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Article

The Role of Agriculture in Climate Change Mitigation—A Polish Example

by
Lucjan Pawłowski
1,*,
Małgorzata Pawłowska
1,
Cezary A. Kwiatkowski
2 and
Elżbieta Harasim
2
1
Faculty of Environmental Engineering, Lublin University of Technology, 20-618 Lublin, Poland
2
Department of Herbology and Plant Cultivation Techniques, University of Life Science, 20-033 Lublin, Poland
*
Author to whom correspondence should be addressed.
Energies 2021, 14(12), 3657; https://doi.org/10.3390/en14123657
Submission received: 12 May 2021 / Revised: 13 June 2021 / Accepted: 15 June 2021 / Published: 19 June 2021
(This article belongs to the Collection Feature Papers in Energy, Environment and Well-Being)
Versions Notes

Abstract

:
Biomass, a basic product of agriculture, is one of the main sinks of carbon in global cycle. Additionally, it can be used as a renewable source of energy, leading to depletion in CO2 emissions. The paper presents the results of estimations on the current and potential share of catch crop cultivation in climate change mitigation, in Poland, where the agricultural sector plays a significant economic role. The estimation of CO2 assimilation in biomass was performed on the basis of our own data on yields of commonly used catch crops, obtained in randomly selected 80 farms in Poland, and the content of carbon in the biomass. Calculation of energy potential of the biomass was conducted, assuming its conversion into biogas, on the basis of our own data on catch crop yields and the literature data on their biomethane potentials. The results have shown that catch crops in Poland, which are cultivated to an area of 1.177 mln ha sequestrate 6.85 mln t CO2 yr−1. However, considering the total area of fields used for spring crop cultivation, it is possible to increase the sequestration to 18.25 mln t CO2 yr−1, which constitutes about 6% of the annual emission of CO2 in Poland. Biomethane yields per hectare of particular crops ranged from 965 to 1762 m3 CH4 ha−1, and were significantly lower compared to maize, which is commonly in use in biogas plants. However, due to high biomethane potential and favorable chemical composition, catch crops can be a valuable co-substrate for the feedstocks with a high C:N ratio. The potential recovery of energy produced from aboveground biomass of catch crops harvested in Poland during the year is 6327 GWh of electricity and 7230 GWh of thermal energy. Thus, it is advisable to promote catch crops on a wide scale due to substantial environmental benefits of their cultivation.

1. Introduction

An increase in the concentration of greenhouse gases (GHGs) in the atmosphere is unquestioned phenomenon. According to IPCC reports [1,2] it results from increased emission of these gases from anthropogenic sources. One of the most important greenhouse gases (GHGs) is carbon dioxide (CO2), which is responsible for 56.4% of the greenhouse effect.
Global annual emissions of CO2 are continuously growing: from 9.3 mL t CO2 in 1751 (at the beginning of the Industrial Age) through 28.1 mln t CO2 in 1800; 198.7 mln t CO2 in 1850; 1.95 Gt CO2 in 1900; 5.79 Gt CO2 in 1950; 30.2 Gt CO2 in 2000 to 39.9 Gt CO2 in 2020 [3,4]. Growing CO2 emissions from anthropogenic sources are responsible for the increase in CO2 concentration in the atmosphere from about 277 ppm in 1751 through 283 ppm in 1800, 285 ppm in 1850, 296 ppm in 1900, 311 ppm in 1950, 368 ppm in 2000, 391 ppm in 2010, 402 ppm in 2015, and to 418 ppm in 2020 [5].
Human activity, mostly electricity, heat production, transport, and cement production, but also agriculture and the changes in the land use (mainly deforestation) are considered to be the main reasons of the GHG emissions. Agriculture, forestry, and other land use release ca. 24% of total global amount of GHGs and are located just behind the electricity and heat production sector (25%) [6]. The ever-growing demand for energy, food and tangible goods does not give rise to optimism in terms of reducing GHGs concentrations, unless the effective, universally acceptable methods of GHGs emission reduction or capture from the atmosphere in particular sectors of human activities are implemented.
While taking the action to mitigate the climate change, it would be helpful to analyze the global natural CO2 fluxes. There are four important global natural CO2 fluxes in the terrestrial ecosystems:
  • CO2 absorption by plants through photosynthesis;
  • CO2 emissions as a result of organisms and plants respiration and decomposition of biomass;
  • CO2 absorption by oceans;
  • CO2 emissions by degassing of ocean water.
Photosynthesis is responsible for absorbing about 451 Gt CO2 yr−1 and covering it into biomass. Simultaneously 435 Gt CO2 yr−1 is emitted from terrestrial ecosystems as a result of organisms’ and plants’ respiration and decomposition of biomass [2]. Oceans are the second most important terrestrial ecosystem which absorbs about 293 Gt CO2 yr−1 and simultaneously emits about 278 Gt CO2 yr−1 through degassing [1,7]. As a major remedial measure, ecological organizations recommend limiting the use of fossil fuels as a source of energy. However, the one-sided approach to only reducing the CO2 emissions through decreasing the use of fossil fuels by replacing it, for example with biomass, often leads to creation of socio-economic problems [8,9,10]. Negative examples are production of biodiesel from the oil obtained from coconut palms grown in Indonesia on the land acquired by burning off tropical forests or the production of ethanol from corn [11,12]. Promotion of biofuels was based on the assumption that the amount of CO2 emitted during biofuels combustion is equal to the amount absorbed by plants from the atmosphere in photosynthesis process. Although this statement is true, it does not account for additional energy used during cultivation, harvesting and processing plants into biofuels. Moreover, such assumption omits the fact that in order to create a plantation, another ecosystem was destroyed—such as tropical forest or peatland—which would absorb greater amounts of CO2 from the atmosphere. These reasons for GHGs emission are included in the “land use changes” [13].
While the use of plant biomass as a source of energy is widely recommended for mitigation of CO2 emissions [14,15], the potential role of whole agriculture in mitigation of CO2 concentration in the atmosphere is not sufficiently recognized. However, an increase in CO2 absorption by plants in the photosynthesis process by only about 9.4% would allow the complete neutralization of the whole annual anthropogenic CO2 emissions [16]. This allows the supposition that agricultural activity consisting in cultivation of cover and catch crops is a potential way to decrease the CO2 concentration in the atmosphere.
Historically, agriculture has traditionally been based on natural methods and fertilizers, catch crop and fallow application, and crop rotation have been widely used, which has resulted in biodiversity. Over time, the use of some traditional practices was significantly reduced or replaced by other systems to maximize yields and profits, disregarding respect for the environment [17,18]. Such an attitude was particularly visible in the policy of many countries in Central and Eastern European, even in the 1980s and 1990s, sometimes leading to corruption in the agro-industrial sector [19]. These practices were implemented and not only on large farms. Many individual farmers departed from traditional farming methods, opting for a simpler and more effective increase in plant yields through the use of mineral fertilization.
These transformations resulted in a number of unfavorable environmental transformations, including soil and ground water contamination, climate change, as well as the alarming depletion of organic matter in soil. About 45% of the mineral soil in Europe is classified as having low or very low organic carbon content (0 to 2%) [20], while the soil in southern Europe has significantly lesser carbon content [21]. It is commonly known that soil organic matter determines the quality of the soil, and directly influences the greenhouse effect, as the carbon released from humic compounds migrates to the atmosphere.
Attempts are being made to remedy the long-term omissions in the environmental impact of agriculture. One of the tools used for this purpose are programs supporting sustainable agriculture, focused on ecological production of healthier food, accompanied by reducing one’s environmental footprint. These actions are driven by socio-economic and technological forces but also supported by public policies, such as the Common Agricultural Policy (CAP) realized in European Union [22]. Supporting the practices contributing to increase the biodiversity, mitigation the climate changes, and limiting soil degradation are the main aims of this program. However, the research of [23] showed that adopted programs do not meet the expectations. That can be explained by the low interest in increasing the share of field margins, buffer strips, and fallow lands, which are the most beneficial actions according to the ecologists. However, because of long-term exclusion of land from cultivation, these practices can be applied in agriculture only to a limited extent and farmers are interested mainly in implementation of catch crops and green covers [24]. The purpose of using catch crops is to prevent a decrease in soil productivity by changing its properties, mainly increasing nitrogen resources, as a result of introducing plant biomass into them, which constitute an additional crop, between successive plantings of main crops [25,26,27,28].
Wide-spreading of cover and catch crops cultivation can make a significant contribution to reduction of the CO2 concentration in the atmosphere by carbon immobilisation in biomass and in soil humic substances, and additionally, could provide a source of valuable substrate for biofuels production, using the biological or thermochemical technologies.
There are many studies on the impact of the catch crops on soil productivity, chemical properties and nitrogen budget [29,30,31,32], but a little is known about the share of catch crops in the carbon budget and their potential in green energy production, although, in recent years, the European Union has begun to examine the scale and motives of farmers to use catch and cover crops in selected regions in some EU countries: Spain; Romania; France, and the Netherlands [33]. However, the analysis does not relate to the value of carbon absorption or the share the catch crop biomass in energy production.
In our multi-faceted studies, we attempted to estimate the significance of catch crops cultivation in Poland, where agriculture plays an important role, as arable land accounts for approx. 44% of the country’s area [34]. The aim of the study was to assess the carbon sequestration potential in the biomass of catch crops cultivated in Poland and to estimate the energy potential contained in the above-ground biomass of these plants when used for biogas production.

2. Materials and Methods

2.1. Estimation of Biomass Yield and Carbon Sequestration

The calculations of potential CO2 sequestration in catch crops in Poland were conducted on the basis of the data given in Statistical Yearbooks of Agriculture of the years 2005, 2012, and 2018 [35,36,37], and the results of surveys conducted among Polish farmers (2017–2018), and the research data given in the relevant literature. The real and theoretical possibilities of increasing the share of catch crops cultivated in Poland in binding atmospheric carbon were estimated taking into account the results of the analysis of the trend in catch crops acreage changes in Poland in the recent years [35,36,37] and the available area of spring crop in Poland in 2018 [37]. The obtained values of potential annual CO2 absorption in plant biomass were compared with the annual CO2 emissions in Poland noted in 2018 [38].
The yields of carbon dioxide sequestration (YCO2, t CO2 ha−1) in catch crops were calculated on total dry biomass yields of catch crops (t ha−1).
The data on the catch crop biomass yields (Yb) were obtained in 2018. Samples of the biomass were taken out from 80 farms in Poland (selected randomly from among farmers cultivating catch crops). The samples were taken from the most popular cultivated catch crops in Poland: white mustard (Sinapis alba L.); tansy phacelia (Phacelia tanacetifolia L.); red clover (Trifolium pretense L.); winter rye (Secale cereale L.); winter vetch (Vicia villosa Roth.); narrow-leafed lupine (Lupinus angustifolius L.), mixtures of: yellow lupine (Lupinus luteus L.) with serradella (Ornithopus sativus Brot.), oats (Avena sativa L.) with spring vetch (Vicia sativa L.) and field pea (Pisum sativum subsp. arvense (L.) Asch.); spring vetch (Vicia sativa L.) with field pea (Pisum sativum subsp. arvense (L.) Asch.) At the agronomically optimal harvest date of the particular catch crop plant, the samples of plant material were collected from an identical area (1.0 m2) of each cover-cropped plot in order to determine total biomass yield (aboveground parts and roots). Samples were taken at 2 randomly selected places in each plot, within a 0.50 × 1.00 m frame. The collected samples were segregated into above and belowground fractions and dried under laboratory conditions at a temperature of 40 °C to determine the dried weight. Next, the yields of particular biomass fractions were calculated (Yb, t ha−1). The carbon (C) content in plant tissues was determined for each fraction [39] and expressed as CO2 content (by multiplying the value of C content ×3.67). The CO2 sequestration efficiency was calculated taking into account the biomass yield of particular catch crops (whose dry matter was between 85 and 88%, depending on the plant species) and the carbon content in biomass expressed as a CO2 basis (t CO2 ha−1). Then, annual CO2 sequestration (t CO2 ha−1·yr−1) was calculated based on the yield of the individual plants (Yb, t ha−1).

2.2. Estimation of Biomethane Yield of Catch Crop Biomass

Potential production of biomethane (YCH4, m3 CH4 ha−1) obtained from the dry weight of biomass harvested from 1 hectare of the crop area was calculated on the approximate value of biomethane potential (BMP, m3 CH4 t−1), estimated on the basis of literature data and the values of biomass yield (Yb, t ha−1) of each crop [40]. Assuming the cultivation area of each crop types, estimated on the basis of statistical data [41], own surveys, and the potential production of biomethane (YCH4) obtained for each crop, the total annual amount of biomethane production (QCH4, mln m3 CH4 yr−1) was calculated.

2.3. Statistical Analysis

Analysis of variance (ANOVA) was used to statistical elaboration of the data, while Tukey’s test was applied to determine Honest Significant Difference (HDS) values at p < 0.05. For the results presented in Table 1 and Table 2 the following parameters were calculated: SD—standard deviation and (r)—coefficient correlation between the dry biomass of catch crop (plants + roots) and carbon content in biomass (plants + roots). The analysis was done by using Statistica PL 13.3.

3. Results and Discussion

Six species of plants cultivated in monocultures and three mixture cultures of catch crops were taken into consideration in this study (Table 1). These plants belong to the catch crops commonly used by farmers in Central and Eastern Europe [25]. The popularity of these crops results from their short growing period (fast growth rate) and good adaptation to climatic and soil conditions. An important factor which determines the crop selection is the aim of its cultivation. Catch crops most frequently are used as green manure (the biomass is mixed with soil by plowing), mulch (the plants are cut in autumn and left on the field surface to early spring) undergoing slow mineralization [42,43] or as a fodder for animals. Most of the catch crops under examination were cultivated as a stubble crop, which is sown in the second half of summer, after harvesting the early main crop. Two of them were used as a winter crop, which is sown in autumn after the main crop is harvested, and harvested in the spring of the following year, and only one was cultivated as an undersown crop, which is sown in spring simultaneously with the main crop or during its growing season and remaining after its harvest until autumn of the same year [44].
The yields of biomass of particular crops cultivated in Poland and yields of CO2 sequestration in their biomass are presented in Table 1.
The lowest CO2 sequestration in the above-ground biomass (4.07 t CO2 ha−1) was observed in case of for narrow-leaf lupine, and the highest (6.56 t CO2 ha−1) for white mustard. Sequestration of CO2 in the below-ground biomass was significantly smaller than in above-ground parts of the plants. The lowest value of this parameter (0.11 t CO2 ha−1) was found in case of red clover and the highest one (0.77 t CO2 ha−1) in the case of the mixture of yellow lupine with serradella. Taking into account total sequestration, the most efficient was white mustard, having absorbed 6.76 t CO2 ha−1, while the lowest sequestration 4.52 t CO2 ha−1 was shown by narrow-leafed lupine.
The analysis of statistical data [35,36,37] shows that the acreage of catch crops in Poland is still growing. In 2005, the acreage of these crops amounted to 700 thousand hectares, in 2012 it increased to about 860 thousand hectares, and in 2019 it reached the value of ca. 1230 million hectares. It can be related to the law regulations according to which the farmers using more than 15 ha of arable land are required to provide at least 5% of the area as ecological focus areas (EFA) which are the part of the CAP. Cultivation of catch crops is one of the ways to fulfill this obligation. The area of catch crops in Poland is dominated by stubble crops, which constitute about 75% of the total amount. The other types of crops: winter cover crops and undersown crops constitute 15% and 10% of the total area of catch crops, respectively.
Our research (Table 2) shows that four species of plants clearly dominate in the catch crops in Poland, another three species are moderately popular, and the other two species are not very popular. About 6% of the area of catch crops is covered by “other species” that have low popularity among the farmers in Poland (a single species corresponds to less than 1% of crops). Our further calculations did not take into account the area covered by this group of plants, assuming that the area covered by catch crops in Poland is 1177 million ha. It was calculated that the catch crops growing in this area sequester 6.85 million t CO2 annually. The characteristics of the annual sequestration of CO2 by the catch crops cultivated in Poland are depicted in Table 2.
Assuming that the current trend of popularity of individual catch crops among Polish farmers will be maintained, it can be stated that it is possible to increase the area of crops by 30% in the next 10 years. It will increase the area of catch crops up to approximately 1530 million ha and the annual sequestration of CO2 to 8.88 million t CO2 yr−1. This seems to be a realistic forecast, especially with greater promotion of such activity. However, not all the carbon sequestration potential of the Polish farmland is currently used. The catch crops can be applied on the fields where spring crops, i.e., cereals, potatoes, and vegetables, are cultivated. These areas were estimated to be 3136 mln t in 2018. Taking into account that 1230 million ha was occupied by catch crop cultivation in 2018 [41], ca. 1900 mln ha (potential increase of 150%) are still potentially available for use for carbon sequestration purpose. This makes it possible to enhance the CO2 absorption by a further 9.37 million t CO2 annually up to 18.25 mln t CO2 annually. For comparison, total annual CO2 emissions in Poland in 2018 were 305.75 mL ton CO2 [19] and total agricultural GHGs emissions in Poland expressed as an equivalent of CO2 were 30.05 mln t [41]. Thus, catch crops can retain ca. 6% of the annual emission of CO2 in Poland and can compensate for over 50% of the national GHGs emissions from agriculture. Additionally, absorption of carbon in catch crops corresponds to 20% of the amount of carbon contained in the biomass of cereals (wheat, rye, barley, oats, triticale), which are the dominate crops cultivated in Poland [41]. Our previous studies [45] showed that cereals cultivated in Poland absorb 23.8 mln t of carbon annually which is equal to 87.3 mln t of CO2.
The potential increase in the available area for catch crops cultivation in Poland is multiply higher than in cases of other countries. For example, the value of these parameters, estimated on the base of the total area where cereals, protein and industrial crops are cultivated, in the Overijssel region in Denmark is 90%, but in some of Spanish, Romanian and French regions is only below 20% [33]. Such different values are related to the popularity of catch crops in the considered regions. In France, where catch crops are very popular [33], the possibilities for increasing their cultivation area are less than in countries where these practices are less used.
The forecast of the scale of CO2 sequestration due to catch crop cultivation in Poland may be even more favorable, taking into account the focus on the cultivation of the most efficient species, i.e., white mustard, tansy phacelia, winter rye, mixtures of spring vetch with field pea or oats with spring vetch and field pea (which entails the highest parameters of CO2 sequestration) (Table 1).
The main purpose of catch crops using (soil enrichment or green biomass production) should be taken into account when selecting the plant species. When the production of green biomass (as fodder or energy source) is more important than soil improvement, species with high aboveground carbon retention are recommended. For these plants, the total carbon sequestration is strongly correlated with the above-ground yield of biomass (Table 3). The strongest significant correlation between the amount of above-ground biomass of catch crops and the CO2 sequestration was observed in cases of white mustard, tansy phacelia and spring vetch mixed with field pea. On the other hand, there is lower dependence in the case of below-ground biomass—only in the case of legumes or their mixtures with cereals (producing high root biomass) is the correlation coefficient statistically significant (Table 3).
The absorption of carbon in plant biomass cannot be considered as the sufficient way to reduce the concentration of CO2 in the atmosphere. However, increasing the production of any type of biomass leads to an increase in the amount of carbon absorbed from the atmosphere and contributes to lowering its concentration in the air, mitigating the greenhouse effect. Moreover, the biomass of catch crops, also the above-ground one, usually remains in the field, and a significant part of carbon included in the plant tissues is incorporated into the soil humic substance. These compounds are hardly biodegradable and highly persistent, remaining in the soil even for over 100 years [46].
However, the assessment of the net soil sequestration of biomass carbon is difficult, because this phenomenon depends on many factors, including physical, chemical and biological soil properties, climatic conditions, land use and management [47] as well as the chemical composition of biomass. These factors influence which part of the biomass remaining in the soil will be transformed into humic compounds or mineralized [48,49,50].
Significant differences in properties of the biomass of shoots and roots determine the rate and pathways of the conversions. For example [51] estimated that the root biomass is more efficiently converted into persistent organic compounds in the soil than the biomass of the aboveground parts. A humification coefficient which refers to the part of organic matter that is converted to the humic substance for roots is 0.42, while for above-ground crop residues it is only 0.22.
The biomass of catch crops which is used as green fertilizer improves the quality of the soil and increases the yields of main agricultural crops used to production of foods or fodders. However, shoot biomass can be also applied as a raw material for the production of biofuels through anaerobic digestion or thermal processes. The use of the biomass for biogas production is very beneficial from the environmental point of view, because it provides the fuels with low impact on the environment. Additionally, digestate still plays a role in carbon sequestration because it constitutes a source of exogenous soil organic matter.
Some researchers emphasize the advantages of use the catch crop biomass, especially the legumes for biogas production. The study in [52,53], which examined the sorgum which can be used as a catch crop, indicated the possibility of increasing the biomethane yield per hectare, even to a value similar for maize, which is commonly used in biogas plants in CEE. They found the biomethane yield of sorgum biomass to be even up to 6500 m3 ha−1. The yields of biomethane per hectare calculated for particular catch crops cultivated in Poland obtained in our study (Table 4) are not in line with this observation. The value of this parameter for particular crops ranged from 965 m3 CH4 ha−1 (which was the lowest value) to 1762 m3 CH4 ha−1 (which was the highest one). The average value of biomethane production from biomass of catch crops was 1382 m3 CH4 ha−1. This value is a few times lower in comparison with the methane yields obtained from maize, that is between 7500 and 10,200 m3 CH4 ha−1 [54].
There is no doubt that catch crops cannot be compared with maize in terms of suitability for biogas production, due to the significantly lower annual biomass yield (which was found even at 21 t ha−1 at full sunlight on the fields in Southwest Germany) [55]. Although the biomethane amount produced during anaerobic decomposition of 1 ton of dry biomass of some catch crops (Table 4) is even higher than in case of maize, which ranged from 262 to 318 m3 t−1 of dry weight of maize silage [56,57]. The highest potential of biomethane of 313 m3 CH4 t−1 of dry biomass was found in the case of red clover, while the lowest one was 222 m3 biogas t−1 biomass for white mustard. Therefore, it can be stated that this type of biomass can complement the base of the feedstock used in this renewable energy sector.
Catch crops, especially legumes, such as clovers, peas, lupines, vetches have low C:N ratios which usually range from 8 to 15 [58,59]. Thus, the biomass of these crops can balance the high value of this parameter observed in the case of maize raw biomass or silage, the commonly used feedstock in biogas plants, which ranges from 29 to 48 [60,61,62,63]. Besides the nitrogen content, the advantage of catch used using in biogas production refers to high content of micronutrients which are necessary for activity of microorganisms growing in anaerobic digester [64], leading to an increase in biogas production. The results of the study of Fahlbusch et al. [64] showed that faba bean, amaranth, and ryegrass have a higher concentration of essential trace elements for the microorganisms responsible for biogas production than maize, triticale, or winter rye intercropped with vetch. The researchers also emphasize the other benefits of catch crops, such as enrichment of biodiversity on the field and improvement of soil quality due to application of the residual product of biogas production to the soil as a source of macronutrient and trace elements.
Assuming that all the above-ground biomass will be used as feedstock in digestion chambers, the total potential amount of biomethane produced from the above-ground biomass is estimated to be 1657 mln m3 yr−1, with the highest share of white mustard and tansy phacelia (20 and 16%, respectively) and the lowest one in case of winter vetch and narrow-leafed lupine (5 and 4%, respectively).
Assuming the efficiency of biogas conversion to energy at the level achieved by typical cogeneration systems (CHP) amounts to 40% electric energy and 45% thermal energy, potential recovery of electrical energy is 2.1 kWhel and thermal energy is 2.4 kWht from 1 m3 of biogas containing approx. 55% CH4. Thus, 6327 GWh of electricity and 7230 GWh of thermal energy can be recovered from the biogas produced from the entire above ground biomass of catch crops collected in Poland during the year (which is estimated on 3012.7 million m3 assuming the CH4 concentration in biogas of 55%). For comparison, the total annual volume of biogas produced in 28 agricultural biogas plants operating in Poland in 2013 was 112.4 million m3, of which 227.89 GWhel yr−1 and 246.56 GWht yr−1 were produced, while the total annual volume of biogas produced in 116 agricultural biogas plants operating in Poland in 2020 was 325.4 million m3, of which 689.12 GWhel yr−1 were recovered [70] and about 780 GWht yr−1 (estimated value, as the law regulations applicable from 2015 on renewable energy sources have abolished the obligation of producers to provide information about amount of heat generated from agricultural biogas. The total biogas volume obtained in biogas plants in 2020 corresponds to 11% of the total potential volume of biogas produced annually from biomass of aboveground catch crops cultivated in Poland). This value shows how large the energy potential contained in the catch crops’ biomass is.
Environmental and practical advantages and catch crops should be recognized and supported by programs promoting green energy production, as well as rural development, which is the second pillar of the European Union’s Common Agricultural Policy (CAP). Increasing the production of catch crop biomass for energy purposes helps to achieve the long-term goals of this program, such as achieving sustainable management of natural resources, actions to limit climate change, achieving balanced territorial development of rural economies and communities, including the creation of new jobs in the sector of production of raw materials for energy purposes or their processing.

4. Conclusions

The interest in catch cropping as a way to increase atmospheric CO2 sequestration has been growing recently. This is of great importance in the context of the global warming phenomenon.
Our estimation has shown that all catch crops cultivated in Poland sequester 6.85 mln t CO2 ha−1. However, taking into account that the potential of cultivating these plants in Poland is not sufficiently exploited, it can be said that an additional 2 million hectares of fields could be used for catch cropping in the future, which will make it possible to increase the CO2 sequestration over 2.5 times. The predicted extent of CO2 sequestration may be even more favorable when cultivation of the most efficient species (in terms of carbon sequestration), such as: white mustard, tansy phacelia, winter rye, spring vetch and field pea mixture, oats, spring vetch and field pea mixture, are applied.
The wide spreading of catch crop cultivation has other advantages as well. The biomass can be used as green fertilizer which improves the quality of soil and thus improves the yields of the main agricultural crops contributing to the increased safety of the food supply, as valuable fodder for animals, and a renewable source of energy, that can be gained by conversion of crop biomass to biofuels such as biomethane or syngas, in anaerobic digestion or thermal processes. The results of our estimation showed that potential production of energy from biogas generated from aboveground biomass of catch crops annually harvested in Poland is a few times higher than the energy obtained from the total volume of biogas produced in this time in all the biogas plants operating in Poland, which indicates a significant, unused profitable source of renewable energy.
Our estimations were based on data collected selectively from several dozen farms in Poland, statistical and literature data. Thus, our results are approximated. The uncertainties are related to the determination of the area occupied by individual types of plants, the impact of soil diversity and microclimatic conditions on the biomass yield, as well as the determination of the methanogenic potential of catch crop biomass. However, the obtained results indicate that even with these limitations of calculations, it is worth considering the cultivation of catch crops not only as a way to improve soil quality and carbon sequestration, but also as a new source of renewable energy with high availability.
The future research in this area should be focus on optimization of catch crop selection in order to obtain the highest yields in various climatic and soil conditions. It could be assumed that, if the farmers had focused only on highly-yielding and low-water demanding species of catch crops, with low-water requirement, the scale of sequestration, and especially the potential effect of obtained energy, would be greater. Additionally, the future studies should also cover the carbon budget related to the catch crop production and applications, including assimilation of CO2 in catch crop biomass, humification of organic matter in soil, following biomass ploughing under (‘soil as a CO2 bank’), carbon emission during energy production from biogas obtained from catch crop biomass, and accumulation of carbon introduced to the soil with residuals after anaerobic digestion of catch crop biomass.
Taking into account the high worldwide demand for energy, unstable and uncertain fossil fuels sources as well as the concern over global climate change, greater attention should be paid to the increase in the role of agriculture in climate change mitigation and providing raw materials for renewable energy production. Increasing the area of sowing catch crops in typically agricultural regions of Central and Eastern Europe may be an effective and relatively cheap way to mitigate adverse climate changes. However, the role of agriculture in climate change mitigation is potentially significant but the knowledge and awareness of the society in this regard are unsatisfactory. Dissemination of catch crops cultivation requires the re-education of farmers and the application of mechanisms of their financial and advisory support. However, to improve the quality of the environment due to the decrease in the CO2 concentration in the atmosphere and increase in carbon sequestration in soil, such investments are necessary.

Author Contributions

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

Funding

This research was funded 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 (RKU/DS/4), University of Life Sciences in Lublin.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Table
Table 1. Biomass yields and carbon dioxide sequestration by catch crops cultivated in Poland.
Table 1. Biomass yields and carbon dioxide sequestration by catch crops cultivated in Poland.
Catch CropsCatch Crops TypeBiomass Yield, Yb
(t ha−1)		Average CO2 Sequestration in Biomass YCO2 (t CO2 ha−1)
In Above- Ground BiomassIn Below- Ground BiomassIn Above- Ground BiomassIn Below- Ground BiomassTotal
White mustardstubble crop3.94–4.58 a0.12–0.14 a6.56 a ± 0.39 **0.20 a ± 0.046.76 a ±0.43
Tansy phaceliastubble crop3.61–4.35 a0.07–0.09 a6.13 a ± 0.270.12 a ± 0.036.25 a ± 0.30
Yellow lupine + serradellastubble crop3.20–3.60 b0.27–0.33 b5.24 b ± 0.210.77 b ± 0.136.01 a ± 0.34
Oats + spring vetch + field peastubble crop3.04–3.40 b0.14–0.18 a4.96 b ± 0.190.25 a ± 0.085.21 b ± 0.27
Red cloverundersown crop2.48–2.90 c0.06–0.08 a4.14 c ± 0.160.11 a ± 0.024.25 c ± 0.18
Winter ryewinter cover crop3.62–4.52 a0.12–0.14 a6.27 a ± 0.250.20 a ± 0.046.47 a ± 0.29
Spring vetch + field peastubble crop2.98–3.94 b0.18–0.24 b5.33 b ± 0.220.65 b ±0.125.98 a ± 0.34
Winter vetchwinter cover crop2.78–3.16 c0.11–0.13 a4.57 b ± 0.180.18 a ± 0.064.75 c ± 0.24
Narrow-leafed lupinestubble crop2.39–2.89 c0.26–0.32 b4.07 b ± 0.130.45 b ± 0.144.52 c ± 0.27
HSD(0.05)-0.350.070.590.090.76
The mean values within a column followed by different letters (a,b…) are significantly different; ** SD—standard deviation.
Table
Table 2. Total annual sequestration of CO2 by catch crops cultivated in Poland.
Table 2. Total annual sequestration of CO2 by catch crops cultivated in Poland.
Catch CropsArea
(ha)		Total Annual CO2 Sequestration
(mln t CO2 yr−1)		Predicted Total Annual CO2 Sequestration Assuming 30% Increase in Catch Crop Area
(mln t CO2 yr−1)		Predicted Total Annual CO2 Sequestration Assuming Total Potential Area for Catch Crop Cultivation
(mln t CO2 yr−1)
White mustard225,000 a *1.52 a ± 0.11 **1.98 a ± 0.174.05 a ± 0.28
Tansy phacelia188,000 b1.18 b ± 0.071.53 b ± 0.133.14 b ± 0.21
Yellow lupine + serradella163,000 b0.98 b ± 0.051.27 b ± 0.102.61 c ± 0.17
Oats + spring vetch + field pea150,000 b0.78 b ± 0.041.02 b ± 0.082.08 c ± 0.15
Red clover113,000 c0.48 c ± 0.030.62 c ± 0.051.28 d ± 0.09
Winter rye106,000 c0.69 b ± 0.040.89 d ± 0.071.84 d ± 0.12
Spring vetch + field pea100,000 c0.60 b ± 0.030.78 d ± 0.061.60 d ± 0.09
Winter vetch69,000 d0.33 d ± 0.020.43 e ± 0.040.88 e ± 0.11
Narrow-leafed lupine63,000 d0.29 d ± 0.020.37 e ± 0.030.77 e ± 0.07
Total1177,0006.858.8818.25
HSD(0.05)35.20.170.190.24
* The means values within a column followed by different letters (a,b…) are significantly different; ** SD—standard deviation.
Table
Table 3. Correlation coefficients (r) between above and below-ground biomass yields and CO2 sequestration in the biomass.
Table 3. Correlation coefficients (r) between above and below-ground biomass yields and CO2 sequestration in the biomass.
Catch CropAbove-Ground BiomassBelow-Ground Biomass
White mustard0.80 *0.45
Tansy phacelia0.77 *0.40
Yellow lupine + serradella0.59 *0.54 *
Oats + spring vetch + field pea0.65 *0.55 *
Red clover0.52 *0.33
Winter rye0.61 *0.29
Spring vetch + field pea0.77 *0.52 *
Winter vetch0.420.46
Narrow-leafed lupine0.57 *0.57 *
* significant correlation coefficient (0.05).
Table
Table 4. Estimated biomethane potential and yield of biomass of catch crops cultivated in Poland.
Table 4. Estimated biomethane potential and yield of biomass of catch crops cultivated in Poland.
Catch CropsBiomethane Potential, BMP
(m3 t−1 of Dry Biomass)		Calculated Biomethane Yield per Hectare, YCH4
(m3 ha−1)		Total Potential Amount of Biomethane Produced from aAbove Ground Biomass, QCH4
(mln m3 yr−1)
White mustard222 11456328
Tansy phacelia234 21434270
Yellow lupine + serradella237 31242202
Oats + spring vetch + field pea292 4 (averaged)1448217
Red clover313 51296146
Winter rye281 21762187
Spring vetch + field pea301 6 (averaged)1604160
Winter vetch270 7123485
Narrow-leafed lupine237 796561
Average:1382-
Total:-1657
1 [65]: assuming CH4 concentration in biogas at 50%. 2 [66] 3 [67]: yellow lupine 237; no data found for serradella (both belong to Fabaceae). 4 [65]: oats 256 m3/t of dry biomass; [67] vetch 270 m3/t of dry biomass; [68] peas 351 m3/t of dry biomass. 5 [69]: assuming vs. content 92%. 6 [67]: vetch 270 m3/t of dry biomass; [69] peas 351 m3/t of dry biomass. 7 [67].
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Pawłowski, L.; Pawłowska, M.; Kwiatkowski, C.A.; Harasim, E. The Role of Agriculture in Climate Change Mitigation—A Polish Example. Energies 2021, 14, 3657. https://doi.org/10.3390/en14123657

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Pawłowski L, Pawłowska M, Kwiatkowski CA, Harasim E. The Role of Agriculture in Climate Change Mitigation—A Polish Example. Energies. 2021; 14(12):3657. https://doi.org/10.3390/en14123657

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Pawłowski, Lucjan, Małgorzata Pawłowska, Cezary A. Kwiatkowski, and Elżbieta Harasim. 2021. "The Role of Agriculture in Climate Change Mitigation—A Polish Example" Energies 14, no. 12: 3657. https://doi.org/10.3390/en14123657

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