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Review

Catch and Cover Crops’ Use in the Energy Sector via Conversion into Biogas—Potential Benefits and Disadvantages

by
Alicja Słomka
1 and
Małgorzata Pawłowska
2,*
1
Independent Researcher, 20-824 Lublin, Poland
2
Faculty of Environmental Engineering, Lublin University of Technology, 20-618 Lublin, Poland
*
Author to whom correspondence should be addressed.
Energies 2024, 17(3), 600; https://doi.org/10.3390/en17030600
Submission received: 7 December 2023 / Revised: 17 January 2024 / Accepted: 18 January 2024 / Published: 26 January 2024
(This article belongs to the Special Issue Carbon Neutrality through Green Innovations – the Role of the Renewable Energy II)
Versions Notes

Abstract

:
The development of civilization is related to an increase in energy demand, while its production is still based mainly on fossil fuels. The release of carbon into the environment, which disturbs the balance of the global system, is the consequence of using these fuels. One possible way to reduce the carbon footprint of the energy sector is the widespread use of cover crops’ biomass for energy production. The aim of this paper is to critically review the knowledge on the dissemination of catch and cover crops’ cultivation in different regions of the world, and the yield, chemical composition and biomethane potential of their biomass. Additionally, the environmental benefits, as well as the challenges and opportunities associated with this biomass use in the energy sector, are considered. The review showed that the aboveground biomass of cover and catch crops is a valuable source for the production of bioenergy in biogas plants. However, the key role of these crops is to prevent soil degradation. Therefore, changes in biomass target use must be preceded by a multi-aspect analysis that allows their impact on the environment to be assessed.

1. Introduction

Energy consumption increases with civilization development [1]. Due to the fact that fossil fuels are still the dominant source of energy on a global scale, the share of which is estimated at 61% [2], the production of greenhouse gases (GHGs) increases with the increase in energy consumption, which deepens the phenomenon of global warming. It is expected that if the current growth rate is maintained “Global warming is likely to reach 1.5 °C between 2030 and 2052” [3]. To prevent these predictions from becoming reality, the world is implementing initiatives to reduce GHGs in the atmosphere, like the Paris Agreement, 2015. The countries that signed it committed to making efforts to achieve a rapid decline in global greenhouse gas emissions and achieving “climate neutrality” in the second half of the 21st century [4]. “Climate neutrality” refers to achieving net zero emissions of greenhouse gases (not just CO2) by offsetting these emissions through their absorption [4].
When looking for ways to reduce the concentration of GHGs in the atmosphere, special attention should be focused on activities in sectors with the highest emissions, such as the energy sector, which includes the production of electricity, heat and transport. It is estimated that over 73% of GHG emissions come from this source [5].
There are many strategies to reduce GHG emissions from the energy sector. They are based on different assumptions, such as the prevention of emissions by reducing energy consumption, increasing energy efficiency in various areas, e.g., by using innovative solutions and materials in construction, shortening supply chains, modifying process lines, rational use of waste, and, above all, by changing energy sources, from fossil to renewable or unconventional, like nuclear power [6,7].
In this paper, we focus on biomass as one of the renewable sources of energy. Currently, biomass is not an important source of energy. When considering the use of biomass in the sector of energy production from renewable sources, Europe is the leader. As much as 23.9% of the electricity generated from all renewable energy sources was produced from biomass, while the corresponding share of this source in other regions of the world was much lower, e.g., in Asia, 8.7%, and in the Americas, 8.6%. However, there are huge reserves of raw materials in the bioenergy sector. Attention has been paid to biomass from agriculture. The possibility of increasing the yield of major crops has been emphasized, which would meet both the nutritional needs and the requirements of the energy market in the field of biofuel production [8]. This sector also produces significant amounts of waste biomass (straw, manure) and unused biomass of grasses and other plants grown for soil protection purposes, so-called catch and cover crops (CCCs). It is worth paying attention to this last type of biomass. The practice of using CCC in agriculture is popular in different part of the world. It is a well-known and old practice with a long history. It is difficult to indicate its origins, although descriptions of research results on the impact of the use of these plants on soil properties and yields can be found in papers published at the beginning of the 20th century, e.g., Penny [9] in 1903 described the effect of using soya bean as a cover crop, and Robinson [10] in 1908 wrote about the fertilizing value of hairy vetch. These plants are grown to protect the soil against external factors, mainly rainfall, which may cause leaching of nutrients, loss of soil due to erosion, or changes in chemical and physicochemical properties due to intensive mineralization of organic matter [11,12,13]. However, it is to be expected that interest in growing CCC should increase in the coming years, especially among farmers in EU countries. Under the Common Agricultural Policy, farmers who introduce solutions leading to carbon sequestration and reducing GHG emissions from the agricultural sector can apply for funding. Greening, including the cultivation of catch crops/green cover, is one of the options that can be used to meet the requirements for applying for funding. According to Kathage et al. [13], who reviewed the literature on the practice of CCCs’ cultivation in Europe, the use of CCCs may bring environmental benefits resulting from the reduction in climate change, but also from improving the properties of soil enriched with exogenous organic matter. The authors conclude, based on sources [14,15], in technical terms, a wider cultivation of CCCs in different regions of the world is possible, which creates prospects for the energy use of surpluses of this biomass.
The multifaceted role of CCCs’ cultivation has been the subject of numerous experimental and overview studies. They mainly focused on the improvement of soil properties and possibility of climate change mitigation. For example, Wooliver and Jagadamma [16] showed the undoubted positive effects of CCCs’ cultivation on the climate and soil quality on the basis of the results of a meta-analysis that took into account data from across 49 studies conducted in Europe and both Americas. The other review papers [17,18,19,20] also analyzed the impact of CCCs on the physical and chemical properties of soil and carbon sequestration, but additionally paid attention to the diversity and activity of soil microorganisms. There is also a review paper of Ruis et al. [21] that collected data from 389 studies on the yield of CCC biomass production in various regions of the world. However, there are only a few reviews that regarded the energetic use of catch crop biomass. Launay et al. [22] studied the environmental consequences of energy cover crop usage, analyzing them mainly from the perspective of soil quality related to the decrease in the amount of green biomass left in the field, and the replacement of this biomass by digestate, which is a by-product of biogas production.
The aim of this paper is to review the knowledge on the dissemination of catch and cover crops’ cultivation in different regions of the world, the yield and chemical properties of CCC biomass, important from the point of view of energy production, and its biomethane potential. The paper also draws attention to the benefits and limitations that can accompany the wider use of CCC biomass for energy production.

2. Materials and Methods

The literature review was performed using the Web of Science and Google Scholar databases as well as official data prepared and presented in the reports and Internet databases from international organizations, such as the International Panel of Climate Change (IPCC), United States Department of Agriculture (USDA), European Statistical Office (Eurostat) and World Bioenergy Association (WBA).
Looking for information about types of plants used as CCCs, and the amount and composition of biomass, we looked for the latest data sources, narrowing the search to the last 10 years. In cases where there was an absence of appropriate information in papers from this period, older sources were used. In order to learn the history of research on the role of CCCs in agricultural practice, databases from the beginning of the 20th century were used.
To gather the data, various combinations of the following keywords were used: “catch crop”, “cover crop”, “biomass yield”, “energy”, “ecological footprint”, “climate change” “environmental impact”, “carbon sequestration”, “soil organic carbon”, green manure”, “anaerobic digestion”, “biogas production”, “biomass combustion”, “calorific value”, and names of particular plant species mentioned in the paper.

3. Characteristics and Spatial Distribution of Catch and Cover Crops

Catch and cover crops are defined as crops sown in pure or mixed sowings between two main crops [23]. A short vegetation period is a key feature of these plants [24]. Depending on the sowing date, one can distinguish stubble catch crops sown after harvesting the main crop at the end of summer and harvested or ploughed in the fall of the same year or left in the field until spring; winter crops sown after harvest at the end of summer and harvested in the spring of the following year; and undersowing catch crops, plants that tolerate shade well, which are sown together with the main crop and after harvesting and left in the field until autumn [25]. The biomass of these plants is used in various ways on farms. It can be used for forage purposes—directly grazed (forage) or processed to fodder in a form of hay or silage; introduced into the soil to improve its chemical properties and its structure (soil improver), for example, as a source of nutrients and organic matter after direct ploughing; or left in the field as a form of mulch after previous mowing or damage by frost. It can also play a protective role in relation to the soil surface, preventing water or wind erosion, or protect nitrogen resources in the soil by incorporating it and retaining it in the biomass in the period after the harvest of the main crop plants [26,27,28]. The latter function is fulfilled by plants with increased nitrogen fixation efficiency (expressed in a low C:N ratio) belonging to the Fabaceae family, such as peas, lupins, seradela, vetch, clover, alfalfa). Through the symbiosis with nitrogen-fixing bacteria growing inside the root nodule cells of these plants, atmospheric nitrogen becomes available to plants and may be incorporated into their biomass [29,30]. Thanks to the production of specific root secretions, many CCC plants have a phytosanitary effect, which involves stimulating the development of beneficial soil microflora and microfauna and limiting the development of pathogens and pests [17,31,32]. In addition, introducing additional plant species, especially mixtures, between the main crops increases biodiversity.
In agricultural practice, many plants are in use that, when grown between the main crops, improve the properties of the soil and contribute to increasing its fertility and yields. Table 1 summarizes the types of plants used as CCCs in the world that are most frequently mentioned in the literature.
Looking at the information given in Table 1, many of the plants used as cover or catch crops, for example, common vetch, hairy vetch, common oat and common buckwheat, are cultivated in various regions of the world, while some, due to unique habitat requirements, are used only in specific regions. The latter include plants, for example, pearl millet, Japanese millet, finger millet, Italian millet, niger and sunn hemp, that grow in the tropical or subtropical zone. The most numerously represented families of plants from the CCC group are Fabaceae (alt. Leguminosae) and Poaceae (alt. Gramineae).
Cover crops are cultivated in monocultures or in mixtures. According to the literature review given by [21], the five highest yielding monoculture cover crops in humid temperate regions are sorghum, sunn hemp, millet, rye and annual ryegrass, while in semiarid regions these are cowpea, barley, triticale, oat and rye. Some research has shown that crop mixtures can be superior compared to pure stands in terms of their aboveground biomass yield and nutrient catching due to their higher durability under unfavorable conditions [68,69,70]. Additionally, Heuermann et al. [71] observed higher belowground biomass yield in the case of more diversified mixtures compared to the simpler ones. They examined four plant species, different in terms of root morphology: white mustard (Sinapis alba), lacy phacelia (Phacelia tanacetifolia), bristle oat (Avena strigosa) and Egyptian clover (Trifolium alexandrinum), which were grown both in pure stands and in a mixture. As the results of research conducted by other scientists indicate, mixed crops do not always have a clear advantage over monocultures in terms of yield [72]. Florence and McGuire [73] conducted a review of the literature, which showed that over 80% of researchers who compared cultivations of monocultures and mixtures did not find a significant difference between the compared biomass parameters. It can therefore be assumed that the beneficial effect of the species mixture is determined by additional factors, perhaps the appropriate selection of species, for example, in terms of the diversity of the root system, which is indicated by the research conducted by [71].
The composition of the mixtures is selected depending on the habitat conditions, and type and function of the crop. For example, in regions with a temperate climate, winter cover crops often used are mixtures, like: hairy vetch and crimson clover; hairy vetch and rye; hairy vetch and triticale; and crimson clover and rye [74,75,76], and in a dry climate, one can use mixtures of cowpea, foxtail millet and sunflower as well as mixtures of various combinations of plants such as: millet, triticale, red clover and fodder radish [77,78].
The wide variety of plant species that are adapted to different habitat conditions allows the most favorable CCC species for a given climatic and soil zone to be found. However, particular plant species differ in terms of biomass composition, which can influence their suitability for energy production, especially via biochemical processes.

4. Catch and Cover Crop Biomass Bioconversion into Energy

4.1. Features of CCC Biomass Important Because of Biochemical Conversion into Biofuels

Biomass of CCCs, similarly to the other types of biomass, can be used as a source of energy after conversion into biofuels, such as biomethane, alcohol, bio-oil and biohydrogen (via biochemical, chemical or thermochemical processes) or directly after combustion. The application of appropriate bifunctional catalytic materials (e.g., Bronsted–Lewis acid), which allow biofuels and chemicals to be produced from lignocellulosic biomass, offer the great opportunities [79]. The choice of the method for converting biomass into energy depends largely on the dry matter content of the raw material. Biological methods, such as methane fermentation, can be used when the moisture content of the feedstock allows intensive development of microorganisms and is not less than 60% dry weight (d.w.) (in the case of dry anaerobic digestion systems) or over 85% (in the case of wet anaerobic digestion systems). However, direct combustion of biomass is justified when the moisture is low, e.g., for straw it is recommended that it is not higher than 25% [80]. Thermochemical methods are instead recommended for dry biomass. When biomass is converted by widely known processes such as pyrolysis (under anoxygenic conditions) or gasification (under oxygen-deficient conditions), the water content should be in the range of 10–20% d.w. in the case of gasification [81], and 15–35% d.w. in the case of pyrolysis [82]. However, a high moisture content does not exclude the possibility of using biomass for the production of biofuels in thermochemical processes. Hydrothermal conversion processes such as hydrothermal liquefaction (HTL), hydrothermal carbonization (HTC) and hydrothermal gasification HTG (supercritical water gasification) allow the conversion of wet biomass into biofuels, such as bio-crude oil, hydrochar and a mixture of combustible gases, respectively. These processes are carried out at temperatures of 100–700 °C and high pressures of 5–40 MPa in a liquid media or hot supercritical water [83]. However, technologies based on these processes are not yet widely used at a technical scale. Thus, in the case of wet biomass biological methods, such as anaerobic digestion, are still preferable. Additionally, well-known technology and the possibility of it using in on a small scale, e.g., on large agricultural farms, are an important arguments for the use of these methods.
The water content of the raw biomass of CCCs varies significantly depending on many factors, among them, plant species and growth stage. Research conducted on corn by [84] in a Mediterranean climate showed that the highest dry matter content, approx. 54% of d.w., was observed in the phase of maturity, while the dry matter content in biomass harvested at the end of the vegetative stage was about 15%. According to the study of Piskier [85], the content of total solids in the corn straw was still high and varied between 40 and 55%. A significant increase in dry matter content from 26.1 to 38.5% d.w. between the early dent and black layer stages of corn growth was observed by Rabelo et al. [86]. Changes in dry matter content in phacelia (Phacelia tanacetifolia) biomass between pre-flowering and end-flowering phases ranged from 32.6 to 54.6% d.w. [87]. Tekeli et al. [88] found that the dry matter content in Persian clovers increased from 6.8 to 12.2% d.w. between the stages of pre-bud and full-bloom.
The results of these studies show that the raw biomass of CCC plants is characterized by high water content even in the later stages of development, such as full-bloom, which indicates its greater suitability for being processed into biofuels using biological methods. If one chooses such methods, there is no need to remove water. Therefore, preparing biomass is less energy-intensive and expensive than preparing it for processing in thermochemical processes, such as pyrolysis and gasification, which require preliminary partial dewatering [89].
However, when choosing biochemical methods, an important feature of biomass is its high biodegradability, which determines the high efficiency of the conversion of chemical energy contained in organic compounds into useful forms of energy. Biodegradability depends on the chemical composition of organic matter, which in turn depends on such factors as the plant species, part of plant, growth stage, harvest time, climate conditions, soil properties and fertilization. Biomass with a high content of non-structural and water-soluble carbohydrates, as well as a low content of lignin, is highly biodegradable [90,91]. Microorganisms can degrade the labile fraction of organic substrates, avoid resistant molecules (e.g., lignin) and produce stabilized metabolites [92,93]. A high lignin content not only reduces biogas production due to difficult biodegradability of this compound, but also due to the reduction in the hydrolysis of cellulose by creating a physical barrier for cellulases and reducing their availability because of the sorption of these enzymes on lignin [94]. As shown in the data presented in Table 2, the lignin content in the raw biomass of plants used as cover crops ranges from 1.42 to 20% d.w., which shows its significantly different biodegradability. Chaves et al. [95] examined grass and legume and found that lignin content in perennial ryegrass (Lolium perenne) varied from 2.38 to 4.35%, while in white clover (Trifolium repens), red clover (Trifolium pratense) and lucerne (Medicago sativa), it was 5.87, 6.23 and 6.12% d.w., respectively.
The chemical composition of biomass varies significantly depending on the stage of plant growth. A decrease in the content of crude protein was observed in phacelia biomass between the pre-flowering and end-flowering phases, from 19.8 to 14.8% d.w., and crude fat, from 2.8 to 1.4% d.w. At the same time an increase in the contents of acid detergent fiber (ADF) and neutral detergent fiber (NFD) was observed, from 28.6 to 32.3% d.w. and 39.0 to 45.0% d.w., respectively [87]. Tekeli et al. [88] stated that the content of crude protein in Persian clovers between the stages of pre-bud and full-bloom decreased from 20.7 to 17.9% d.w., while the content of crude cellulose increased from 14.5 to 17.8% d.w.
For comparison, according to Wojcieszak et al. [96], the content of lignin unsuitable for biogas production in various parts of corn (cobs, leaves, stalks, husks), which is a popular substrate used for biogas production in Europe, collected 5–6 months after sowing, ranged from 13.1 to 20.1% d.w. However, corn intended for silage production should be harvested earlier, which translates into a lower lignin content in the biomass. Nowicka et al. [97] claimed that the lignin content in the corn silage they tested was 2.6% d.w., while the content of polysaccharides, which after hydrolysis can be processed by microorganisms, was 20.1% and 14.6% d.w. in the case of cellulose and hemicellulose, respectively.
Table 2. Contents of cellulose, hemicellulose and lignin in biomass of selected cover crops.
Table 2. Contents of cellulose, hemicellulose and lignin in biomass of selected cover crops.
SpeciesCellulose
(% d.w.)		Hemicellulose
(% d.w.)		Lignin
(% d.w.)		References
Grasses37.85 27.339.65[98]
Grass silage 34.1524.272.78[99]
Sunflower (Helianthus annuus L.)34.065.187.72[98]
Fodder radish (Raphanus sativus L.), flowering stage8.517.69.43[100]
Fodder radish (Raphanus sativus L.), maturation stage18.9914.5410.63
Pearl millet (Pennisetum glaucum), flowering stage22.4429.874.7
Pearl millet (Pennisetum glaucum), maturation stage12.9627.6410.56
Orchard grass, cocksfoot (Dactylis glomerata L.)52.342.96.6[101]
Abruzzi rye (Secale cereal L.)25.2625.172.56[102]
Black oat (Avena Strigosa Schreb)46.227.849.12[103]
Black oat (Avena strigosa Schreb)25.1720.821.77[102]
Winter barley (Hordeum vulgare L.).19.3620.881.42[102]
Field (winter) pea (Pisum sativum L.) different varieties26.8–38.75.1–11.8l.d.[104]
Field (winter) pea (Pisum sativum L.) 17 different genotypes20.3–36.169.18–10.84.86–10.2[105]
Crimson clover (Trifolium incarnatum)26–61.34.5–8.0[103]
Crimson clover (Trifolium incarnatum)17.3312.653.37[106]
Crimson clover (Trifolium incarnatum)25.589.533.35[102]
Crimson clover (Trifolium incarnatum)29.1–36.88 *10.8–11.12 *7.5–10.10 *[105]
Hairy vetch (Vicia villosa Roth)26.84–33.5310.84–11.638.3–11.2[107]
Hairy vetch (Vicia villosa Roth)28.410.127.57[103]
Hairy vetch (Vicia villosa Roth)27.2414.294.86[102]
Common vetch (Vicia sativa L.).13.425.87.3[108]
White lupine (Lupinus albus L.) silage40.3413.67.63[109]
Broad bean (Vicia faba L.) silage28.1218.597.22[109]
Switchgrass (Panicum virgatum)39.5–4520.3–31.512–20[110]
l.d.—lack of data; * values grown with harvest timings
Considering the data given in Table 2, it can be stated that cellulose dominates in crude fiber fraction, reaching up to 50% d.w., while the content of lignin in CCC biomass is usually lower than 10% d.w. Late harvest usually leads to an increase in the lignin content in CCC biomass, causing a decrease in biodegradability. Thus, the time of plant harvesting is a very important factor that influences the biomass’ suitability for biogas production.
Other important chemical properties indicating the suitability of plant biomass for energy use in biochemical conversion processes are related to the content of carbon and nitrogen and the mutual ratio of these parameters C/N, as well as the content of macroelements such as P, K, Ca, Mg and numerous microelements, which influence the functioning of microorganisms responsible for the biodegradation process of biomass.
The carbon content in the dry matter of catch crops in the aboveground part usually ranges from 40 to 50% [111,112]. The nitrogen concentration in plant biomass varies depending on the species, ranging from 13.6 to 52 g N kg dry d.w.−1 in the biomass of brassicas and grasses, respectively. Higher concentrations of nitrogen, from 43 to 84 g N kg of d.w.−1, are found in legumes [53,112]. Kwiatkowski et al. (2019) [41] found the nitrogen content in the biomasses of white mustard and lacy phacelia to be 38.6–39.321 g N/kg dry d.w.−1 and 2.74–3.21 g N kg d.w.−1, respectively. Studies carried out in France and Denmark showed that the total nitrogen amount in catch crops’ biomass harvested on 1 hectare ranged from 10 to 171 kg N ha−1 for legumes, and from 9 to 89 kg N ha−1 for non-legumes, while the C:N ratio ranged widely, from 9 to 40, thus sometimes going beyond the range considered optimal for microorganisms’ growth, which is estimated to be between 20 and 35 [113,114]. According to the study of Szwarc et al. [99], the C:N ratio of grass silage was ca. 23.
The concentrations of other important nutrients in the catch crops’ biomasses, belonging to grasses, legumes and brassicas, were: phosphorus—2–8.2 g kg d.w.−1, potassium—15–52.8 g kg d.w.−1, magnesium—0.9–4 g kg d.w.−1 [41,53], calcium—21.4–26.6 g kg d.w.−1 [41] and sulfur—1–9 kg d.w.−1 [53]. An excessive content of sulfur in biomass poses a threat to the proper course for both the biochemical and thermal methods of biomass conversion into energy. In the case of anaerobic digestion, problems are related to the production of H2S, which inhibits the growth of microorganisms [115]. The solution is to modify the composition of the substrate so that the optimal value of C:S in the feedstock is over 40 [116]. In the case of combustion, the high content of sulfur in the fuel leads to SO2 production. In general, the content of sulfur in plant biomass is low [117].

4.2. Energy Potential of CCC Biomass Converted into Biogas

The basis for the economic assessment of the suitability of plant biomass for use in the energy sector is the value of energy that can be produced from biomass harvested per hectare of crop area per year (MJ ha−1 yr−1). This value is calculated based on the yield of the raw material (Mg ha−1 yr−1), which is an energy carrier, and its energy value (MJ Mg−1). In the case of the methane fermentation process, the measure of the suitability of CCC biomass for biogas production is its specific methane yield or biomethane potential, which is the volume of methane obtained per mass unit of substrate (m3 CH4 Mg−1) and biomass yield obtained per hectare per year (Mg ha−1 yr−1). On this basis, the methane yield per hectare per year (m3 CH4 ha−1 yr−1) is calculated. Assuming the lower heating value of methane (35.8 MJ Nm−3), the energy of biomass per mass unit or cropping area unit is estimated.
According to Möller and Müller [118], during anaerobic fermentation, up to 95% of the carbon contained in the substrate is converted into gaseous components of biogas (CH4 and CO2). In the case of energy catch crops examined by Bareha et al. [111], the amount of carbon converted into biogas during this process ranged from 43 to 74%, while in the case of animal manure, it is 36–41%. The degree of conversion depends on many factors, including the content of water-soluble organic compounds, polysaccharides, lignin, C:N ratio, the kind of biomass pretreatment, e.g., grinding or ensiling, and the operational conditions of anaerobic digestion.
Th value of the specific methane yield of the aboveground biomass of different CCC plants is similar. According to Graß et al. [119], the methanogenic potential of the biomass of plant species, such as turnip rape, rye, winter pea, maize, sorghum and sunflower cultivated in different combinations in double-cropping systems in Germany harvested in the vegetative phase, only slightly differed among the particular species. Thus, the yield of biomass was a key factor determining the potential of these plants for biogas production in the fermentation process. The similarity of the specific methane yield values is also indicated by the data presented in Table 3.
According to them, the values of this parameter of the raw biomass of different cover crops ranged from 140 to 490 m3 CH4 Mg−1 VS (VS—volatile solids), and the highest value was observed in the case of ryegrass. The literature values reported by Amon et al. [87], which ranged from 213 to 442 m3 Mg−1 VS, are in this scope. In addition, the results of the studies of Molinuevo-Salces et al. [30], carried out in Denmark on 10 types of catch crops (single species: white mustard, yellow lupin, oil seed radish, lupin, bean; mixed species: white mustard and common vetch, oil seed rape and winter vetch, perennial rye and Persian clover, winter ryegrass and winter vetch, triticale and winter vetch) were comparable with the values given above. The methane potential was between 229 and 450 m3 Mg−1 VS, and its highest values were obtained in the case of a mixture of rapeseed and winter vetch (399–415 m3 Mg−1 VS), and oilseed radish (368–450 m3 Mg−1 VS) cultivated in one of the locations tested in the study (Holstebro), while the lowest values were obtained in the case of white mustard (239–252 m3 Mg−1 VS), regardless of the location of the crops. The biomethane potential of the raw biomass of CCCs usually does not differ from the potential of raw corn biomass, which is 256 ± 15 m3 Mg−1 VS [129].
A serious limitation in the energetic use of the biomass of plants is the difficulty in maintaining its chemical properties for a long time. The biomasses of CCC plants harvested in the low-maturity phase, useful for biogas production, have a high water content, and are low in their resistance to biodegradation during storage. This is an unfavorable feature when taking into account the efficiency of methane production because it leads to carbon losses before the process of organic matter conversion into biogas. However, research indicates that this problem can be dealt with by the use of ensiling, commonly practiced as a method of preserving plant biomass for animal feed. This process involves the transformation of organic matter in the fermentation process carried out by lactic acid bacteria [130]. During the process, organic matter is lost. However, as reported by Borreani et al. [131] based on the results of their literature review, these losses may vary widely from 1 to 34% depending on the process conditions. According to Villa et al. [132], a properly conducted ensiling process allows for the conservation of up to 93% of the gross energy of biomass. The process leads to a change in the chemical composition that is beneficial for methanogens, which involves the production of organic acids that are easily accessible to them. According to Franco et al. [133], the most preferable features of feedstock subjected to ensiling are the high content of accessible carbohydrates, low buffering capacity and low moisture. The research conducted by Van Vlierberghe et al. [134] showed that the high moisture in CCC biomass leads to the production of leachate, and thereby causes losses in the amount of valuable substrates for biogas production. Their study confirmed that the addition of co-substrates with a high water retention capacity, such as bio-waste and manures, allows the organic matter losses to be limited and the high biogas potential of the silage to be maintained.
Herrmann et al. [135] showed that the reference values of the methane yields of silages of different crop species, such as Italian ryegrass, fodder radish, phacelia, annual ryegrass, spring barley, rapeseed, buckwheat, alfalfa, clover/grass mixtures, alfalfa/grass mixture, oat/fodder vetch mixture, mustard, Bokhara clover and buckwheat/phacelia mixture, related to maize silage, ranged from 57 to 109%, and the lowest value was observed in the case of the alfalfa/grass mixture, and the highest one in the case of the oat/fodder vetch and clover/grass mixtures. The mean methane yield of maize silage determined in this study was 354.6 m3 Mg−1 VS. Hutňan [136] found a lower value for the specific methane yield of maize silage, which was in the range 206–283 m3 Mg−1 VS.
In Europe, aboveground catch biomass rarely exceeds 5 Mg d.w. ha−1 [53]. The biomass yield of CCCs can vary depending on crop species, soil properties and climatic conditions. According to Hansen et al. [53], the production of biomass in Denmark remains highly variable, and it ranges from 3 to 15 Mg d.w. ha−1 for summer energy crops, and from 2 to 16 Mg d.w. ha−1 for winter energy crops. According to their observations, it was difficult to obtain a dense and uniform cover in the summer season due to the lack of water in the soil, while the low number of sunny days was the limiting factor in autumn.
Many studies have been conducted to maximize CCC biomass yield, e.g., by modification in the selection of the plants used in double-cropping systems in order to increase their potential in energy production [119,124,127,137,138].
Energy production per unit mass of substrate is an important element in the economic assessment of biomass use for energy production. However, there are more factors influencing the dissemination of new technologies or their modifications. This issue is the subject of the next chapter.

5. Status and Prospects of CCC Use in the Energy Sector

Current research shows the greater potential for energy production from biomass via methane fermentation systems compared to combustion technology [139,140]. The production of cover crops’ biomass for biogas production is promoted in various countries to increase renewable energy production [26,30,141,142]. In France, the concept of the “energy cover crop” was even introduced, which is defined as a “crop grown between two cash crops, and its biomass is harvested and anaerobically digested to produce biogas” [22]. In this country, already in the 1980s, attention was paid to the possibility of using catch crops as one of the sources of biomass, additional to crop residues, animal waste and agro-industrial waste [143]. The production of cover crops for energy purposes has also been promoted by the Luxembourg Institute of Science and Technology [144] and research on the viability of the cultivation of energy cover crops as sources of second-generation biofuels has started in Spain [145].
In Denmark, where in 2009 regulations stating that by 2020, 50% of the manure from Danish livestock should be converted to energy were established [146], and catch crops are widely grown due to regulations on reducing the risk of environmental pollution with nitrogen compounds [147], the use of the aboveground biomass of CCCs as a potential co-substrate for the anaerobic digestion of manure, improving the C:N of the feedstock and increasing biogas production, is very popular. The benefits and drawbacks of popularizing the use of CCC for the production of biofuels have also been considered by specialists in the United States [34,148,149].
The advantages and disadvantages of the energy use of CCC biomass are considered economically and environmentally. An economic factor related to the possibility of farmers obtaining funds as a result of selling biomass to biogas plants favors such a use. However, the economic balance depends mainly on the volume of biomass yields. Szerencsits et al. [26] also point out the advantages of this solution resulting from the fact that the cultivation of catch crops for energy purposes does not constitute competition for arable land in regions where only one main crop can be grown per year, as is the case with typical energy crops.
Due to the low variability in the methane yield of the biomass of crop species harvested during the vegetative stage, as mentioned in the previous section, the economic profitability of using CCC biomass for energy production is primarily determined by the biomass yield per hectare [119,141,150]. Therefore, the species most useful for energy purposes are those with high aboveground yields. Molinuevo-Salces et al. [150] claim that the biomass yield of catch crops below 1 Mg of biomass per 1 ha per year is a contraindication to the use of this crop for energy because the profit obtained from the sale of energy barely compensates for the costs of its production. According to Szerencsits et al. [26], an average of 1300 m³ of CH4 can be obtained from 1 ha of catch crops in Austria, assuming that the biomass yield is 4.5 Mg d.w. ha−1. After deducting the energy invested in the production of biomass and its conversion into biogas, the value of energy corresponds to 1000 m3 CH4 ha−1.
Molinuevo-Salces et al. [150] indicated an economically justified method of using CCC for biogas production, consisting of the combined harvesting of their biomass together with the remaining straw from the main crop. Based on the analysis of seven types of CCC collected in this way, the researchers calculated that the yields of raw material for biogas plants ranged from 3.2 to 3.6 Mg d.w. ha−1 yr−1, of which CCC biomass accounted for approximately 10%.
The logistical problem related to the use of CCC biomass for energy purposes is the seasonality of its production. The key question is to find a way to store biomass that will guarantee the availability of the material throughout the year and will not significantly affect changes in its properties. Ensiling as a method of storing various types of catch crops was tested, showing that anaerobic fermentation of ensiled samples leads to an increase in the decomposition rate in the initial stages of the process. This may be an economic benefit for the biogas plant as it allows the hydraulic retention time (HRT) in the bioreactor to be shortened [150]. Studies by Franco et al. [151] conducted on catch crops (mixtures of sunflower, sorghum, peas, vetch and Egyptian clover) with a low content of total solids (10.1%) showed that ensiling is an effective method of preparing biomass for storage. It allows the methane potential of biomass to be maintained at a high level. Potential tests conducted on biomass stored for 98 days showed that the value of this parameter in the case of open-air stored biomass was only 18% of the potential determined for ensiled biomass.
Biomass, as a renewable raw material replacing fossil fuels, can make a significant contribution to the energy transformation due to the reduction in CO2 emissions from the energy sector, both in heating, electricity production and transport. However, the use of agricultural residues, which offer great potential for sustainable bioenergy is limited by spatial factors related to the distance of the biomass source and the possibility of using the generated energy in the immediate vicinity. The transport of raw material to a biogas plant also involves greenhouse gas emissions, which affects the result of assessing the carbon footprint of energy production from biomass. Modeling studies conducted by Siegrist et al. [152], who analyzed, among others, the impact of the distance of the biomass source from the biogas plant (10, 15 and 20 km), indicate, that transport-based emissions, which were 0.5–0.8 kgCO2-eq per GJ of produced energy from biogas, are negligible compared to the benefits from anaerobic digestion.
However, there are concerns that the energetic use of CCC biomass for purposes other than those for which they are traditionally used in agriculture will have a negative impact on the soil quality and consequently on crop yield, due to the leaching of nitrogen from the soil, reducing the stock of organic matter and nutrients, and depriving the soil of protection against external factors. Most of the plant biomass is removed from the cultivation area. Only the root biomass and a small part of the aboveground parts remain. Additionally, the environmental CCCs functions, relating to participation in cycles of nitrogen and carbon in nature—the elements that are part of the compounds responsible for creating the greenhouse effect—can be impaired. In the case of nitrogen, catch crops reduce the escape of this nutrient from the soil by binding it into biomass. This is important in the season when there are no target crops in the field anymore, and with rainfall, ammonium ions could be washed into deep parts of the soil or undergo nitrification with the release of nitrogen oxides [28].
The share of CCC biomass in carbon sequestration is also important. A significant part of the biomass of these plants remains in the cultivation area and goes to the soil, where it undergoes transformations, the direction of which depends on various factors, including climatic and soil conditions [26]. Actual measurements of carbon content in soils where cover crops were grown, subjected to meta-analysis, the results of which were presented in the review paper by Poeplau and Don [18], show that the cultivation of these plants is associated with an increase in soil carbon resources on average by 0.32 Mg C ha−1 yr−1 within approximately 50 years from the start of cultivation. The researchers calculated that when the saturation with soil organic carbon will be achieved the carbon stock in the soil will increase by 16.7 Mg ha−1, and the potential sequestration of carbon in soil on a global scale will be 0.12 PgC rok−1. We can compare it with the global energy-related CO2 emissions in 2022, which were 36.8 Pg [153]. This equals 10.04 PgC yr−1, while global emissions from agriculture and related land use in 2018 were 9.3 Pg CO2eq [80,154], that is 2.54 PgC. Thus, assuming anthropogenic emissions remained unchanged, potential sequestration would offset 1.2% energy-related CO2 emissions or 5% of agricultural-related emissions.
The potential for carbon sequestration in the soil under CCCs is evidenced by the biomass yield values of these plants. Ruis et al. [21], based on a literature review, reported that the biomass yield of 20 typical-for-temperate-regions CCC species was 3.37 ± 2.96 Mg ha−1, and the values of the yield ranged from 0.67 to 6.30 Mg ha−1 in humid regions, with precipitation >750 mm, and from 0.87 to 6.03 Mg ha−1 in semiarid regions with precipitation <750 mm. The authors indicate that, taking into account the global scale, the yield of CCC biomass is very variable, from values below 1 Mg ha−1 up to 32 Mg ha−1. Generally, biomass production is lower in colder climates where the growing season is relatively short.
However, it cannot be assumed that all the carbon contained in the CCC biomass that remains in the field will be permanently retained in the soil. The factors influencing the amount and rate of carbon sequestration in the soil are not yet sufficiently understood; however, it is known that the transformation of exogenous organic matter (EOM) introduced into the soil into humic substances is determined by the physical, chemical and biological properties of the soil, climatic conditions and land use [155], as well as properties of the EOM source, for example, by its susceptibility to biodegradation. Field research [156] showed that the content of labile soil organic carbon in soil enriched with the CCC biomass of such plants as pea, oat and canola depended on the dose of biomass introduced, and the decomposition rate constant (k) depended on the type of biomass introduced. Shahbaz et al. [157], on the basis of the results of short-term (ca. 2 months) research, found that the incorporation of organic matter into the soil fed with wheat crop residues depended on the biomass dose. They also found that the intensity of organic matter mineralization increased with the EOM dose but was lower in the case of roots than in the aboveground parts of the plant. This suggests that the biomass of the underground parts of plants has a greater potential for carbon sequestration than the biomass of aboveground parts.
Researches indicate that environmental conditions are an important factor that influence the amounts of carbon accumulation in various parts of the plant. Plants adapt their growth strategy to different environments and, in particular, tend to allocate more biomass to root systems under more stressful, nutrient-poor and poor climate conditions [158]. However, individual species inhabiting similar habitats differ in terms of root/shoot ratio (RSR), as indicated by the results of research on the division of the aboveground and underground biomass of various plant species grown in the same soil and climatic conditions [53,159]. For example, oilseed radish, which represents the family Brassicaceae, is characterized by a high accumulation of biomass in the underground parts—the mass of the roots constituted as much as half of the mass of the aboveground parts, while plants of the family Fabaceae accumulated an underground mass that was only from 10 to 15% of the mass of the aboveground biomass and the underground mass of Italian ryegrass (family Poaceae) was around 35%. Differentiation of the distribution of biomass into aboveground and underground parts depending on species allows for the appropriate selection of CCCs, depending on the purposes of their use. Redin et al. [160], on the basis of field tests, found that significantly more C was allocated to grass root systems (11 species of Poaceae family with an average R/S ratio 0.19) than non-grass species (15 species of Fabaceae, 11 Asteraceae, 1 Brasicaceae and 2 Euphorbiaceae with an average R/S ratio 0.13). The average aboveground biomass of grasses was also higher (7.72 Mg ha−1) than that of non-grassy plants (6.78 Mg ha−1). Table 4 summarizes data on the yields of the aboveground and underground biomass of cover crops belonging to several families.
Removing from the field the whole aboveground CCC biomass, which sometimes constitutes as much as 90% of the total matter, poses a risk of disrupting the agricultural and environmental roles of growing these plants
The environmental risks related to the use of CCCs as energy raw materials were investigated by Herbstritt et al. [149], who showed, based on the results of field tests, that an integrated system of using the winter rye energy cover crop (Secale cereale L.) biomass for energy (biomethane production) and feed purposes may be beneficial from the point of view of the impact on the climate, due to the reduction in the overall carbon footprint. Szerencsits et al. [26] also claim that the negative impact of the use of cover crops’ biomass in the energy sector on soil, water and climate quality can be minimized by harvesting cover crops using methods that do not compact the soil and returning the digestate to the field in appropriate amounts. According to their estimates, the risk of nitrous oxide emissions can be reduced by up to 50% compared to the situation if cover crops were left in the field. In addition, the impact on humus content and erosion resistance would be similar or even better than in the case of using cover crops as green manure, if the same amount of biomass left in the soil was assumed. The authors conclude that the ecological footprint of arable farming expressed with Sustainable Process Index (SPI) can be reduced by as much as 50% considering the CH4 production using the cover crops. Even assuming that most of the aboveground biomass is removed from the cultivation area, the root biomass remains there, the amount of which is significant in the case of some plant species (Table 4).
However, a comparison of the environmental effect of using winter rye, grown as a cover crop after maize, as a co-substrate for biogas production with a monosubstrate system based only on maize, indicates higher total impacts on climate change and use of the resource in the case of the first option, because of the more complex infrastructure used for the production of 1 MJ of energy [163]. Other studies have shown that if a part of the biomass undergoing fermentation is replaced with CCC crops, the use of nitrogen fertilizers will increase and, as a result, N2O emissions in the field will increase. Additionally, CO2 emissions will also increase due to the greater demand for energy used during field work [164]. The environmental effect of using CCC for biogas production mainly depends on method of digestate management. When it is not returned to the soil, greenhouse gas emissions can increase even by 80% [165].

6. Conclusions

Cover or catch crops have great potential as a raw material for energy production. Because of the high moisture content of biodegradable organic compounds and macro- and microelements, their biomass is suitable for processing using biological methods. Rapid growth, usually low soil requirements, the lack of competition with main crops used for food production and high susceptibility to ensiling, which makes their long-term storage possible, are the features that support the use of this biomass in the energy sector. Additionally, a possible reduction in the carbon footprint of the energy industry resulting from the use of renewable sources is a significant advantage of the use of CCCs for biogas production. This effect is pronounced when post-fermentation residues, rich in organic matter and more stable than raw aboveground biomass, are returned to the soil as a source of nutrients and precursors for humus production.
This review shows that when considering the popularization of the use of catch crop biomass as an energy raw material, the attention should be focused on multispecies crops because they produce a high amount of aboveground biomass, and therefore, more raw material for use in the energy industry, as well as a high amount of underground biomass, i.e., more precursors for humus production and nutrients for the growth of main crops. However, it is necessary to be aware that due to their function in agroecosystems, the whole amount of the aboveground biomass of CCCs should not be used for energy purposes. Therefore, further multifaceted research is necessary to assess the share of the CCC biomass stream that, under given conditions, can be directed to biogas plants without environmental risk.

Author Contributions

Conceptualization, M.P. and A.S.; methodology, M.P.; investigation, A.S. and M.P.; writing—original draft preparation, A.S. and M.P.; writing—review and editing, M.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflicts of interest.

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Table 1. Typical plants cultivated as cover or catch crops in different parts of the world, and their additional applications (main sources [33,34]).
Table 1. Typical plants cultivated as cover or catch crops in different parts of the world, and their additional applications (main sources [33,34]).
FamilySpeciesApplication/FunctionDistributionReferences with Regard to Use as CCC
Asteraceae
(alt. Compositae)		Niger, Niger seed
(Guizotia abyssinica)		Soil improver, fodder; source of oilAfrica: Ethiopia (n)
Africa (cult., natur.)
Asia (cult. natur.) Australia (cult.) South. Europe (natur.)		[35,36,37]
Sunflower
(Helianthus annuus L.)		Fodder; honey production, oil production, ornamental, human food, North. America (n)
Widely cult.		[36]
BoraginaceaeLacy phacelia, purple tansy
(Phacelia tanacetifolia Benth.)		Soil improver: cover crops; honey production; ornamental function; phyto sanitary functionNorth America (n) Australia (natur.)
Europe (natur.)		[36,37,38,39,40,41]
Brassicaceae
(alt. Cruciferae)		White mustard
(Sinapis alba)		Soil improver (deep root system): cover crops; fodder; source of lipids; medicine herbs; phytosanitary functionEurope
North. Africa
West. Asia		[39,40,42,43,44]
Fodder radish, Oilseed radish
(Raphanus sativus)		Soil improver (deep and bulky root system), cover crop; fodder; phytosanitary functionWidely cult.[37,39,45]
Camelina, false flax
(Camelina sativa L.)		Soil improver: cover crop, green manure, source of oil; fodderAsia (n)
Europe (n),
North America (n) Widely natur.		[35]
Turnip, field mustard, colbaga
(Brassica rapa L.) 		Soil improver: cover crop, human food; fodderWidely cult.[39,45]
Rape, rapeseed, winter canola
(Brassica napus L.)		Soil improver: cover crop; fodderWidely cult.[45,46]
Fabaceae (alt. Leguminosae)Cowpea, field pea
(Vigna unguiculata L. Walp)		Soil improver: green manure, cover crop; catch crop; forage; human foodAfrica (n)
Widely cult.		[47,48]
Sunn hemp, Indian hemp
(Crotalaria juncea L.)		Soil improver: green manure, cover crop; forage; catch crop, nitrogen-fixing, fiber productionAsia (n)
South Africa
Cult. throughout tropics		[47,49]
Yellow lupine
(Lupinus luteus L.)		Soil improver: cover and catch crops; fodder; forage;
medicine herbs		North. Africa (n)
South. Europe (n)
Australia (cult.)
West. Asia (natur.)
South. Africa (natur.)		[50,51]
Narrowleaf lupin, narrow-leaved lupin, blue lupin
(Lupinus angustifolius)		Soil improver: catch crop; fodder; forageNorth. Africa (n)
West. Asia (n)
South. Europe (n)
Australia (cult.)		[52]
White lupine (Lupinus albus L.)Soil improver; cover crop; fodder; forage; ornamental functionAsia (n)
Europe (n)
Widely cult.		[53]
Alfalfa, lucerne (Medicago sativa L.)Soil improver, cover crop, fodderAfrica (n)
Asia (n)
Europa (n)
Widely cult.		[54,55,56]
Common vetch
(Vicia sativa L.) 		Soil improver: catch crop; fodder; forageAfrica (n)
Asia (n)
Europe (n)
Widely cult.		[35,36,37,44,57,58]
Fodder vetch, hairy vetch, winter vetch (Vicia villosa Roth.)Soil improver: catch crop; fodder; forage (but can be toxic to horses)Africa (n)
Asia (n)
Europe (n)
Widely cult.		[39,59,60]
Faba bean, fava bean, broad bean
(Vicia faba L.)		Soil improver: catch crop, cover cropWidely cult.[39,50,58]
Seradela, French serradella
(Ornithopus sativus Brot.)		Soil improver: catch crop; forageNorth. Africa (n)
South. Europe (n)
Australia (cult.)
Europa (cult.)
Africa (natur.)
Egyptian clover, berseem clove
(Trifolium alexandrinum L.)		Soil improver: catch crop; forageAfrica (cult.)
Asia (cult.)
Australia (cult.)
Europa (cult.)
Northern America (cult.)		[39,58]
Reversed clover, Persian clover
(Trifolium resupinatum L.)		Soil improver: catch crop; forage; fodderAfrica (n)
Asia (n)
Europa (n)
Widely cult.		[61]
White clover
(Trifolium repens L.)		Soil improver: catch crop; forage; Africa (n)
Asia (n)
Europa (n)
Widely cult. in temperate regions		[44]
Red clover
(Trifolium pratense L)		Soil improver: catch crop; forage; fodder; honey production, food additiveAfrica (n)
Asia (n)
Europa (n)
Widely cult. and natur. in temperate regions		[43,46,56,62]
Crimson clover
(Trifolium incarnatum)		Soil improver: catch crop; forage; fodder; honey productionAfrica (n)
Asia (n)
Europa (n)
Widely cult. in temperate regions		[50,52,62]
Pea, field pea
(diverse Pisum sativum L.)		Soil improver: catch crop; human foodAfrica (n)
Asia (n)
Europa (n)
Worldwide (cult.)		[36,46,52,58,60]
Poaceae (alt. Gramineae)Black oat, lopsided oat, bristle oat
(Avena strigose)		Soil improver: cover crop, green manure; forage; fodder;
source of oil used in cosmetics		Europe (n)
South America (cult.)
South. part of North America (cult.)
South Africa (cult.)		[35,37,47]
Common oat
(Avena sativa L.)		Soil improver: cover crop; human food; fodder; forageWidely cult.[46,47,56,60]
Rye, common rye, winter rye, stooling rye (Secale cereale)Soil improver: cover crop, green manure,
human food; forage; fodder		Asia (n)
Europa (n)
Widely cult.		[39,47,52]
Triticale (Triticale A. Müntzing)Soil improver: cover crop, green manure; forage, human foodEurope (cult.)
Asia (cult.)
South Africa (cult.)		[47,56]
Italian millet, foxtail millet
(Setaria italica L.)		Forage, fodder, cover, green manureSouth. Asia (n)
Asia (cult.)
Africa (cult.)
South. Part of North America (cult.or natur.)		[47,60]
Finger millet
(Eleusine coracana L. Gaertn.) 		Cover, human food, fodderAsia (cult.)
South Africa (cult)		[47,49]
Pearl millet, bajra
(Pennisetum glaucum)		Soil improver: cover crops, erosion control; forage; fodder; human food; ornamentalAsia (cult.)
Africa (cult.)
North America (cult.) 		[47,49,58,63]
Japanese millet, white millet
(Echinochloa esculenta)		Soil improver: cover crop; fodder; forage; human foodAfrica (cult.)
Asia (cult.)
North America (cult.)
South America (cult.)		[63]
Westerwold ryegrass, Italian ryegrass (Lolium multiflorum Lam.)Soil improver: cover crop, erosion control; fodder; forageAfrica (n)
Asia (n)
Europa (n)
North. America (natur.)		[62,64,65,66]
Perennial ryegrass, English ryegrass (Lolium perenne L.) Soil improver: cover crop, erosion control; fodder; forageAfrica (n)
Asia (n)
Europa (n)
North, America (natur.)
South. America (natur.)
Australasia (natur.)		[62,66,67]
Meadow fescue, English bluegrass (Festuca pratensis Huds.)Soil improver: cover crop, erosion, forageAfrica (n)
Asia (n)
Europa (n)
Widely natur.		[66]
Orchard grass, cocksfoot
(Dactylis glomerata L.)		Soil improver: cover crop; fodder; forage; ornamentalAfrica (n)
Asia (n)
Europa (n)
North. America (cult.)
Australasia (natur.)
South. America (natur.)		[54,62]
PolygonaceaeCommon buckwheat
(Fagopyrum esculentum Moench)		Soil improver: cover crops, green manure; human food; forage; fodder; honey productionAsia (n)
Widely cult. and natur.		[35,37,39,59]
Abbreviations: n—native, cult.—cultivated, natur.—naturalized.
Table 3. Specific methane yields of selected catch and cover crop biomass.
Table 3. Specific methane yields of selected catch and cover crop biomass.
CropPart of the PlantMethane Yield (m3 Mg−1VS)Reference
White mustard (Sinapis alba)Tops352[120]
Oil seed rape (Brassica napus spp. oleifera)Straw420[121]
Radish (Raphanus sativus)Shoots293–304[122]
Rape (Brassica napus arvensis)Tops334[120]
Rape (Brassica napus)Not reported340[123]
Winter rye (Secale cereale montanum)Straw360[121]
Rye (Secale cereale)Whole plants140–275[87]
Triticale (Triticale)Whole plants212–286[87]
Triticale (Triticale)Whole plants396[124]
Faba bean (Vicia faba)Straw440[125]
Faba bean (Vicia faba)Whole plants387[126]
Ryegrass (Lolium sp.)-410[123]
Ryegrass (Lolium sp.)-490[104]
Clover (Trifolium sp.)Vegetative stage210[127]
Clover (Trifolium sp.)Flowering stage140[127]
Grass hay-350[127]
Oat-260[127]
Lupine (Lupinus polyphyllus)Whole plants310–360[120]
Vetch oat (50% Vicia sativa)Whole plants400–410[120]
Red clover (Trifolium pratense)Whole plants310–320[120]
Red clover (Trifolium pratense)Whole plants238–293[128]
Red/white clover–ryegrass Trifolium pratense, Trifolium repens L, Lolium perenne L.)Whole plants281–315[128]
CornCorn stover256 ± 15[129]
Table 4. Relation of above- and belowground dry biomass of cover crops.
Table 4. Relation of above- and belowground dry biomass of cover crops.
FamilySpeciesShootRootsRoot/Shoot Ratio (-)References
BrassicaceaeOilseed radish (Raphanus sativus)2.035 Mg/ha1.053 Mg/ha0.52[53]
Oilseed radish (Raphanus sativus)6.81 Mg/ha0.86 Mg/ha0.13[160]
Oilseed rape (Brassica napus)2.77 Mg/ha0.84 Mg/ha0.30[160]
BoraginaceaePhacelia (Phacelia tanacetifolia)4.36 Mg/ha2.56 Mg/ha0.59[161]
FabaceaeCrimson clover (Trifolium incarnatum)2.088 Mg/ha0.296 Mg/ha0.14[53]
Red clover (Trifolium pratense), diff. populat., av. value43.65 g/meso-cosm6.38 g/meso-cosm0.15[162]
Yellow lupine (Lupinus luteolus)1.06 g/pot0.26 g/pot0.25[159]
Narrowleaf lupin (Lupinus angustifolius)1.43 g/pot0.29 g/pot0.20[159]
White lupin (Lupinus albus)2.842 Mg/ha0.416 Mg/ha0.15[53]
White lupin (Lupinus albus)2.13 g/pot0.51 g/pot0.24[159]
Hairy vetch (Vicia villosa)1.376 Mg/ha 0.142 Mg/ha0.10[53]
Common vetch (Vicia sativa)3.67 Mg/ha1.33 Mg/ha0.36[160]
Pea (Pisum sativum)5.43 Mg/ha0.66 Mg/ha0.12[160]
Blue lupine (Lupinus angustifolius)5.54 Mg/ha0.85 Mg/ha0.15[160]
Indian hemp (Crotalaria juncea)8.90 Mg/ha0.88 Mg/ha0.10[160]
PoaceaeItalian ryegrass (Lolium multiflorum)2.456 Mg/ha0.862 Mg/ha0.35[53]
Triticale (Triticale rimpaui)4.26 Mg/ha1.40 Mg/ha0.32[160]
Ryegrass (Lolium multiflorum)5.48 Mg/ha1.15 Mg/ha0.21[160]
Black oat (Avena strigosa)7.98 Mg/ha1.85 Mg/ha0.23[160]
PolygonaceaeCommon buckwheat
(Fagopyrum esculentum)		3.854 Mg/ha0.373 Mg/ha0.10[53]
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Słomka, A.; Pawłowska, M. Catch and Cover Crops’ Use in the Energy Sector via Conversion into Biogas—Potential Benefits and Disadvantages. Energies 2024, 17, 600. https://doi.org/10.3390/en17030600

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Słomka A, Pawłowska M. Catch and Cover Crops’ Use in the Energy Sector via Conversion into Biogas—Potential Benefits and Disadvantages. Energies. 2024; 17(3):600. https://doi.org/10.3390/en17030600

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Słomka, Alicja, and Małgorzata Pawłowska. 2024. "Catch and Cover Crops’ Use in the Energy Sector via Conversion into Biogas—Potential Benefits and Disadvantages" Energies 17, no. 3: 600. https://doi.org/10.3390/en17030600

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