The carbon implications of large‐scale afforestation of agriculturally marginal land with short‐rotation willow in Saskatchewan

Afforestation with short‐rotation coppice (SRC) willow plantations for the purpose of producing bioenergy feedstock was contemplated as one potential climate change mitigation option. The objectives of this study were to assess the magnitude of this mitigation potential by addressing: (i) the land area potentially available for SRC systems in the province of Saskatchewan, Canada; (ii) the potential biomass yields of SRC plantations; and (iii) the carbon implications from such a large‐scale afforestation program. Digital soils and land‐use data were used to identify, map, and group into clusters of similar polygons 2.12 million hectares (Mha) of agriculturally marginal land that was potentially suitable for willow in the Boreal Plains and Prairies ecozones in Saskatchewan. The Physiological Principles in Predicting Growth (3PG) model was calibrated with data from SRC experiments in Saskatchewan, to quantify potential willow biomass yields, and the Carbon Budget Model of the Canadian Forest Sector (CBM‐CFS3), was used to simulate stand and landscape‐level C fluxes and stocks. Short‐rotation willow plantations managed in 3 year rotations for seven consecutive harvests (21 years) after coppicing at Year 1 produced about 12 Mg ha^?1 yr^?1 biomass. The more significant contribution to the C cycle was the cumulative harvest. After 44 years, the potential average cumulative harvested biomass C in the Prairies was 244 Mg C ha^?1 (5.5 Mg C ha^?1 yr^?1) about 20% higher than the average for the Boreal Plains, 203 Mg C ha^?1 (4.6 Mg C ha^?1 yr^?1). This analysis did not consider afforestation costs, rate of establishment of willow plantations, and other constraints, such as drought and disease effects on biomass yield. The results must therefore be interpreted as a biophysical mitigation potential with the technical and economic potential being both lower than our estimates. Nevertheless, short‐rotation bioenergy plantations offer one potential mitigation option to reduce the rate of CO₂ accumulation in the earth's atmosphere and further research is needed to operationalise such a mitigation effort.     

Using CO2 to enhance carbon capture and biomass applications of freshwater macroalgae

A major limiting factor in the development of algae as a feedstock for the bioenergy industry is the consistent production and supply of biomass. This study is the first to access the suitability of the freshwater macroalgal genus Oedogonium to supply biomass for bioenergy applications. Specifically, we quantified the effect of CO₂ supplementation on the rate of biomass production, carbon capture, and feedstock quality of Oedogonium when cultured in large‐scale outdoor tanks. Oedogonium cultures maintained at a pH of 7.5 through the addition of CO₂ resulted in biomass productivities of 8.33 (±0.51) g DW m^?2 day^?1, which was 2.5 times higher than controls which had an average productivity of 3.37 (±0.75) g DW m^?2 day^?1. Under these productivities, Oedogonium had a carbon content of 41–45% and a higher heating value of 18.5 MJ kg^?1, making it an ideal biomass energy feedstock. The rate of carbon fixation was 1380 g C m^?2 yr^?1 and 1073.1 g C m^?2 yr^?1 for cultures maintained at a pH of 7.5 and 8.5, and 481 g C m^?2 yr^?1 for cultures not supplemented with CO₂. This study highlights the potential of integrating the large‐scale culture of freshwater macroalgae with existing carbon waste streams, for example coal‐fired power stations, both as a tool for carbon sequestration and as an enhanced and sustainable source of bioenergy.     

Comparative assessment of ecosystem C exchange in Miscanthus and reed canary grass during early establishment

Land‐use change to bioenergy crop production can contribute towards addressing the dual challenges of greenhouse gas mitigation and energy security. Realisation of the mitigation potential of bioenergy crops is, however, dependent on suitable crop selection and full assessment of the carbon (C) emissions associated with land conversion. Using eddy covariance‐based estimates, ecosystem C exchange was studied during the early‐establishment phase of two perennial crops, C₃ reed canary grass (RCG) and C₄ Miscanthus, planted on former grassland in Ireland. Crop development was the main determinant of net carbon exchange in the Miscanthus crop, restricting significant net C uptake during the first 2 years of establishment. The Miscanthus ecosystem switched from being a net C source in the conversion year to a strong net C sink (?411 ± 63 g C m^?2) in the third year, driven by significant above‐ground growth and leaf expansion. For RCG, early establishment and rapid canopy development facilitated a net C sink in the first 2 years of growth (?319 ± 57 (post‐planting) and ?397 ± 114 g C m^?2, respectively). Peak seasonal C uptake occurred three months earlier in RCG (May) than Miscanthus (August), however Miscanthus sustained net C uptake longer into the autumn and was close to C‐neutral in winter. Leaf longevity is therefore a key advantage of C₄ Miscanthus in temperate climates. Further increases in productivity are projected as Miscanthus reaches maturity and are likely to further enhance the C sink potential of Miscanthus relative to RCG.     

Carbon and nitrogen dynamics in bioenergy ecosystems: 2. Potential greenhouse gas emissions and global warming intensity in the conterminous United States

This study estimated the potential emissions of greenhouse gases (GHG) from bioenergy ecosystems with a biogeochemical model AgTEM, assuming maize (Zea mays L.), switchgrass (Panicum virgatum L.), and Miscanthus (Miscanthus × giganteus) will be grown on the current maize‐producing areas in the conterminous United States. We found that the maize ecosystem acts as a mild net carbon source while cellulosic ecosystems (i.e., switchgrass and Miscanthus) act as mild sinks. Nitrogen fertilizer use is an important factor affecting biomass production and N₂O emissions, especially in the maize ecosystem. To maintain high biomass productivity, the maize ecosystem emits much more GHG, including CO₂ and N₂O, than switchgrass and Miscanthus ecosystems, when high‐rate nitrogen fertilizers are applied. For maize, the global warming potential (GWP) amounts to 1–2 Mg CO₂eq ha^?1 yr^?1, with a dominant contribution of over 90% from N₂O emissions. Cellulosic crops contribute to the GWP of less than 0.3 Mg CO₂eq ha^?1 yr^?1. Among all three bioenergy crops, Miscanthus is the most biofuel productive and the least GHG intensive at a given cropland. Regional model simulations suggested that substituting Miscanthus for maize to produce biofuel could potentially save land and reduce GHG emissions.     

Initial soil C and land‐use history determine soil C sequestration under perennial bioenergy crops

In the UK and other temperate regions, short rotation coppice (SRC) and Miscanthus x giganteus (Miscanthus) are two of the leading ‘second‐generation’ bioenergy crops. Grown specifically as a low‐carbon (C) fossil fuel replacement, calculations of the climate mitigation provided by these bioenergy crops rely on accurate data. There are concerns that uncertainty about impacts on soil C stocks of transitions from current agricultural land use to these bioenergy crops could lead to either an under‐ or overestimate of their climate mitigation potential. Here, for locations across mainland Great Britain (GB), a paired‐site approach and a combination of 30‐cm‐ and 1‐m‐deep soil sampling were used to quantify impacts of bioenergy land‐use transitions on soil C stocks in 41 commercial land‐use transitions; 12 arable to SRC, 9 grasslands to SRC, 11 arable to Miscanthus and 9 grasslands to Miscanthus. Mean soil C stocks were lower under both bioenergy crops than under the grassland controls but only significant at 0–30 cm. Mean soil C stocks at 0–30 cm were 33.55 ± 7.52 Mg C ha^?1 and 26.83 ± 8.08 Mg C ha^?1 lower under SRC (P = 0.004) and Miscanthus plantations (P = 0.001), respectively. Differences between bioenergy crops and arable controls were not significant in either the 30‐cm or 1‐m soil cores and smaller than for transitions from grassland. No correlation was detected between change in soil C stock and bioenergy crop age (time since establishment) or soil texture. Change in soil C stock was, however, negatively correlated with the soil C stock in the original land use. We suggest, therefore, that selection of sites for bioenergy crop establishment with lower soil C stocks, most often under arable land use, is the most likely to result in increased soil C stocks.     

Modeling spatial and dynamic variation in growth,yield, and yield stability of the bioenergy crops Miscanthus × giganteus and Panicum virgatum across the conterminous United States

C₄ perennial grasses are being considered as environmentally and economically sustainable high yielding bioenergy feedstocks. Temporal and spatial variation in yield across the conterminious United States is uncertain due to the limited number of field trials. Here, we use a semi‐mechanistic dynamic crop growth and production model to explore the potential of Miscanthus × giganteus (Greef et. Deu.) and Panicum virgatum L. across the conterminous United States. By running the model for 32 years (1979–2010), we were able to estimate dry biomass production and stability. The maximum rainfed simulated end‐of‐growth‐season harvestable biomass for M. × giganteus was ca. 40 Mg ha^?1 and ca. 20 Mg ha^?1 for P. virgatum. In addition, regions of the southeastern United States were identified as promising due to their high potential production and stability and their relative advantage when compared with county‐level maize biomass production. Regional and temporal variation was most strongly influenced by precipitation and soil water holding capacity. Miscanthus × giganteus was on average 2.2 times more productive than P. virgatum for locations where yields were ≥10 Mg ha^?1. The predictive ability of the model for P. virgatum was tested with 30 previously published studies covering the eastern half of the United States and resulted in an index of agreement of 0.71 and a mean bias of only ?0.62 Mg ha^?1 showing that, on average, the model tended to only slightly overestimate productivity. This study provides with potential production and variability which can be used for regional assessment of the suitability of dedicated bioenergy crops.     

Crassulacean acid metabolism (CAM) offers sustainable bioenergy production and resilience to climate change

Biomass production on low‐grade land is needed to meet future energy demands and minimize resource conflicts. This, however, requires improvements in plant water‐use efficiency (WUE) that are beyond conventional C3 and C4 dedicated bioenergy crops. Here we present the first global‐scale geographic information system (GIS)‐based productivity model of two highly water‐efficient crassulacean acid metabolism (CAM) candidates: Agave tequilana and Opuntia ficus‐indica. Features of these plants that translate to WUE advantages over C3 and C4 bioenergy crops include nocturnal stomatal opening, rapid rectifier‐like root hydraulic conductivity responses to fluctuating soil water potential and the capacity to buffer against periods of drought. Yield simulations for the year 2070 were performed under the four representative concentration pathway (RCPs) scenarios presented in the IPCC's 5th Assessment Report. Simulations on low‐grade land suggest that O. ficus‐indica alone has the capacity to meet ‘extreme’ bioenergy demand scenarios (>600 EJ yr^?1) and is highly resilient to climate change (?1%). Agave tequilana is moderately impacted (?11%). These results are significant because bioenergy demand scenarios >600 EJ yr^?1 could be met without significantly increasing conflicts with food production and contributing to deforestation. Both CAM candidates outperformed the C4 bioenergy crop, Panicum virgatum L. (switchgrass) in arid zones in the latitudinal range 30°S–30°N.     

Biomass production potential from Populus short rotation systems in Romania

The aim of this study was to assess the potential of biomass production by short rotation poplar in Romania without constraining agricultural food production. Located in the eastern part of Europe, Romania provides substantial land resources suitable for bioenergy production. The process‐oriented biogeochemical model Landscape DNDC was used in conjunction with the forest‐growth model PSIM to simulate the yield of poplar grown in short‐rotation coppice at different sites in Romania. The model was validated on five sites with different climatic conditions in Central Europe. Using regional site conditions, with climatic parameters and organic carbon content in soil being the most important, the biomass production potential of poplar plantations was simulated for agricultural areas across Romania. Results indicated a mean productivity of 12.2 ± 0.5 t ha^?1 year^?1 of poplar coppices on arable land in Romania. The highest yields were simulated for lowland areas in the south‐east and west and for the Mures valley, whereas the lowest yields – due to either temperature or water limitations – were found for the mountainous regions, the Danube valley, and the region west of Bucharest. The amount of abandoned arable land in the past 10 years indicates that around 10% of cropping land in production in 1999 (approximately 1 million ha) is available for bioenergy production systems today. Production of poplar grown in short‐rotation coppices on these areas would result in a yield of approximately 10 million tons of wood per year. The energy that can be generated by conversion of poplar short rotation coppice biomass may contribute up to approximately 8% of the national energy demand if these set‐aside areas are used for lignocellulosic bioenergy.     

Environmental implications of the use of agro‐industrial residues for biorefineries: application of a deterministic model for indirect land‐use changes

Biorefining agro‐industrial biomass residues for bioenergy production represents an opportunity for both sustainable energy supply and greenhouse gas (GHG) emissions mitigation. Yet, is bioenergy the most sustainable use for these residues? To assess the importance of the alternative use of these residues, a consequential life cycle assessment (LCA) of 32 energy‐focused biorefinery scenarios was performed based on eight selected agro‐industrial residues and four conversion pathways (two involving bioethanol and two biogas). To specifically address indirect land‐use changes (iLUC) induced by the competing feed/food sector, a deterministic iLUC model, addressing global impacts, was developed. A dedicated biochemical model was developed to establish detailed mass, energy, and substance balances for each biomass conversion pathway, as input to the LCA. The results demonstrated that, even for residual biomass, environmental savings from fossil fuel displacement can be completely outbalanced by iLUC, depending on the feed value of the biomass residue. This was the case of industrial residues (e.g. whey and beet molasses) in most of the scenarios assessed. Overall, the GHGs from iLUC impacts were quantified to 4.1 t CO₂‐eq.ha^?1_demanded yr^?1 corresponding to 1.2–1.4 t CO₂‐eq. t^?1 dry biomass diverted from feed to energy market. Only, bioenergy from straw and wild grass was shown to perform better than the alternative use, as no competition with the feed sector was involved. Biogas for heat and power production was the best performing pathway, in a short‐term context. Focusing on transport fuels, bioethanol was generally preferable to biomethane considering conventional biogas upgrading technologies. Based on the results, agro‐industrial residues cannot be considered burden‐free simply because they are a residual biomass and careful accounting of alternative utilization is a prerequisite to assess the sustainability of a given use. In this endeavor, the iLUC factors and biochemical model proposed herein can be used as templates and directly applied to any bioenergy consequential study involving demand for arable land.     

Genotype identity has a more important influence than genotype diversity on shoot biomass productivity in willow short‐rotation coppices

Willow (Salix spp.) short‐rotation coppice is commercially grown to produce lignocellulosic biomass to meet renewable bioenergy demands. Most commercial willow coppices are grown in stands of a single genotype, but biomass productivity may be greater in mixed communities, and the productivity in mixed communities may depend on the specific genotypes involved. We assessed the biomass production of four different Salix genotypes (“Björn,” “Jorr,” “Loden,” “Tora”) grown without additional nutrient fertilization during one cutting cycle at three locations in Europe (Uppsala in Sweden, Rostock and Freiburg in Germany) in plots of pure and mixed communities. We evaluated (i) the effect of genotype diversity on shoot biomass productivity, including the evidence for complementarity and selection effects; (ii) the influence of individual genotypes on mixed community productivity; and (iii) the productivity of individual genotypes in response to pure vs. mixed culture. Mean shoot biomass production after the first cutting cycle decreased in the order Rostock (8.7 Mg ha^?1) > Freiburg (6.9 Mg ha^?1) > Uppsala (5.7 Mg ha^?1), with values similar to those for other nonfertilized willow stands after the first growth cycle. Consistently across all three locations, increasing genotype diversity did not significantly affect shoot biomass production. Using Bayesian statistics, the addition of the genotypes “Jorr” and “Loden” was predicted to enhance shoot biomass production, while “Tora” and “Björn” are more likely to reduce shoot biomass production in mixed communities. In addition, we found evidence for a negative selection effect due to the genotype “Tora” performing better in mixed than in pure communities in two of the sites (Freiburg, Uppsala). In conclusion, our results imply that increasing genetic richness has no negative effect on productivity and that there is a potential to design site‐specific genotype mixtures of short‐rotation coppice promoting both high genetic diversity and high biomass production.     

On the global limits of bioenergy and land use for climate change mitigation

Across energy, agricultural and forestry landscapes, the production of biomass for energy has emerged as a controversial driver of land‐use change. We present a novel, simple methodology, to probe the potential global sustainability limits of bioenergy over time for energy provision and climate change mitigation using a complex‐systems approach for assessing land‐use dynamics. Primary biomass that could provide between 70 EJ year^?1 and 360 EJ year^?1, globally, by 2050 was simulated in the context of different land‐use futures, food diet patterns and climate change mitigation efforts. Our simulations also show ranges of potential greenhouse gas emissions for agriculture, forestry and other land uses by 2050, including not only above‐ground biomass‐related emissions, but also from changes in soil carbon, from as high as 24 GtCO₂eq year^?1 to as low as minus 21 GtCO₂eq year^?1, which would represent a significant source of negative emissions. Based on the modelling simulations, the discussions offer novel insights about bioenergy as part of a broader integrated system. Whilst there are sustainability limits to the scale of bioenergy provision, they are dynamic over time, being responsive to land management options deployed worldwide.     

Conversion of open lands to short‐rotation woody biomass crops: site variability affects nitrogen cycling and N2O fluxes in the US Northern Lake States

Short‐rotation woody biomass crops (SRWC) have been proposed as a major feedstock source for bioenergy generation in the Northeastern US. To quantify the environmental effects and greenhouse gas (GHG) balance of crops including SRWC, investigators need spatially explicit data which encompass entire plantation cycles. A knowledge gap exists for the establishment period which makes current GHG calculations incomplete. In this study, we investigated the effects of converting pasture and hayfields to willow (Salix spp.) and hybrid‐poplar (Populus spp.) SRWC plantations on soil nitrogen (N) cycling, nitrous oxide (N₂O) emissions, and nitrate (NO₃^?) leaching at six sites of varying soil and climate conditions across northern Michigan and Wisconsin, following these plantations from pre conversion through their first 2 years. All six sites responded to establishment with increased N₂O emissions, available inorganic N, and, where it was measured, NO₃^? leaching; however, the magnitude of these impacts varied dramatically among sites. Soil NO₃^? levels varied threefold among sites, with peak extractable NO₃^? concentrations ranging from 15 to 49 g N kg^?1 soil. Leaching losses were significant and persisted through the second year, with 44–112 kg N ha^?1 leached in SRWC plots. N₂O emissions in the first growing season varied 30‐fold among sites, from 0.5 to 17.0 Mg‐CO_2eq ha^?1 (carbon dioxide equivalents). N₂O emissions over 2 years resulted in N₂O emissions due to plantation establishment that ranged from 0.60 to 22.14 Mg‐CO_2eq ha^?1 above baseline control levels across sites. The large N losses we document herein demonstrate the importance of including direct effects of land conversion in life‐cycle analysis (LCA) studies of SRWC GHG balance. Our results also demonstrate the need for better estimation of spatial variability of N cycling processes to quantify the full environmental impacts of SRWC plantations.     

Rapid carbon turnover beneath shrub and tree vegetation is associated with low soil carbon stocks at a subarctic treeline

Climate warming at high northern latitudes has caused substantial increases in plant productivity of tundra vegetation and an expansion of the range of deciduous shrub species. However significant the increase in carbon (C) contained within above‐ground shrub biomass, it is modest in comparison with the amount of C stored in the soil in tundra ecosystems. Here, we use a ‘space‐for‐time’ approach to test the hypothesis that a shift from lower‐productivity tundra heath to higher‐productivity deciduous shrub vegetation in the sub‐Arctic may lead to a loss of soil C that out‐weighs the increase in above‐ground shrub biomass. We further hypothesize that a shift from ericoid to ectomycorrhizal systems coincident with this vegetation change provides a mechanism for the loss of soil C. We sampled soil C stocks, soil surface CO₂ flux rates and fungal growth rates along replicated natural transitions from birch forest (Betula pubescens), through deciduous shrub tundra (Betula nana) to tundra heaths (Empetrum nigrum) near Abisko, Swedish Lapland. We demonstrate that organic horizon soil organic C (SOC_org) is significantly lower at shrub (2.98 ± 0.48 kg m^?2) and forest (2.04 ± 0.25 kg m^?2) plots than at heath plots (7.03 ± 0.79 kg m^?2). Shrub vegetation had the highest respiration rates, suggesting that despite higher rates of C assimilation, C turnover was also very high and less C is sequestered in the ecosystem. Growth rates of fungal hyphae increased across the transition from heath to shrub, suggesting that the action of ectomycorrhizal symbionts in the scavenging of organically bound nutrients is an important pathway by which soil C is made available to microbial degradation. The expansion of deciduous shrubs onto potentially vulnerable arctic soils with large stores of C could therefore represent a significant positive feedback to the climate system.     

Soil carbon changes under Miscanthus driven by C4 accumulation and C3 decompostion – toward a default sequestration function

Bioenergy has to meet increasing sustainability criteria in the EU putting conventional bioenergy crops under pressure. Alternatively, perennial bioenergy crops, such as Miscanthus, show higher greenhouse gas savings with similarly high energy yields. In addition, Miscanthus plantations may sequester additional soil organic carbon (SOC) to mitigate climate change. As the land‐use change in cropland to Miscanthus involves a C₃‐C₄ vegetation change (VC), it is possible to determine the dynamic of Miscanthus‐derived SOC (C₄ carbon) and of the old SOC (C₃ carbon) by the isotopic ratio of ¹³C to ¹²C. We sampled six croplands and adjacent Miscanthus plantations exceeding the age of 10 years across Europe. We found a mean C₄ carbon sequestration rate of 0.78 ± 0.19 Mg ha^?1 yr^?1, which increased with mean annual temperature. At three of six sites, we found a significant increase in C₃ carbon due to the application of organic fertilizers or difference in baseline SOC, which we define as non‐VC‐induced SOC changes. The Rothamsted Carbon Model was used to disentangle the decomposition of old C₃ carbon and the non‐VC‐induced C₃ carbon changes. Subsequently, this method was applied to eight more sites from the literature, resulting in a climate‐dependent VC‐induced SOC sequestration rate (0.40 ± 0.20 Mg ha^?1 yr^?1), as a step toward a default SOC change function for Miscanthus plantations on former croplands in Europe. Furthermore, we conducted a SOC fractionation to assess qualitative SOC changes and the incorporation of C₄ carbon into the soil. Sixteen years after Miscanthus establishment, 68% of the particulate organic matter (POM) was Miscanthus‐derived in 0–10 cm depth. POM was thus the fastest cycling SOC fraction with a C₄ carbon accumulation rate of 0.33 ± 0.05 Mg ha^?1 yr^?1. Miscanthus‐derived SOC also entered the NaOCl‐resistant fraction, comprising 12% in 0–10 cm, which indicates that this fraction was not an inert SOC pool.     

Implications of productivity and nutrient requirements on greenhouse gas balance of annual and perennial bioenergy crops

Biomass from dedicated crops is expected to contribute significantly to the replacement of fossil resources. However, sustainable bioenergy cropping systems must provide high biomass production and low environmental impacts. This study aimed at quantifying biomass production, nutrient removal, expected ethanol production, and greenhouse gas (GHG) balance of six bioenergy crops: Miscanthus × giganteus, switchgrass, fescue, alfalfa, triticale, and fiber sorghum. Biomass production and N, P, K balances (input‐output) were measured during 4 years in a long‐term experiment, which included two nitrogen fertilization treatments. These results were used to calculate a posteriori ‘optimized’ fertilization practices, which would ensure a sustainable production with a nil balance of nutrients. A modified version of the cost/benefit approach proposed by Crutzen et al. (2008), comparing the GHG emissions resulting from N‐P‐K fertilization of bioenergy crops and the GHG emissions saved by replacing fossil fuel, was applied to these ‘optimized’ situations. Biomass production varied among crops between 10.0 (fescue) and 26.9 t DM ha^?1 yr^?1 (miscanthus harvested early) and the expected ethanol production between 1.3 (alfalfa) and 6.1 t ha^?1 yr^?1 (miscanthus harvested early). The cost/benefit ratio ranged from 0.10 (miscanthus harvested late) to 0.71 (fescue); it was closely correlated with the N/C ratio of the harvested biomass, except for alfalfa. The amount of saved CO₂ emissions varied from 1.0 (fescue) to 8.6 t CO₂eq ha^?1 yr^?1 (miscanthus harvested early or late). Due to its high biomass production, miscanthus was able to combine a high production of ethanol and a large saving of CO₂ emissions. Miscanthus and switchgrass harvested late gave the best compromise between low N‐P‐K requirements, high GHG saving per unit of biomass, and high productivity per hectare.     

Tissue chemistry and morphology affect root decomposition of perennial bioenergy grasses on sandy soil in a sub‐tropical environment

Second‐generation biofuels and bio‐based products derived from lignocellulosic biomass are likely to replace current fuels derived from simple sugars and starch because of greater yield potential and less competition with food production. Besides the high aboveground biomass production, these bioenergy grasses also exhibit extensive root systems. The decomposition of root biomass greatly influences nutrient cycling and microbial activity and subsequent accumulation of carbon (C) in the soil. The objective of this research was thus to characterize root morphological and chemical differences in six perennial grass species in order to better understand root decomposition and belowground C cycling of these bioenergy cropping systems. Giant reed (Arundo donax), elephantgrass (Pennisetum purpureum), energycane (Saccharum spp.), sugarcane (Saccharum spp.), sweetcane (Saccharum arundinaceum), and giant miscanthus (Miscanthus × giganteus) were established in Fall 2008 in research plots near Gainesville, Florida. Root decomposition rates were measured in situ from root decomposition bags over 12 months along with initial and final root tissue composition. Root potential decomposition rate constant (K) was higher in elephantgrass (3.64 g kg^?1 day^?1) and sweetcane (2.77 g kg^?1 day^?1) than in sugarcane (1.62 g kg^?1 day^?1) and energycane (1.48 g kg^?1 day^?1). Notably, K was positively related to initial root tissue total C (Total C), total fiber glucose (TFG), total fiber xylose (TFX), and total fiber carbohydrate (TFC) concentrations, but negatively related to total fiber arabinose (TFA) and lignin (TL) concentrations and specific root volume (SRV). Among the six species, elephantgrass exhibited root traits most favorable for fast decomposition: high TFG, high TFX, high TFC, high specific root length (SRL), and a low SRV, whereas giant reed, sugarcane, and energycane exhibited slow decomposition rates and the corresponding root traits. Thus, despite similar aboveground biomass yields in many cases, these species are likely to differentially affect soil C accumulation.     

Quantifying above‐ and belowground biomass carbon loss with forest conversion in tropical lowlands of Sumatra (Indonesia)

Natural forests in South‐East Asia have been extensively converted into other land‐use systems in the past decades and still show high deforestation rates. Historically, lowland forests have been converted into rubber forests, but more recently, the dominant conversion is into oil palm plantations. While it is expected that the large‐scale conversion has strong effects on the carbon cycle, detailed studies quantifying carbon pools and total net primary production (NPP_total) in above‐ and belowground tree biomass in land‐use systems replacing rainforest (incl. oil palm plantations) are rare so far. We measured above‐ and belowground carbon pools in tree biomass together with NPP_total in natural old‐growth forests, ‘jungle rubber’ agroforests under natural tree cover, and rubber and oil palm monocultures in Sumatra. In total, 32 stands (eight plot replicates per land‐use system) were studied in two different regions. Total tree biomass in the natural forest (mean: 384 Mg ha^?1) was more than two times higher than in jungle rubber stands (147 Mg ha^?1) and >four times higher than in monoculture rubber and oil palm plantations (78 and 50 Mg ha^?1). NPP_total was higher in the natural forest (24 Mg ha^?1 yr^?1) than in the rubber systems (20 and 15 Mg ha^?1 yr^?1), but was highest in the oil palm system (33 Mg ha^?1 yr^?1) due to very high fruit production (15–20 Mg ha^?1 yr^?1). NPP_total was dominated in all systems by aboveground production, but belowground productivity was significantly higher in the natural forest and jungle rubber than in plantations. We conclude that conversion of natural lowland forest into different agricultural systems leads to a strong reduction not only in the biomass carbon pool (up to 166 Mg C ha^?1) but also in carbon sequestration as carbon residence time (i.e. biomass‐C:NPP‐C) was 3–10 times higher in the natural forest than in rubber and oil palm plantations.     

Development and evaluation of ForestGrowth‐SRC a process‐based model for short rotation coppice yield and spatial supply reveals poplar uses water more efficiently than willow

Woody biomass produced from short rotation coppice (SRC) poplar (Populus spp.) and willow (Salix spp.) is a bioenergy feedstock that can be grown widely across temperate landscapes and its use is likely to increase in future. Process‐based models are therefore required to predict current and future yield potential that are spatially resolved and can consider new genotypes and climates that will influence future yield. The development of a process‐based model for SRC poplar and willow, ForestGrowth‐SRC, is described and the ability of the model to predict SRC yield and water use efficiency (WUE) was evaluated. ForestGrowth‐SRC was parameterized from a process‐based model, ForestGrowth for high forest. The new model predicted annual above ground yield well for poplar (r² = 0.91, RMSE = 1.46 ODT ha^?1 yr^?1) and willow (r² = 0.85, RMSE = 1.53 ODT ha^?1 yr^?1), when compared with measured data from seven sites in contrasting climatic zones across the United Kingdom. Average modelled yields for poplar and willow were 10.3 and 9.0 ODT ha^?1 yr^?1, respectively, and interestingly, the model predicted a higher WUE for poplar than for willow: 9.5 and 5.5 g kg^?1 respectively. Using regional mapped climate and soil inputs, modelled and measured yields for willow compared well (r² = 0.58, RMSE = 1.27 ODT ha^?1 yr^?1), providing the first UK map of SRC yield, from a process‐based model. We suggest that the model can be used for predicting current and future SRC yields at a regional scale, highlighting important species and genotype choices with respect to water use efficiency and yield potential.     

Carbon sequestration in soil in a semi‐natural Miscanthus sinensis grassland and Cryptomeria japonica forest plantation in Aso,Kumamoto, Japan

Although Miscanthus sinensis grasslands (Misc‐GL) and Cryptomeria japonica forest plantations (Cryp‐FP) are proposed bioenergy feedstock systems, their relative capacity to sequester C may be an important factor in determining their potential for sustainable bioenergy production. Therefore, our objective was to quantify changes in soil C sequestration 47 years after a Misc‐GL was converted to a Cryp‐FP. The study was conducted on adjacent Misc‐GL and Cryp‐FP located on Mt. Aso, Kumamoto, Japan. After Cryp‐FP establishment, only the Misc‐GL continued to be managed by annual burning every March. Mass C and N, δ¹³C, and δ¹⁵N at 0–30 cm depth were measured in 5 cm increments. Carbon and N concentrations, C:N ratio, δ¹³C, and δ¹⁵N were measured in litter and/or ash, and rhizomes or roots. Although C input in Misc‐GL by M. sinensis was approximately 36% of that in Cryp‐FP by C. japonica, mean annual soil C sequestration in Misc‐GL (503 kg C ha^?1 yr^?1) was higher than that in Cryp‐FP (284 kg C ha^?1 yr^?1). This was likely the result of larger C input from aboveground litter to soil, C‐quality (C:N ratio and lignin concentration in aboveground litter) and possibly more recalcitrant C (charcoal) inputs by annual burning. The difference in soil δ¹⁵N between sites indicated that organic C with N had greater cycling between heterotrophic microbes and soil and produces more recalcitrant humus in Misc‐GL than in Cryp‐FP. Our data indicate that in terms of soil C sequestration, maintenance of Misc‐GL may be more advantageous than conversion to Cryp‐FP in Aso, Japan.     

Bioenergy crop productivity and potential climate change mitigation from marginal lands in the United States: An ecosystem modeling perspective

Growing biomass feedstocks from marginal lands is becoming an increasingly attractive choice for producing biofuel as an alternative energy to fossil fuels. Here, we used a biogeochemical model at ecosystem scale to estimate crop productivity and greenhouse gas (GHG) emissions from bioenergy crops grown on marginal lands in the United States. Two broadly tested cellulosic crops, switchgrass, and Miscanthus, were assumed to be grown on the abandoned land and mixed crop‐vegetation land with marginal productivity. Production of biomass and biofuel as well as net carbon exchange and nitrous oxide emissions were estimated in a spatially explicit manner. We found that, cellulosic crops, especially Miscanthus could produce a considerable amount of biomass, and the effective ethanol yield is high on these marginal lands. For every hectare of marginal land, switchgrass and Miscanthus could produce 1.0–2.3 kl and 2.9–6.9 kl ethanol, respectively, depending on nitrogen fertilization rate and biofuel conversion efficiency. Nationally, both crop systems act as net GHG sources. Switchgrass has high global warming intensity (100–390 g CO₂eq l^?1 ethanol), in terms of GHG emissions per unit ethanol produced. Miscanthus, however, emits only 21–36 g CO₂eq to produce every liter of ethanol. To reach the mandated cellulosic ethanol target in the United States, growing Miscanthus on the marginal lands could potentially save land and reduce GHG emissions in comparison to growing switchgrass. However, the ecosystem modeling is still limited by data availability and model deficiencies, further efforts should be made to classify crop‐specific marginal land availability, improve model structure, and better integrate ecosystem modeling into life cycle assessment.