Site-adapted production of bioenergy feedstocks on poorly drained cropland through the cultivation of perennial crops. A feasibility study on biomass yield and biochemical methane potential

Biomass and Bioenergy 119 (2018) 429–435

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Short communication

Site-adapted production of bioenergy feedstocks on poorly drained cropland through the cultivation of perennial crops. A feasibility study on biomass yield and biochemical methane potential

T

Th. Ruf∗, C. Emmerling University of Trier, Faculty of Regional and Environmental Sciences, Dept. of Soil Science, Campus II, D - 54286, Trier, Germany

ARTICLE INFO

ABSTRACT

Keywords: Maize Perennial energy crops Planosols Pot experiment Stagnosols Waterlogging

Soil compaction depicts a major threat to soil fertility in extensive maize cultivation systems on soils that show conditions of waterlogging in autumn. Perennial energy crops may reduce the vulnerability for soil compaction by earlier harvest dates and improved soil stability. However, the performance of such crops to be grown on Stagnosols and Planosols is currently an open issue. Within the framework of a two-year experiment, we investigated the potential of five perennial energy crops and maize to be cultivated under periodically waterlogged soil conditions and effects on biomass and biomethane yield. Dry matter yields of perennial energy crops were 50% – 100% higher compared to maize. Contrarily, maize showed a weak yield performance under pronounced waterlogged conditions. Biochemical methane potentials were approx. 310 LN CH4 kg−1 VS for maize and on average 275 LN CH4 kg−1 VS for perennial energy crops, whereby different soil moisture regimes had no significant influence. Thus, our results reveal first indication for a sustainable biomass production on soils with periodically waterlogged conditions through the cultivation of perennial energy crops.

1. Introduction The cultivation of bioenergy crops on arable land has increased during the last decades with maize as most economic crop for biomethanation under temperate conditions [1,2]. In the context of increasing maize cultivation, the apprehension of a creeping deterioration of soil fertility, resulting from a depletion of soil organic matter, soil compaction and erosion has initiated legislative regulations that are focussing on a diversification of cultivated crops (e.g. EU Regulation No. 1307/2013). In Europe, soil compaction under maize cultivation potentially results from an unfavorable combination of pedogenic factors, prevailing weather conditions and crop-specific management efforts [3]. Stagnosols, Planosols, and related soil types that show periodic water stagnation during the winter half years are particularly sensitive for soil compaction by heavy machinery during harvesting of maize. From the perspective of soil protection, abundant autumn precipitation prior to the harvest period and the severely limited drainage of these soil types, resulting in high soil moisture contents and thus low carrying capacity, make them to least favorable sites for maize cultivation. This also means that these sites show great potential to establish a more sustainable biomass production. It is hypothesized that perennial energy

∗

crops (PECs) would have the capability to distinctly reduce vulnerability for soil compaction by combining earlier harvest dates in late summer with typically lower soil moisture contents and the stabilization of the soil by the permanent rooting system [4]. Indeed, the suitability of several PECs for anaerobic digestion were tested in a number of studies [5–11]. Although none of the tested crops could outperform maize with respect to methane yields per area, these studies revealed that some of the alternative crops do not lag far behind, do not require special preparations or technologies for digestion and that several plants provide specific ecosystem services [12,13]. To our knowledge, no study dealt with the general suitability and performance of PECs to be cultivated on soil that show periodically soil water stagnation, except for Reed Canary Grass [14,15]. In every respect, outliving of periods of waterlogging with associated altered soil chemical properties depicts a challenge for plants due to inadequate gas exchanges in the soil-atmosphere continuum [16–19]. Thus, plants’ energy metabolism may be impaired by oxygen starvation resulting in an accumulation of plant-toxic substances in cytoplasm. Nonetheless, numerous plant species can tolerate these unfavorable environmental conditions due to morphological and biochemical adaptation mechanisms [17,20], such as the development of aerenchym tissue and modified energy (adenosine triphosphate) supply of the root cells. However, consequences of

Corresponding author. E-mail address: [email protected] (T. Ruf).

https://doi.org/10.1016/j.biombioe.2018.10.007 Received 24 May 2018; Received in revised form 28 September 2018; Accepted 5 October 2018 0961-9534/ © 2018 Elsevier Ltd. All rights reserved.

Biomass and Bioenergy 119 (2018) 429–435

T. Ruf, C. Emmerling

such conditions on biomass development and biochemical methane potential (BMP) need to be addressed. In the framework of a two-year pot experiment, we examined the suitability of five promising PEC species and maize, to gain evidence about their performance if cultivated under periodically waterlogged conditions. We measured plant heights, quantified biomass yields and analyzed the BMP to evaluate the qualification of these crops to substitute maize on Planosols and Stagnosols.

Table 1 Characterisation of the experimental setup. Pots Type Volume Target bulk density

L g cm−3

Soila Origin

2. Material and methods In the framework of this feasibility study a pot experiment was conducted that allowed for a precise adjustment of soil water contents. The general drawbacks of pot experiments compared to field studies were accepted against the background of much better standardized conditions and working independently from natural precipitation. A total set of 72 Kick-Brauckmann pots were used whereby 60 pots were assigned to the main experiment aiming to determine growth parameters as well as the BMP and further 12 pots for monitoring of soil physical and hydraulic properties.

Stagnic cambisol developed from Devonian clay- and sandstones with small levels of loess; collected at 49.889°N, 6.712°E sieved to 6 mm

Preparation pH-value Soil organic carbon Soil nitrogen C to N ratio

2.1. Experimental setup 72 Kick-Brauckmann pots had been stepwise filled and compacted with a sandy loam. FD-sensors were vertically installed at a medium depth in the pots for determination of volumetric water contents (Table 1). Six different plants species were cultivated in the time from April 2015 to May 2017. Five PECs were selected resulting from (i) their promising BMPs and biomass yields determined in previous studies (c.f. section 1) and (ii) a literature research about the natural habitat of these species. Cup Plant (Silphium perfoliatum), Tall Wheatgrass (Agropyron elongatum ‚Szarvasi-1’), Giant Knotweed (Fallopia japonicum x bohemica ‚IGNISCUM Candy’), and Reed Canary Grass (Phalaris arundinacea) were obtained by vegetative propagation dividing the rootstocks of mature plants. Jerusalem Artichoke (Helianthus tuberosus ‘Gute Gelbe’) was cultivated from tubers. After five weeks of precultivation, 10 similarly developed plants of each species, with respect to the number of shoots and habitus, were selected and repotted in each one Kick-Brauckmann pot. At the time of repotting, each two maize grains (Zea mays ‘Ronaldinio’, KWS Saat AG, Einbeck, Germany) were seeded also in Kick-Brauckmann pots. For four weeks, the soil was kept moist in order to enable proliferation.

−1 −1

5.87 13.0 0.95 13.7

% % %

57.8 22.7 19.4 Ls4 (acc. to [23]) SL (acc. to [24])

Bulk densityb

g cm−3

1.32 ( ± 0.08)

% % %

49.9 ( ± 2.9) 30.4 ( ± 2.8) 20.4 ( ± 3.1)

%

10.3 ( ± 0.2)

Instrumentation for soil moisture monitoring

Fertilization Type Dates Amountsc N (47% as NO3-N, 53% as NH4-N) P (as P2O5) K (as K2O) Mg (as MgO) S

• • • • •

Harvesting Cutting height Single cut regime Double cut regime

The pore size distribution to relate volumetric soil water contents to soil water and air characteristics was determined by sampling (100 cm³ sample rings) from 12 pots, also planted with PECs, using a pressure plate apparatus at 60, 300 and 15000 hPa (Table 1) according to Richards et al. [21]. Determination of particle size distribution was done by combining wet sieving and sedimentation after Köhn [22].

g kg g kg

Grain size distribution Sand fraction Silt fraction Clay fraction Texture class

Pore size distributionb Total pore volume Field capacity Plant avail. water content Not plant avail. water content

2.2. Determination of soil physical and hydraulic properties

Kick-Brauckmann 7.0 1.25

FD-sensors (ECH2O EC-5, Decagon Devices, Pullman, United States) connected to a Delta-T Data-Logger (Delta-T Devices Ltd., Cambridge, Great Britain) ‚Vollkorndünger Perfekt’ (Raiffeisen GmbH, Frankfurt, Germany) 02.06.15; 11.04.16; 07.03.2017 2.10 g pot equals approx. 122 kg ha−1 0.30 g 2.32 g 0.17 g 1.12 g

pot pot pot pot

equals equals equals equals

approx. approx. approx. approx.

17 kg ha−1 134 kg ha−1 10 kg ha−1 65 kg ha−1

5 cm above the soil surface Maize, Cup Plant, Jerusalem Artichoke Tall Wheatgrass, Giant Knotweed, Reed Canary Grass

a

Parameters determined at the beginning of the experiment. Shown values represent mean values ( ± S.D.) of 4 sampling dates (13.06.15; 07.11.15; 22.04.16; 29.01.17) with each 18 sampling rings (3 pots with each 6 rings). c Recalculated to kg ha−1 taking into consideration the higher plant densities in the pots compared to field conditions. b

2.3. Soil moisture variants Soil types that are characterized by waterlogging show stagnant soil water under mid European conditions particularly in autumn, winter, and spring until higher evaporation and transpiration rates lead to disappearance of stagnant soil moisture. Our experimental setup aimed to constitute different duration and intensities of stagnant soil water in both years. Therefore, two different soil moisture variants with each 5 pots per plant species were established.

• The “Excess soil moisture” variant (in the following referred to as

“EM”) aimed to represent periodically waterlogged soil conditions. In the context of this study, the term ‘excess soil moisture’ defines soil water contents above field capacity (pF 1.8) leading to a

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Biomass and Bioenergy 119 (2018) 429–435

T. Ruf, C. Emmerling

deprivation of air capacity [25,26].

frost were removed. Statistical analysis of the data was done using RStudio programming language version 3.3.2 [28]. For plants that were treated in double cut regimes, the sum (biomass yield) or the weighted mean values (total solids, volatile solids, specific biogas yield, specific methane yield) of both cuts were used for statistical analysis. Testing for differences in growth parameters and biogas potential between EM and NM variant of a specific crop was done using two sample-t-tests and Wilcoxon signed rank test, depending on normality assumption and homoscedasticity, which was prior tested using Shapiro-Wilk and Levene-tests, respectively [29,30].

• The “Non-excess soil moisture” variant (in the following referred to as “NM”) served as control and aimed to represented soil conditions that allowed for unrestricted percolation of soil water without times of waterlogging.

2.4. Crop management Starting from June 2015, watering of the plants in the pots was done to reach predefined nominal values representing the EM and the NM variants. The compensation of the water deficit was done manually as required by the plants using a measuring beaker by watering on the top of the soil. Each year, all pots received a single fertilizer dressing at practice relevant levels which was applied to the soil surface and shallowly mixed in at the beginning of vegetation (Table 1). Harvesting of the plant biomass was done manually using a shears after measuring plant height using a folding ruler. Cutting regime was selected similar to agricultural practice (Table 1). Harvested biomass was directly weighted, dry matter and organic dry matter content determined at 105 °C and 550 °C, respectively. An aliquot of the biomass was dried directly after the harvest at 40 °C to be stored for the batch tests.

3. Results and discussion 3.1. Soil water dynamics in the pot experiment The EM variant was always adjusted to higher soil water contents relative to the NM variant (Table 3). During summers, both variants were kept comparably dry at soil water contents slightly above the permanent wilting point (PWP). During autumns, the soil water contents were elevated. In the NM variant, the soil moisture was adjusted to values below the field capacity (FC). In contrast to that, the FC was permanently exceeded in the EM variant for several weeks, particularly in the winter and spring of the second year of the experiment. During that time, the water level in the pots was approximately 10–15 cm below the soil surface. An exceedance of the FC was realized for 43.9% of the total experimental time in the EM variant but only for 7.6% in the NM variant.

2.5. Analytical methods of batch tests The biogas yields were determined in duplicates in accordance with [27]. Detailed information about the analysis is presented in Table 2. Biogas yields of every measuring date were standardized to normal conditions (273 K, 101.3 kPa) and corrected for water vapor pressure. For every measurement, volumes of CH4 and CO2 produced per gram of inoculum were subtracted from gas volumes produced by the samples. Volumes (LN) of CH4 were referred per kilogram of volatile substance (VS) to obtain specific methane yields.

3.2. Impact of excess soil moisture on growth and yield All PECs showed positive responses with respect to the growth parameters, like plant height and biomass development (Fig. 1). By contrast, maize was significantly negative affected by WCs above FC during spring 2016 and showed germination failure, seedling death, leaf chlorosis and stunted development, consequently leading to very low yields. Though, elevated soil moisture contents below FC, as present in 2015, had beneficial effects on maize growth. These results correspond well with results of [26,31,32] who observed a high susceptibility of maize growth towards waterlogging, particularly during

2.6. Data and statistical analysis Data about water contents in the pots were first checked for plausibility. Failures of individual sensors and data logged during periods of

Table 2 Process parameters of batch tests to determine the biochemical methane potential (BMP) of cultivated energy crops. Process parameters and setup Batch reactors Batch test duration Temperatur Substrate:Inoculum ratio Shaking Measurement interval Volume determination Determination of CH4 concentration Determination of CO2 concentration Sensor calibration Inoculum Origin Preparation Total solids Volatile solids Substrates Preparation Total solids Volatile solids Reference substrate

100 cm³ all glass syringes (Fortuna Optima, Poulten & Graf, Wertheim, Germany) 50 days Mesophilic (38 ± 1 °C), thermostatic incubator 0.46 ± 0.07 (on the basis of volatile solids) Manually; 1st week: daily, from 2nd week on every two days 1st week: daily, from 2nd week on as needed per biogas production Reading the graduation of the syringe NDIR dual wavelength flow sensor (S-AGM Plus 1010; Sensors Europe GmbH, Erkrath, Germany) NDIR dual wavelength flow sensor (S-AGM Plus 1032; Sensors Europe GmbH, Erkrath, Germany) Weekly interval (Präzisionsprüf- und Kalibriergas UN 1954, Air Products, Bochum, Germany) Secondary fermenter of a commercial, mesophilic biogas plant fed with cattle sludge, maize, whole cereal silage, grass silage, and, in small amounts of Miscanthus. Regular adding of trace elements and enzymes. Sieved to 2 mm. Reduction of residual gas potential by keeping at 38°C for 8–10 days prior to batch assays. Adjustment of total solids content to about 5% of FM using water. 5.34 ± 0.72% w/w (det. via water loss at 105°C for 24h) 3.65 ± 0.51% w/w (det. via loss of ignition at 550°C for 6h) Manually harvested, dried at 40°C until constant weight was reached, chopped using a cutting mill to 1 mm particle size (Fritsch pulverisette 15, Idar-Oberstein, Germany). determined via water loss at 105°C for 24h using an aliquot of the harvested biomass determined via loss of ignition at 550°C for 6h using an aliquot of the harvested biomass Microgranular cellulose (Sigma-Aldrich, Steinheim, Germany)

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accessions; however, riparian ecosystems in Northern America represent the natural habitat of Jerusalem Artichoke [47]. In our experiment, no crop showed such a clearly positive response than Jerusalem Artichoke which nearly doubled yields in the EM variant. Tall Wheatgrass cultivar ‘Szarvasi-1’ shows several morphological characteristics like a thick cuticle and a mesomorphic position of the stomatal guard cells that (i) are typical for drought tolerant species and (ii) may increase water use efficiency which were both breeding objectives in designing a robust energy crop for semi-arid lands [48]. Nonetheless, Csete [48] also described that the flood tolerance is “relatively good” but only for well-established stands which was also proven in the framework of our study. Moist soils, for example along rivers, lake shores and wetlands depict the natural habitat of Reed Canary Grass [14,49]. Wrobel [50] reported that Reed Canary Grass is one of the most productive grasses on poorly drained soils in temperate regions. A high tolerance of its roots against poor soil aeration which results from high air permeability within shoot and root tissue due to intercellular spaces (aerenchyma) had early been described by Bittman et al. [51] and Freyman [52]. Our results of the biomass development clearly showed that Reed Canary Grass has a large potential to be grown under waterlogged conditions.

Table 3 Summary of volumetric soil water contents (WC) on a monthly basis for both variants during the experimental period. Besides median values of volumetric soil water content, proportional exceedances of the permanent wilting point (WC < PWP, pF < 4.2, WC < 10.3 Vol. %) and field capacity (WC > FC, pF > 1.8, WC > 30.4 Vol. %) are shown. Variant

Excess soil moisture (‘EM’)

Non-excess soil moisture (‘NM’)

Month

Median WC

WC < PWP

WC > FC

Median WC

WC < PWP

WC > FC

Vol. %

% of time

% of time

Vol. %

% of time

% of time

25.6 24.9 10.9 10.2 16.2 26.5 26.9 29.7 29.4 31.7 36.3 34 28.1 31.3 32.6 22.3 15 23 28.6 31.1 34.1 35 33.1 30.5

0 0 44.2 55.0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

50.0 0 0 0 0 0 0 15.1 42.6 92.8 100.0 86.0 47.9 56.7 83.6 24.4 0 0 0 99.9 100.0 100.0 100.0 55.6

25.9 15.5 10.5 9.8 10.3 17.7 16.3 21.2 21.3 25.9 27.3 20.4 16.3 16.7 17.2 13.3 18.1 19.8 23.5 25.6 25.4 24 23 22.7

0 0 47.7 80.0 53.6 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

50 0 0 0 0 0 0 0 0 0 23.3 0 0 0 0 0 0 0 0 0 0 0 0 0

May ‘15 Jun. ‘15 Jul. ‘15 Aug. ‘15 Sep. ‘15 Oct. ‘15 Nov. ‘15 Dec. ‘15 Jan. ‘16 Feb. ‘16 Mar. ‘16 Apr. ‘16 May ‘16 Jun. ‘16 Jul. ‘16 Aug. ‘16 Sep. ‘16 Oct. ‘16 Nov. ‘16 Dec. ‘16 Jan. ‘17 Feb. ‘17 Mar. ‘17 Apr. ‘17

3.3. Biochemical methane potential and influences of excess soil moisture Biogas yield showed a clear dependency from plant species (Table 4). In contrast to that, an effect of both soil moisture treatments on the BMP could not statistically be proven. The methane concentrations of biogas were steadily above 50% and also showed neither a dependency from plant species nor from soil moisture regime. As benchmark, the BMP of maize was determined with in mean 315 LN CH4 kg−1 VS resembling the results of former studies that reported BMPs in the range from 312 to 365 LN CH4 kg−1 VS [8,53,54]. The BMP of Cup Plant determined in this study was in mean 260 LN CH4 kg−1 VS and therewith 20 LN CH4 kg−1 VS higher than values published by Haag et al. [5] and Mast et al. [8] which may be traced back to a 15 days longer retention time in this study. For Jerusalem Artichoke we measured in mean a BMP of 275 LN CH4 kg−1 VS. This value is quite similar to 284 LN CH4 kg−1 VS determined by Gunnarson et al. [55] after 49 days of fermentation. A considerably higher BMP was reported by Lehtomäki et al. [7] associated with a threefold longer retention time. A BMP of 205 LN CH4 kg−1 VS determined for Giant Knotweed in this study lies quite in the middle between 152 and 270 LN CH4 kg−1 VS which have previously been reported [7,11]. Lehtomäki et al. [7] described a clear dependency of the BMP from (i) retention time in the reactor and (ii) harvest date which results from increasing shares of poorly degradable lignin with progressing maturity. For Tall Wheatgrass cultivar ‘Szarvasi-1’ we determined in mean 287 LN CH4 kg−1 VS. This value is about 10% higher than those reported by Herrmann et al. [6] and Dickeduisberg et al. [56] which both determined 260 LN CH4 kg−1 VS but after only 30 and 35 days of digestion, respectively. Reed Canary Grass typically shows favorable C/N-ratios for anaerobic digestion [57] which result in BMPs of up to 430 LN CH4 kg−1 VS [7]. However, the importance of suitable cutting regimes to avoid lignification at the onset of flowering was already highlighted [58,59]. The mean BMP of Reed Canary Grass in our study was 282 LN CH4 kg−1 VS, whereby first cuts always showed higher values than second cuts.

early crop stages. The reasons for the beneficial effects of stagnant soil water on growth parameters of PECs were to our knowledge not a specific issue of research until now. However, the natural habitat, morphology and anatomy of the species may provide initial indications for the observed ecophysiological behavior. In this study, the biomass yields of Cup Plant increased significantly with higher water availability in the EM variant. Assefa et al. [33] already highlighted “because of its (…) tolerance of poorly drained soils cup plant can serve as an alternative (…) on soils that tend to flood in the spring …“, although the Cup Plant is commonly recommended for cultivation on drier sites [34,35]. However, a 33% lower water use efficiency of the Cup Plant compared to maize is hardly consistent with drought tolerance [36]. Indeed, the natural habitat of the Cup Plant is generally characterized by high soil moisture contents and humidity [37–40]. A natural habitat of Giant Knotweed similar to that of the Cup Plant is reported by Refs. [41,42] which is further indicated by hollow stems, large lamina and a thin cuticle layer. Forman et al. [43] describe, that this species shows highest growth at sites “where water is readily available”. In line with this, Mantovani et al. [44] revealed that biomass yields of cultivar ‘IGNISCUM Candy’ were significantly elevated by 34% with an increase in soil water availability from 70% to 100% of field capacity which coincides with our results. The tolerance of Jerusalem Artichoke against excess moisture conditions has been inconsistently evaluated. Shoemaker [45] stated that “Jerusalem Artichoke is adapted to all soils except those too wet” whereas Küppers-Sonnenberg [46] pointed out that the Jerusalem Artichoke can tolerate longer periods of stagnant water. The contradictory statements may perhaps be traced back to different varieties or plant

3.4. Prospects for field conditions Summarizing, the PECs have shown a high tolerance against stagnant soil moisture conditions and a good conversion of high water availability into biomass. The much higher biomass development in the

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Fig. 1. Mean values (n = 5, ± S.D.) of plant height (a) and biomass yields (b) subdivided for both experimental years. Plant heights and biomass yields of the first and second cut (where applicable) are shown on top of each other. Significant differences (sum of both cuts) between the variants (‘EM’, ‘NM’) of a certain species are indicated as follows: °: < 0.10, *: < 0.05, **: < 0.01, ***: < 0.001.

EM variant in this pot experiment would certainly be relativized under field conditions; a typical limitation of pot experiments. Thus, field experiments are inevitably to determine the methane potential per hectare. They should be conducted on soils with different textures and for several years to reflect different climatic and soil moisture conditions. In fact, the depth of the layer with low permeability decides upon (i) the potential amount of soil water resources and (ii) the survival of the plants. Thus, for successful cultivation of PECs on Stagnosols and Planosols, particular attention will need to be paid to the site-specific soil conditions. Waterlogged conditions or even flooding at times with higher temperatures may significantly impair the plants. At higher temperatures metabolic activities of roots and microbes are distinctly higher which result in fast oxygen depletion and hypoxic or anoxic soil conditions in the rhizosphere [16,17]. By this reason, the oxygen saturation and redoxpotential of the soil should be monitored in future experiments. The specific adaptation mechanisms of the plants also need to be investigated. Indeed, it appears quite likely that a sharp, species-specific boundary between advantageous and disadvantageous soil moisture contents and the levels of stagnating water below the ground surface exists. This particularly holds for the period of crop establishment [16,26].

Moreover, specific interactions of certain species with soil climatic conditions of Stagnosols and Planosols (e.g. slower warming up in spring, high water vapor saturation of the boundary layer close to the soil) and suitable, climate-friendly ways of fertilization emerge. 4. Conclusion Summarizing, our results suggest that (i) the PECs investigated in this feasibility study have the capability to outlive periods of waterlogged soil conditions in the winter half year and (ii) that these conditions moreover have prevalently positive effects on the yield performance. In contrast to that, maize significantly suffered from intense waterlogging, particularly during early growth stages. The results of this experimental approach, despite the constraints concerning time and number of replicates, emphazise that cropping of PECs on Stagnosols, Planosols and related soil types could contribute to a more sustainable agricultural biomass production and a diversification of the agricultural landscape. The fast sprouting and early biomass development of PECs may be a key factor in substantially outperforming maize with respect to methane yields per hectare. The risk of soil compaction during harvest operations may distinctly be reduced by earlier harvest dates, when soils typically show lower soil water contents and a

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Biomass and Bioenergy 119 (2018) 429–435 335.4 ( ± 6.68) 347.1* ( ± 4.54) 282.4 ( ± 16.69) 273.4 ( ± 8.95) 278.3° ( ± 11.11) 252.4 ( ± 26.61) 222.4 ( ± 8.46) 219.6 ( ± 7.42) 291.2° ( ± 16.56) 267.7 ( ± 10.25) 277.0 ( ± 4.06) 277.3 ( ± 5.69)

generally more firm soil structure by reduced management efforts. From our perspective, there is a considerable need for research on this topic. Acknowledgement

288.2 ( ± 9.19) 282.1 ( ± 14.86) 249.9* ( ± 2.95) 236.3 ( ± 4.47) 268.9 ( ± 20.92) 301.2* ( ± 9.10) 189.8 ( ± 4.99) 192.1 ( ± 8.85) 301.5 ( ± 17.67) 285.6 ( ± 7.68) 284.4 ( ± 7.35) 290.2 ( ± 7.32)

We kindly acknowledge the scholarship foundation of the Federal State of Rhineland-Palatinate which has granted a graduate scholarship for this project. The research assistants Ramesh Sangumani, Ruth Dederichs, Jennifer Makselon, Sandra Höhbauer, Julia Pape, and Dennis Lüders are thanked for support during growing of plants, processing of yields and laboratory trials.

656.4 ( ± 32.77) 668.9 ( ± 12.31) 527.5 ( ± 39.32) 493.5 ( ± 13.26) 549.7* ( ± 19.21) 483.4 ( ± 46.88) 428.2 ( ± 16.31) 414.9 ( ± 13.37) 543.9° ( ± 36.15) 491.4 ( ± 10.04) 527.3 ( ± 9.06) 521.0 ( ± 10.97)

Appendix A. Supplementary data Supplementary data related to this article can be found at https:// doi.org/10.1016/j.biombioe.2018.10.007. References

27.35 ( ± 3.18) 26.88 ( ± 1.59) 24.01 ( ± 2.06) 20.82 ( ± 1.08) 27.12*** ( ± 0.70) 20.34 ( ± 0.47) 24.43*** ( ± 0.85) 17.69 ( ± 1.10) 32.87 ( ± 1.81) 34.22 ( ± 0.29) 26.89 ( ± 2.30) 25.64 ( ± 2.54)

94.98 ( ± 0.83) 94.25 ( ± 0.43) 87.11 ( ± 1.86) 87.09 ( ± 1.45) 89.36 ( ± 1.75) 89.35 ( ± 1.37) 92.10 ( ± 0.46) 90.31 ( ± 1.04) 92.88 ( ± 1.57) 94.36 ( ± 2.39) 90.84* ( ± 1.16) 88.54 ( ± 1.42)

96.70 ( ± 2.89) 95.06 ( ± 1.67) 93.17** ( ± 1.43) 89.22 ( ± 0.84) 94.45* ( ± 0.70) 90.56 ( ± 0.29) 93.95** ( ± 1.32) 91.03 ( ± 0.42) 94.65 ( ± 0.85) 93.53 ( ± 2.00) 92.76° ( ± 1.19) 91.25 ( ± 0.80)

570.7 ( ± 11.18) 551.8 ( ± 31.95) 509.4** ( ± 6.21) 485.9 ( ± 8.30) 523.0 ( ± 42.13) 587.1* ( ± 19.04) 389.8 ( ± 3.28) 389.4 ( ± 16.79) 587.5 ( ± 30.04) 559.9 ( ± 10.58) 533.1 ( ± 14.97) 547.5 ( ± 10.98)

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Reed Canary Grass

Tall Wheatgrass

Giant Knotweed

Jerusalem Artichoke

Cup Plant

EM NM EM NM EM NM EM NM EM NM EM NM Maize

22.60 ( ± 1.09) 20.76 ( ± 3.05) 23.80 ( ± 4.57) 26.86* ( ± 2.66) 27.08 ( ± 1.39) 32.79* ( ± 3.30) 20.96 ( ± 1.77) 23.26* ( ± 0.56) 29.15 ( ± 2.02) 29.92 ( ± 1.63) 28.56 ( ± 1.32) 27.24 ( ± 1.86)

2016 2015 2015 2015 2015

2016

% of TS % of FM

2016

LN Gas kg−1 VS

2016

LN CH4 kg−1 VS

CH4 Biogas Volatile solids Total solids Variant Species

Table 4 Characterisation of the plant substrates and results of the batch test for both sampling years. Mean values ( ± S.D., n = 5) of both variants (‘EM’: excess soil moisture; ‘NM’: non-excess soil moisture) are shown. For plant species harvested in double cut regimes, weighted mean values of both cuts are presented. Significant differences between both variants of a certain species are indicated as follows: °: < 0.10,*: < 0.05,**: < 0.01,***: < 0.001.

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