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Wood pellets on the German pellet market mainly consist of spruce wood, but other wood species are increasingly used during pellet production. Moreover, additives such as starch or kaolin can be added to improve fuel quality and combustion behavior. Consequently, wood pellets vary in their physical and chemical properties and this variation might affect combustion behavior in stoves and boilers. In this study, eight different wood species were used for pellet production at the Technology and Support Centre (TFZ). Three types of starch and one type of kaolin were added to spruce wood without bark, using two additivation levels (starch: 0.5 w-% and 1.8 w-%, kaolin: 0.17 and 0.34 w-%). All pellet assortments were analyzed according to international standards for solid biofuels and additional methods. Fuels were combusted in a 6-kW pellet stove and a 15-kW pellet boiler. Selection of wood species had a significant impact on CO emissions for the pellet stove ranging between 85 and 4189 mg/m3 and on TPM emissions ranging between 23 and 263 mg/m3. Similar combustion behavior was observed for the pellet boiler but on a lower emission level. Kaolin reduced CO and TPM emissions as well as slag formation when the additivation level was set according to stoichiometric calculations. When kaolin was overdosed, it had a negative impact on TPM emissions mainly in the pellet boiler, probably due to entrainment. Regarding particles from incomplete combustion, a negative impact of overdosed kaolin was observed. Overall, at the current state of the art, the pellet stove showed larger sensitivity towards variable pellet qualities compared to the automatic pellet boiler. This calls for further improvements in fuel design and stove technology.

Food waste (FW) from households represents a major fraction of municipal waste and it is often collected in separate biowaste bins. Until waste collection is carried out, storage conditions in the biowaste bin influence FW properties. To draw conclusions for an optimized waste utilization in anaerobic digestion (AD), the aim of this study was to evaluate the impact of storage duration (20 to 40 days) and temperature (5 °C and 20 °C) on inherent energy potentials of household FW during aerobic and anaerobic storage. Therefore, physico-chemical parameters of recipe-based FW samples with reproducible initial compositions were monitored. After 20 days of aerobic storage, water contents (WC) were reduced from 61.9% to 39.5% (20 °C) and from 63.9% to 50.3% (5 °C) while organic dry matter (oDM) concentrations were lowered by 4.3% (20 °C) and 1.1% (5 °C). Increased pH-values of 6.6 (initially 5.5) were only measured for FW stored aerobically at 20 °C. In total, the energy potential was decreased by 31% (20 °C) and by 16% (5 °C). Thus, storage temperature and duration are crucial parameters for optimized aerobic FW storage leading to higher energy yields in AD. Instead, anaerobic storage of FW decreased pH-values to5 while increasing WC in all samples (up to 67% at 20 °C). As oDM concentrations were preserved almost completely, the energy potential losses were only marginal proving that energy contents of FW could be preserved at household level. Consequently, energy yields in AD of FW could be increased through anaerobic storage conditions.

In Germany, wood pellets must fulfill high quality standards to be utilized in small-scale furnaces. According to current legislation, i.e., the 1st BImSchV, only ENplus A1–certified pellets may be used. Despite these strict requirements, physical fuel parameters can vary within the permissible range and variation usually increases due to fuel transport and storage. Fluctuations in physical fuel parameters may have a strong influence on combustion, especially in pellet stoves with a low degree of automation regarding fuel control and air supply. During this study, pellet length, moisture content, and the content of fines of spruce wood pellets were varied artificially at three levels for each parameter. All pellets were analyzed according to international standards for solid biofuels. Fuels were combusted in a 6 kW pellet stove and selected assortments in a 15 kW pellet boiler. For the pellet stove, pellet length had a significant influence on gaseous but not on total particulate matter (TPM) emissions. Both, rather short (10.3 mm) and long (17 mm) pellets caused an increase in emissions. In contrast, for the boiler, no significant effect of pellet length on gaseous and TPM emissions was observed. A low moisture content (3 w-%) led to increased CO, organic gaseous carbon (OGC), and TPM emissions in the pellet stove. This could be especially relevant for furnaces with very simple or no heat output control as it is common in most pellet stoves. High content of fines (3 to 10 w-%) led to a significant increase in gaseous emissions (CO up to fivefold, OGC up to tenfold) and TPM emissions (up to 1.4-fold) in the pellet stove most likely due to elevated particle entrainment. Overall, the state-of-the-art pellet stove showed larger sensitivity towards variable pellet qualities compared to the automatically stoked pellet boiler; thus, improvements in fuel design and stove technology are recommended. Overall, this study has provided important insights into the influence of pellet length, fines content, and moisture content on emissions from pellet stoves and pellet boilers, which can help drive future technical development in the areas of fuel flexibility and emission reduction.

The aim of this study was the modification of agricultural biofuels to allow for low emission combustion in domestic biomass boilers without slagging or other ash related problems. Four agricultural biomasses were additivated with kaolin. Fuel blends were produced from wheat straw and woody biomass. All fuels were combusted in a 30 kW biomass boiler while emissions and slag formation were measured. Due to additivation or fuel blending, total particle and CO emissions were reduced by 53–77% and 69–95%, respectively. At the same time, slag formation (determined as the mass of agglomerates > 3.15 mm) strongly declined. For fuels containing high amounts of sulphur or chlorine, kaolin additivation may lead to increased HCl and SOx emissions. Moreover, NOx-formation may be higher during combustion of additivated fuels (up to 21%). Overall, additivation with kaolin or blending of fuels might be an interesting option for the utilization of agricultural biomasses. The selection of the optimal upgrading method should consider the chemical and physical biomass properties and address the minimum amount of additive or blending ratio to achieve a sufficient reduction of emissions and slagging.

Wood chip quality is essential for failure-free and low-emission combustion in automatically stoked small-scale biomass boilers (

Within six case studies, different drying and sieving process steps were employed for the removal of adhering soil and other extraneous impurities from wood chips. Consequently, it was systematically investigated to which extent this strategy can be used to jointly mitigate the risk of bottom ash slagging and high pollutant emission levels during combustion in an automatically stoked small-scale boiler. Throughout all combustion tests, slag and emission formation were recorded. Formation of agglomerates in the bottom ash was not observed in the fuel bed. However, fuel processing resulted in an increase of the ash shrinking–softening range up to 230 K indicating a lower slagging risk in the bottom ash. An asymptotic trend for the ash melting temperatures was observed as a function of the molar (Si + P + K)/(Ca + Mg) ratio. It was also found that potassium is less efficiently retained in the bottom ash with lower Si content in the fuel. Lower moisture contents in the wood chips typically resulted in lower CO emissions and higher boiler efficiencies for the investigated range of moisture content. The sieving of the unprocessed wood chips reduced NOx emission levels up to 28%. However, fuel processing did not necessarily reduce the level of particulate matter emissions.

Ashes from biomass heat (and power) plants that apply untreated woody biofuels may be suitable for use as fertilizers if certain requirements regarding pollutant and nutrient contents are met. The aim of this study was to examine if both bottom and cyclone ashes from 17 Bavarian heating plants and one ash collection depot are suitable as fertilizers (n = 50). The range and average values of relevant nutrients and pollutants in the ashes were analyzed and evaluated for conformity with the German Fertilizer Ordinance (DüMV). Approximately 30% of the bottom ashes directly complied with the heavy metal limits of the Fertilizer Ordinance. The limits were exceeded for chromium(VI) (62%), cadmium (12%) and lead (4%). If chromium(VI) could be reduced by suitable treatment, 85% of the bottom ashes would comply with the required limit values. Cyclone ashes were high in cadmium, lead, and zinc. The analysis of the main nutrients showed high values for potassium and calcium in bottom ashes, but also relevant amounts of phosphorus, making them suitable as fertilizers if pollutant limits are met. Quality assurance systems should be applied at biomass heating plants to improve ash quality if wood ashes are used as fertilizers in agriculture.

The storage of wood chips is important for the biomass supply chain as it compensates for temporal differences in production and consumption. Typical storage-related problems are dry matter and energy losses due to microbial activity. In extensive field trials, we investigated the storage of spruce wood chips from forest residues (FRC) and from energy roundwood (ERC) with and without rain protection under Central European conditions. Additionally, we examined the storage of unchipped piles. The results indicate that the investigated factors, i. e. storage duration, season, assortment and rain protection, have a statistically significant influence on moisture content and dry matter loss of wood chips. During five months of storage, the highest decline in moisture content was 22.6 %-points, the highest dry matter loss 11.1 %. In winter, energy losses reached up to 11.3 %. In summer, energy contents did not change or even increased slightly (max. 4.7 %). Pile temperature and dry matter losses were significantly positively correlated in FRC. Formation of different layers within the piles could be detected. Storage performance was better in unchipped than in chipped energy roundwood. Storage of unchipped forest residues was not beneficial concerning energy content, but fuel quality increased due to reduced ash and fine particle content. Clear best practice recommendations could be drawn regarding wood chip storage under Central European conditions. During winter, FRC should be stored with rain protection or as short as possible while during a dry and warm summer, wood chips can be stored with only few restrictions.

Chipper fine tuning, i. e. the selection of suitable feedstocks and machine settings for comminution, might help to optimize wood chip production in terms of chip quality and energy efficiency. A total of 46 wood chip samples were produced with an electrically powered stationary drum chipper. Feedstocks derived from small diameter stem wood (O

The introduction of wood chips into the market for small-scale heating appliances of below 100 kW has become more challenging due to stricter regulations on emission levels for carbon monoxide and particle emission, e.g. in Germany. Therefore, it is important to identify high wood chip qualities that are suitable for failure-free and low-emission combustion. In this investigation, several wood chip assortments produced from stem wood and forest residues were analysed and combusted in up to two different wood chip boilers with a nominal heat capacity of 50 and 99 kW at full load. Some combustion tests were performed at part load (i.e. 30% heat output). Throughout experiments, the emission behaviour was measured (CO, NOX and total particulate matter (TPM)). In total, seven different wood chip assortments were tested in the large boiler, whereas ten wood chip assortments were combusted in the smaller furnace. Wood chips from stem wood are characterized by lower ash contents (0.27–1.05 w%, d.b.), lower nitrogen contents (0.04–0.14 w%, d.b.) and lower aerosol forming elements (379–1075 mg/kg, d.b.) compared to forest residues (A = 0.58–2.05 w%¸ N = 0.13–0.32 w%, aerosols = 728–2199 mg/kg, d.b.). Combustion of stem wood caused lower emissions compared to forest residues at full-load operation. Overall, part-load operation caused higher CO and particle emissions. In addition, increases of fine woody particles lead to increases in CO emissions. The influence of moisture content was more pronounced especially for CO emissions.

Air pressure resistance during ventilation was measured experimentally for a high variety of conventional wood chip samples that had been produced using various raw materials, chippers and chipper settings (n = 95). Physical fuel properties such as bulk density or particle size distribution were determined according to international standards for solid biofuels. In addition, particle length and particle shape were determined using a continuously measuring image analysis device. Pressure resistance (in Pa m−1) was determined using a custom-built flow cylinder (2 m height, 0.4 m3 filling volume) and related to air flow rate. Pressure resistance increased exponentially with air flow rate for all samples. Thereby, pressure resistance was highly variable among wood chip samples, ranging from 10 to 107 Pa m−1 at low air flow rates (0.1 m3 s−1 m−2) and from 197 to 1240 Pa m−1 at higher air flow rates (0.5 m3 s−1 m−2). Four common equations to describe pressure resistance (Ramsin, Shedd, Hukill & Ives, Ergun) were compared. Multiple linear regression analysis was applied to express pressure resistance as a function of selected fuel quality parameters. Sensitivity analyses of the resulting equations were performed. Results indicate that pressure resistance strongly depends on bulk density, particle size and particle shape as these parameters determine the number, size and shape of free air voids within wood chip fillings.

Aerobic and anaerobic storage of wood chips from coniferous forest residues, coniferous energy roundwood and short rotation coppice were investigated regarding dry matter losses, energy losses and changes in fuel properties using several approaches, i.e. a field trial with 90 m3 wood chip piles, small-scale storage containers (0.5 m3) incl. greenhouse gas measurements and miniature bunker silos (1.5–2 m3). After storage, selected fuels were combusted in a 30 kW wood chip boiler and emissions (CO, NOX, TPM) were measured. Dry matter losses in fleece-covered piles were 7%–9% after 153 days of storage. Drying compensated for energy losses. Container trials achieved full anaerobic conditions in the lab reducing dry matter losses to 0.4–2% while preserving fuel properties. Methane emissions during anaerobic storage in containers were low. Anaerobic conditions could not be achieved in outdoor bunker silos due to insufficient airtightness. The mass fraction of H2O in fuels increased in bunker silos by 4.2–10.4% due to precipitation and dry matter losses up to 22% were recorded. During combustion, CO and TPM emissions were often increased significantly with materials that were stored in silos or piles compared to technically dried fuels, most likely due to changes in their woody or physical-mechanical structure and their mass fraction of H2O. Anaerobic storage might in theory be an interesting option to reduce dry matter losses but could not sufficiently be realized during this study using miniature bunker silos. For many applications, aerobic storage in fleece-covered piles seems preferable to anaerobic storage.

The ERA-NET Project “BioFlex!” aims at the utilisation of agricultural fuels for failure free and low emission combustion in domestic biomass boilers. Within the project, the effect of kaolin additivation of four agricultural biomass fuels as well as of fuel blends of wheat straw with woody biomass was assessed regarding combustion behaviour in small scale boilers. By additivation of kaolin as well as by fuel blending, total particle emissions and CO emissions were reduced by 53 to 77 % and 69 to 95 %, respectively. In addition, slag formation (determined as amount of agglomerates > 3.15 mm) of the agricultural fuels was also reduced. Thus, additivation with kaolin or blending of agricultural fuels might be an interesting option for the utilisation of “difficult” biomass fuels. However, regarding the pelletisation process itself, a distinct increase in wearing off of the dies and the rollers of the pellet mill was observed. This also has to be considered during production., Proceedings of the 26th European Biomass Conference and Exhibition, 14-17 May 2018, Copenhagen, Denmark, pp. 408-417

The removal of fine particles such as needles or small pieces of wood and bark by screening might improve storage behaviour of wood chips due to increased pile ventilation and decreased degradation by microbes. Field trials on storage of screened and unscreened wood chips (forest residues, energy roundwood) in large piles (100 m3, each) were performed for 5 month during the summer of 2017. To monitor changes in fuel quality and dry matter losses, a total of 36 balance bags were placed into each wood chip pile. Highest pile temperatures were measured in the unscreened forest residue wood chip pile (63.1 °C). Screening strongly reduced pile temperature and, thus, might reduce the risk of pile self ignition. Best drying (-20.9 w-%) was observed for the screened forest residue chips. Overall, wood chip piles of forest residues showed better drying, but wood chip piles of energy roundwood revealed smaller dry matter losses., Proceedings of the 26th European Biomass Conference and Exhibition, 14-17 May 2018, Copenhagen, Denmark, pp. 896-899

Fuel qualities of wood chips and energy consumption during their production vary depending on raw materials and machine settings. Thereby, requirements for small combustion units

Newly developed gravimetric or electric methods may be suitable alternatives to provide a rapid and accurate moisture content determination of wood chips. In total, nine different rapid determination devices were selected for testing including two infrared dryers, four dielectric instruments, two conductivity methods and one time-domain-reflectometry method. Oven drying according to ISO 18134-2 was used as reference. Testing was carried out on different raw materials of wood chips and at different levels of moisture content. On average, the mean measuring deviation from the reference value ranged from -2.6 w-% (± 1.7 w-% SD) to 3.8 w-% (± 4.7 w-% SD). Thereby, best results were obtained with the infrared dryer MA35 for a large range of different moisture contents. In contrast, the electric methods were less accurate but allowed for larger sample sizes. Furthermore, instrument accuracy strongly increased with decreasing moisture content. The high accuracy of the oven drying method could not be reached by any of the tested devices. However, by using a high amount of samples the average moisture content of all measurements approximated the reference value. Therefore, some instruments could be recommended for quality assurance during production., Proceedings of the 24th European Biomass Conference and Exhibition, 6-9 June 2016, Amsterdam, The Netherlands, pp. 634-638

Biological degradation processes during storage of fresh wood chips in large piles may cause high dry matter (DM) losses and a decline in fuel quality. To monitor storage effects, two field trials were conducted i. e. one during winter and one during summer. For each trial, four piles of wood chips were established using two raw materials and two treatments: wood chips from forest residues (FRC) and from energy roundwood (ERC) covered with a fleece and uncovered. Additionally, two piles of unchipped raw material were established during each trial to compare storage effects of chipped and unchipped material. During winter storage, drying of wood chips was low. DM losses amounted up to 1.6 w-% and 0.7 w-% per month for FRC and ERC, respectively. During summer, fuel quality of wood chips in piles improved due to large drying effects. However, DM losses of ERC were twice as high compared to winter. Storage of unchipped material did not improve drying but led to lower DM decomposition compared to storage in wood chip piles. Thereby, the loss of bark material and needles increased total DM losses in unchipped storage piles but positively affected fuel quality parameters., Proceedings of the 24th European Biomass Conference and Exhibition, 6-9 June 2016, Amsterdam, The Netherlands, pp. 53-59

• The CO(2) efflux of adult trees is supplied by recent photosynthates and carbon (C) stores. The extent to which these C pools contribute to growth and maintenance respiration (R(G) and R(M), respectively) remains obscure. • Recent photosynthates of adult beech (Fagus sylvatica) and spruce (Picea abies) trees were labeled by exposing whole-tree canopies to (13) C-depleted CO(2). Label was applied three times during the year (in spring, early summer and late summer) and changes in the stable C isotope composition (δ(13) C) of trunk and coarse-root CO(2) efflux were quantified. • Seasonal patterns in C translocation rate (CTR) and fractional contribution of label to CO(2) efflux (F(Label-Max)) were found. CTR was fastest during early summer. In beech, F(Label-Max) was lowest in spring and peaked in trunks during late summer (0.6 ± 0.1, mean ± SE), whereas no trend was observed in coarse roots. No seasonal dynamics in F(Label-Max) were found in spruce. • During spring, the R(G) of beech trunks was largely supplied by C stores. Recent photosynthates supplied growth in early summer and refilled C stores in late summer. In spruce, CO(2) efflux was constantly supplied by a mixture of stored (c. 75%) and recent (c. 25%) C. The hypothesis that R(G) is exclusively supplied by recent photosynthates was rejected for both species.

Respiration is a substantial driver of carbon (C) flux in forest ecosystems and stable C isotopes provide an excellent tool for its investigation. We studied seasonal dynamics in δ¹³C of CO₂ efflux (δ¹³C(E)) from non-leafy branches, upper and lower trunks and coarse roots of adult trees, comparing deciduous Fagus sylvatica (European beech) with evergreen Picea abies (Norway spruce). In both species, we observed strong and similar seasonal dynamics in the δ¹³C(E) of above-ground plant components, whereas δ¹³C(E) of coarse roots was rather stable. During summer, δ¹³C(E) of trunks was about -28.2‰ (Beech) and -26.8‰ (Spruce). During winter dormancy, δ¹³C(E) increased by 5.6-9.1‰. The observed dynamics are likely related to a switch from growth to starch accumulation during fall and remobilization of starch, low TCA cycle activity and accumulation of malate by PEPc during winter. The seasonal δ¹³C(E) pattern of branches of Beech and upper trunks of Spruce was less variable, probably because these organs were additionally supplied by winter photosynthesis. In view of our results and pervious studies, we conclude that the pronounced increases in δ¹³C(E) of trunks during the winter results from interrupted access to recent photosynthates.

Stable carbon (C) isotopes, in particular employed in labeling experiments, are an ideal tool to broaden our understanding of C dynamics in trees and forest ecosystems. Here, we present a free-air exposure system, named isoFACE, designed for long-term stable C isotope labeling in the canopy of 25 m tall forest trees. Labeling of canopy air was achieved by continuous release of CO2 with a δ13C of −46.9‰. To this end, micro-porous tubes were suspended at c. 1 m distance vertically through the canopy, minimizing CO2 gradients from the exterior to the interior and allowing for C labeling exposure during periods of low wind speed. Target for CO2 concentration ([CO2]) increase was ambient +100 μmol mol−1. Canopy [CO2] stayed within 10% of the target during more than 57% of the time and resulted in a drop of δ13C in canopy air by 7.8‰. After 19 labeling days about 50% of C in phloem sugars and stem CO2 efflux were turned over and 20–30% in coarse root CO2 efflux and soil CO2. The isoFACE system successfully altered δ13C of canopy air for studying turn-over of C pools in forest trees and soils, highlighting their slow turn-over rates.

Ecuadorian mountain rainforests are declining dramatically due to deforestation. Exploitation of remaining forests has led to low abundances of native, valuable timber species. Enrichment planting of selected native tree species into forest gaps is a strategy that may increase their abundance and maintain biodiversity. However, the development of successful planting strategies requires knowledge of environmental demands on, and ecological requirements of, native species during their establishment. This knowledge is currently lacking for mid- and late-successional species in Central American forests. Two deciduous, mid-successional (Cedrela montana, Tabebuia chrysantha) and two evergreen, late-successional native tree species (Nectandra membranacea, Podocarpus sprucei) were planted into felling gaps. Photosynthetic performance and growth in height of these species were assessed along light gradients during seedling establishment to test whether species-specific light responses were related to plant successional traits. Both mid-successional species benefited from higher light levels in gaps up to 30% canopy openness60°. In larger gaps, C. montana exhibited a significant decline in growth. As expected, growth of the late-successional species was only marginally increased at higher light levels. Nevertheless, the photosynthetic apparatus of N. membranacea displayed rapid acclimation to higher light conditions in gaps. Plant response to felling gaps may not always be predicted based on successional status. Our results suggest that the four investigated species may coexist in the same gap by occupying different niches along light gradients. This arrangement may offer an ecological basis to increase the abundance of valuable timber species through enrichment planting in Ecuador mountain rainforests.

Throughput rate and specific energy consumption of chippers may strongly depend on raw materials, chipper size, chipper type and on specific machine settings such as knife sharpness, screen size or discharge system. Field trials (n = 36) were conducted at various wood chip production sites in Bavarian forests. In addition, stationary experiments with PTO-driven chippers = 150 kW (1 x drum, 1 x disk, 1 x spiral chipper, n = 16) and with an electric drum chipper (n = 46) were conducted at TFZ. Throughput rate (lcm/h, tdm/h) and fuel consumption (l/h, l/lcm, l/tdm) were recorded and related to raw material and chipper settings. Both throughput rate and energy consumption during field trials strongly related to chipper size. Specific fuel consumption ranged from 0.17 to 0.76 l/lcm of bulk material and from 1.13 to 4.10 l/tdm (dry matter), respectively. Mean energy consumed during chipping was 0.50 ± 0.15 % of the energy stored within one ton of dry matter. Observations from field trials could be replicated during stationary experiments, e.g. by highlighting a strong increase in fuel consumption when wood chips were produced with blunt knives., Proceedings of the 22nd European Biomass Conference and Exhibition, 23-26 June 2014, Hamburg, Germany, pp. 483-488

The hypothesis was tested that O(3)-induced changes in leaf-level photosynthetic parameters have the capacity of limiting the seasonal photosynthetic carbon gain of adult beech trees. To this end, canopy-level photosynthetic carbon gain and respiratory carbon loss were assessed in European beech (Fagus sylvatica) by using a physiologically based model, integrating environmental and photosynthetic parameters. The latter were derived from leaves at various canopy positions under the ambient O(3) regime, as prevailing at the forest site (control), or under an experimental twice-ambient O(3) regime (elevated O(3)), as released through a free-air canopy O(3) fumigation system. Gross carbon gain at the canopy-level declined by 1.7%, while respiratory carbon loss increased by 4.6% under elevated O(3). As this outcome only partly accounts for the decline in stem growth, O(3)-induced changes in allocation are referred to and discussed as crucial in quantitatively linking carbon gain with stem growth.

The terrestrial carbon (C) cycle has received increasing interest over the past few decades, however, there is still a lack of understanding of the fate of newly assimilated C allocated within plants and to the soil, stored within ecosystems and lost to the atmosphere. Stable carbon isotope studies can give novel insights into these issues. In this review we provide an overview of an emerging picture of plant-soil-atmosphere C fluxes, as based on C isotope studies, and identify processes determining related C isotope signatures. The first part of the review focuses on isotopic fractionation processes within plants during and after photosynthesis. The second major part elaborates on plant-internal and plant-rhizosphere C allocation patterns at different time scales (diel, seasonal, interannual), including the speed of C transfer and time lags in the coupling of assimilation and respiration, as well as the magnitude and controls of plant-soil C allocation and respiratory fluxes. Plant responses to changing environmental conditions, the functional relationship between the physiological and phenological status of plants and C transfer, and interactions between C, water and nutrient dynamics are discussed. The role of the C counterflow from the rhizosphere to the aboveground parts of the plants, e.g. via CO2 dissolved in the xylem water or as xylem-transported sugars, is highlighted. The third part is centered around belowground C turnover, focusing especially on above- and belowground litter inputs, soil organic matter formation and turnover, production and loss of dissolved organic C, soil respiration and CO2 fixation by soil microbes. Furthermore, plant controls on microbial communities and activity via exudates and litter production as well as microbial community effects on C mineralization are reviewed. A further part of the paper is dedicated to physical interactions between soil CO2 and the soil matrix, such as CO2 diffusion and dissolution processes within the soil profile. Finally, we highlight state-of-the-art stable isotope methodologies and their latest developments. From the presented evidence we conclude that there exists a tight coupling of physical, chemical and biological processes involved in C cycling and C isotope fluxes in the plant-soil-atmosphere system. Generally, research using information from C isotopes allows an integrated view of the different processes involved. However, complex interactions among the range of processes complicate or currently impede the interpretation of isotopic signals in CO2 or organic compounds at the plant and ecosystem level. This review tries to identify present knowledge gaps in correctly interpreting carbon stable isotope signals in the plant-soil-atmosphere system and how future research approaches could contribute to closing these gaps.

Respiration is a substantial driver of carbon (C) flux in forest ecosystems and stable C isotopes provide an excellent tool for its investigation. We studied seasonal dynamics in δ¹³C of CO₂ efflux (δ¹³C(E)) from non-leafy branches, upper and lower trunks and coarse roots of adult trees, comparing deciduous Fagus sylvatica (European beech) with evergreen Picea abies (Norway spruce). In both species, we observed strong and similar seasonal dynamics in the δ¹³C(E) of above-ground plant components, whereas δ¹³C(E) of coarse roots was rather stable. During summer, δ¹³C(E) of trunks was about -28.2‰ (Beech) and -26.8‰ (Spruce). During winter dormancy, δ¹³C(E) increased by 5.6-9.1‰. The observed dynamics are likely related to a switch from growth to starch accumulation during fall and remobilization of starch, low TCA cycle activity and accumulation of malate by PEPc during winter. The seasonal δ¹³C(E) pattern of branches of Beech and upper trunks of Spruce was less variable, probably because these organs were additionally supplied by winter photosynthesis. In view of our results and pervious studies, we conclude that the pronounced increases in δ¹³C(E) of trunks during the winter results from interrupted access to recent photosynthates.

Critical elements such as chloride (Cl), potassium (K), silicon (Si) or heavy metals might determine high temperature corrosion, emission behavior and slagging of solid biofuels during combustion. Using the fact that plant material contains distinctly less iron (Fe) than mineral soils and vice versa manganese (Mn) in plant tissue is significant higher than in soil material and that the Al-content of plant biomass is limited, indexes could be created to classify wood chips into "contaminated with soil" and uncontaminated "biomass" by building ratios of Fe/Mn and Al/200 Thus, "biomass fuel indexes for the contamination with mineral soil" (BFICS) were developed. Plausibility checks were carried out, using tree biomass of various species und soil datasets from other research projects. They showed that for Pinus sylvestris an adapted Al-index is necessary. 2.7 % of the tree samples, Pinus sylvestris excluded, exceed the threshold value of 1 for the Fe/Mn-index and only 3.3 % exceed 1 for Al/200. The newly developed BFICS are useful to determine the source of critical elements for combustion, for aerosol formation, to detect high shares of non-combustible material and to prove the effectivity of pre-treatments like screening., Proceedings of the 24th European Biomass Conference and Exhibition, 6-9 June 2016, Amsterdam, The Netherlands, pp. 630-633

Critical chemical elements such as nitrogen (N), sulphur (S), chloride (Cl), potassium (K) or heavy metals might determine high temperature corrosion, emission behavior and slagging of solid biofuels during combustion. Recently developed biomass fuel indexes may help to pre-evaluate fuel behavior during combustion. In total, 92 samples of wood chips (n = 75), of unchipped material (n = 11) and of twigs and needles (n = 6) were collected and analyzed for elemental composition (As, Ca, Cd, Cl, Cr, Cu, Hg, K, Mg, N, Na, Ni, Pb, S, Si, Zn). Biomass fuel indexes were calculated from the results to pre-evaluate NOX and particle emissions, high temperature corrosion, K release and slagging behavior. Wood chips from natural wood showed low concentrations of critical chemical elements. With increasing shares of mineral soil, critical elements also increased. Biomass fuel indexes such as the sum of aerosol forming elements (K, Na, Pb, Zn) or the molar Si/(Ca+Mg) ratio for ash-melting behavior indicated that biomass fuels without contamination with mineral soil, road salt or other anthropogenic waste can be considered uncritical for combustion., Proceedings of the 24th European Biomass Conference and Exhibition, 6-9 June 2016, Amsterdam, The Netherlands, pp. 573-577