Biochemical methane potential (BMP) of six perennial

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Aug 27, 2018 - Axel Schmidt1 & Sébastien Lemaigre2 & Philippe Delfosse2 & Herbert von ...... accordance to the weather conditions and soil fertility levels.
Biomass Conversion and Biorefinery https://doi.org/10.1007/s13399-018-0338-2

ORIGINAL ARTICLE

Biochemical methane potential (BMP) of six perennial energy crops cultivated at three different locations in W-Germany Axel Schmidt 1 & Sébastien Lemaigre 2 & Philippe Delfosse 2 & Herbert von Francken-Welz 3 & Christoph Emmerling 1 Received: 9 May 2018 / Revised: 27 August 2018 / Accepted: 29 August 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract To mitigate the effects of climate change, the emission of greenhouse gases (GHG) should be reduced. In the near future, an increased use of biomass for energy production will be prescribed by law in the European Union (EU) to achieve the target of emission reduction. One possibility to use biomass energetically is its transformation into biogas. The most widely grown crop for this purpose is maize. Although, the increased use of maize as biogas feedstock, due to generous subsidies, is linked to several ecological and (socio)economic problems. The most important are soil erosion, depletion and compaction, high demand and subsequent leaching of fertilizers/biocides, heavy workload, and competition for land between food and energy production. In contrast, the use of second-generation perennial energy crops (PECs) as biogas feedstock can be an auspicious approach to improve the environmental footprint of biomass production. Therefore, we evaluated and compared the biochemical methane potential (BMP) of five different perennial species (cup plant, virginia mallow, tall wheatgrass, giant knotweed, reed canary grass) and a wild plant mix (composite of 25 in parts perennial plants) on three different sites in West-Germany. In terms of methane production per area, tall wheatgrass and reed canary grass exceeded the productivity of maize under favorable conditions. Hence, we recommend both species as biogas feedstock for Central Europe or comparable climates. Additionally, other species might be suitable for biomethanation from an overall perspective, to enhance (agro)biodiversity in rural areas. Keywords Biochemical methane potential . Anaerobic digestion . Biogas feedstock . Perennial energy crops

* Axel Schmidt [email protected] Sébastien Lemaigre [email protected] Philippe Delfosse [email protected] Herbert von Francken-Welz [email protected] Christoph Emmerling [email protected] 1

Faculty of Regional and Environmental Sciences, Soil Science Department, University of Trier, Behringstr.21, 54296 Trier, Germany

2

Environmental Research and Innovation Department (ERIN), Luxembourg Institute of Science and Technology (LIST), 41, rue du Brill, L-4422 Belvaux, Luxembourg

3

Energy and Agriculture Department, Center of Rural Services Rhineland-Palatinate, Westpark 11, 54634 Bitburg, Germany

Abbreviations 2G Second generation % Percent per weight ADF Acid detergent fiber ADL Acid detergent lignin AEC Annual energy crops amsl Above mean sea level BMP Biochemical methane potential c.v. Cultivated variety DLR Agricultural Service Center, Rhineland-Palatinate (Dienstleistungszentrum ländlicher Raum) EU European Union FM Fresh matter GHG Greenhouse gas ha Hectare = 100 m ∙ 100 m = 10,000 m2 NDF Neutral detergent fiber PEC Perennial energy crops t Tons (= 1 Mg = 1000 kg) TS Total solids VS Volatile solids

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1 Introduction Continued emission of greenhouse gases (GHG) will cause further warming and changes in the climatic system and increase severe impacts for humanity and ecosystems. To limit climate change risks, a substantial and sustained reduction of the GHG emissions is required [1]. Against this background, the goal to mitigate the effects of climate change has become part of the legislation in Europe. For example, the European Renewable Energy Directive 2009/28/EC demands a share of at least 20% of the consumed energy to be from renewable resources [2] and biomass should cover 10% of total energy consumption by 2020 in the EU [3]. In the field of energy generation from biomass in Europe, biogas production has become a widely used technology. In contrast to other environmentally friendly energy production methods like solar and wind power, the electrical and thermal output of biogas plants can be adjusted to the current demands [4, 5]. During the process of anaerobic digestion, up to three times more net energy yield can be obtained compared to the production of bioethanol or biodiesel. Additionally, biogas plants contribute to nutrient cycling, can be built more locally and a large variety of organic material can be used as feedstock for biomethanation [6]. This capability makes energy from biogas a valuable complement in the field of energy production from renewable resources while contributing to the upcoming circular economy. Furthermore, additional technical installations can purify the produced methane and supply it to the natural gas grid. The possibility to use existing infrastructure for transport and storage of this energy carrier could be an enormous advantage compared to wind and solar power [7]. These advantages and government subsidies have promoted the construction of biogas plants in Europe in the last decade. A large share of the feedstock for these installations is currently maize (Zea mays) due to its high biomass production and good biological decomposability. Additionally, most farmers are familiar with the production and ensiling of this crop and can use existing methods and machinery [8]. Nonetheless, the production of maize is connected to several ecological problems. The most considerable ones are soil erosion, high fertilizer demands, susceptibility to pests and subsequent leaching of nutrients and biocides to the groundwater and adjacent waterbodies [9]. The permanent cultivation of maize as monoculture (on temporal and spatial scale) can deplete soils irreparably and the reduction of soil organic matter deteriorates the carbon footprint [10]. Furthermore, a concentrated maize production also reduces the (agro)biodiversity and alter the characteristic landscape of a region [9]. To make use of the advantages of biogas production without the drawbacks of maize cultivation, the usage of perennial energy crops (PEC) as feedstock can be an auspicious approach due to considerable advantages in terms of ecological aspects and

ecosystem services [11]. In contrast to annual energy crops (AEC), they require less energy and financial input, can generate higher yields than short rotation coppices, can be cultivated with existing farm equipment and can be profitable when cultivated for at least 4 years [12]. The most PECs are considered to be low-input cultures. This means that they have low requirements concerning nutrients, plant protection, work, and in some cases water supply [13–16]. In terms of GHG emissions, the cultivation of PECs can be favorable compared to AEC’s [17]. This property is based on two pathways: On the one hand, GHG emissions are lower due to a smaller input of GHG intensive substances and activities (e.g., fertilizer, fuel consumption). On the other hand, most of such plants are able to function as GHG sink on the basis of carbon sequestration in the soil [17]. Investigations of Adler et al. [18] showed, that compared to AEC’s, reduced workload and subsequent fuel saving have the largest greenhouse reduction potential, followed by soil sequestration. Concerning soil protection, the year-round cover with plants or plant residues and the intensive rooting reduces the risk of soil erosion, which is an often underrated loss of production resources [10]. Furthermore, PECs have the potential to improve the soil quality and earthworm activity compared to annual energy crops [19–21] and enhance the biodiversity of invertebrates due to absence of chemical plant protection and yearly seed bed preparation [22]. Additional advantages are for instance increased rural area employment and agriculture income diversification, improved landscaping and reduced nutrient leaching to the ground water or adjacent water bodies [17]. PECs must meet several requirements to be suitable as biogas feedstock. The most important parameter is the net energy yield per hectare. This trait arises from high biomass yields per area (BMP per hectare), an extensive biodegradability under anaerobic conditions like in fermenters of biogas plants (specific BMP) and low cultivation requirements [8]. Especially, the share of lignocellulosic compounds can be very recalcitrant against anaerobic microbiological breakdown and contribute therefore sparsely to the methane yield. Additionally, the biomass must not have inhibitory properties that interferes the growth and conversion performance of the microbial community. Concerning economic demands, potential biogas feedstock should have low requirements for fertilizers, biocides, climatic conditions, and workload. In this field, the multi-annuality is an important advantage. The omission of yearly seedbed preparation and sowing (or planting) can make their cultivation considerable less work intensive. Within our experiments, we tested six different crops, which were considered to fulfill the above-mentioned requirements and therefore might be an auspicious biogas feedstock. These were the perennial species cup plant (Silphium perfoliatum), virginia mallow (Sida hermaphrodita), reed canary grass (Phalaris arundinacea), tall wheatgrass (Elymus

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elongatus cv. ‘Szarvasi-1’), and giant knotweed (Reynoutria japonica cv. ‘Igniscum Candy’). Additionally, a wild plant mix consisting of annual, biennial and perennial flowering species (mixture of 25 different plant species, Table 1) was included in the investigation. To assess the potential of these crops under different conditions, we tested them on three different sites in Rhineland-Palatinate, West-Germany. The first site was favorable, productive land suitable for intensive agricultural crop production, the second was elevated agricultural land with skeleton-rich soils, and the third was a sandy, dry site. To our knowledge, there are no studies available, which compare such a wide range of second-generation energy crops under different climatic and pedogenic conditions. The crops were produced with a high level of practical orientation. They had already exceeded the establishment stage and were grown on plots that were large enough to minimize edge effects. The objective of this study was to point out which species under which conditions is able to compete with maize in terms of methane yield. Additionally, we wanted to give farmers reliable information to select appropriate biogas feedstock in accordance with the local conditions.

Table 1 Composition of perennial wild plant mixture seeds (‘Biogas mehrjährig‘, Saaten Zeller, Eichenbühl, Germany) Species

Life period

Inula helenium Malva alcea Medicago sativa Onobrychis viciifolia

Perennial Perennial Perennial Perennial

Tanacetum vulgare Fagopyron esculentum Helianthus annus Malva mauritanica Malva verticillata Melilotus albus Melilotus officinalis Althaea officinalis Cichorium intybus Dipsacus sylvestris Daucus carota Foeniculum vulgare

Perennial Annual Annual Annual Annual Perennial Perennial Perennial Perennial Biennial Biennial Biennial - perennial

Echium vulgare Malva sylvestris Verbascum thapsus Silena alba Silena diocia Anthemis tinctoria Artemisia vulgaris Centaurea nigra

Biennial - perennial Biennial - perennial Biennial Biennial - perennial Biennial - perennial Biennial - perennial Perennial Perennial

2 Methods 2.1 Crop production The different 2G perennial bioenergy crops were grown within a multiannual trial by the Agricultural Service Center, Rhineland-Palatinate (Dienstleistungszentrum Ländlicher Raum, DLR) at three different locations in Middle West of Germany. Study site Altrich is located near Wittlich, 40 km NE of Trier, Klosterkumbd is located in the Hunsrück region near the city of Simmern, and study site Speyer is located in the Upper Rhine Plain near the city of Speyer. The three locations differed substantially concerning soil fertility and climate (Table 2). At every study site, four test plots with 7.5 m in length and 4.5 m in width (= 33.75 m2) were established for each of the six crops. All crops were planted/sown in spring 2011. Only reed canary grass was sown later in spring 2013 in order to replace the formerly planted switchgrass (Panicum virgatum) which did not establish adequately. Due to relinquishment of the experimental site in Speyer, only cup plant and virginia mallow could be used from this site. For the BMP test, representative samples (pooled samples from five randomized points per test plot) were taken shortly before harvest. In this process, the plant material was cut at 10 cm above ground. Sowing, planting, fertilization, and harvesting have been carried out by technicians of the DLR. After seedbed preparation, pesticides were not applied on the perennial crops. Fertilizing was performed in order to reach a level of 160 kg ha−1 nitrogen. Harvest was conducted once a year for cup plant, virginia mallow, and wild plant mix and twiceyearly for tall wheatgrass, reed canary grass, and giant knotweed. For species with more than one annual harvest, composite samples were commingled according to the particular yields. Sampling for this study was conducted in 2014 (Table 5). Sampled biomass was chopped (particle size around 3 cm) and subsequently ensiled in sealed plastic bags (Allpax, Germany) and stored under vacuum at room temperature until analyses were carried out. The fermentation gas produced during the ensiling process was drained by opening the bag and resealing it again under vacuum. This procedure had to be repeated ordinarily twice to reach a stable ensiled sample. Total solids (TS) and volatile solids (VS) of the silages were measured after 24 h drying in an oven at 105 °C, and after 6 h in a furnace at 550 °C, respectively. The TS values were not corrected for losses of volatile components. VS correction using standard values was not possible because, to our knowledge, there is no data available for the silages of the examined plant species. Investigations in that domain gave hints that the error is lower than 5% [23, 24]. Although, it is important to keep this source of uncertainty in mind when discussing the results and future research should focus on the TS overestimation of silages of the particular species.

Biomass Conv. Bioref. Table 2 Location

Overview of study sites Altitude [m amsl]

Mean annual temperature [°C]

Mean annual precipitation Soil type [mm]

Soil texture of plowhorizon

Klosterkumbd 365

7.8

664

Clay-rich loam (Lt3)

Altrich

170

8.9

710

Stagno-Cambisols derived from Devonian Slate Luvisols derived from Loess

Speyer

99

10.0

583

Fluvisol

Silty sand (Su2)

2.2 BMP measurements Biogas and biomethane productions were measured according to VDI Guideline 4630 [25]. To be able to compare the biogas production of the different energy crops under identical conditions, the test duration was set to 42 days for all samples. The BMP of each sample was tested in triplicates. Bottles of 2 L capacity (heavy- duty polypropylene bottles, Nalgene 2126-2000, Thermo Scientific) were used as anaerobic digesters, which were floatingly arranged in heated water baths at mesophilic temperature range (37 °C). Each digester lid was equipped with fittings (Nalgene 2162-0531, thermos Scientific) and connected through tubing (Tygon R-3603, Saint-Gobain) and a reflux condenser with a 10-L gas-bag (Tecobag, Tesseraux Spezialverpackungen GmbH). Digester lids and venting ports of the gas bags were assembled gas tight using bi-component DP405 adhesive glue (3M Scotch-Weld, USA). The digesters were filled with an ensiled energy crop sample (approx. 10gVS) and the inoculum (approx. 25gVS) at the beginning of the test. The precise amount of both was weighed when filling the digesters. The inoculum was collected from the mesophilic anaerobic digester from the municipal wastewater treatment plant in Schifflange (SIVEC, Luxembourg) and incubated for 5 days at 37 °C to reduce its endogenous gas production. Sewage sludge is fully suitable and recommended for anaerobic digestion batch tests in the laboratory as the species-rich biocoenosis was exposed to a wide variety of organic matters contained in the wastewater. Furthermore, its steady composition and properties were a good basis to reproduce batch tests under constant conditions [26] and especially to assess BMP values of various energy crops [27]. The measuring interval for the produced biogas was defined as 1 day during the first week and 7 days during the remaining time of the experiment. The biogas volume was measured with a wet drum-type gasmeter (TG05 wet-type, Ritter, Germany, accuracy: 2 ml) and normalized (273 K, 1013 hPa) depending on the temperature and pressure conditions during BMP test. The content of methane and carbon dioxide (expressed in volume percentage) was determined using specific infrared sensors (Dynament, UK). Additionally, triplicates of the inoculum alone and the inoculum fed with microcrystalline cellulose (Sigma-Aldrich) were measured

Silty loam (Lu)

simultaneously to the anaerobic digestion of the energy crop silages in order to determine the endogenous gas production of the inoculum alone and to check its activity. For every measurement, averages of the biomethane productions inherent to the inoculum were subtracted from the biomethane volumes produced by the energy crops silages digested within the inoculum. At the end of the digestion process, biomethane productions were cumulated. The specific BMP values were calculated with regard to the amount of wet matter added in the digesters (BMPWW), and then expressed per unit of volatile solids (BMPVS) using the VS content determined on subsample triplicates (Table 3). Then, the BMP per hectare was calculated from the specific BMP value and the corresponding biomass yield.

2.3 Fiber analysis The gravimetrically measured amounts of hemi-cellulose, cellulose, and lignin of the samples taken in Altrich were calculated from different fractions measured with a fiber analyzer (Fibretherm FT 12, Gerhardt, Germany) according to the manufacturer instructions. These fractions were NDF (neutral detergent fiber), ADF (acid detergent fiber), and ADL (acid detergent lignin). The NDF fraction represents hemi-cellulose, lignin and lignin-N-compounds. The ADF fraction coincides with cellulose, lignin, and lignin-N-components. After treatment with sulfuric acid (72%), only the raw lignin remains in the samples (ADL fraction). From these fractions, the different fiber concentrations were calculated as follows: 1. Hemi-cellulose = NDF – ADF 2. Cellulose = ADF – ADL 3. Lignin = ADL

2.4 Data processing and analysis Calculation of means, standard deviation, and coefficient of variation was carried out using Excel 2013 (Microsoft, 2013). Due to absence of normal distribution and homogeneity of variances, nonparametric Mann-Whitney U test and KruskalWallis H test (more than two independent variables) were used to compare means. If the Kruskal-Wallis H test detected

Biomass Conv. Bioref. Table 3 Conditions of the performed BMP test

Parameters

Value

Inoculum Origin

MWTP (Schifflange, Luxembourg), mesophilic anaerobic digester

Total solids Volatile solids

2.9% FM 53.5% TS

Activity Degassing period prior to assays

Checked with microcrystalline cellulose 5 days at 37 °C

Control substrate Type

Microcrystalline cellulose

Total solids Volatile solids

96.23% FM 96.23% FM 10 g FM and 6 g VS ∙ kg inoculum−1

Amount and concentration at start-up of the experiment BMP Substrate

387.8 ± 10.36 mL∙ gVS−1

Type

Silages of cup plant, virginia mallow, reed canary grass, tall wheatgrass, wild plant mix, giant knotweed Wet, chopped into 3 cm pieces 21.1–39.9% FM 94.1–85.1% TS

State Total solids Volatile solids Experimental conditions Measurement replicates Measurement system Type of gas analyzed Biogas composition

3 Volumetric. drum-type gas meter Biogas Methane and carbon dioxide by specific infrared sensors

Operational conditions Reactor capacity Temperature

Total volume: 2 L, working volume: 1.6 L Mesophilic (37 °C), thermostatic water bath

Stirring Duration Headspace gas

Manual, daily 42 days No flushing at start-up

pH/alkalinity adjustment Mineral medium Inoculum to substrate ratio (on VS basis)

No adjustment No mineral medium added 2.5: 1

Results are expressed as mean ± standard error MWTP municipal wastewater treatment plant, FM fresh matter

significant differences in means, the Mann-Whitney U test was used pairwise as post-hoc test to detect differences in sub-groups. Significant differences in sub-groups were marked with different letters. All tests were performed using SPSS Statistics 22 (IBM, 2013). A significance probability level of α = 0.05 was used for all statistical tests. For the analysis of the digestion kinetics, a 3-parameter logistic regression [27, 28] was used to fit a curve on the cumulated BMP using Sigma Plot 12.5 (Systat Software, 2011). As measure for the potential for residual gas production, the relative differences between the modeled BMP after

42 days and 100 days were calculated. Large differences indicate an incomplete digestion within the test duration. They can be a hint that the fermentation needs more time or that the digestibility is generally low. This can be a revealing information for operators of biogas plants to control the digestion process.

2.5 Ecosystem services and energy yield assessment To facilitate a comparable view on the tested species in terms of ecosystem services, we reviewed available literature. On

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this basis, we assessed the cultivation of these crops concerning their ability to store carbon, protect soils, protect water bodies and being a food source for pollinators. For the first category, we defined a net carbon storage as positive (+), a net carbon consumption as negative (−) and a neutral behavior as intermediate (o). For the remaining three categories, a rating could not be made on basis of numerical values. Therefore, we defined the presence of soil/water protecting properties and pollinator promotion as positive (+), harmful effects on soil/water and no food source for pollinators as negative (−) and neutral effects on soils/water bodies as intermediate (o). In the case of extraordinary positive or negative properties, we defined the ratings very positive (++) and very negative (−−). Additionally, we rated the energy yield of the tested crops at the three different test sites. We defined BMP per hectare values above 3500 m3N ∙ ha−1 as positive (+), values between 2000 and 3500 m3N ∙ ha−1 as intermediate (o) and values below 2000 m3N ∙ ha−1 as negative (−).

3 Results and discussion The most important parameter to assess energy crops in terms of economic aspects is the methane yield per hectare, which is the product of the biomass yield per hectare and the specific BMP. We compared these values between the different sites and species. Due to its high biomass yield and comprehensive biodegradability, maize (Zea mays) is the most important biogas feedstock and therefore widely used as reference value to assess other energy crops. Since it was not possible to use maize samples from the respective study sites, we calculated an average value based on published mean values of the specific BMP and the BMP per hectare of maize as reference to assess the performance of the investigated 2G energy crops. Therefore, it should be kept in mind that this reference value is not as proper as a comparison with samples from the same location. Nevertheless, the calculated reference value is based on data from fertile, productive locations and enables a conservative and reliable comparison.

3.1 General results The mean biomass yield of all species at all sites ranged between 0.56 and 15.26 tVS ∙ ha−1 (Table 5). When comparing them, two aspects were discernible: There were large differences between the different species on the particular study sites (Altrich, 10.67–15.45 tVS ∙ ha−1; Klosterkumbd, 4.19– 15.09 tVS ∙ ha−1; Speyer, 0.56–4.67 tVS ∙ ha−1) and between the same species on different study sites (e.g., cup plant, 4.67– 13.90 tVS ∙ ha−1; virginia mallow 0.56–10.67 tVS ∙ ha−1). This emphasized the strong influence of both, the species and the site conditions (soil fertility, climate) on the biomass quality.

The results represent clearly the different soil types at the three different sites. The deep Luvisols derived from Loess in Altrich with its high cation exchange capacity and porosity showed the highest biomass productivity. In contrast, the stony Stagno-Cambisol derived from Devonian Slate in Klosterkumbd (low mountain range) were less productive because of their shallowness. Additionally, the high clay content could lead to reductive conditions due to impeded draining and consequently inhibits rooting of plants, which were not morphologically adapted to anoxic conditions. The sandy Fluvisols in Speyer were the less fertile soils in the test. Even though Fluvisols can be rich in nutrients, the high sand content promoted drought which interfered the plant growth (Table 2). Additionally, the harvest date is a parameter, which can influence the biomass quantity and quality. While normally, lignification increases with later harvest and subsequently decreases the methane yield, the share of storage carbohydrates increases with growing time. Though, the effect of maturity on the digestibility of biomass vigorously depends on the plant species and should be considered when choosing a decent harvest time and the number of cuts per year [8]. The relatively early harvests at Speyer, due to achievement of the target TS concentration, might have reduced the biomass yield crucially. On both sites with all six investigated crops (Altrich and Klosterkumbd), reed canary grass and tall wheatgrass produced the largest amounts of biomass while virginia mallow and wild plant mix produced least. The content of total solids (in % of fresh matter) and volatile solids (in % of total solids) is presented in Fig. 1. The TS values ranged between 21.08% (Giant knotweed in Altrich) and 39.90% (virginia mallow in Speyer). Differences occurred between species as well as sites. Beside the fact that the TS values (except cup plant) were slightly higher in Klosterkumbd compared to Altrich, no systematic rules were discernible. In contrast, the differences between the volatile solids were marginal. The lowest content was 85.06% (cup plant in Speyer) and the largest 94.09% (wild plant mix in Altrich). The specific BMP at all sites and species was between 132.08 and 389.49 LN ∙ kgVS−1 (Table 6). The differences on the respective sites were less pronounced than for biomass yield (Altrich, 146.85–389.49 LN ∙ kgVS−1; Klosterkumbd, 132.08–336.31 LN ∙ kgVS−1; Speyer, 315.37–345.21 LN ∙ kgVS−1). The same effect could be observed for the particular species (cup plant, 288.31–345.21 LN ∙ kgVS−1; virginia mallow, 213.40–315.37 LN ∙ kgVS−1). The inter-species comparison on the study sites Altrich and Klosterkumbd pointed out that reed canary grass and tall wheatgrass exhibited significantly the highest, while giant knotweed showed the significantly lowermost specific BMP (Table 6). Compared to typical values for maize (333 LN ∙ kgVS−1, Table 4), the specific BMP of most of the 2G-energy crops was beneath or in the lower range. Only reed canary grass and tall wheatgrass in

Biomass Conv. Bioref. Fig. 1 Mean shares (plot, n = 4, measured in triplicates) of (a) total solids in percent of fresh matter and (b) volatile solids in percent of total solids of the six investigated plant species at the three study sites. Error bars represent standard error

Altrich, tall wheatgrass in Klosterkumbd and cup plant in Speyer reached values above 333 LN ∙ kgVS−1 (Table 6). When multiplying both values to get the BMP per hectare, large differences became apparent (Altrich, 1775–5877 m3N ∙ ha−1, Klosterkumbd, 894–5075 m3N ∙ ha−1, Speyer, 177–1612 m3N ∙ ha−1, Table 6). The wide variation in terms of biomass yield within the same species seemed to affect the BMP per hectare stronger than the specific BMP (cup plant, 1612–4022 m3N ∙ ha−1; virginia mallow, 177–3350 m3N ∙ ha−1). Hence, the area related methane yield varied about multiple magnitudes within the same species. This highlights the need to choose the judicious energy crops under adequate conditions concerning soil fertility and climate to reach satisfactory biomass yields and subsequent methane yields per hectare. When ranking the area related methane production statistically, reed canary grass and tall wheatgrass showed the significantly highest BMP per hectare in Altrich and tall wheatgrass reached the significantly highest BMP in Klosterkumbd (Table 6). In each case they gained values above 5000 m3N ∙ ha−1, which was in the range (96.66%, 103.62%, and 89.47% respectively, Table 6) of the mean reported BMP per hectare of maize (Table 4). Comparing the three sites statistically (only valid for cup plant and virginia mallow) concerning their specific BMP, the differences were significant with the highest values in Speyer followed by Altrich and Klosterkumbd (Table 6). However, including all six species in the comparison between Klosterkumbd and Altrich the differences between the sites were not statistically significant. Concerning the BMP per hectare, Altrich showed significantly higher methane yields Table 4 Composition of the maize reference value

than Klosterkumbd (for the comparison of all six species and the comparison of cup plant and virginia mallow). The productivity in Altrich was also significantly higher than in Speyer but the differences between Klosterkumbd and Speyer were not statistically relevant. Nevertheless, other aspects should be taken into account when rating biogas feedstock, especially perennial crops. Compared to annual plants the input costs for perennial crops decline with increasing cultivation period. This is based on the fact that sowing or planting must be conducted only once for the hole lifetime. Additionally, no plant protection is required except in some cases during establishment period when the juvenile plants have less competitive power. Although most PECs have low requirements for nutrients, fertilizing is mandatory to maintain high biomass yields over long periods. This can be conducted at least partially with the residues of the fermentation process. Beside the fewer demands of PECs, some positive effects on the environment should be regarded. The most important is the possibility of carbon sequestration in soils, prevention of soil erosion due to year-round coverage of the soil surface and ecosystem services like providing habitat and source of nutrition for diverse pollinators and many other invertebrates [34]. Table 7 shows a summary of reported ecosystem services provided by the tested energy crops. Beside the soil fertility on the different sites, also the local climatic conditions are very important, because they affect the quantity and quality of the produced biomass strongly. At all three tested sites, the spring was very dry. Cumulated from March to June the water supply was beneath the long-term

Reference

specific BMP [LN ∙ kgVS−1]

BMP per hectare [m3N ∙ ha−1]

Mayer et al. 2014 [27] Döhler 2013 [29] Amon et al. 2007 [30] Grieder et al. 2012a [31] Grieder et al. 2012b [32] Kaiser & Gronauer 2007 [33] Average

416 – 308 306 298 338 333

6934 4945 7475 5555 3450 – 5672

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average. In contrast, June and August were extraordinary wet. Additionally, the climate at the three sites in 2014 was extra ordinary warm compared to the long-term average (Fig. 2). These conditions may have led to poor biomass production in spring, which might be compensated slightly by the wet summer. Although, this compensation could only work if the soils were able to store enough water to enable the plants to survive dry periods. While these exceptional conditions can be seen as a drawback of this study, we opine that exactly that underlines the potential of the mentioned species to grow even under unfavorable circumstances. Hence, the productivity of the Fig. 2 Mean monthly temperatures in °C and precipitation in mm at (a) Klosterkumbd, (b) Altrich, and (c) Speyer in 2014. Long-term average temperatures and precipitation are presented [47]

PECs tested in this study might be underestimated in contrast to long-term investigations, but this fact shows that the executed comparison was conservative and reliable. 3.1.1 Digestion kinetics and potential of residual gas production From the differences between the cumulated methane production after 42 days and the extrapolated cumulated BMP after 100 days (curve-fitting according to a 3-parameters sigmoidal model), we derived information concerning the digestibility of

Biomass Conv. Bioref.

the different plant samples [28]. A large difference can be an evident hint, that the digestion process was not completed within the BMP test period. During our experiments, different methane production kinetics were observed between the tested species and between the same species at different sites. Figure 3 shows exemplary the digestion kinetics of reed canary grass in Klosterkumbd and Altrich. It can be seen, that after the digestion time of 42 days, the slope of the curve is still above zero. This implies persuasively, that the fermentation is still going on. This is an important information for the digestion process of this crop in regular biogas plants to be able to adjust feeding rates and frequencies to the properties of the feedstock. An adequate hydraulic retention time enables a sufficient degradation of plant material and maximizes the methane yield. Furthermore, it minimizes the methane emissions into the atmosphere from fermentation residues stored in open storage tanks or after field application [48]. This is fundamental because methane can function as a potent greenhouse gas. The composition of the feedstock is the most important factor influencing the biodegradability. Therefore, the tested plant species differ concerning their digestion time and residual gas production. The comparison of the modeled cumulated methane production after 42 and 100 days showed that cup plant has the lowest residual gas production with differences below 1 %. Virginia mallow and wild plant mix showed slightly higher values below 5 %, while Giant knotweed, reed canary grass and tall wheatgrass reached relevant differences between 5.57 and 13.70% (Table 8). There appeared not only differences between the particular species but also between the different study sites. Altogether, there was no systematic hierarchy that biomass from one location entails higher or lower residual gas production. Additionally, we tested the fiber contents of the plant material exemplary for the site Altrich. The highest content of lignin was found in Giant knotweed with 18.2% (Table 9) which has also the

Fig. 3 Measured and modeled (3-parameter logistic regression) specific BMP exemplary of reed canary grass in Klosterkumbd and Altrich

highest residual gas production (Table 8) and lowest specific BMP in Altrich (Table 6). Nevertheless, the lignin content alone is not a sufficient measure to predict the residual gas production or the BMP. Concerning the extent of the biodegradation in our experiments, we conclude, that for most samples the duration of the BMP test was sufficient. For the other samples, the specific BMP might be slightly underestimated.

3.2 Species related results In this chapter, we discuss the results of the conducted experiments separately for each species and compare them with data provided by available scientific literature. Additionally, we collated further information concerning C-to-N-ratio, fiber contents, ensilability, fertilizer requirements, ecosystem services, and environmental effects to enable a more comprehensive view on the suitability as biogas feedstock. Due to large differences in terms of available articles, the scope of information differs between the tested crops. If there is no information concerning the mentioned categories given in this chapter, appropriate scientific data was to our knowledge not available. 3.2.1 Cup plant The biomass yield of cup plant in Altrich was 13.90 tVS ∙ ha−1. This is in the range reported by Šiaudinis et al. 2015 (ca. 6–12 tVS ∙ ha−1) and Gansberger et al. 2015 (up to 15 tVS ∙ ha−1). The productivity on the other two sites was quite lower with values of 6.24 tVS ∙ ha−1 in Klosterkumbd and 4.67 tVS ∙ ha−1 in Speyer. The contents of total solids and volatile solids ranged only slightly between the three sites (Fig. 1). The enormous difference in terms of biomass yield might have been caused by the lower soil fertility and the different harvest dates (Klosterkumbd 26.06., Altrich 16.09., Speyer 14.05., Table 5). Another reason could have been the challenging climatic conditions at the less productive locations. Especially in May, the water supply was insufficient due to sparse precipitation in Klosterkumbd and Speyer (Fig. 1a), c)). Although cup plant is considered to be drought tolerant due to its extensive root system and dew water collection by leaf cups (minimum water demand 400–500 mm ∙ y−1), for competitive biomass yields more water is needed [15]. However, especially when water availability is the limiting factor for growth, maize is a more favorable crop due to its better water use efficiency (cup plant 33 kg ∙ ha−1 ∙ mm−1, maize 50 kg ∙ ha−1 ∙ mm−1, [36]. For sufficient biomass production, cup plant prefers humic and fertile soils with adequate nitrogen content and optimal soil reaction [49, 50]. The highest specific BMP of cup plant was detected in Speyer (345.21 LN ∙ kgVS−1). On both other sites, it was nearly identical with values of 289.37 and 288.31 LN ∙ kgVS−1 respectively. These values were higher than those reported by

Biomass Conv. Bioref. Table 5

Cumulated biomass yields [tVS ∙ ha−1] and harvest dates (in parentheses) at the three locations

Study site

Cup Plant

Virginia mallow

Reed canary grass

Klosterkumbd

6.24 (26.06.)

4.19 (26.06.)

12.44 (26.06./14.10.)

15.09 (26.06./14.10.)

4.63 (26.06.)

8.55 (26.06./14.10.)

Altrich

13.90 (16.09.)

10.67 (31.07.)

15.45 (26.05./10.09.)

15.26 (26.05./10.09.)

12.31 (31.07.)

12.09 (26.06./10.09.)

Speyer

4.67 (14.05.)

0.56 (14.05.)









Haag et al., 2015, and Herrmann et al., 2016, with 251 and 237 LN ∙ kgVS−1 [37, 51]. This shows that under more adverse conditions like in Speyer, the lower quantity of biomass was accompanied with slightly better quality in terms of specific BMP. Concerning the BMP per hectare on the different sites, cup plant in Altrich showed by far the highest productivity with 4022 m3N ∙ ha−1. In Klosterkumbd and Speyer, the methane yields per hectare were lower with 1799 m3N ∙ ha−1 and 1612 m3N ∙ ha−1 respectively due to the lower biomass production. Haag et al., 2015, and Mast et al., 2014, reported BMP per hectare values of 2578–3161 m3N ∙ ha−1 and 3318–4301 m3N ∙ ha−1 [9, 37]. With regard to their results, the fertile site in Altrich produced comparatively high methane yields, while the productivity in Klosterkumbd and Speyer was very low. Beside the economic consideration, some environmental aspects should be taken into account. Additionally to the general advantages of PECs, cup plant has an early onset of growth and therefore protects soils from eroding agents better than later growing plants [36]. Furthermore, during the longlasting flowering period from July to September, it functions as nutrition source for many pollinating insects, especially bees [14]. The long-term cultivation of cup plant is also increasing the abundance of earthworms [20, 21]. In terms of anaerobic degradability, a carbon-to-nitrogen ratio below 40 is recommended, at best at 20 [52, 53]. Depending on the maturity stage, ratios of 19–30 were reported and confirm the suitability of cup plant for biogas production [54, 55]. Additionally, the methane production kinetics gave no hint of negative impacts on the process stability, which coincides with the results of Haag et al. [37]. The residual gas production after the fermentation time of 42 days was beneath 1 %, which is an indication for a sufficient, rapid microbial degradation. For the suitability as biogas feedstock, the possibility to preserve the biomass through ensiling is an important aspect. We observed no difficulties during the ensiling process, which might be based on the high amount of water-soluble carbohydrates in cup plant, that enables the formation of lactic acid to stabilize the silage and prevent rotting of the material [15]. Summed up, we can state that cup plant is a potential alternative to maize on humic and fertile soils. Although it reaches maximally about 71% of the mean methane yield of maize

Tall wheatgrass

Wild plant mix

Giant knotweed

(Table 6), its economic and ecologic advantages may compensate this lower productivity (Table 7).

3.2.2 Virginia mallow The biomass yield of virginia mallow was 13.90 tVS ∙ ha−1 in Altrich, 4.19 tVS ∙ ha−1 in Klosterkumbd and 0.56 tVS ∙ ha−1 in Speyer. Beside the differences in terms of soil fertility and climatic conditions, the results reflect the different harvest dates at the three sites (Altrich, 31.07; Klosterkumbd, 26.06.; 14.05.). While at first glance the variation of the reaping date may seem to impair the comparison, it should be kept in mind that due to the practical orientation of the test, the decision to harvest was made based on the target TS concentration. Although, distinct contrasts were also reported in literature. Šiaudinis et al. 2015 measured 4.4–7.0 tVS ∙ ha−1, Borkowska and Molas 2013, 12 tVS ∙ ha−1, Oleszek et al. 2013, 7.2–12.6 tVS ∙ ha−1 (one swath) or rather 13.5–18 tVS ∙ ha−1 (two swaths) and Jablonowski et al. 2017, up to 25 tVS ∙ ha−1 [38, 40, 41, 56]. This wide range indicates differences in temperatures as well as water and nutrient supply and soil reaction [50]. In our experiments, the yields were in total accordance to the weather conditions and soil fertility levels of the three experimental sites. Due to a lower water-useefficiency of virginia mallow, the biomass productivity in general is considered to be smaller than for cup plant [55]. The specific BMP of virginia mallow was 313.96 and 315.37 LN ∙ kgVS−1 in Altrich and Speyer respectively. In Klosterkumbd, the value was considerably lower with 213.40 LN ∙ kgVS−1. Comparable to cup plant, the results showed that the decline of biomass yield was not correlated with the specific BMP. Furthermore, under less favorable conditions, the drop of biomass productivity was partially compensated by better specific methane yields. Concerning the BMP per area, virginia mallow reached values of 3350 m3N ∙ ha−1 in Altrich, 894 m3N ∙ ha−1 in Klosterkumbd, and 177 m3N ∙ ha−1 in Speyer. While the yield on fertile soils and under preferable climatic conditions like in Altrich was about 60% of maize (Table 6), the very low results in Klosterkumbd and especially Speyer showed that virginia mallow might not be suitable for such sites due to paltry biomass production.

Biomass Conv. Bioref. Table 6 Specific BMP, BMP per hectare, and BMP relative to maize (Table 4) of all tested energy crops at all study sites. Values are means of four field replicates and three repeated measurements ± SE. Nonparametric Kruskal-Wallis H test (p < 0.05, n = 4) was used to compare BMP and BMP per hectare of the different crops at each location and to Energy crop

Altrich

compare BMP and BMP per hectare between the three locations. Pairwise Mann-Whitney U test (p < 0.05, n = 4) was applied as post-hoc test for the comparison between the three locations. Different letters mark significant differences between crops at each site BMP per hectare [m3N ∙ ha−1]

Spec BMP [LN ∙ kgVS−1]

BMP per hectare relative to maize (= 5672 m3N ∙ ha−1) [%]

Cup Plant

289.37

±

24.54

c

4022

±

341.10

c

70.91

Virginia mallow Reed canary grass

313.96 354.86

± ±

19.08 18.78

c,d d

3350 5483

± ±

203.57 290.18

c d

59.06 96.66

Tall Wheatgrass

389.49

±

25.26

d

5877

±

381.18

d

103.62

Wild plant mix Giant Knotweed

218.22 146.85

± ±

5.36 9.55

b a

2686 1775

± ±

65.95 115.42

b a

47.36 31.30

H-test crop (n = 4) Cup Plant

0.001 271.92

±

19.82

c

0.001 1696

±

123.69

c

31.72

Virginia mallow

213.40

±

27.91

b

894

±

116.95

a

15.76

Reed canary grass Tall Wheatgrass Wild plant mix

314.92 336.31 208.44

± ± ±

5.03 20.49 8.49

c c b

3918 5075 965

± ± ±

62.59 309.14 39.30

d e a

69.07 89.47 17.02

Giant Knotweed

132.08

±

4.59

a

1129

±

39.20

b

19.91

H-test crop (n = 4) Cup Plant

0.001 345.21

±

14.85

a

0.001 1612

±

69.37

b

28.42

Virginia mallow H-test crop (n = 4)

315.37 ± 0.430 Spec. BMP

15.72

a

±

8.80

a

3.11

0.001

Sp > Alt > Klo

0.000

Alt>Klo,Sp

Altrich *Altrich

H-test location (n = 8) Pairwise U-test (n = 8) Klosterkumbd Klosterkumbd

0.080 0.028

Alt > Klo Alt > Klo

0.001 0.000

Alt > Klo Alt > Klo

*Altrich *Klosterkumbd

Speyer Speyer

0.028 0.000

Sp > Alt Sp > Klo

0.000 0.105

Alt > Sp Klo = Sp

Klosterkumbd

Speyer

177 0.021

BMP per hectare

*Only cup plant and virginia mallow were compared

In terms of anaerobic digestibility, the harvest date and subsequently the maturity is an important factor. During our experiments in Altrich, we determined fiber contents of 40.1% cellulose and 7.8% lignin (Table 9). In contrast, [57] reported mean crude fiber (mainly cellulose and lignin) contents of only 17.7–22.3%TS. This difference might be based on a premature ripening of the biomass due to drought stress during spring. Although this content can be diminished when harvesting twice a year to reduce the lignification of the stems, repeated mowing lowers the productivity in the following years and shortens the life of plantations [38]. The C-to-N ratio of virginia mallow is typically between 20 and 30, and therefore well suited for biogas production [38, 53, 55]. Additionally, 40 days lasting batch tests have shown that more than 75% of the total methane was produced within the first 14 days. Therefore, it can be concluded that virginia mallow is suitable for the digestion in biogas plants even if the hydraulic retention time is relatively low [38].

One of the advantages of biogas production is the recirculation of nutrients by use of biogas manure as fetilizer. The investigations of Barbosa and Molas 2014, Nabel et al. 2014, and Nabel et al. 2016 showed that virginia mallow responds positively to the application of digestate comparable to mineral fertilizer. The optimal dose was considered as 40 kg N ∙ ha−1. Furthermore, incompletely digested parts like fibers built up a pool of soil organic matter and the slow release of nitrogen serves as long-term fertilizer and limits the risk of leaching [16, 39, 42]. Additionally, recalcitrant components of the plant material might play an important role in carbon sequestration. In total, we conclude that virgina mallow is only suitable for biogas production on fertile soils at sufficient moisture level. Based on our results, we would prefer cup plant on such sites due to its better pollinator promoting properties and higher productivity. Nevertheless, virginia mallow could play a role as a minor component in biogas production when seeking for more agro-biodiversity.

Biomass Conv. Bioref. Table 7

Review of selected properties of the tested crops concerning ecosystem services and energy yield (+ = positive, − = negative, o = intermediate)

Plant

Energy yield†

Ecosystem services* Carbon storage

Soil protection

Water protection

Food source for pollinators

Clayey soil (Altrich)

Loamy soil (Kloster-kumbd)

Sandy soil (Speyer)

Maize









+

+

+

Cup plant

+

+

+

++

+





Virginia mallow

+

+

+

+

o





Reed canary grass Tall wheatgrass

+ +

+ +

+ +

– –

+ +

+ +

– –

Wild plant mix Giant knotweed

+ +

+ +

+ +

+ –

o –

– –

– –

*Based on selected references [8, 9, 14–21, 34–46] †Based on the present investigation (except maize)

3.2.3 Reed canary grass The biomass yield of reed canary grass was 15.45 tVS ∙ ha−1 in Altrich and 12.44 tVS ∙ ha−1 in Klosterkumbd, which was in full accordance to the soil fertility level and the climatic conditions. This was approximately twice the amount measured in other experiments from Shinners et al. [58] and Sanderson and Adler [43] with values of 7.06 tVS ∙ ha−1 and 7.7–10.2 tVS ∙ ha−1 respectively. While in our experiments nitrogen levels of 160 kg N ∙ ha−1 were aimed, other authors tested the performance of reed canary grass with lower nitrogen levels. They showed that the biomass yield increases linearly up to 80 kg N ∙ ha−1. At this dose, an optimum was reached. Higher levels of nitrogen fertilizer led to lower biomass yields, which may have reduced the yields during our experiments. These results confirmed the very low input requirements in terms of nitrogen of reed canary grass due to efficient internal nitrogen recycling from shoots to rhizomes [43, 58–60]. The specific BMP of reed canary grass in Altrich and Klosterkumbd was relatively high with 354.86 and 314.92 LN ∙ kgVS−1 respectively. The differences between these results can be explained by the earlier harvest in Altrich at lower TS contents (Klosterkumbd, 26.06./14.10., 31.19% TS; Altrich 26.05./10.09., 24.92% TS). These values were larger than test results reported by Triolo et al. 2012, (110–280 LN ∙ kgVS−1), but slightly smaller than Oleszek 2014, (406 LN ∙ kgVS−1) measured during tests with cultivated varieties of reed canary grass [61, 62].

Table 8 Relative difference of the modeled (3-parameter logistic regression) specific BMP between day 42 and 100 as an estimate for the residual gas production

The measured BMP per hectare was 5483 m3N ∙ ha−1 in Altrich and 3918 m3N ∙ ha−1 in Klosterkumbd. These values were the second-best on both sites and in the range or rather larger than during comparable investigations in Finland and (3800–4200 m3N ∙ ha−1) [8]. The results of the residual gas potential showed that after a longer fermentation time of 100 days, higher methane yields were expectable (Altrich + 13.70%, Klosterkumbd + 9.04%; Fig. 3, Table 8). A typical C-to-N ratio of reed canary grass is 28, and therefore in a suitable range for anaerobic digestion [8]. Although, Triolo et al. [61] stated that phytomass with lignin concentrations above 100 g ∙ kgVS−1 leads to low BMP levels. In our experiments, the lignin content of reed canary grass in Altrich was 8.8% and therefore beneath this margin (Table 9). In contrast to Triolo et al. [61], Lehtomäki et al. [8] stated that even silages of reed canary grass with lignin contents of about 19 %VS can be an auspicious biogas plant feedstock. In conclusion, we strongly recommend the use of reed canary grass as biogas feedstock. Compared to all other investigated species, except tall wheatgrass, it reaches methane yields on fertile soils comparable to maize. Considering the fact that reed canary grass was planted 2 years later than the other crops and may not have passed the establishment stage, it might be possible that even higher biomass and methane yields can be reached in the following years. Against the background of exceptional low precipitation rates in spring, this result is highly remarkable. Even at the site Klosterkumbd with worse soil conditions and less rain, it reached nearly 70%

Study site

Cup plant

Virginia mallow

Wild plant mix

Giant Knotweed

Reed canary grass

Tall Wheatgrass

Klosterkumbd Altrich Speyer

0.66% 0.70% 0.34%

3.83% 1.40% 0.53%

3.16% 4.41%

9.41% 12.63%

13.70% 9.04%

13.33% 5.57%

Biomass Conv. Bioref. Table 9 Contents (% TS) of hemi-cellulose, cellulose, and lignin of the tested plant species in Altrich Plant species

Hemi-cellulose [%]

Cellulose [%]

Lignin [%]

Cup plant

11.5

30.4

8.2

Virginia mallow

9.1

40.1

7.8

Wild plant mix Giant knotweed

17.9 3.1

31.2 32.8

10.6 18.2

Reed canary grass

11.7

32.6

8.8

Tall wheatgrass

11.6

41.8

10.1

compared to one-cut management, because methane production decreases with increasing lignin content [5, 35]. On basis of these results, we recommend tall wheatgrass seriously as biogas feedstock. In terms of methane per hectare, it was comparably productive like maize on both sites (Klosterkumbd: 89.47%, Altrich 103.62%). It seems that tall wheatgrass is not only productive like maize and easier to grow, but has also many advantages concerning ecologic aspects. 3.2.5 Wild plant mix

of the methane productivity of maize. Considering the advantages of PECs in an overall view, reed canary grass can even in this case reach superiority compared to maize concerning economic performance.

3.2.4 Tall wheatgrass Cumulative biomass yields of two cuts of tall wheatgrass were 15.26 t ∙ ha−1 in Altrich and 15.09 t ∙ ha−1 in Klosterkumbd. These values were in full accordance to the results of experiments in Hungary, where 9–23 tVS ∙ ha−1 were attained under different fertilizer regimes [63]. Also Dickeduisberg et al. [35] measured 18.4 tVS ∙ ha−1 in Germany under two-cut management. The little difference between the both sites might be a valuable hint, that the soil fertility and climatic conditions in Klosterkumbd were sufficient to gain satisfactory yields of this crop. Tall wheatgrass reached on both sites the highest specific BMP with values of 389.49 and 336.31 LN ∙ kgVS−1 in Altrich and Klosterkumbd respectively. The slightly better biodegradability in Altrich might be based on the lower content of total solids (Klosterkumbd: 30.41%, Altrich: 24.33%, Fig. 1) and the more favorable soil and climatic conditions. This is much more than reported by Hermann et al. [5] with 258 LN ∙ kgVS−1 and only slightly lower than measured by Lalak et al. [64] with values up to 400 LN ∙ kgVS−1, which were attained after pretreatment with funghi. The measured BMP per hectare of tall wheatgrass was 5877 m 3 N ∙ ha −1 in Altrich and 5075 m 3 N ∙ ha −1 in Klosterkumbd and thus, higher than or rather almost comparable to the reference value for maize. Furthermore, this species showed the highest methane productivity at each site. Additionally, according to the determination of the residual gas potential, 5.57 and 13.33% higher methane yields per hectare can be anticipated in Altrich and Klosterkumbd respectively. The results were comparable to other experiments in Germany, where 5705 m3N ∙ ha−1 were reached in a two-cut regime. Hereby, the highest productivity was achieved on the one hand by high biomass yields and on the other hand by the relatively high specific BMP due to lower lignification

The biomass yield of wild plant mix was 12.31 tVS ∙ ha−1 and 4.63 tVS ∙ ha−1 in Altrich and Klosterkumbd respectively. A distinct variation of yields between 2.9 and 22.5 tVS ∙ ha−1 and strong fluctuations over years were also reported by Cossel and Lewandowski [45]. However, due to the heterogeneity of species in wild plant mix and their composition as consequence of site conditions and age of the plantation, the comparability to other investigations is limited. The measured specific BMP was 218.22 LN ∙ kgVS−1 in Altrich and 208.44 LN ∙ kgVS−1 in Klosterkumbd. These values were similar to the 221.6 LN ∙ kgVS−1 reported by Hermann et al. [51]. The BMP per hectare was the second lowest in Altrich with 2686 m3N ∙ ha−1 and the lowest in Klosterkumbd with 965 m3N ∙ ha−1. Concerning the specific and area related methane production, we cannot commend wild plant mix as biogas feedstock. Nevertheless, wild plant mix has numerous ecological benefits compared to the cultivation of AEC’s and the biomass is well suitable for ensiling. Moreover, the mixture of diverse plant species within one field can enhance the biodiversity on arable land and promote organisms like pollinating insects. All these effects heavily depend on the seed composition and the succession of the different species with time. Hence, the properties of biomass from wild plant mixtures can be very divers and is therefore hard to compare with other biogas feedstock. 3.2.6 Giant knotweed The biomass yield of Giant knotweed was 12.09 tVS ∙ ha−1 in Altrich and 8.55 tVS ∙ ha−1 in Klosterkumbd, which reflects the higher soil fertility level in Altrich. The result in Klosterkumbd was similar to another study, which reported yields of 8.6 tVS ∙ ha−1 [65]. The specific BMP was on each site the lowest with 146.85 LN ∙ kgVS−1 and 132.08 LN ∙ kgVS−1 respectively. These results were much lower than reported by Meerbeek et al., 2015, Hermann et al., 2016b, and Lehtomäki, 2008, with 279, 269 and 170 LN ∙ kgVS−1 respectively [8, 51, 65]. The BMP per hectare was relatively low with 1175 m3N ∙ −1 ha and 1129 m3N ∙ ha−1. This is insofar remarkable, because

Biomass Conv. Bioref.

giant knotweed has a better transpiration coefficient than maize and should have been less affected by the drought conditions during the experiments [44, 46]. Even under less productive climatic conditions in Finland, more than three times higher results of 3800 m3N ∙ ha−1 were achieved [8]. The comparable low methane yields may be explained by the fact that the wild form of Fallopia sachalinensis prefers moist riparian habitats and partially by the relatively high residual gas production, which shows that after a longer fermentation time of 100 days 12.63% (Altrich) and 9.41% (Klosterkumbd) higher methane yields can be expectable (Table 8). On basis or our results, we dissuade the use of giant knotweed as biogas feedstock. Additionally to the poor performance during the BMP tests, the invasivity of this species seems to be a potential problem. The wild form of Fallopia sachalinensis is known to propagate by seeds and vegetatively [46]. Although the tested variety ‘Igniscum Candy’ is considered to be less invasive, we observed the generative growth of rhizomes in neighboured fields during our experiments. Matthews et al. [46] observed the same effect in the Netherlands also. In contrast, a propagation of germinable seeds or stem fragments with fermentation residues constitutes no danger due to the destructive forces of the biocoenosis in biogas plants [65].

4 Conclusions Within the tested PECs, tall wheatgrass in Klosterkumbd and Altrich and reed canary grass (although planted 2 years later) in Altrich exceeded the reference methane yield of maize. Therefore, we can strongly recommend both species alternatively to maize as biogas feedstock on basis of our investigations. Although, plant species, which did not reach the methane production of maize, might be also suitable for biomethanation from an overall perspective. Hereby, considerable economic advantages in terms of lower input of workload, energy, fertilizer and plant protection might be able to compensate the lower productivity of energy crops. Hence, further investigations should focus on a cost-benefit analysis to determine these effects. Additionally, the positive environmental external effects like protection against erosion, less use of fertilizers and biocides, carbon sequestration, increased agro-diversity, prevention of water pollution as well as habitat and food source for many organisms should be internalized during the assessment of energy crops. In this context, the task of incentives should not be only a quantitative promotion of bioenergy, but the support of environmentally and economically appropriate energy crops. In this domain, the EU introduced explicit measures to remunerate the provision of ecosystem services by farmers, the so-called greening payment. However, within the tested crops only cup plant is currently part of this program. Hence, we suggest to include the other tested species—except giant knotweed—as well. Against the background that the tested species

were considered to be low-input culture, the profuse nitrogen fertilization is a drawback of this study. Therefore, it was not possible to determine the performance of the tested PECs to grow under poor provision of nitrogen, which is an important growing factor. Although, when comparing them with the commonly used maize, a sufficient nitrogen supply is reasonable to have similar test conditions. Nevertheless, low-input bioenergy crops should be cultivated on abandoned fields, which are less appropriate for food production and therefore diminish the competition between energy and food production. Newly introduced energy crops have not a long-time breeding history like maize, which went through thousands of years of cultivation and breeding selection. Therefore, there might be a high potential in plant breeding for all investigated species. This includes the increase of biomass yield, specific BMP as well as BMP per area and the reduction of negative effects like, e.g., a possible invasivity or the susceptibility to pests. There is little knowledge concerning phytosanitary properties of newly introduced 2G energy crops. Hence, future investigations should focus on this topic to prevent harvest losses of themselves and other agricultural crops. In accordance with other studies, the biomass of all six investigated energy crops showed no problems during ensiling. Nevertheless, potential losses during this process should be investigated in further experiments to take them into account. When enhancing the agro-biodiversity through introduction of new biogas feedstock, the impact of composites of them on the fermentation process stability requires further research to avoid inhibitory effects or to aim possible synergetic effects. Acknowledgements The authors would like to thank Sebastian Thielen, Otto Lang, and Dr. Martin Armbruster for the opportunity to take samples from the field trials of the DLR. We would also like to express gratitude to Anaïs Noo and Bénédicte De Vos for their valuable support conducting the BMP measurements. Funding information This work has been financially supported by the Ministry of Education, Science, Youth & Culture Rhineland-Palatinate, Germany, within the Research Initiative: Trier Centre of Sustainable Studies (TriCSS), 04/2013 – 12/2016, University of Trier. Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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