• Energy Research

Eight willow (Salix) clones (Inger, Klara, Linnea, Resolution, Stina, Terra Nova, Tora, Tordis) were planted on two soil types in Denmark. The biomass quality was evaluated after 3 years of growth by measuring differences in concentrations of 14 elements associated with ash behavior during combustion, and total ash content. Three-year-old shoots of Tordis and Tora performed in general the best with relatively lower mean concentrations of K, Ca, Na, S, and total ash content than other clones across the two experimental sites. Terra Nova was the least suited for combustion as it contained up to 22, 27, 35, and 23 % higher concentrations of K, S, Ca, and total ash than the other clones. In addition to clone and site, appropriate management could further improve the fuel quality of willow biomass. When shoots of Inger were harvested annually (1-year shoots) high concentrations of K and Cl were found in all three consecutive harvests, but concentrations decreased significantly when rotation length was extended beyond 1 year of growth. Significant decreases of Mg, Na, P, S, and Zn were also registered from 2- to 3-year-old shoots. No difference in quality of biomass was found between two plant densities (8000 and 12,000 trees ha−1) of the clones Inger and Tora after the first 3-year rotation when grown at the site with a coarse sandy soil. The study indicates considerable diversity in concentration of elements within commercially available willow cultivars and suggests breeders and growers to select clones not only according to biomass yield potential but also according to biomass quality.

Empirical greenhouse gas (GHG) flux estimates from diverse peatlands are required in order to derive emission factors for managed peatlands. This study on a drained fen peatland quantified the annual GHG balance (Carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), and C exported in crop yield) from spring barley (SB) and reed canary grass (RCG) using static opaque chambers for GHG flux measurements and biomass yield for indirectly estimating gross primary production (GPP). Estimates of ecosystem respiration (ER) and GPP were compared with more advanced but costly and labor-intensive dynamic chamber studies. Annual GHG balance for the two cropping systems was 4.0 ± 0.7 and 8.1 ± 0.2 Mg CO2-Ceq ha(-1) from SB and RCG, respectively (mean ± standard error, n = 3). Annual CH4 emissions were negligible (<0.006 Mg CO2-Ceq ha(-1)), and N2O emissions contributed only 4-13 % of the full GHG balance (0.5 and 0.3 Mg CO2-Ceq ha(-1) for SB and RCG, respectively). The statistical significance of low CH4 and N2O fluxes was evaluated by a simulation procedure which showed that most of CH4 fluxes were within the range that could arise from random variation associated with actual zero-flux situations. ER measured by static chamber and dynamic chamber methods was similar, particularly when using nonlinear regression techniques for flux calculations. A comparison of GPP derived from aboveground biomass and from measuring net ecosystem exchange (NEE) showed that GPP estimation from biomass might be useful, or serve as validation, for more advanced flux measurement methods. In conclusion, combining static opaque chambers for measuring ER of CO2 and CH4 and N2O fluxes with biomass yield for GPP estimation worked well in the drained fen peatland cropped to SB and RCG and presented a valid alternative to estimating the full GHG balance by dynamic chambers.

Abstract Production of a broad spectrum of products from biomass is of key importance for an economically viable and sustainable biorefinery sector. The aim of this study was to determine the total crude protein yield and theoretical extractable true protein of potential biorefinery feedstocks optimized for supplying biomass to biorefineries. Field experiments during 2013–2014 with perennial crops (pure grasses: cocksfoot, festulolium, reed canary, tall fescue, two miscanthus species and two grass-legume mixtures) and annual crops in optimized rotations (winter rye, sugar beet, maize, triticale, hemp and grass-clover) were compared to traditional crops common in Danish agriculture (maize, barley, wheat and triticale). Theoretical extractable true protein was determined according to the Cornell Net Carbohydrate and Protein System, which fractionates the crude protein based on solubilities. The easily extractable fraction of the true protein was denoted as neutral-extractable. Concentration of crude protein was on average 164–191 g kg−1 DM per cut of pure grasses and grass-legume mixtures, with the summer cuts having the lowest values. Pure grasses produced the highest crude protein yield per hectare annually, ranging from 2595 to 3693 kg ha−1 irrespective of the year, of which 920–1640 kg ha−1 was neutral-extractable protein. Whilst the neutral-extractable true protein per hectare of festulolium and tall fescue was superior to those of all other crops, the neutral extractable true protein per hectare of grass-legume mixtures and of winter rye and maize double crop was similar to those of reed canary and cocksfoot. On a mass basis, 34–46% of crude protein in pure grasses was neutral-extractable, depending on the year. The potential extractability of crude protein may be increased by 14–35% if the cell wall-bound protein can be extracted too.

Climate change impacts rainfall patterns which may lead to drought stress in rain-fed agricultural systems. Crops with higher drought tolerance are required on marginal land with low precipitation or on soils with low water retention used for biomass production. It is essential to obtain plant breeding tools, which can identify genotypes with improved drought tolerance and water use efficiency (WUE). In C3 plant species, the variation in discrimination against 13C (Δ13C) during photosynthesis has been shown to be a potential indicator for WUE, where discrimination against 13C and WUE were negatively correlated. The aim of this study was to determine the variation in the discrimination against 13C between species and cultivars of three perennial C3 grasses (Dactylis glomerata (cocksfoot), Festuca arundinacea (tall fescue) and Phalaris arundinacea (reed canary grass)) and test the relationships between discrimination against 13C, season-long water use WUEB, shoot and root biomass production in plants grown under well-watered and water-limited conditions. The grasses were grown in the greenhouse and exposed to two irrigation regimes, which corresponded to 25% and 60% water holding capacity, respectively. We found negative relationships between discrimination against 13C and WUEB and between discrimination against 13C and shoot biomass production, under both the well-watered and water-limited growth conditions (p < 0.001). Discrimination against 13C decreased in response to water limitation (p < 0.001). We found interspecific differences in the discrimination against 13C, WUEB, and shoot biomass production, where the cocksfoot cultivars showed lowest and the reed canary grass cultivars highest values of discrimination against 13C. Cocksfoot cultivars also showed highest WUEB, shoot biomass production and potential tolerance to water limitation. We conclude that discrimination against 13C appears to be a useful indicator, when selecting C3 grass crops for biomass production under drought conditions.

AbstractPerennial crops replacing annual crops are drawing global attention because they harbor potential for sustainable biomass production and climate change mitigation through soil carbon sequestration. At present, it remains unclear how long perennial crops can sequester carbon in the soil and how soil carbon stock dynamics are influenced by climate, soil, and plant properties across the globe. This study presents a meta-analysis synthesizing 51 publications (351 observations at 77 sites) distributed over different pedo-climatic conditions to scrutinize the effect of perennialization on organic carbon accumulation in soil compared with two annual benchmark systems (i.e., monoculture and crop rotation). Results showed that perennial crops significantly increased soil organic carbon stock by 16.6% and 23.1% at 0–30 cm depth compared with monoculture and crop rotation, respectively. Shortly after establishment (< 5 years), perennial crops revealed a negative impact on soil organic carbon stock; however, long duration (> 10 years) of perennialization had a significant positive effect on soil organic carbon stock by 30% and 36.4% at 0–30 cm depth compared with monoculture and crop rotation, respectively. Compared with both annual systems, perennial crops significantly increased soil organic carbon stock regardless of their functional photosynthetic types (C3, C4, or C3-C4 intermediates) and vegetation type (woody or herbaceous). Among other factors, pH had a significant impact on soil organic carbon; however, the effect of soil textures showed no significant impact, possibly due to a lack of observations from each textural class and mixed pedoclimatic effects. Results also showed that time effect of perennialization revealed a sigmoidal increase of soil organic carbon stock until about 20 years; thereafter, the soil carbon stocks advanced towards a steady-state level. In conclusion, perennial crops increased soil organic carbon stock compared with annual systems; however, the time since conversion from annual to perennial system decisively impacted soil organic carbon stock changes.

Abstract Optimal fertilization of short rotation coppice (SRC) willow is important both in terms of economic yield and environmental effect. We measured biomass yield and nutrient uptake in two willow clones, Inger and Tordis, grown on a coarse sandy soil and within six different fertilization regimes. Fertilization treatments were carried out during two two-year harvest rotations, beginning in the 2nd growth year of the plantation. Willow was fertilized as follows with names referring to type of fertilizer and total quantities of nitrogen (kg ha −1 ) in first and second year within both rotations: 1) Control 0+0 , 2) NPK 120+0 , 3) Slurry 180+0 , 4) NPK 120+120 , 5) NPK 240+0 , 6) Slurry 360+0 . Fertilization affected biomass yield significantly but interacted with rotation and clone. In first rotation, fertilization increased dry matter (DM) yield across clones significantly from 3.7 Mg ha −1 y −1 for Control 0+0 to 6.5, 6.4 and 5.6 for Slurry 360+0 , NPK 120+120 and NPK 240+0 , respectively. In second rotation, yield increased from 6.2 Mg ha −1 y −1 to 8.8, 8.2, 7.8 and 7.4 for Slurry 360+0 , NPK 240+0 , Slurry 180+0 and NPK 120+120 , respectively. Biomass dry matter yield per ha increased linearly at 15 kg kg −1 of applied total-N in both rotations. The yield increase in response to fertilization was generally larger in Inger than in Tordis. In general, element concentration in the harvested biomass was either unaffected or slightly reduced by fertilization. In conclusion, yield response to fertilization appears to be primarily related to the quantity of N applied but the effect depended on fertilizer type, harvest rotation and willow clone.

Peatlands cover 3 % of the Danish land area, but drainage of these areas contributes to approximately 25 % of the total agricultural greenhouse gas (GHG) emissions. Paludiculture, defined as agriculture on wet or rewetted peatlands, has been proposed as a strategy to mitigate GHG emissions while keeping up production. However, little is known about the net GHG effects during establishment and how it is influenced by soil biogeochemical conditions. In this study, we determined annual carbon balances of five Danish peatlands, three fens and two bogs, for the first year of cultivation with reed canary grass (RCG) with two annual cuts in a mesocosm set-up under controlled conditions. Biomass yields were highly variable, ranging between 0.8 and 7.4 t dry matter (DM) ha-1 yr-1, and significantly higher on fen peat soils. Ecosystem respiration (Reco) fluxes of CO2 were naturally highest from sites with high biomass establishment. Methane emissions were site-specific, ranging between 0.03 and 1.85 t CH4 (CO2eq ha-1 yr-1), and affected by biomass growth, as well as bulk density and the iron content within soil. Nitrous oxide fluxes were negligible, despite nitrogen (N) fertilisation with 200 kg N ha-1 yr-1. Driven by net primary production (NPP) we found that the fen sites were GHG sinks, with a global warming potential (GWP) of -1.3 to -11.5 t CO2eq ha-1 yr-1 during the first year of rewetting and RCG establishment. The bogs remained sources of carbon (5.3 t CO2eq ha-1 yr-1). Our results highlighted that the nutrient-rich Danish fen peatlands showed a potential for GHG mitigation by paludiculture under extensive agricultural management, while this was not the case for the bog sites due to poor biomass establishment. In conclusion, we found the highest GHG mitigation potential by rewetting and RCG paludiculture on nutrient-rich fen peatlands.

This study evaluated the effects of harvesting managements with two-cuts (2C) and three-cuts (3C) per year for subsequent specific methane yield (SMY) and methane yield per hectare (MYPH) of festulolium and tall fescue cultivated on a riparian fen peatland in a block-designed field experiment (n = 3). For the 2C managements, three timings of the first cut were implemented corresponding to growth stages of pre-heading (2C-early), inflorescence emergence (2C-mid), and flowering (2C-late). Anaerobic digestion batch assays with biomass samples were run for 68 days, showing that 90% of total methane (CH4) was produced within 38 days. Specific methane yield ranged from 315 to 464 NL CH4 kg−1 volatile solids (mean, 393 NL). On average, SMY of the final cut biomass was 13% lower than the first cut biomass. Methane yield per hectare ranged from 5277 to 6963 Nm3 CH4 ha−1 (mean, 6265 Nm3) and was predominantly influenced by biomass yield since SMY only deviated modestly in relation to harvest management (crop maturity). Methane yield per hectare of festulolium under 3C and 2C-late management were significantly higher than 2C-early and 2C-mid managements, whereas the harvesting managements did not influence MYPH of tall fescue. The levels of SMY and MYPH in the present study represented high-end of reported values due to a combination of high activity of the biogas inoculum and a high productivity of festulolium and tall fescue at the riparian fen peatland.

AbstractDrought is a great challenge to agricultural production, and cultivation of drought‐tolerant or water use‐efficient cultivars is important to ensure high biomass yields for bio‐refining and bioenergy. Here, we evaluated drought tolerance of four C3 species, Dactylis glomerata cvs. Sevenop and Amba, Festuca arundinacea cvs. Jordane and Kora, Phalaris arundinacea cvs. Bamse and Chieftain and Festulolium pabulare cv. Hykor, and two C4 species Miscanthus × giganteus and M. lutarioriparius. Control (irrigated) and drought‐treated plants were grown on coarse and loamy sand in 1 m2 lysimeter plots where rain was excluded. Drought periods started after harvest and lasted until 80% of available soil water had been used. Drought caused a decrease in dry matter yield (DM; P < 0.001) for all species and cultivars during the drought period. Cultivars Sevenop, Kora and Jordane produced DM at equal levels and higher than the other C3 cultivars in control and drought‐treated plots both during and after the drought period. Negative correlations were observed between stomatal conductance (gs) and leaf water potential (P < 0.01) and positive correlations between gs and DM (P < 0.05) indicating that gs might be suitable for assessment of drought stress. There were indications of positive associations between plants carbon isotope composition and water use efficiency (WUE) as well as DM under well‐watered conditions. Compared to control, drought‐treated plots showed increased growth in the period after drought stress. Thus, the drought events did not affect total biomass production (DMtotal) of the whole growing season. During drought stress and the whole growing season, WUE was higher in drought‐treated compared to control plots, so it seems possible to save water without loss of biomass. Across soil types, M. lutarioriparius had the highest DMtotal (15.0 t ha−1), WUEtotal (3.6 g L−1) and radiation use efficiency (2.3 g MJ−1) of the evaluated grasses.