin and very thick stem is less valuable for textile use. The thicknesses of the stems at both locations are between 1.19 to 1.78 mm, which is suitable for obtaining fibres of satisfactory quantity and corroborates the research of other authors [29,30,32].  Values with the same letter are not significant at a level of 5%; as the significance value decreases, the letters become lower since they are in alphabetical order.  . From the analysis of fibre fineness and tenacity, it can be concluded that these factors affected the investigated properties independently, which is consistent with other authors [3]. In addition, the obtained results of fibre properties are in accordance with the research of other authors [4,33,34].
The means of the morphological properties of fibre flax, and the physical-mechanical properties of fibres dependent on nitrogen rates, are given in Table 4 for Zagreb and in Table 5 for Križevci for all three years.
The results of previous research on fertilizing flax with nitrogen are different, and depend on the added amounts of nitrogen, application time, soil nitrogen supply and weather conditions. However, the authors found that there was no significant increase in the properties of the flax and obtained fibres when larger amounts of nitrogen (30 kg/ha) were added, and 30 kg/ha of nitrogen is recommended for obtaining an optimal fibre quality [8,15,17,35,36]. According to the obtained values of the investigated properties, there were no significant differences between the variants in which 30, 60 or 90 kg/ha of nitrogen were added. Importantly, only in 2009, at Zagreb, the thickest stem was achieved with 60 kg/ha of nitrogen, and at Krizevci, the longest fibre length was achieved with 60 and 90 kg/ha of nitrogen. In general, by increasing the fertilization from 30 to 60 kg/ha of nitrogen, the values increase slightly, and after the application of 90 kg/ha of nitrogen they decrease.
Fibre length, fineness and tenacity are the most important processing properties and determine the quality and suitability of flax fibres as a textile raw material for yarn and fabric manufacturing. Furthermore, measurements of fibre properties are in compliance with measurements of plant properties.

Conclusions
This paper presents a systematic and interdisciplinary evaluation from a three-year study on the effect of different amounts of nitrogen on the properties of flax plants and fibres through the valorisation of their morphological (technical stem length, stem thickness) and physical-mechanical properties (length, fineness, tenacity). For the purpose of evaluating the influence of cultivars, location and nitrogen fertilization on plant and fibre properties, a number of parameters were taken into consideration that affect plant growth and development as well as fibre properties.
Based on the analysis of the results, individual conclusions can be made depending on the investigated properties: Nitrogen-fertilized variants had longer technical stem lengths and fibre lengths, thicker stems, and slightly coarser and stronger fibres. Since there is no significant increase in the properties of flax and obtained fibres when larger amounts of nitrogen of 30 kg/ha were added, there is no need to add more than 30 kg/ha of nitrogen in the fertilization of flax.
Based on the results of the morphological and textile-technological properties of flax, the cultivars Viola and Agatha achieved higher values at Križevci, and it is not necessary to add more than 30 kg of N/ha.