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.     

Comparing annual and perennial crops for bioenergy production – influence on nitrate leaching and energy balance

Production of energy crops is promoted as a means to mitigate global warming by decreasing dependency on fossil energy. However, agricultural production of bioenergy can have various environmental effects depending on the crop and production system. In a field trial initiated in 2008, nitrate concentration in soil water was measured below winter wheat, grass‐clover and willow during three growing seasons. Crop water balances were modelled to estimate the amount of nitrate leached per hectare. In addition, dry matter yields and nitrogen (N) yields were measured, and N balances and energy balances were calculated. In willow, nitrate concentrations were up to approximately 20 mg l^?1 nitrate‐N during the establishment year, but declined subsequently to <5 mg l^?1 nitrate‐N, resulting in an annual N leaching loss of 18, 3 and 0.3 kg ha^?1 yr^?1 N in the first 3 years after planting. A similar trend was observed in grass‐clover where concentrations stabilized at 2–4 mg l^?1 nitrate‐N from the beginning of the second growing season, corresponding to leaching of approximately 5 kg ha^?1 yr^?1 N. In winter wheat, an annual N leaching loss of 36–68 kg ha^?1 yr^?1 was observed. For comparison, nitrate leaching was also measured in an old willow crop established in 1996 from which N leaching ranged from 6 to 27 kg ha^?1 yr^?1. Dry matter yields ranged between 5.9 and 14.8 Mg yr^?1 with lowest yield in the newly established willow and the highest yield harvested in grass‐clover. Grass‐clover gave the highest net energy yield of 244 GJ ha^?1 yr^?1, whereas old willow, winter wheat and first rotation willow gave net energy yields of 235, 180 and 105 GJ ha^?1 yr^?1. The study showed that perennial crops can provide high energy yields and significantly reduce N losses compared to annual crops.     

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.     

Comparing predicted yield and yield stability of willow and Miscanthus across Denmark

To achieve the goals of energy security and climate change mitigation in Denmark and the EU, an expansion of national production of bioenergy crops is needed. Temporal and spatial variation of yields of willow and Miscanthus is not known for Denmark because of a limited number of field trial data. The semi‐mechanistic crop model BioCro was used to simulate the production of both short‐rotation coppice (SRC) willow and Miscanthus across Denmark. Predictions were made from high spatial resolution soil data and weather records across this area for 1990–2010. The potential average, rain‐fed mean yield was 12.1 Mg DM ha^?1 yr^?1 for willow and 10.2 Mg DM ha^?1 yr^?1 for Miscanthus. Coefficient of variation as a measure for yield stability was poorest on the sandy soils of northern and western Jutland, and the year‐to‐year variation in yield was greatest on these soils. Willow was predicted to outyield Miscanthus on poor, sandy soils, whereas Miscanthus was higher yielding on clay‐rich soils. The major driver of yield in both crops was variation in soil moisture, with radiation and precipitation exerting less influence. This is the first time these two major feedstocks for northern Europe have been compared within a single modeling framework and providing an important new tool for decision‐making in selection of feedstocks for emerging bioenergy systems.     

Simulating switchgrass biomass production across ecoregions using the DAYCENT model

The production potential of switchgrass (Panicum virgatum L.) has not been estimated in a Mediterranean climate on a regional basis and its economic and environmental contribution as a biofuel crop remains unknown. The objectives of the study were to calibrate and validate a biogeochemical model, DAYCENT, and to predict the biomass yield potential of switchgrass across the Central Valley of California. Six common cultivars were calibrated using published data across the US and validated with data generated from four field trials in California (2007–2009). After calibration, the modeled range of yields across the cultivars and various management practices in the US (excluding California) was 2.4–41.2 Mg ha^?1 yr^?1, generally compatible with the observed yield range of 1.3–33.7 Mg ha^?1 yr^?1. Overall, the model was successfully validated in California; the model explained 66–90% of observed yield variation in 2007–2009. The range of modeled yields was 2.0–41.4 Mg ha^?1 yr^?1, which corresponded to the observed range of 1.3–41.1 Mg ha^?1 yr^?1. The response to N fertilizer and harvest frequency on yields were also reasonably validated. The model estimated that Alamo (21–23 Mg ha^?1 yr^?1) and Kanlow (22–24 Mg ha^?1 yr^?1) had greatest yield potential during the years after establishment. The effects of soil texture on modeled yields tended to be consistent for all cultivars, but there were distinct climatic (e.g., annual mean maximum temperature) controls among the cultivars. Our modeled results suggest that early stand maintenance of irrigated switchgrass is strongly dependent on available soil N; estimated yields increased by 1.6–5.5 Mg ha^?1 yr^?1 when residual soil mineral N was sufficient for optimal re‐growth. Therefore, management options of switchgrass for regional biomass production should be ecotype‐specific and ensure available soil N maintenance.     

Productivity and water use efficiency of Agave americana in the first field trial as bioenergy feedstock on arid lands

Agave species are high‐yielding crassulacean acid metabolism (CAM) plants, some of which are grown commercially and recognized as potential bioenergy species for dry regions of the world. This study is the first field trial of Agave species for bioenergy in the United States, and was established to compare the production of Agave americana with the production of Agave tequilana and Agave fourcroydes, which are produced commercially in Mexico for tequila and fiber. The field trial included four experimental irrigation levels to test the response of biomass production to water inputs. After 3 years, annual production of healthy A. americana plants reached 9.3 Mg dry mass ha^?1 yr^?1 (including pup mass) with 530 mm of annual water inputs, including both rainfall and irrigation. Yields in the most arid conditions tested (300 mm yr^?1 water input) were 2.0–4.0 Mg dry mass ha^?1 yr^?1. Agave tequilana and Agave fourcroydes were severely damaged by cold in the first winter, and produced maximum yields of only 0.04 Mg ha^?1 yr^?1 and 0.26 Mg ha^?1 yr^?1, respectively. The agave snout weevil (Scyphophorus acupunctatus) emerged as an important challenge for A. americana cropping, killing a greater number of plants in the higher irrigation treatments. Physiological differences in A. americana plants across irrigation treatments were most evident in the warmest season, with gas exchange beginning up to 3 h earlier and water use efficiency declining in treatments with the greatest water input (780 mm yr^?1 water input). Yields were lower than previous projections for Agave species, but results from this study suggest that A. americana has potential as a bioenergy crop and would have substantially reduced irrigation requirements relative to conventional crops in the southwestern USA. Challenges for pest management and harvesting must still be addressed before an efficient production system that uses Agave can be realized.     

Genotype × environment interaction analysis of North American shrub willow yield trials confirms superior performance of triploid hybrids

Development of dedicated bioenergy crop production systems will require accurate yield estimates, which will be important for determining many of the associated environmental and economic impacts of their production. Shrub willow (Salix spp) is being promoted in areas of the USA and Canada due to its adaption to cool climates and wide genetic diversity available for breeding improvement. Willow breeding in North America is in an early stage, and selection of elite genotypes for commercialization will require testing across broad geographic regions to gain an understanding of how shrub willow interacts with the environment. We analyzed a dataset of first‐rotation shrub willow yields of 16 genotypes across 10 trial environments in the USA and Canada for genotype‐by‐environment interactions using the additive main effects and multiplicative interactions (AMMI) model. Mean genotype yields ranged from 5.22 to 8.58 oven‐dry Mg ha^?1 yr^?1. Analysis of the main effect of genotype showed that one round of breeding improved yields by as much as 20% over check cultivars and that triploid hybrids, most notably Salix viminalis × S. miyabeana, exhibited superior yields. We also found important variability in genotypic response to environments, which suggests specific adaptability could be exploited among 16 genotypes for yield gains. Strong positive correlations were found between environment main effects and AMMI parameters and growing environment temperatures. These findings demonstrate yield improvements are possible in one generation and will be important for developing cultivar recommendations and for future breeding efforts.     

Miscanthus biomass productivity within US croplands and its potential impact on soil organic carbon

Interest in bioenergy crops is increasing due to their potential to reduce greenhouse gas emissions and dependence on fossil fuels. We combined process‐based and geospatial models to estimate the potential biomass productivity of miscanthus and its potential impact on soil carbon stocks in the croplands of the continental United States. The optimum (climatic potential) rainfed productivity for field‐dried miscanthus biomass ranged from 1 to 23 Mg biomass ha^?1 yr^?1, with a spatial average of 13 Mg ha^?1 yr^?1 and a coefficient of variation of 30%. This variation resulted primarily from the spatial heterogeneity of effective rainfall, growing degree days, temperature, and solar radiation interception. Cultivating miscanthus would result in a soil organic carbon (SOC) sequestration at the rate of 0.16–0.82 Mg C ha^?1 yr^?1 across the croplands due to cessation of tillage and increased biomass carbon input into the soil system. We identified about 81 million ha of cropland, primarily in the eastern United States, that could sustain economically viable (>10 Mg ha^?1 yr^?1) production without supplemental irrigation, of which about 14 million ha would reach optimal miscanthus growth. To meet targets of the US Energy Independence and Security Act of 2007 using miscanthus as feedstock, 19 million ha of cropland would be needed (spatial average 13 Mg ha^?1 yr^?1) or about 16% less than is currently dedicated to US corn‐based ethanol production.     

Miscanthus × giganteus leaf senescence,decomposition and C and N inputs to soil

Energy crops are currently promoted as potential sources of alternative energy that can help mitigate the climate change caused by greenhouse gases (GHGs). The perennial crop Miscanthus × giganteus is considered promising due to its high potential for biomass production under conditions of low input. However, to assess its potential for GHG mitigation, a better quantification of the crop's contribution to soil organic matter recycling under various management systems is needed. The aim of this work was to study the effect of abscised leaves on carbon (C) and nitrogen (N) recycling in a Miscanthus plantation. The dynamics of senescent leaf fall, the rate of leaf decomposition (using a litter bag approach) and the leaf accumulation at the soil surface were tracked over two 1‐year periods under field conditions in Northern France. The fallen leaves represented an average yearly input of 1.40 Mg C ha^?1 and 16 kg N ha^?1. The abscised leaves lost approximately 54% of their initial mass in 1 year due to decomposition; the remaining mass, accumulated as a mulch layer at the soil surface, was equivalent to 7 Mg dry matter (DM) ha^?1 5 years after planting. Based on the estimated annual leaf‐C recycling rate and a stabilization rate of 35% of the added C, the annual contribution of the senescent leaves to the soil C was estimated to be approximately 0.50 Mg C ha^?1yr^?1 or 10 Mg C ha^?1 total over the 20‐year lifespan of a Miscanthus crop. This finding suggested that for Miscanthus, the abscised leaves contribute more to the soil C accumulation than do the rhizomes or roots. In contrast, the recycling of the leaf N to the soil was less than for the other N fluxes, particularly for those involving the transfer of N from the tops of the plant to the rhizome.     

Yield potential of Miscanthus energy crops in the Loess Plateau of China

Growing second‐generation energy crops on marginal land is conceptualized as one of the primary means of future bioenergy development. However, the extent to which marginal land can support energy crop production remains unclear. The Loess Plateau of China, one of the most seriously eroded regions of the world, is particularly rich in marginal land. On the basis of the previous field experiment of planting Miscanthus species in Qingyang of the Gansu Province, herein, we estimated the yield potential of Miscanthus lutarioriparius, the species with the highest biomass, across the Loess Plateau. On the basis of the radiation model previously developed from Miscanthus field trials, annual precipitation was introduced as an additional variable for yield estimate in the semiarid and semihumid regions of the Loess Plateau. Of 62 million hectares (Mha) of the Loess Plateau, our model estimated that 48.7 Mha can potentially support Miscanthus growth, with the average yield of 17.8 t ha^?1 yr^?1. After excluding high‐quality cropland and pasture and land suitable for afforestation, a total of 33.3 Mha of presumably marginal land were left available for producing the energy crop at the average yield of 16.8 t ha^?1 yr^?1 and the total annual yield of 0.56 billion tons. The analysis of environmental factors indicated that erosion, aridity, and field steepness were the primary contributors to the poor quality of the marginal land. The change of land uses from traditional agriculture to energy crop production may prevent further erosion and land degradation and consequently establish a sustainable economy for the region.     

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.     

Carbon sequestration potential in perennial bioenergy crops: the importance of organic matter inputs and its physical protection

To date, only few studies have compared the soil organic carbon (SOC) sequestration potential between perennial woody and herbaceous crops. The main objective of this study was to assess the effect of perennial woody (poplar, black locust, willow) and herbaceous (giant reed, miscanthus, switchgrass) crops on SOC stock and its stabilization level after 6 years from plantation on an arable field. Seven SOC fractions related to different soil stabilization mechanisms were isolated by a combination of physical and chemical fractionation methods: unprotected (cPOM and fPOM), physically protected (iPOM), physically and chemically protected (HC‐μs + c), chemically protected (HC‐ds + c), and biochemically protected (NHC‐ds + c and NHC‐μs + c). The continuous C input to the soil and the minimal soil disturbance increased SOC stocks in the top 10 cm of soil, but not in deeper soil layers (10–30; 30–60; and 60–100 cm). In the top soil layer, greater SOC accumulation rates were observed under woody species (105 g m^?2 yr^‐1) than under herbaceous ones (71 g m^?2 yr^‐1) presumably due to a higher C input from leaf‐litter. The conversion from an arable maize monoculture to perennial bioenergy crops increased the organic C associated to the most labile organic matter (POM) fractions, which accounted for 38% of the total SOC stock across bioenergy crops, while no significant increments were observed in more recalcitrant (silt‐ and clay‐sized) fractions, highlighting that the POM fractions were the most prone to land‐use change. The iPOM fraction increased under all perennial bioenergy species compared to the arable field. In addition, the iPOM was higher under woody crops than under herbaceous ones because of the additional C inputs from leaf‐litter that occurred in the former. Conversion from arable cropping systems to perennial bioenergy crops can effectively increase the SOC stock and enlarge the SOC fraction that is physically protected within soil microaggregates.     

Can the agricultural AquaCrop model simulate water use and yield of a poplar short‐rotation coppice?

We calibrated and evaluated the agricultural model AquaCrop for the simulation of water use and yield of a short‐rotation coppice (SRC) plantation with poplar (Populus) in East Flanders (Belgium) during the second and the third rotation (first 2 years only). Differences in crop development and growth during the course of the rotations were taken into account during the model calibration. Overall, the AquaCrop model showed good performance for the daily simulation of soil water content (R² of 0.57–0.85), of green canopy cover (R² > 0.87), of evapotranspiration (ET; R² > 0.76), and of potential yield. The simulated, total yearly water use of the SRC ranged between 55% and 85% of the water use of a reference grass ecosystem calculated under the same environmental conditions. Crop transpiration was between 67% and 93% of total ET, with lower percentages in the first than in the second year of each rotation. The observed (dry mass) yield ranged from 6.61 to 14.76 Mg ha^?1 yr^?1. A yield gap of around 30% was observed between the second and the third rotation, as well as between simulated and observed yield during the third rotation. This could possibly be explained by the expansion of the understory (weed) layer; the relative cover of understory weeds was 22% in the third year of the third rotation. The agricultural AquaCrop model simulated total water use and potential yield of the operational SRC in a reliable way. As the plantation was extensively managed, potential effects of irrigation and/or fertilization on ET and on yield were not considered in this study.     

A comparison of isoprene and monoterpene emission rates from the perennial bioenergy crops short‐rotation coppice willow and Miscanthus and the annual arable crops wheat and oilseed rape

Biogenic volatile organic compounds (BVOC) emissions from bioenergy crops may differ from those of conventional crops. We compared emission rates of isoprene and a number of monoterpenes from the lignocellulosic bioenergy crops short‐rotation coppice (SRC) willow and Miscanthus, with the conventional crops wheat and oilseed rape. BVOC emission rates were measured via dynamic vegetation enclosure and GC‐MS analysis approximately monthly between April 2010 and August 2012 at a location in England and from SRC willow at two locations in Scotland. The largest BVOC emission rates were measured from willow in England and varied between years. Isoprene emission rates varied between μg g^?1 h^?1. Of the monoterpenes detected from willow, α‐pinene emission rates were highest (μg g^?1 h^?1), followed by μg g^?1 h^?1 for δ‐3‐carene, μg g^?1 h^?1 for β‐pinene and μg g^?1 h^?1 for limonene. BVOC emission rates measured in Scotland were much lower. Low emission rates of isoprene and α‐pinene were measured from Miscanthus in 2010 (μg g^?1 h^?1 and μg g^?1 h^?1, respectively) but were not detected in subsequent years. Emission rates from wheat of isoprene were negligible but relatively high for monoterpenes (μg g^?1 h^?1 and μg g^?1 h^?1 for α‐pinene and limonene, respectively). No significant emission rates of BVOCs were measured from oilseed rape. The measured emission rates followed a clear seasonal trend. Crude extrapolations based solely on data gathered here indicate that isoprene emissions from willow could correspond to 0.004–0.03% (UK) and 0.76–5.5% (Europe) of current global isoprene if 50% of all land potentially available for bioenergy crops is planted with willow.     

An evaluation of the global potential of bioenergy production on degraded lands

In this article the global potential of energy crop production on degraded lands was estimated using detailed, spatially explicit data about the area, type and extent of degradation derived from the Global Assessment of Land Degradation Dataset, and by combining this dataset with various spatially explicit data sets. Next, an estimate was made of the possible yield of perennial energy crops on the degraded areas as a function of the type and degree of degradation. Lightly degraded areas were not included, as these areas might be suitable for conventional food production. The total global potential energy production on degraded lands was assessed to be slightly above 150 and 190 EJ yr^?1, for grassy and woody energy crops, respectively. Most of this potential, however, is on areas currently classified as forest, cropland or pastoral land, leaving a potential of around 25 and 32 EJ yr^?1 on other land cover categories. Most of the potential energy crop production on degraded land is located in developing regions. China has a total potential of 30 EJ yr^?1, of which 4 EJ yr^?1 from areas classified as other land. Also USA, Brazil, West Africa, East Africa, Russia and India have substantial potentials of 12–18 EJ yr^?1, with up to 30% of the potential from areas classified as other land.     

Environmental costs and benefits of growing Miscanthus for bioenergy in the UK

Planting the perennial biomass crop Miscanthus in the UK could offset 2–13 Mt oil eq. yr^?1, contributing up to 10% of current energy use. Policymakers need assurance that upscaling Miscanthus production can be performed sustainably without negatively impacting essential food production or the wider environment. This study reviews a large body of Miscanthus relevant literature into concise summary statements. Perennial Miscanthus has energy output/input ratios 10 times higher (47.3 ± 2.2) than annual crops used for energy (4.7 ± 0.2 to 5.5 ± 0.2), and the total carbon cost of energy production (1.12 g CO₂‐C eq. MJ^?1) is 20–30 times lower than fossil fuels. Planting on former arable land generally increases soil organic carbon (SOC) with Miscanthus sequestering 0.7–2.2 Mg C4‐C ha^?1 yr^?1. Cultivation on grassland can cause a disturbance loss of SOC which is likely to be recovered during the lifetime of the crop and is potentially mitigated by fossil fuel offset. N₂O emissions can be five times lower under unfertilized Miscanthus than annual crops and up to 100 times lower than intensive pasture. Nitrogen fertilizer is generally unnecessary except in low fertility soils. Herbicide is essential during the establishment years after which natural weed suppression by shading is sufficient. Pesticides are unnecessary. Water‐use efficiency is high (e.g. 5.5–9.2 g aerial DM (kg H₂O)^?1, but high biomass productivity means increased water demand compared to cereal crops. The perennial nature and belowground biomass improves soil structure, increases water‐holding capacity (up by 100–150 mm), and reduces run‐off and erosion. Overwinter ripening increases landscape structural resources for wildlife. Reduced management intensity promotes earthworm diversity and abundance although poor litter palatability may reduce individual biomass. Chemical leaching into field boundaries is lower than comparable agriculture, improving soil and water habitat quality.     

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.     

Bioenergy crop greenhouse gas mitigation potential under a range of management practices

Perennial grasses have been proposed as viable bioenergy crops because of their potential to yield harvestable biomass on marginal lands annually without displacing food and to contribute to greenhouse gas (GHG) reduction by storing carbon in soil. Switchgrass, miscanthus, and restored native prairie are among the crops being considered in the corn and agricultural regions of the Midwest and eastern United States. In this study, we used an extensive dataset of site observations for each of these crops to evaluate and improve the DayCent biogeochemical model and make predictions about how both yield and GHG fluxes would respond to different management practices compared to a traditional corn‐soy rotation. Using this model‐data integration approach, we found 30–75% improvement in our predictions over previous studies and a subsequent evaluation with a synthesis of sites across the region revealed good model‐data agreement of harvested yields (r² > 0.62 for all crops). We found that replacement of corn‐soy rotations would result in a net GHG reduction of 0.5, 1.0, and 2.0 Mg C ha^?1 yr^?1 with average annual yields of 3.6, 9.2, and 17.2 Mg of dry biomass per year for native prairie, switchgrass, and miscanthus respectively. Both the yield and GHG balance of switchgrass and miscanthus were affected by harvest date with highest yields occurring near onset of senescence and highest GHG reductions occurring in early spring before the new crops emergence. Addition of a moderate length rotation (10–15 years) caused less than a 15% change to yield and GHG balance. For policy incentives aimed at GHG reduction through onsite management practices and improvement of soil quality, post‐senescence harvests are a more effective means than maximizing yield potential.     

Plant roots and GHG mitigation in native perennial bioenergy cropping systems

Native perennial bioenergy crops can mitigate greenhouse gases (GHG) by displacing fossil fuels with renewable energy and sequestering atmospheric carbon (C) in soil and roots. The relative contribution of root C to net GHG mitigation potential has not been compared in perennial bioenergy crops ranging in species diversity and N fertility. We measured root biomass, C, nitrogen (N), and soil organic carbon (SOC) in the upper 90 cm of soil for five native perennial bioenergy crops managed with and without N fertilizer. Bioenergy crops ranged in species composition and were annually harvested for 6 (one location) and 7 years (three locations) following the seeding year. Total root biomass was 84% greater in switchgrass (Panicum virgatum L.) and a four‐species grass polyculture compared to high‐diversity polycultures; the difference was driven by more biomass at shallow soil depth (0–30 cm). Total root C (0–90 cm) ranged from 3.7 Mg C ha^?1 for a 12‐species mixture to 7.6 Mg C ha^?1 for switchgrass. On average, standing root C accounted for 41% of net GHG mitigation potential. After accounting for farm and ethanol production emissions, net GHG mitigation potential from fossil fuel offsets and root C was greatest for switchgrass (?8.4 Mg CO2e ha^?1 yr^?1) and lowest for high‐diversity mixtures (?4.5 Mg CO2e ha^?1 yr^?1). Nitrogen fertilizer did not affect net GHG mitigation potential or the contribution of roots to GHG mitigation for any bioenergy crop. SOC did not change and therefore did not contribute to GHG mitigation potential. However, associations among SOC, root biomass, and root C : N ratio suggest greater long‐term C storage in diverse polycultures vs. switchgrass. Carbon pools in roots have a greater effect on net GHG mitigation than SOC in the short‐term, yet variation in root characteristics may alter patterns in long‐term C storage among bioenergy crops.     

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.