The preservation of biomasses through ensiling has a long history, and its sustainability has many aspects. One that is rarely considered is that the seeds of a wide variety of plants can enter the ensiling process with the plant biomass. This concise review provides an overview of the probability of seed survival in various types of silage since the 1930s. All data extracted from the reviewed studies are made available in a repository. The key finding from the 90 years of research is that ensiling can reduce the viability of plant seeds, but the seeds of some plant species can survive ensiling. Thus, silage production is both (1) a potential tool to ensure the sustainable, i.e., weed-free, use of plant biomass in agricultural production, including animal production chains, and (2) a potential gateway for weed spread, especially with regard to the closed material loops in circular bioeconomy approaches. The search for seed-borne factors and ensiling conditions that promote seed survival or killing is still ongoing and should be the subject of future research.

Rewetting of peatlands requires the development of new biomass utilization pathways. The supply of strategic elements with key importance for the development of priority technologies, such as germanium (Ge), silicon (Si) and rare earth elements, from fenland plants is one option. To provide a first estimation of the potential, concentrations of strategic elements were determined in nine biomass samples covering typical fenland vegetation in northeast Germany. Subsequently, a simplified estimation of potential revenue from strategic element recovery was made. The analysed plant species can be classified as high or intermediate Si plant accumulators with highest contents of more than 16.0 g Si kg −1 dry mass (DM) in sedges and common reeds. Ge concentrations were lower with reed canary grass containing the highest amounts of 465.3 µg Ge kg −1 DM. Simultaneous acquisition of Ge and Si could provide higher total element yields and revenues of up to 500 $ ha −1 . In contrast, the potentials for supplying rare earth elements appeared to be very low, with common reed containing the highest sum of rare earth elements of 437.4 µg kg −1 DM. Biomass from rewetted fenlands is capable of accumulating strategic elements. More knowledge is required to understand the factors affecting their accumulation.

IntroductionPlant seeds from weeds and energy crops have the potential to survive anaerobic digestion (AD). Species able to form physically dormant, i.e., hardseeded (HS) seeds seem particularly resistant. However, it is not yet known to what extent the storage of the digestate after AD affects seed viability.MethodsSeed survival of five HS and six non-HS (NHS) species was investigated in three combinations of digestate storage (DS) and AD. First, untreated seeds were exposed to DS for maximum 12 weeks. To simulate short-circuited AD (ADshort) in biogas reactors, seeds were second subjected to lab-scale AD for 1 day before DS. Third, seeds of six species were exposed to full-scale AD (ADfull) followed by DS. Seed viability was determined using a combination of germination tests and tetrazolium staining. Viability was modeled as a function of exposure time.Results and discussionSeed viability was affected by DS, AD and AD + DS, but responses varied greatly between species and treatments. With increasing exposure time, viability decreased after a lag-phase, remained stable or even increased. The NHS species Cichorium intybus, Daucus carota, Echium vulgare, and Verbascum thapsus were most susceptible, with seed-killing close to 100% if DS was involved. The HS species Malva sylvestris, Melilotus albus and Melilotus officinalis were most resistant. They survived all treatments and were alive after 35 days of ADfull plus 3 months of DS. The resistance potential of the HS species Abutilon theophrasti and Malva alcea and of the NHS species Chenopodium album and two tomato varieties was intermediate. None of them survived ADfull + DS, but except A. theophrasti they were viable after ADshort + DS. With few exceptions, seed-killing by AD + DS was higher than that by AD alone. In conclusion, DS and AD + DS have the potential to reduce seed viability, but do not completely inactivate all species. Therefore, digestate can be contaminated with viable seeds and may lead to the spread of weeds, especially after a short-circuited AD. In order to ensure the sustainable use of digestates in terms of weeds, we recommend to investigate the factors contributing to seed inactivation and the quantity of seed introduced to AD.

When plant biomass is anaerobically digested, seeds may survive the energy production process and contaminate the digestate. Hard-seeded (HS), i.e., physically dormant, species were found to be difficult to inactivate. Here, we aimed to verify this finding from lab-scale experimental reactors (ERs) in a full-scale commercial reactor (CR). In addition, we tested seed survival in a pH-buffered water bath (WB). Seeds were exposed to CR, ER and WB treatments at 42 °C for a maximum of 36 days. The viability of seeds was checked by measuring germination and response to tetrazolium staining and modeled as a function of exposure time using a dose–response approach. CR killed seeds more effectively than ER and WB treatments. The non-HS reference species, Chenopodium album, was completely inactivated by all treatments. Responses of the HS species ranged from complete inactivation to complete insensitivity. The most resistant was Malva sylvestris. The least resistant species were inactivated mainly by temperature, while additional mortality factors were effective in the more resistant species. We concluded that mesophilic AD in CRs can reduce the risk of seed contamination in the digestate for non-HS but not for HS species. Moreover, WB treatments seem suitable to estimate the minimum mortality of non-HS species in CR.

Xylo-oligosaccharides are sugar oligomers with 2~7 xylose units considered non-digestible fibers that can be produced from biodegradable and low-cost biomass like wheat straw. An integrated approach consisting of hydrothermal pretreatment, alkaline treatment, enzymatic treatment and the combinations thereof was applied to overcome the recalcitrance structure of the wheat straw and allow selective fractioning into fermentable sugars and xylo-oligosaccharides. The hydrolysates and processed solids were chemically characterized by High-performance liquid chromatography and Ion chromatography, and the results were expressed as function of the severity factor and statistically interpreted. The concentration of fermentable sugars (glucose, xylose, arabinose) was the highest after the combination of alkaline and enzymatic treatment with xylanase (18 g/L sugars), while xylo-oligosaccharides (xylotriose and xylotetraose) were released in lower amounts (1.33 g/L) after the same treatment. Refining experiments were carried out to obtain a purified fraction by using anion and cation exchange chromatography. The polymer adsorber resin MN-502 showed efficient removal of salts, phenols and furan derivatives. However, the xylo-oligosaccharides yields were also slightly reduced. Although still requiring further optimization of the treatments to obtain higher purified oligomer yields, the results provide information on the production of xylo-oligosaccharides and fermentable sugars from wheat straw for potential use in food applications.

Anaerobic digestion (AD) is a keystone in the circular economy concept. The produced biogas can be demand-driven controlled and regionally adaptable, but sustainable feedstocks are required. Sorghum is a valuable alternative considering its bioenergy potential and wide adaptability to different environmental conditions. In this study, AD of sorghum silage was performed in a single-stage continuously stirred tank reactor (CSTR) and in a two-stage leach-bed reactor (LBR) system. The two-stage LBR system revealed higher methane yields (434 L-N kg(VS)(-1)) compared to the single-stage CSTR (308 L-N kg(VS)(-1)). Methane content of biogas ranged from 53 to 59% in both systems. Only the CSTR exhibited acids accumulation, led by propionic acid concentration (2.0 g L-1). Buffer capacity of the two-stage LBR system was superior to the CSTR. Technical disruptions and constant ammonium content declination were shown in both systems, associated to inherent characteristics of fibrous sorghum. AD of sorghum biomass, however, can be effectively performed under controlled conditions, generating high methane yields in both types of systems.

The use of wildflower species as biogas feedstock carries the risk that their seeds survive anaerobic digestion (AD) and cause weed problems if spread with the digestate. Risk factors for seed survival in AD include low temperature, short exposure and hardseededness (HS). However, it is not possible to predict how AD will affect seed viability of previously unstudied species. In laboratory-scale reactors, we exposed seeds of eight species from a mixture of flowering wild plants intended as biogas feedstock and three reference species to AD at two mesophilic temperatures. Half of the species were HS, the other was non-HS (NHS). Viability was determined using a combination of tetrazolium and germination tests. Viability and germinability were modeled as functions of exposure time using a dose-response approach. Responses to AD varied considerably among species, and none of the considered influencing factors (time, temperature, HS) had a consistent effect. Seed lots of a species differed in inactivation times and seed-killing efficacy. The HS species Melilotus officinalis, Melilotus albus, and Malva sylvestris were particularly AD-resistant. They were the only ones that exhibited biphasic viability curves and tended to survive and germinate more at 42°C than at 35°C. Viability of the remaining species declined in a sigmoidal curve. Most NHS species were inactivated within a few days (Cichorium intybus, Daucus carota, Echium vulgare, and Verbascum thapsus), while HS species survived longer (Malva alcea). AD stimulated germination in the HS species A. theophrasti and its AD-resistance overlapped with that of the most resistant NHS species, C. album and tomato. In all seed lots, germinability was lost faster than viability, implying that mainly dormant seeds survived. After the maximum exposure time of 36 days, seeds of HS species and Chenopodium album were still viable. We concluded that viability responses to mesophilic AD were determined by the interplay of AD-conditions and species- and seed-lot-specific traits, of which HS was an important but only one factor. For the use of wildflowers as biogas feedstock, we recommended long retention times and special care with regard to HS species.

Lignocellulosic rice processing residue was pretreated in liquid hot water (LHW) at three different temperatures (140, 160, and 180 °C) and two pretreatment times (10 and 20 min) in order to assess its effects on hydrolysates composition, matrix structural changes and methane yield. The concentrations of acetic acid, 5-hydroxymethylfurfural and furfural increased with pretreatment severity (log Ro). The maximum methane yield (276 L kg−1 VS) was achieved under pretreatment conditions of 180 °C for 20 min, with a 63% increase compared to untreated biomass. Structural changes resulted in a slight removal of silica on the upper portion of rice husks, visible predominantly at maximum severity. However, the outer epidermis was kept well organized. The results indicate, at severities 2.48 ≤ log Ro ≤ 3.66, a significant potential for the use of LHW to improve methane production from rice processing residue.

Sorghum is currently being introduced in the temperate regions of Europe. It is characterized by good digestibility and high biomass yields, which make it a useful crop for anaerobic digestion. In this study, six commercial sorghum varieties comprising four different cultivars of Sorghum bicolor L. Moench and two interspecific sorghum hybrids (Sorghum sudanense L. × S. bicolor L. Moench) were harvested on two different dates during the years 2016 and 2017 at two diverse soil‐climate sites in Germany. The fresh harvested material and silages were analyzed to examine the ensilability of the different varieties with contrasting maturity characteristics. Subsequently, methane production experiments were performed to determine the specific methane yield (SMY) of the samples. The sorghum fresh matter (FM) varied among the sorghum types, including the parameters total solids (TS; 22.69%–46.93%FM), water‐soluble carbohydrates (2.68%–11.38%TS), and nitrates (0%–0.35%TS). The excellent ensiling ability of all the sorghum types analyzed was confirmed by evaluating the fermentation profile (pH range of 3.7–4.6; dominant presence of lactic acid [LA]; acetic acid [AA] in the range of 0.70%–2.38%TS; insignificant amount of butyric acid). The SMY ranged between 231.25 and 321.31 Ln kg−1 VS and tended to decrease with the increasing harvest time and maturity. LA and AA were positively correlated with the SMY, while the neutral detergent fiber content was negatively correlated with it. The SMY—a key parameter reflecting the crop biomass quality for biogas production—was slightly higher for S. bicolor than for the sorghum hybrids. However, the results of this study confirmed that if the final purpose is biomethanation, the ensilability of different sorghum types imposes no restriction. Furthermore, different sorghum types offer a wide harvest window, which can be useful for cropping schemes, ensiling, and methane production.

An interactive version of Fig. 2. (HTML 408 kb)

After nearly two decades of subsidized and energy crop-oriented development, agricultural biogas production in Germany is standing at a crossroads. Fundamental challenges need to be met. In this article we sketch a vision of a future agricultural biogas plant that is an integral part of the circular bioeconomy and works mainly on the base of residues. It is flexible with regard to feedstocks, digester operation, microbial communities and biogas output. It is modular in design and its operation is knowledge-based, information-driven and largely automated. It will be competitive with fossil energies and other renewable energies, profitable for farmers and plant operators and favorable for the national economy. In this paper we discuss the required contribution of research to achieve these aims.

During the last years, a demand for regionally produced biogas feedstocks was created by government subsidies to biogas production in Germany—contrary to the trend of specialization of agricultural production towards global commodity markets. The question arose whether this trend could contribute to an increased cultivation of uncommon crops and diversification of cropping patterns, owing to comparably different and less restricted feedstock requirements. In the cooperative research project “EVA,” a multi-site experimental crop rotation field trial was conducted over 8 years at eight sites, representing the variety of soil-climatic conditions in Germany. The aim of the trial was to assess a variety of established and novel crops for anaerobic digestion. This paper presents the key findings of the trial. Special emphasis is given to biomass productivity and profitability. The chances for the approach “diversification of cropping patterns via energy cropping” are discussed. Results show that maize (average 4-year dry matter yield varied site-specific between 14.22 and 25.12 t ha−1) is clearly the most efficient crop for biogas production in Central Europe. Some cropping options for biogas feedstock production, such as winter triticale (whole crop, average yield of 6.71 to 15.17 t ha−1) or perennial fodder mixtures (average yield of 7.51 to 19.44 t ha−1) are feasible choices for farmers in some regions, which could contribute to diverse cropping systems.

Table S4. Abundance of microbial key families, phages and, metaproteins related to microbial immune response. This excel sheet contains the detailed grouping of all metaproteins by their families as well as by their belonging to phages, and microbial immune response. It was the basis for Additional file 7: Table S5. (XLSX 18377 kb)

BACKGROUND:In biogas plants, complex microbial communities produce methane and carbon dioxide by anaerobic digestion of biomass. For the characterization of the microbial functional networks, samples of 11 reactors were analyzed using a high-resolution metaproteomics pipeline.RESULTS:Examined methanogenesis archaeal communities were either mixotrophic or strictly hydrogenotrophic in syntrophy with bacterial acetate oxidizers. Mapping of identified metaproteins with process steps described by the Anaerobic Digestion Model 1 confirmed its main assumptions and also proposed some extensions such as syntrophic acetate oxidation or fermentation of alcohols. Results indicate that the microbial communities were shaped by syntrophy as well as competition and phage-host interactions causing cell lysis. For the families Bacillaceae, Enterobacteriaceae, and Clostridiaceae, the number of phages exceeded up to 20-fold the number of host cells.CONCLUSION:Phage-induced cell lysis might slow down the conversion of substrates to biogas, though, it could support the growth of auxotrophic microbes by cycling of nutrients.

Synchytrium endobioticum is one of the most important pathogens of potato and is known for its persistent propagation structures. Under favourable conditions, infection of highly susceptible potato cultivars leads to clearly visible cauliflower‐like tissue warts, the typical symptom of potato wart disease. However, unfavourable infection conditions or low infection pressure may result in symptoms being overlooked. Thus, the introduction of pathogen structures into stages of the processing industry cannot be ruled out. As the amounts of processed potato products continue to rise, phytosanitary risks from processing discarded potatoes and potato waste in biogas plants have to be considered. Hence, the resilience of resting spores against mesophilic anaerobic digestion was analysed in stirred tank reactors. Laboratory‐scale results show that S. endobioticum not only withstands mesophilic anaerobic digestion but also subsequent storage of the digestate for at least 4 weeks. Large numbers of viable resting spores were detectable by microscopic assessment in all samples. Viability was proved and verified additionally by bioassay. Consequently, potatoes, potato waste and processing water from potato processing industries used in biogas plants pose a phytosanitary risk if the accruing digestates are returned as fertilizer to arable land.

In this study, microbiomes of 36 full-scale anaerobic digesters originated from 22 different biogas plants were compared by terminal restriction fragment length polymorphism (TRFLP) analysis. Regarding the differences in microbial community composition, a weighting of the environmental parameters could be derived from higher to lower importance as follows: (i) temperature, (ii) TAN and NH3 concentrations and conductivity, and (iii) the chemical composition of the supplied feedstocks. Biotic interactions between specific bacterial and archaeal community arrangements were revealed, whereby members of the phyla Bacteroidetes and Cloacimonetes combined with the archaeal genus Methanothrix dominated the conversion of homogeneous feedstocks, such as waste water sludge or industrial waste. As most of the detected TRFs were only found in a certain number of anaerobic digestion plants, each plant develops its unique microbiome. The putative rare species, the specialists, are potentially hidden drivers of microbiome functioning as they provide necessary traits under, e.g., process-inconvenient conditions.

Crop rotations are part of current agricultural practice, since they and their effects can contribute to a sustainable agricultural cropping system. However, in current Life Cycle Assessment (LCA) studies, crop rotation effects are insufficiently considered, since these effects are difficult to measure. LCA studies from crop production typically take only one vegetation period into account. As a result, the consideration of how the assessed crop is influenced by the previous crop (crop rotation effects) including: (1) nutrient carryover, (2) reduction in operational requirements and (3) different intensity and timing of farming activities, is outside of the system boundary. However, ignoring these effects may lead to incorrect interpretation of LCA results and consequently to poor agricultural management as well as poor policy decisions. A new LCA tool called the “Model for integrative Life Cycle Assessment in Agriculture (MiLA)” is presented in this work. MiLA has been developed to assess GHG emissions and cumulative energy demands (CED) of cropping systems by taking the characteristics of crop cultivation in rotation into account. This tool enables the user to analyze cropping systems at farm level in order to identify GHG mitigation options and energy-efficient cropping systems. The tool was applied to a case study, including two crop rotations in two different regions in Germany with the goal of demonstrating the effectiveness of this tool on LCA results. Results show that including crop rotation effects can influence the GHG emission result of the individual crop by −34% up to +99% and the CED by −16 up to +89%. Expanding the system boundary by taking the whole crop rotation into account as well as providing the results based on different functional units improves LCA of energy crop production and helps those making the assessment to draw a more realistic picture of the interactions between crops while increasing the reliability of the LCA results.

Biogas production can cause environmental problems due to a biased alignment of one energy crop used as a feedstock, e.g., maize in Germany. Diversification of crop rotations and resource-efficient management can be the key to sustainable crop management. Four crop rotations on eight sites across Germany were evaluated in terms of their resource efficiency (area use, energy, and economic efficiency) to derive options. Analysis revealed high variation in all indicators under review, with a high variance explanation by the interaction between crop rotation and regional characteristics. Furthermore, results indicate that high area-specific methane yields do not equate to high energy efficiency. Crop management adaptation is a useful tool for optimizing resource efficiency.