Quality of Sugar Beets under the E ﬀ ects of Digestate Application to the Soil

: Management of digestate from production of biogas has a great environmental importance. One of feedstock for biogas generation is beet pulp, a side product of sugar beet processing plant. In the paper a closed loop of beet pulp utilization at sugar beet plantation is presented. E ﬀ ects of soil application of digestate obtained from digestion of sugar beet pulp were compared with standard mineral fertilizers. The ﬁeld experiment was performed in three successive growing seasons. The studies were concentrated on quality of sugar beets grown under e ﬀ ects of two fertilization treatments—soil application of digestate cv. standard mineral fertilizers. It was found that some important quality indices (weight of single sugar beet root, content of sucrose in root tissues) were higher for beet harvested from digestate treatment compared to standard mineral fertilization (control). The concentration of harmful component (amide nitrogen) in sugar beets grown under conditions of digestate soil application was lower than in the control. It can be concluded that soil application of digestate from processing of sugar beet pulp can be treated as environmentally sound and e ﬀ ective method of its management.


Introduction
Beet pulp, a by-product of sugar factories for many years, has been used as feed of high nutritional value, especially for dairy cows. However, recently noted considerable reduction of cattle number in Poland as well as breaking ties between crop and animal production in many farms, made in a sugar beet processing plants urgent necessity of finding an alternative and effective method for beet pulp management.
One of alternatives with high potential from economical as well as from an environmental aspect seems to be processing of pulp in gasifiers.
In the Institute, research works have been carried out to develop technology of anaerobic digestion of sugar beet pulp to produce biogas of high calorific value [1].
Digestion of plant wastes as stock material is a common practice in many countries [2][3][4][5][6][7][8][9] and the final product of digestion i.e., biogas is a valuable source of renewable energy which can be converted to heat, power or to liquid fuels. Under optimal conditions during the course of the gasification process biogas contains: 25-85% of methane, 14-48% carbon dioxide, 0.08-0.5% hydrogen sulphide, 0-5% hydrogen, 0-2.1% carbon monoxide, nitrogen 0.6-7.5% and 0-1% of oxygen. Final composition of biogas depends on the kind of stock material introduced to the installation and applied method of gasification [10,11].

Materials and Methods
Studied material was digestate generated from gasification of sugar beet pulp and sugar beets (Beta vulgaris cv. Fighter) collected from experimental plots.
Research works were conducted at experimental field owned by the Institute in three growing seasons i.e., 2013, 2014 and 2015.
Area of single plot was 18.75 m 2 and the field trial was performed in triplicate in modified Latin square design with buffer zones (any variability of soil conditions has been detected).
Two treatments have been studied-standard complex fertilizer application at the rate of 120 kg N ha −1 and digestate in the form mixture of liquid and solid fractions taken directly from the installation every season in the rate equivalent to nitrogen in fertilizers.
Samples of sugar beet roots were taken (30 roots from each treatment every year) and their technological parameters were assessed in the laboratory of the Institute.
Experimental results were analysed by Statistica®ver. 12.0 PL software (Cracow, Poland) in two-factorial design of ANOVA using Fisher's test (P ≤ 0.05) and presented as homogenous groups.

Results and Discussion
In Poland, there are not environmental regulations specific for soil application of digestate and therefore herein law regulations for soil application of sewage sludge were taken into account. These regulations were issued in Poland by Minister of the Environment on 6 February 2015. These regulations concern sludge parameters but not having specific regulations parameters of digestate were compared to permissible levels required for sewage sludge application and presented in Table 1. It can be stated that in any single case level of potential soil contaminant in applied digestate was not exceeded and this waste can be used as amendment in appropriate rates limited only by content of nitrogen.
As for other crops also to obtain high yield of sugar beet weather conditions play an essential role. On Figures 1-3 patterns of most important weather parameters are presented. It can be seen that seasons of the study were variable as far as presented weather parameters are concerned.     Weather requirements of sugar beet in the phase of crop intensive growth (June-October) are more complex. Biomass accumulation in the roots is highly dependent on temperature and high level of soil moisture, whereas for sucrose accumulation dominant role plays high insolation rate. When at that time number of hours with direct sun radiation is high, sugars are accumulated intensively whereas high precipitation in this period reduce sugar content.
Therefore, it can be concluded that the appropriate pattern of weather condition for sugar beet growth in the initial growth phase occurred in 2013 but in the season 2015 in this period sugar beets met worse conditions. In the season 2013, weather also favoured accumulation of biomass in roots, contrary to 2014. When solar radiation period at the phase of sugar accumulation in roots in studied growing seasons was compared, it can be stated that all three seasons was almost equal.
For growers studied parameters of technological quality of sugar beets are not so important because from their point of view yield and sugar content are crucial.
Contrary, for sugar factory, all presented below sugar beets quality parameters have a key importance because they determine technological value of stock materials i.e., sugar beets for processing.
Results presented on Figures 4-11 show that tested major parameters were affected by the season (factor A) and also by fertilization treatments (factor B). In the all graphs, lower-case letters designate homogeneous group of mean values for factor B and interaction A × B; whereas capital letters point significant difference for factor A.
Most important quality parameters are weight of sugar beet together with sucrose content and in some sugar factories also content of α-amino acids is routinely analysed. In the experiment the lowest weight of single root was noted in season 2014 and the highest in season 2013 what reflected the effect of weather conditions.
It was found that the weight of single root in 2014 growing season was not affected by studied fertilization treatments-average values were almost equal. Different impact of fertilization treatments on single root weight was found in the seasons 2013 and 2015. In 2013 significantly higher weight of single root was found for NPK treatment whereas in 2015 roots harvested from plots Sugar beet is a crop of high water requirements and for high yields it needs appropriate pattern of temperature during growing period In early phases of plant growth (April-May) water availability and appropriate temperature are critical factors, while insolation has a minor importance. When weather conditions at this phase are optimal growing period lasts longer.
Weather requirements of sugar beet in the phase of crop intensive growth (June-October) are more complex. Biomass accumulation in the roots is highly dependent on temperature and high level of soil moisture, whereas for sucrose accumulation dominant role plays high insolation rate. When at that time number of hours with direct sun radiation is high, sugars are accumulated intensively whereas high precipitation in this period reduce sugar content.
Therefore, it can be concluded that the appropriate pattern of weather condition for sugar beet growth in the initial growth phase occurred in 2013 but in the season 2015 in this period sugar beets met worse conditions. In the season 2013, weather also favoured accumulation of biomass in roots, contrary to 2014. When solar radiation period at the phase of sugar accumulation in roots in studied growing seasons was compared, it can be stated that all three seasons was almost equal.
For growers studied parameters of technological quality of sugar beets are not so important because from their point of view yield and sugar content are crucial.
Contrary, for sugar factory, all presented below sugar beets quality parameters have a key importance because they determine technological value of stock materials i.e., sugar beets for processing.
Results presented on Figures 4-11 show that tested major parameters were affected by the season (factor A) and also by fertilization treatments (factor B). In the all graphs, lower-case letters designate homogeneous group of mean values for factor B and interaction A × B; whereas capital letters point significant difference for factor A.
Processes 2020, 8, x FOR PEER REVIEW 6 of 12 amended with digestate showed higher weight than roots from control treatment ( Figure 4). This variability resulted in lack of significant differences when treatment effect was taken into account. Applied digestate is a source of nutrient for growing crop but contrary to manures, composts and sewage sludge which have been extensively studied in the past research on digestates is not complex so far. Under conditions of Southern Sweden it was found that soil chemical properties were only slightly modified when soil was amended with some organic by-products including digestates [21]. Odlare et al. [21] reported that among studied amendments i.e., pig manure, cow manure, compost, inorganic fertilizer and digestate from household wastes soil treated with the last one manifested highest biomass and activity of studied microorganisms.
It was found improvement of quality of soils treated with anaerobic digestates in short term. Besides the beneficial impact on microbial biomass and content of essential nutrients also reducing soil bulk density and increasing its hydraulic conductivity have been pointed. However, it has to be mentioned that literature on this topic is relatively scarce [27,28].
Content of sucrose accumulated in sugar beet roots should be regarded as relatively high because reported in Poland content of sucrose usually ranges between 14 and 19% (30). In the current study it ranged between 17.3 and 18.6%, for season 2014 and 2013, respectively. It was found that differences of sugar content in sugar beet roots were significant in relations to the season of studies and the highest was in 2013. Also fertilization treatment significantly affected sugar content and roots collected from plots amended with digestate showed higher by 0.5% sucrose content than roots from NPK treatment ( Figure 5).
Data presented in Figure 6 show that content of dry matter in tissues of beet roots ranged from 22.3 to 24.2%. I was revealed that differences of this key parameter in seasons 2013 and 2014 as well as average in three seasons were not proven and only in 2015 beets harvested from plots amended with digestate accumulated significantly more dry matter in roots than roots from NPK treatment.
It can be pointed out that content of dry matter in sugar beet roots obtained in the experiment was relatively high. Dobrzycki [29] reported that 25% of DM in sugar beet roots is a typical value for sugar beets of high industrial quality.   What is worth to mention is the fact that over 80% of dry matter consist of sucrose irrespectively of fertilization treatment under studies what indicate high efficiency of sugar accumulation in sugar beets storage organs. What is worth to mention is the fact that over 80% of dry matter consist of sucrose irrespectively of fertilization treatment under studies what indicate high efficiency of sugar accumulation in sugar beets storage organs.   Soluble ash fraction represents mineral compounds contained in the juice and weakly bond to root tissues. During sugar extraction process soluble ash through draft juice reaches molasses. During processing this fraction is considered as beneficial for extraction because soluble ash consisting mainly of sodium and potassium stabilises pH of draft juice in a basic range [30].
Obtained results of analytical procedures showed generally low variability of average ash content in seasons of studies in sugar beet roots because it ranged from 0.28 to 0.35%. In two seasons of studies (2013 and 2014) roots from plots amended with digestate showed significant decrease of soluble ash fraction comparing to beet from the other studied treatment (NPK (Figure 7). Two analysed nitrogen species (Figures 8 and 9) are harmful for the technological process in sugar factories processing sugar beets. Amino acids negatively affect pH of juices and reactions between them and simple sugars undergo intensively and their products are undesirable colourful substrates. They are defined as "harmful no-sugar compounds". Amide compounds during thermal processes in during sugar extraction are degraded to ammonia what pollutes condensate which is pumped to steam boiler what makes the necessity of applying water purification operation. Also occurrence of amide compounds can result in presence of pyrrolidone carboxylic acid which negatively affect technological quality of juice. Moreover, amino acids molecules are substrates of so called Maillard reaction which gives brown products undesirable during white sugar production [31].  Variability of concentration of amide nitrogen content in sugar beet roots was related to the fertilization treatment in two seasons of studies (2014 and 2015). In tissues of sugar beets collected from plots amended with digestate significantly lower concentration of this nitrogen species was noted. It can be stated this method of soil fertilization of sugar beets using digestate resulted in lower content of harmful amide nitrogen comparing to standard mineral fertilizers (Figure 9).  Variability of concentration of amide nitrogen content in sugar beet roots was related to the fertilization treatment in two seasons of studies (2014 and 2015). In tissues of sugar beets collected from plots amended with digestate significantly lower concentration of this nitrogen species was noted. It can be stated this method of soil fertilization of sugar beets using digestate resulted in lower content of harmful amide nitrogen comparing to standard mineral fertilizers (Figure 9).   Sum of both studied elements (sodium and potassium) represents so called "natural alkalinity". Sugar beet uses them to maintain pH value of juice between 5.8 and 6.2 what favours accumulation of high amount of sucrose. Sodium and potassium in ash represented a dominant fraction. During processing in sugar factory both elements play essential role in maintaining appropriate pH of extract of processed stock material. On the other hand, mentioned alkali metals compounds show high affinity to sucrose and increase sugar concentration in molasses what may result in higher losses of sucrose [30].
Relatively high variability of sodium content in seasons of studies was found because it ranged from 0.012 to 0.164% in 2013 and 2014, respectively. In two seasons (2014 and 2015) as well as on an Most important quality parameters are weight of sugar beet together with sucrose content and in some sugar factories also content of α-amino acids is routinely analysed. In the experiment the lowest weight of single root was noted in season 2014 and the highest in season 2013 what reflected the effect of weather conditions. It was found that the weight of single root in 2014 growing season was not affected by studied fertilization treatments-average values were almost equal. Different impact of fertilization treatments on single root weight was found in the seasons 2013 and 2015. In 2013 significantly higher weight of single root was found for NPK treatment whereas in 2015 roots harvested from plots amended with digestate showed higher weight than roots from control treatment (Figure 4). This variability resulted in lack of significant differences when treatment effect was taken into account.
Applied digestate is a source of nutrient for growing crop but contrary to manures, composts and sewage sludge which have been extensively studied in the past research on digestates is not complex so far. Under conditions of Southern Sweden it was found that soil chemical properties were only slightly modified when soil was amended with some organic by-products including digestates [21]. Odlare et al. [21] reported that among studied amendments i.e., pig manure, cow manure, compost, inorganic fertilizer and digestate from household wastes soil treated with the last one manifested highest biomass and activity of studied microorganisms.
It was found improvement of quality of soils treated with anaerobic digestates in short term. Besides the beneficial impact on microbial biomass and content of essential nutrients also reducing soil bulk density and increasing its hydraulic conductivity have been pointed. However, it has to be mentioned that literature on this topic is relatively scarce [27,28].
Content of sucrose accumulated in sugar beet roots should be regarded as relatively high because reported in Poland content of sucrose usually ranges between 14 and 19% (30). In the current study it ranged between 17.3 and 18.6%, for season 2014 and 2013, respectively. It was found that differences of sugar content in sugar beet roots were significant in relations to the season of studies and the highest was in 2013. Also fertilization treatment significantly affected sugar content and roots collected from plots amended with digestate showed higher by 0.5% sucrose content than roots from NPK treatment ( Figure 5).
Data presented in Figure 6 show that content of dry matter in tissues of beet roots ranged from 22.3 to 24.2%. I was revealed that differences of this key parameter in seasons 2013 and 2014 as well as average in three seasons were not proven and only in 2015 beets harvested from plots amended with digestate accumulated significantly more dry matter in roots than roots from NPK treatment.
It can be pointed out that content of dry matter in sugar beet roots obtained in the experiment was relatively high. Dobrzycki [29] reported that 25% of DM in sugar beet roots is a typical value for sugar beets of high industrial quality.
What is worth to mention is the fact that over 80% of dry matter consist of sucrose irrespectively of fertilization treatment under studies what indicate high efficiency of sugar accumulation in sugar beets storage organs.
Soluble ash fraction represents mineral compounds contained in the juice and weakly bond to root tissues. During sugar extraction process soluble ash through draft juice reaches molasses. During processing this fraction is considered as beneficial for extraction because soluble ash consisting mainly of sodium and potassium stabilises pH of draft juice in a basic range [30].
Obtained results of analytical procedures showed generally low variability of average ash content in seasons of studies in sugar beet roots because it ranged from 0.28 to 0.35%. In two seasons of studies (2013 and 2014) roots from plots amended with digestate showed significant decrease of soluble ash fraction comparing to beet from the other studied treatment (NPK (Figure 7).
Two analysed nitrogen species (Figures 8 and 9) are harmful for the technological process in sugar factories processing sugar beets. Amino acids negatively affect pH of juices and reactions between them and simple sugars undergo intensively and their products are undesirable colourful substrates. They are defined as "harmful no-sugar compounds". Amide compounds during thermal processes in during sugar extraction are degraded to ammonia what pollutes condensate which is pumped to steam boiler what makes the necessity of applying water purification operation. Also occurrence of amide compounds can result in presence of pyrrolidone carboxylic acid which negatively affect technological quality of juice. Moreover, amino acids molecules are substrates of so called Maillard reaction which gives brown products undesirable during white sugar production [31].
It was found that concentration of α-amino acids nitrogen varied from 0.001 to 0.009% irrespectively of fertilization treatment but ANOVA results showed that in the season 2015 this nitrogen species content was significantly lower than in two other seasons of studies ( Figure 8).
Variability of concentration of amide nitrogen content in sugar beet roots was related to the fertilization treatment in two seasons of studies (2014 and 2015). In tissues of sugar beets collected from plots amended with digestate significantly lower concentration of this nitrogen species was noted. It can be stated this method of soil fertilization of sugar beets using digestate resulted in lower content of harmful amide nitrogen comparing to standard mineral fertilizers ( Figure 9). Sum of both studied elements (sodium and potassium) represents so called "natural alkalinity". Sugar beet uses them to maintain pH value of juice between 5.8 and 6.2 what favours accumulation of high amount of sucrose. Sodium and potassium in ash represented a dominant fraction. During processing in sugar factory both elements play essential role in maintaining appropriate pH of extract of processed stock material. On the other hand, mentioned alkali metals compounds show high affinity to sucrose and increase sugar concentration in molasses what may result in higher losses of sucrose [30].
Relatively high variability of sodium content in seasons of studies was found because it ranged from 0.012 to 0.164% in 2013 and 2014, respectively. In two seasons (2014 and 2015) as well as on an average, sodium concentration in sugar beets from digestate treatment was significantly lower than in roots of beets fertilized with NPK ( Figure 10). Similar pattern was found in the case of second analysed alkali metal (potassium) i.e., significantly lower concentration under the effect of digestate comparing to application of standard mineral fertilizers i.e., NPK despite of the fact that in one season (2015) contrary effect was observed ( Figure 11).
So it can be concluded that average concentration of sodium and potassium in sugar beets harvested in three subsequent seasons from plots amended with digestate can be regarded as beneficial for sugar processing.