Assessment of the energetic and mechanical properties of pellets produced from agricultural biomass

This paper presents an assessment of the energetic and mechanical properties of pellets produced from agricultural biomass. For the production of pellets the following raw materials were used: wheat straw, rape straw, and maize straw. Additionally, the mixtures of wheat-rape straw, wheat-maize straw, and rape-maize straw (each accounting for 50% of the mass) were applied. The studied resources were ground with the use of a universal shredder driven by a 7.5 kW electric engine. A pelleting machine fitted with a fixed flat matrix with two driven thickening rolls was used to produce the pellets. Analyses of the moisture and calorific value of resources as well as the bulk density and mechanical strength of pellets were performed according to biding standards. The moisture of resources ranged from 16.5% to 18.5% for rape and maize straw, respectively. The average calorific value fluctuated between 15.3 MJ kg⁻¹ for a mixture of wheat and rape straw to 16.2 MJ kg⁻¹ for maize straw. The bulk density and mechanical strength of pellets depended on the type of resources used. The lowest bulk density was recorded for wheat straw pellets (386–420 kg m⁻³), and the highest (561–572 kg m⁻³) for maize straw pellets. The lowest mechanical strength of pellets was noted for rape (95.4–96.8%), whereas the highest was for pellets made from a wheat and maize straw mixture (96.8–98.9%).     

Enhanced substrate degradation and methane yield with maleic acid pre-treatments in biomass crops and residues

Organic acids are envisaged as alternative catalysts to strong mineral acids, in pre-treatment of ligno-cellulosic biomass for anaerobic digestion (AD). To evaluate this hypothesis, an untreated control and four pre-treatments (25 °C for 24 h) involving two levels of maleic acid (34.8 and 69.6 kg m⁻³), alone and combined with sulphuric acid (4 kg m⁻³), were studied in three agricultural substrates: Arundo (aka giant reed), Barley straw and B133 fibre sorghum. Methane production was assessed in a batch AD assay (35 °C for 51 days) with 4 g L⁻¹ of volatile solid (VS) load. Fibre composition and structure were investigated through chemical analysis and Fourier transform infrared (FTIR) spectrometry. Arundo and B133 that were the most and least recalcitrant substrate, respectively, staged the highest and lowest increase in methane with high maleic acid: +62% over 218 cm³ g⁻¹ of VS in untreated Arundo; +36% over 284 cm³ g⁻¹ of VS in untreated B133. Barley straw showed an intermediate behaviour (+41% over 269 cm³ g⁻¹ of VS). H₂SO₄ addition to maleic acid did not improve CH₄ output. The large increase in methane yield determined by pre-treatments was reflected in the concurrent decrease of fibre (between 14 and 39% depending on fibrous component). Based on FTIR spectra, bands assigned to hemicellulose and cellulose displayed lower absorbance after pre-treatment, supporting the hypothesis of solubilisation of structural carbohydrates and change in fibre structure. Hence, maleic acid was shown a suitable catalyst to improve biodegradability of ligno-cellulosic biomass, especially in recalcitrant substrates as Arundo.     

Pretreatment of cassava stems and peelings by thermohydrolysis to enhance hydrolysis yield of cellulose in bioethanol production process

The potential of wastes obtained from the cultivation of Manihot esculenta Crantz as raw material for bioethanol production was studied. The objective was to determine the optimal conditions of hemicellulose thermohydrolysis of cassava stems and peelings and evaluate their impact on the enzymatic hydrolysis yield of cellulose. An experimental design was conducted to model the influence of factors on the pentose, reducing sugar and phenolic compound contents. Residues obtained from the optimal pretreatment conditions were hydrolysed with cellulase (filter paper activity 40 FPU/g). The hydrolysates from pretreatment and enzymatic hydrolysis were fermented respectively using Rhyzopus spp. and Sacharomyces cerevisiae. The yield of enzymatic hydrolysis obtained under the optimal conditions were respectively 73.1% and 86.6% for stems and peelings resulting in an increase of 39.84% and 55.40% respectively as compared to the non-treated substrates. The ethanol concentrations obtained after fermentation of enzymatic hydrolysates were 1.3 and 1.2 g/L respectively for the stem and peeling hydrolysates. The pentose and phenolic compound concentrations obtained from the multi-response optimization were 10.2 g/L; 0.8 g/L and 10.1 g/L; 1.3 g/L respectively for stems and peelings. The hydrolysates of stems and peelings under these optimal conditions respectively gave ethanol concentrations of 5.27 g/100 g for cassava stems and 2.6 g/100 g for cassava peelings.     

Development of a membrane bioreactor for enzymatic hydrolysis of cellulose

Cellulose hydrolysis is an important step in the production of bioethanol from lignocellulose. Using enzymes, as a biocatalyst, is expected to have a lower utility cost compared to the conventional acidic hydrolysis because it is carried out at milder conditions and does not require subsequent treatment step. The major obstacle to the practical realization of the potentials of enzymatic hydrolysis is the high cost of the enzymes and the slow reaction rate due to the inhibition of the enzyme by the products. In this work, a membrane bioreactor was simulated to tackle these two obstacles and enhance the reaction rate. It was found that for a 5000 kg h⁻¹ lignocellulosic feed, to achieve 50% hydrolysis conversion, a 125 m³ membrane bioreactor containing 923 kg m⁻³ cellulase need to be used. The amount of the enzyme that escapes from the system and needs replacement was estimated at 92 kg h⁻¹. The membrane reactor model was further tested using the competitive product inhibition model for the hydrolysis of totally amorphous Carboxymethylcellulose (CMC). It was shown that the reactor volume required to achieve a conversion of 50% was significantly less than that required for the lignocelluloses, even at a lower membrane mass transfer coefficient.     

Grinding energy and physical properties of chopped and hammer-milled barley,wheat, oat,and canola straws

In the present study, specific energy for grinding and physical properties of wheat, canola, oat and barley straw grinds were investigated. The initial moisture content of the straw was about 0.13–0.15 (fraction total mass basis). Particle size reduction experiments were conducted in two stages: (1) a chopper without a screen, and (2) a hammer mill using three screen sizes (19.05, 25.4, and 31.75 mm). The lowest grinding energy (1.96 and 2.91 kWh t⁻¹) was recorded for canola straw using a chopper and hammer mill with 19.05-mm screen size, whereas the highest (3.15 and 8.05 kWh t⁻¹) was recorded for barley and oat straws. The physical properties (geometric mean particle diameter, bulk, tapped and particle density, and porosity) of the chopped and hammer-milled wheat, barley, canola, and oat straw grinds measured were in the range of 0.98–4.22 mm, 36–80 kg m⁻³, 49–119 kg m⁻³, 600–1220 kg m⁻³, and 0.9–0.96, respectively. The average mean particle diameter was highest for the chopped wheat straw (4.22-mm) and lowest for the canola grind (0.98-mm). The canola grinds produced using the hammer mill (19.05-mm screen size) had the highest bulk and tapped density of about 80 and 119 kg m⁻³; whereas, the wheat and oat grinds had the lowest of about 58 and 88–90 kg m⁻³. The results indicate that the bulk and tapped densities are inversely proportional to the particle size of the grinds. The flow properties of the grinds calculated are better for chopped straws compared to hammer milled using smaller screen size (19.05 mm).     

Physical properties of rice husk and bran briquettes under low pressure densification for rural applications

Agriculture generates large amount of by-products that could be used to produce energy and reduce the amount of fuelwood required to meet the daily cooking needs, especially in developing countries. Rice is a major crop grown in West Africa and rice husk is a by-product of the milling process. The goal of this study was to develop a low cost system to produce biomass briquettes from rice husks in the context of a rural village. A manual press generating a pressure of 4.2 MPa was developed and used. The influence of the briquette formulation (type of binder, binder content, water addition, and bran content) was studied. The binders investigated were cassava wastewater, rice dust, and okra stem gum. The physical properties (density, moisture content, calorific value, durability, and compressive strength) were tested to identify the briquettes with the highest quality, i.e. greatest physical integrity. The briquettes made with rice dust had the highest durability (91.9%) and compressive strength (2.54 kN), while the briquettes made with cassava starch wastewater had the greatest density (441.18 kg m⁻³). Water added to the rice husk before densification positively influenced the briquette quality while bran seemed to mostly increase the density, but not necessarily the briquette quality. The briquette formulation did not significantly influence the calorific value. With a higher heating value of 16.08 MJ kg⁻¹ dry basis, rice husk briquettes represent an interesting alternative to fuelwood.     

Biogas production from catch crops: Increased yield by combined harvest of catch crops and straw and preservation by ensiling

The combination of catch crop cultivation with its use for biogas production would increase renewable energy production in the form of methane, without interfering with the production of food and fodder crops. The low biomass yield of catch crops has been shown as the main limiting factor for using these crops as co-substrate in biogas plants, since the profit obtained from the sale of methane barely compensates the harvest costs. Therefore, a new agricultural strategy to harvest catch crops together with the residual straw of the main crop was investigated, in order to increase the biomass and the methane yield per hectare. Seven catch crops harvested together with stubble from the previous main crop were evaluated. The effects of stubble height, harvest time and ensiling as a storage method for the different catch crops/straw blends were studied. Biomass yields as TS ranged between 3.2 and 3.6 t ha⁻¹ y⁻¹of which the catch crop constituted around 10% of the total biomass yield. Leaving the straw on the field until harvest of the catch crop in the autumn could benefit methane production from the straw both due to increased biomass yield and an increased organic matter bioavailability of the straw taking place on the field during the autumn months. Ensiling as a storage method could be feasible in terms of energy storage and guaranteeing the feedstock availability for the whole year. This new agricultural strategy may be a good alternative for economically feasible supply of catch crops and straw for biogas production.     

Energy and CO2 analysis of poplar and maize crops for biomass production in north Italy

The rising price of fossil fuel and the increasing environmental concern encourage the use of biomasses as energy sources. Aim of this study was to compare two poplar SRC and vSRC (6 and 3 years rotation cycle) with an annual crop (maize), used for biomass production in north Italy.The average of the biomass production was 13.9 Mg DM ha⁻¹ per year for the SRC and vSRC poplar and 19.2 Mg DM ha⁻¹ for the maize.The energy consumption for the poplar cultivations was about 15 GJ ha⁻¹ per year, which represented only the 6% of the energy biomass product (about 257 GJ ha⁻¹ per year).The input value of the maize was higher (26.8 GJ ha⁻¹ per year). In this case, the input value was about the 7% of the energy content in the biomass product (about 370 GJ ha⁻¹ per year).During the vSRC cultivation an amount of 8090 kg CO₂ eq ha⁻¹ was emitted, 6420 kg CO₂ eq ha⁻¹ for the SRC and 26,370 kg CO₂ eq ha⁻¹ for the maize.Compared to the maize, the poplar SRC (or vSRC) crops are interesting from an energetic point of view, while maize requires less manpower, but it has major problems related to the landscape biodiversity.     

Effect of chitosan on reactor performance and population of specific methanogens in a modified CSTR treating raw POME

In this study, chitosan was periodically added to a CSTR treating raw POME in order to retain more working microorganisms at high OLRs. The data indicated that the CSTR with chitosan addition can be operated with reactor stability at an OLR of 26.5 kg m⁻³ d⁻¹ which is approximately 7.5 kg m⁻³ d⁻¹ higher than that for the control CSTR without chitosan addition. In the control CSTR, the biogas production did not increase with increased OLR, and the overall process was limited by slow methanogenic rates. For the CSTR with chitosan addition, the biogas production was 9.39 m³ m⁻³ d⁻¹ with a methane volume fraction of 68% at an OLR of 26.5 kg m⁻³ d⁻¹. Corresponding to this increase in methane production, it was found that Methanosarcinales numbers were significantly higher (P < 0.05) in the CSTR with chitosan addition than in the control CSTR.     

Comparison of entrapment and biofilm mode of immobilisation for bioethanol production from oilseed rape straw using Saccharomyces cerevisiae cells

Cell immobilisation provides the opportunity to reduce the cost of producing bioethanol from lignocellulosic biomass such as oilseed rape (OSR) straw, in addition to enhancing operational stability. Bioethanol fermentation of OSR straw hydrolysate by free and immobilised Saccharomyces cerevisiae was studied. Cells were either entrapped in alginate beads or Lentikat^® discs or immobilised as a biofilm on spent grains, Leca, or reticulated foam. The overall aims of the research were to compare bioethanol yields produced from free and immobilised systems, and to identify the most suitable immobilisation technique in terms of bioethanol yield and longevity of the immobilised cell system. Cell entrapment in alginate beads and Lentikat^® discs resulted in significantly higher bioethanol yields compared to when cells were free in suspension or immobilised as a biofilm on a support material. The maximum amount of bioethanol produced by cells immobilised in alginate beads and Lentikat^® discs were 169.26 ± 0.24 and 165.13 ± 0.67 g bioethanol kg⁻¹ OSR straw after 3 h and 7 h of fermentation, respectively. Due to the high mechanical stability and bioethanol yield, immobilisation of S. cerevisiae in Lentikat^® discs was considered the most appropriate immobilisation technique for bioethanol production.     

Process optimization for bioethanol production from cassava starch using novel eco-friendly enzymes

Although cassava (Manihot esculenta Crantz) is a potential bioethanol crop, high operational costs resulted in a negative energy balance in the earlier processes. The present study aimed at optimizing the bioethanol production from cassava starch using new enzymes like Spezyme^® Xtra and Stargen™ 001. The liquefying enzyme Spezyme was optimally active at 90 °C and pH 5.5 on a 10% (w/v) starch slurry at levels of 20.0 mg (280 Amylase Activity Units) for 30 min. Stargen levels of 100 mg (45.6 Granular Starch Hydrolyzing Units) were sufficient to almost completely hydrolyze 10% (w/v) starch at room temperature (30 ± 1 °C). Ethanol yield and fermentation efficiency were very high (533 g/kg and 94.0% respectively) in the Stargen + yeast process with 10% (w/v) starch for 48 h. Raising Spezyme and Stargen levels to 560 AAU and 91.2 GSHU respectively for a two step loading [initial 20% (w/v) followed by 20% starch after Spezyme thinning]/initial higher loading of starch (40% w/v) resulted in poor fermentation efficiency. Upscaling experiments using 1.0 kg starch showed that Stargen to starch ratio of 1:100 (w/w) could yield around 558 g ethanol/kg starch, with a high fermentation efficiency of 98.4%. The study showed that Spezyme level beyond 20.0 mg for a 10% (w/v) starch slurry was not critical for optimizing bioethanol yield from cassava starch, although an initial thinning of starch for 30 min by Spezyme facilitated rapid saccharification-fermentation by Stargen + yeast system. The specific advantage of the new process was that the reaction could be completed within 48.5 h at 30 ± 1 °C.     

Impact of operational conditions on development of the hydrogen-producing microbial consortium in an AnSBBR from cassava wastewater rich in lactic acid

Biohydrogen production from cassava starch wastewater was evaluated in anaerobic sequencing batch biofilm reactor (AnSBBR) using different inoculum (mixed cultures from naturally fermented wastewater and anaerobic sludge thermally treated) and feeding strategies (batch and fed-batch). The highest hydrogen productivity (2.4 LH₂ L⁻¹ d⁻¹) and yield (11.7 molH₂ kg⁻¹Carbohydrates) were verified in low and intermediate organic load rates (12 and 14 g L⁻¹ d⁻¹) and longer cycle time (4 h), respectively. The productivity was favored by fed-batch strategy, and yield by batch. The hydrogen production was verified in both inoculum sources. However, in the assays inoculated from naturally fermented wastewater, with higher organic load rate (18 g L⁻¹ d⁻¹) and intermediate cycle time (3 h) no hydrogen was observed, regardless the feeding strategy, indicating that the inhibitory effects of the indigenous microorganisms present in cassava starch wastewater were more expressive in these conditions. The operational conditions applied to hydrogen production in AnSBBR from cassava starch wastewater may influence the microflora development in the reactor. In this study three possible scenarios were verified: hydrogen-producing bacteria (HPB) growth; hydrogen-producing bacteria inhibition or coexistence between ones and lactic acid bacteria (LAB), which are autochthones of this wastewater.     

An evaluation of cassava, sweet potato and field corn as potential carbohydrate sources for bioethanol production in Alabama and Maryland

The recent emphasis on corn production to meet the increasing demand for bioethanol has resulted in trepidation regarding the sustainability of the global food supply. To assess the potential of alternative crops as sources of bioethanol production, we grew sweet potato (Ipomoea batatas) and cassava (Manihot esculentum) at locations near Auburn, Alabama and Beltsville, Maryland in order to measure root carbohydrate (starch, sucrose, glucose) and root biomass. Averaged for both locations, sweet potato yielded the highest concentration of root carbohydrate (ca 80%), primarily in the form of starch (ca 50%) and sucrose (ca 30%); whereas cassava had root carbohydrate concentrations of (ca 55%), almost entirely as starch. For sweet potato, overall carbohydrate production was 9.4 and 12.7 Mg ha⁻¹ for the Alabama and Maryland sites, respectively. For cassava, carbohydrate production in Maryland was poor, yielding only 2.9 Mg ha⁻¹. However, in Alabama, carbohydrate production from cassava averaged 10 Mg ha⁻¹. Relative to carbohydrate production from corn in each location, sweet potato and cassava yielded approximately 1.5× and 1.6× as much carbohydrate as corn in Alabama; 2.3× and 0.5× for the Maryland site. If economical harvesting and processing techniques could be developed, these data suggest that sweet potato in Maryland, and sweet potato and cassava in Alabama, have greater potential as ethanol sources than existing corn systems, and as such, could be used to replace or offset corn as a source of biofuels.     

Evaluation of methane production from maize silage by harvest of different plant portions

Biogas production is mainly based on the anaerobic digestion of cereals silages and maize silage is the most utilized. Regarding biogas production, the most important portion of the plant is the ear. The corn ear, due to high starch content, is characterized by a higher biogas production compared to the silage of the whole plant.In this paper, we present the results of experimental field tests carried out in Northern Italy that aim to evaluate the anaerobic methane potential (BMP) of different portions of ensiled maize hybrids. The BMP production is evaluated considering the possibility of harvesting and ensiling: the whole plant; the plant cut at 75 cm of height; the ear only; the plant without the ear. For the different solutions, the results are reported as specific BMP and as average biogas production achievable per hectare. The methane production by harvesting and ensiling the whole plant (10,212 and 10,605 m³ ha⁻¹, for maize class 600 and 700, respectively) is higher than the ones achievable by the other plant portions (7961 and 7707 m³ ha⁻¹, from the ear; 9523 and 9784 m³ ha⁻¹, from the plant cut at 75 cm; 3328 and 3554 m³ ha⁻¹, from the plant without the ear, for maize class 600 and 700, respectively). The harvest of the whole plant, although it is the most productive solution, could not be the best solution under an economic and environmental point of view. Harvesting only the ear can be interesting considering the new Italian subsidy framework and for the biogas plants fed by biomass transported over long distances.     

Strategies to improve the biohydrogen production from cassava wastewater in fixed-bed reactors

The biological production of hydrogen from cassava starch wastewater (CSW) was evaluated in an anaerobic fixed-bed reactor (UAFBR). The assays were carried out to evaluate the effects of organic loading rate (OLR) increase and strategies of inoculation (AS – anaerobic sludge thermally treated and NF – naturally fermented cassava starch wastewater) on UAFBR performance. The OLR increase (10–20 g L⁻¹ d⁻¹) associated with hydraulic retention time (HRT) decrease (4–2 h) improved the volumetric hydrogen production rate (VHPR, from 229 to 550 mLH₂.L⁻¹.d⁻¹), molar hydrogen flow rate (MHFR, from 1.0 to 2.5 mmolH₂.h⁻¹) and hydrogen yield (HY, from 0.2 to 0.3 molH₂.mol⁻¹Carb) from CSW due to increase in substrate availability. Both inoculation alternatives (AS and NF) were effective for the selection of acidogenic microorganisms, which demonstrates that NF could be considered a simple and economic alternative for the acquisition of inoculum for continuous acidogenic reactors. Hydrogen production decreased after 10 days of operation when the specific organic loading rate (SOLR) reached reduced values (<1 gCarb.g⁻¹VSS.d⁻¹), which impairs hydrogen production. For all assays, methane was present in the biogas after the 20th day of operation mainly due to biomass accumulation, which alters the biota of the reactor. Although many factors could influence the process performance in UAFBR for the production of biohydrogen, the accumulation of biomass have been pointed as the main factor in the determination of the production time, thus demanding the implementation of systematic practices to remove the excess of biomass to maintain the SOLR in levels adequate for hydrogen production.     

Nitrogen fertilization affects silicon concentration,cell wall composition and biofuel potential of wheat straw

Nitrogen is an essential input factor required for plant growth and biomass production. However, very limited information is available on how nitrogen fertilization affects the quality of crop residues to be used as lignocellulosic feedstock. In the present study, straw of winter wheat plants grown at six different levels of nitrogen supply ranging from 48 to 288 kg nitrogen ha⁻¹ was analyzed for major cell wall components and mineral elements. Enzymatic digestion of the straw was carried out to evaluate the saccharification efficiency. The nitrogen concentration in the straw dry matter increased linearly from 0.32% to 0.71% over the range of nitrogen treatments. Cellulose and hemicellulose were not affected by the nitrogen supply while lignin peaked at medium rates of nitrogen application. The nitrogen treatments had a distinct influence on the silicon concentration, which decreased from 2.5% to 1.5% of the straw dry matter when the nitrogen supply increased from 48 to 192 kg ha⁻¹. No further decline in Si occurred at higher rates of nitrogen application. The most abundant metals in the straw were potassium and calcium and their concentrations almost doubled over the range of nitrogen supplies. The enzymatic saccharification efficiency was negatively correlated with the rate of nitrogen supply. We conclude that the level of nitrogen supply to wheat plants alters the composition of cell wall components in the straw and that this may result in reduced saccharification efficiency.     

Comparative net energy ratio analysis of pellet produced from steam pretreated biomass from agricultural residues and energy crops

A process model was developed to determine the net energy ratio (NER) for the production of pellets from steam pretreated agricultural residue (wheat straw) and energy crops (i.e., switchgrass in this case). The NER is a ratio of the net energy output to the total net energy input from non-renewable energy sources into a system. Scenarios were developed to measure the effects of temperature and level of steam pretreatment on the NER of steam pretreated wheat straw and switchgrass pellets. The NERs for the base case at 6 kg h⁻¹ are 1.76 and 1.37 for steam-pretreated wheat straw and switchgrass-based pellets, respectively. The reason behind the difference is that more energy is required to dry switchgrass pellets than wheat straw pellets. The sensitivity analysis for the model shows that the optimum temperature for steam pretreatment is 160 °C with 50% pretreatment (i.e. 50 % steam treated material is blended with the raw biomass and then pelletised). The uncertainty results for NER for steam pretreated wheat straw and switch grass pellets are 1.62 ± 0.10 and 1.42 ± 0.11, respectively.     

Fuel ethanol production from sweet sorghum bagasse using microwave irradiation

Sweet sorghum is a hardy crop that can be grown on marginal land and can provide both food and energy in an integrated food and energy system. Lignocellulose rich sweet sorghum bagasse (solid left over after starch and juice extraction) can be converted to bioethanol using a variety of technologies. The largest barrier to commercial production of fuel ethanol from lignocellulosic material remains the high processing costs associated with enzymatic hydrolysis and the use of acids and bases in the pretreatment step. In this paper, sweet sorghum bagasse was pretreated and hydrolysed in a single step using microwave irradiation. A total sugar yield of 820 g kg⁻¹ was obtained in a 50 g kg⁻¹ sulphuric acid solution in water, with a power input of 43.2 kJ g⁻¹ of dry biomass (i.e. 20 min at 180 W power setting). An ethanol yield based on total sugar of 480 g kg⁻¹ was obtained after 24 h of fermentation using a mixed culture of organisms. These results show the potential for producing as much as 0.252 m³ tonne⁻¹ or 33 m³ ha⁻¹ ethanol using only the lignocellulose part of the stalks, which is high enough to make the process economically attractive.     

Pretreatment of energy crops with sodium hydroxide and cellulolytic enzymes to increase biogas production

This work presents the influence of alkali pretreatment on the enzymatic hydrolysis and efficiency of anaerobic digestion of lignocellulosic biomass pretreated both in a one- (chemical or enzymatic) and two-step (chemical and enzymatic) process. In this study two species of energy crops were used Miscanthus giganteus and Sida hermaphrodita. The aim of this work was to compare biogas production and methane yield during fermentation of pretreated and untreated energy crops. The results show that alkali pretreatment is necessary for the effective biogas generation from plant material due to high delignification level and significant hemicellulose degradation. The two-step hydrolysis process consisting on the alkali and enzymatic step leads to the release of high concentrations of glucose (about 20 g L⁻¹). The best results were achieved for M. giganteus with biogas production yield of 421.5 Ndm³ kg TS⁻¹ and with methane production yield of 257 Ndm³ kg TS⁻¹.     

Carbon and nutrients of Scots pine stands on sandy soils in Lithuania in relation to bioenergy sustainability

Sustainable forestry is based on the principle that harvesting practices should avoid negative influence on soil fertility, wood production and long-term soil carbon (C) stocks. We examined C and nutrient concentrations and stocks of Scots pine (Pinus sylvestris L.) stands on Arenosols in south-western Lithuania. The stands were 10, 20, 40, 50 and 65 years of age. C concentrations were relatively constant, while the concentrations of N, P, K, Ca, Mg and S often varied between compartments and stand ages.The total aboveground stocks of nitrogen (N) were estimated to be in the range of 185–260 kg ha⁻¹, and 78–189 kg ha⁻¹ for calcium (Ca), 75–104 kg ha⁻¹ for potassium (K), 22–33 kg ha⁻¹ for phosphorus (P), 21–41 kg ha⁻¹ for magnesium (Mg) and 16–28 kg ha⁻¹ for sulphur (S). Corresponding stocks of the crown alone were 139–207 kg ha⁻¹ of N, 54–88 kg ha⁻¹ of Ca, 44–79 kg ha⁻¹ of K, 15–26 kg ha⁻¹ of P, 15–23 kg ha⁻¹ of Mg, and 11–15 kg ha⁻¹ of S. Biomass, C and nutrient stocks in the crown did not change with age, whereas the stemwood stocks increased with stand age. The total removals of C and N over a whole 100-year rotation were simulated to be 129 Mg ha⁻¹ and 449 kg ha⁻¹, respectively. An example scenario was created to compare the magnitude of potential nutrient removals with the atmospheric influx, soil stocks, and the internal litterfall flux. We suggest that intensified utilisation of these stands for bioenergy may be sustainable.