Switchgrass Biofuel Production on Reclaimed Surface Mines: II. Feedstock Quality and Theoretical Ethanol Production

Switchgrass (Panicum virgatum L.), a warm-season perennial grass, is an important bioenergy crop candidate because it produces high biomass yields on marginal lands and on reclaimed surface mined sites. In companion studies, dry matter (DM) yields for Cave-in-Rock, Shawnee, and Carthage cultivars varied from 4.2 to 13.0 Mg ha^?1averaged over 6 years at the reclaimed Hampshire site, and fertilization increased yields of Cave-in-Rock at Black Castle and Coal Mac sites from 0.3 to 2 Mg ha^?1 during the first 3 years. The objective of these experiments was to compare the impacts of cultivar and soil amendments on biomass quality and theoretical ethanol production of switchgrass grown on surface mines with differing soil characteristics. Biomass quality was determined for fiber, ash, lignin, digestibility, and carbohydrate contents via near-infrared reflectance spectroscopy, and carbohydrates were used to calculate theoretical ethanol yield (TEY; L Mg^?1) and multiplied by biomass yield to calculate theoretical ethanol production (TEP; L ha^?1). Cultivars at the Hampshire site did not differ in TEY and ranged from 426 to 457 L Mg^?1. Theoretical ethanol production from Cave-in-Rock at Hampshire was 7350 L ha^?1, which was higher than other cultivars because of its greater biomass production. This TEP was higher than in other studies which predicted 4000 to 5000 L ha^?1. At the Black Castle and Coal Mac sites, fertilizer applications slightly affected biomass quality of switchgrass and TEY, but provided greater TEP as a function of increased yield. Similar to other findings, total switchgrass biomass production has more impact than compositional differences on TEP, so maximizing biomass production is critical for maximizing potential biofuel production. With appropriate soil substrates, fertilization, planning, and management, large areas of reclaimed surface mines can be converted to switchgrass stands to produce high biomass quality and yields to support a bioethanol industry.     

Switchgrass and Giant Miscanthus Biomass and Theoretical Ethanol Production from Reclaimed Mine Lands

Switchgrass (Panicum virgatum L.) and giant miscanthus (Miscanthus x giganteus Greef & Deuter ex Hodkinson & Renvoize) are productive on marginal lands in the eastern USA, but their productivity and composition have not been compared on mine lands. Our objectives were to compare biomass production, composition, and theoretical ethanol yield (TEY) and production (TEP) of these grasses on a reclaimed mined site. Following 25 years of herbaceous cover, vegetation was killed and plots of switchgrass cultivars Kanlow and BoMaster and miscanthus lines Illinois and MBX-002 were planted in five replications. Annual switchgrass and miscanthus yields averaged 5.8 and 8.9 Mg dry matter ha^?1, respectively, during 2011 to 2015. Cell wall carbohydrate composition was analyzed via near-infrared reflectance spectroscopy with models based on switchgrass or mixed herbaceous samples including switchgrass and miscanthus. Concentrations were higher for glucan and lower for xylan in miscanthus than in switchgrass but TEY did not differ (453 and 450 L Mg^?1, respectively). In response to biomass production, total ethanol production was greater for miscanthus than for switchgrass (5594 vs 3699 L ha^?1), did not differ between Kanlow and BoMaster switchgrass (3880 and 3517 L ha^?1, respectively), and was higher for MBX-002 than for Illinois miscanthus (6496 vs 4692 L ha^?1). Relative to the mixed feedstocks model, the switchgrass model slightly underpredicted glucan and slightly overpredicted xylan concentrations. Estimated TEY was slightly lower from the switchgrass model but both models distinguished genotype, year, and interaction effects similarly. Biomass productivity and TEP were similar to those from agricultural sites with marginal soils.     

Productivity, 15N dynamics and water use efficiency in low‐ and high‐input switchgrass systems

Sustainable and environmentally benign switchgrass production systems need to be developed for switchgrass to become a large‐scale dedicated energy crop. An experiment was conducted in California from 2009 to 2011 to determine the sustainability of low‐ and high‐input irrigated switchgrass systems as a function of yield, irrigation requirement, crop N removal, N translocation from aboveground (AG) to belowground (BG) biomass during senescence, and fertilizer ¹⁵N recovery (FNR) in the AG and BG biomass (0–300 cm), and soil (0–300 cm). The low‐input system consisted of a single‐harvest (mid‐fall) irrigated until flowering (early summer), while the high‐input system consisted of a two‐harvest system (early summer and mid‐fall) irrigated throughout the growing season. Three N fertilization rates (0, 100, and 200 kg N ha^?1 yr^?1) were applied as subtreatments in a single application in the spring of each year. A single pulse of ¹⁵N enriched fertilizer was applied in the first year of the study to micro‐plots within the 100 kg N ha^?1 subplots. Average yields across years under optimal N rates (100 and 200 kg ha^?1 yr^?1 for low‐ and high‐input systems, respectively) were 20.7 and 24.8 Mg ha^?1. However, the low input (372 ha mm) required 47% less irrigation than the high‐input system (705 ha mm) and achieved higher irrigation use efficiency. In addition, the low‐input system had 46% lower crop N removal, 53% higher N stored in BG biomass, and a positive N balance, presumably due to 49% of ¹⁵N translocation from AG to BG biomass during senescence. Furthermore, at the end of 3 years, the low‐input system had lower fertilizer ¹⁵N removed by harvest (26%) and higher FNR remaining in the system in BG biomass plus soil (31%) than the high‐input system (45% and 21%, respectively). Based on these findings, low‐input systems are more sustainable than high‐input systems in irrigated Mediterranean climates.     

Targeted subfield switchgrass integration could improve the farm economy,water quality,and bioenergy feedstock production

Progress on reducing nutrient loss from annual croplands has been hampered by perceived conflicts between short‐term profitability and long‐term stewardship, but these may be overcome through strategic integration of perennial crops. Perennial biomass crops like switchgrass can mitigate nitrate‐nitrogen (NO₃‐N) leaching, address bioenergy feedstock targets, and – as a lower‐cost management alternative to annual crops (i.e., corn, soybeans) – may also improve farm profitability. We analyzed publicly available environmental, agronomic, and economic data with two integrated models: a subfield agroecosystem management model, Landscape Environmental Assessment Framework (LEAF), and a process‐based biogeochemical model, DeNitrification‐DeComposition (DNDC). We constructed a factorial combination of profitability and NO₃‐N leaching thresholds and simulated targeted switchgrass integration into corn/soybean cropland in the agricultural state of Iowa, USA. For each combination, we modeled (i) area converted to switchgrass, (ii) switchgrass biomass production, and (iii) NO₃‐N leaching reduction. We spatially analyzed two scenarios: converting to switchgrass corn/soybean cropland losing >US$ 100 ha^?1 and leaching >50 kg ha^?1 (‘conservative’ scenario) or losing >US$ 0 ha^?1 and leaching >20 kg ha^?1 (‘nutrient reduction’ scenario). Compared to baseline, the ‘conservative’ scenario resulted in 12% of cropland converted to switchgrass, which produced 11 million Mg of biomass and reduced leached NO₃‐N 18% statewide. The ‘nutrient reduction’ scenario converted 37% of cropland to switchgrass, producing 34 million Mg biomass and reducing leached NO₃‐N 38% statewide. The opportunity to meet joint goals was greatest within watersheds with undulating topography and lower corn/soybean productivity. Our approach bridges the scales at which NO₃‐N loss and profitability are usually considered, and is informed by both mechanistic and empirical understanding. Though approximated, our analysis supports development of farm‐level tools that can identify locations where both farm profitability and water quality improvement can be achieved through the strategic integration of perennial vegetation.     

N fertilizer and harvest impacts on bioenergy crop contributions to SOC

Belowground root biomass is infrequently measured and simply represented in models that predict landscape‐level changes to soil carbon stocks and greenhouse gas balances. Yet, crop‐specific responses to N fertilizer and harvest treatments are known to impact both plant allocation and tissue chemistry, potentially altering decomposition rates and the direction and magnitude of soil C stock changes and greenhouse gas fluxes. We examined switchgrass (Panicum virgatum L.) and corn (Zea mays L.,) yields, belowground root biomass, C, N and soil particulate organic matter‐C (POM‐C) in a 9‐year rainfed study of N fertilizer rate (0, 60, 120 and 180 kg N ha^?1) and harvest management near Mead, NE, USA. Switchgrass was harvested with one pass in either August or postfrost, and for no‐till (NT) corn, either 50% or no stover was removed. Switchgrass had greater belowground root biomass C and N (6.39, 0.10 Mg ha^?1) throughout the soil profile compared to NT‐corn (1.30, 0.06 Mg ha^?1) and a higher belowground root biomass C:N ratio, indicating greater recalcitrant belowground root biomass C input beneath switchgrass. There was little difference between the two crops in soil POM‐C indicating substantially slower decomposition and incorporation into SOC under switchgrass, despite much greater root C. The highest N rate decreased POM‐C under both NT‐corn and switchgrass, indicating faster decomposition rates with added fertilizer. Residue removal reduced corn belowground root biomass C by 37% and N by 48% and subsequently reduced POM‐C by 22% compared to no‐residue removal. Developing productive bioenergy systems that also conserve the soil resource will require balancing fertilization that maximizes aboveground productivity but potentially reduces SOC sequestration by reducing belowground root biomass and increasing root and soil C decomposition.     

Nitrogen and harvest effects on soil properties under rainfed switchgrass and no‐till corn over 9 years: implications for soil quality

Nitrogen fertilizer and harvest management will alter soils under bioenergy crop production and the long‐term effects of harvest timing and residue removal remain relatively unknown. Compared to no‐tilled corn (NT‐C, Zea mays L.), switchgrass (Panicum virgatum L.) is predicted to improve soil properties [i.e. soil organic C (SOC), soil microbial biomass (SMB‐C), and soil aggregation] due to its perennial nature and deep‐rooted growth form, but few explicit field comparisons exist. We assessed soil properties over 9 years for a rainfed study of N fertilizer rate (0, 60, 120, and 180 kg N ha^?1) and harvest management on switchgrass (harvested in August and postfrost) and NT‐C (with and without 50% stover removal) in eastern NE. We measured SOC, aggregate stability, SMB‐C, bulk density (BD), pH, P and K in the top 0–30 cm. Both NT‐C and switchgrass increased SMB‐C, SOC content, and aggregate stability over the 9 years, reflecting improvement from previous conventional management. However, the soils under switchgrass had double the percent aggregate stability, 1.3 times more microbial biomass, and a 5–8% decrease in bulk density in the 0–5 and 5–10 cm depths compared to NT‐C. After 9 years, cumulative decrease in available P was significantly greater beneath NT‐C (?24.0 kg P ha^?1) compared to switchgrass (?5.4 kg P ha^?1). When all measured soil parameters were included in the Soil Management Assessment Framework (SMAF), switchgrass improved soil quality index over time (ΔSQI) in all depths. NT‐C without residue removal did not affect ΔSQI, but 50% residue removal decreased ΔSQI (0–30 cm) due to reduced aggregate stability and SMB‐C. Even with best‐management practices such as NT, corn stover removal will have to be carefully managed to prevent soil degradation. Long‐term N and harvest management studies that include biological, chemical, and physical soil measurements are necessary to accurately assess bioenergy impacts on soils.     

Evaluation of eastern gamagrass as dual-purpose complementary bioenergy and forage feedstock to switchgrass

Switchgrass (SG) is considered a model bioenergy crop and a warm-season perennial grass (WSPG) that traditionally served as forage feedstock in the United States. To avoid the sole dependence on SG for bioenergy production, evaluation of other crops to diversify the pool of feedstock is needed. We conducted a 3-year field experiment evaluating eastern gamagrass (GG), another WSPG, as complementary feedstock to SG in one- and two-cut systems, with or without intercropping with crimson clover or hairy vetch, and under different nitrogen (N) application rates. Our results showed that GG generally produced lower biomass (by 29.5%), theoretical ethanol potential (TEP, by 2.8%), and theoretical ethanol yield (TEY, by 32.9%) than corresponding SG under the same conditions. However, forage quality measures, namely acid detergent fiber (ADF), crude protein (CP), and elements P, K, Ca, and Mg were significantly higher in GG than those in SG. Nitrogen fertilizer significantly enhanced biomass (by 1.54 Mg ha⁻¹), lignin content (by 2.10 g kg⁻¹), and TEY (787.12 L ha⁻¹) in the WSPGs compared to unfertilized treatments. Intercropping with crimson clover or hairy vetch did not significantly increase biomass of the WSPGs, or TEP and TEY in unfertilized plots. This study demonstrated that GG can serve as a complementary crop to SG and could be used as a dual-purpose crop for bioenergy and forage feedstock in farmers' rotations.     

Switchgrass for forage and bioenergy: harvest and nitrogen rate effects on biomass yields and nutrient composition

Switchgrass (Panicum virgatum L.) may have value as forage and a bioenergy feedstock. Our objective was to evaluate how harvest system and N fertilizer rates affected biomass yield and nutrient composition of young stands of switchgrass (cv. Alamo) in the southern Great Plains, USA. Nitrogen fertilization increased biomass yields from 10.4, 10.8, and 12.2 Mg ha^?1 at 0 kg N?ha^?1 to 13.7, 14.6, and 21.0 Mg ha^?1 at 225 kg N?ha^?1 when harvested after seed set (October), after frost (December), and twice per year after boot stage (July) and frost, respectively. Nutrient concentrations and removal were generally twice as great when biomass was harvested twice versus once per year. Precipitation strongly affected biomass yields across the two years of these experiments. When late-summer precipitation is available to support regrowth in this environment, harvesting switchgrass twice per year will result in greater biomass yields. Harvesting twice per year, however, will increase fertilization requirements and reduce feedstock biomass quality. Switchgrass harvested during mid-summer after boot stage was of poor forage quality. To have value as a dual-purpose forage and bioenergy feedstock, switchgrass would need to be utilized during spring to early summer while in a vegetative stage.     

Are the benefits of yield responses to nitrogen fertilizer application in the bioenergy crop Miscanthus × giganteus offset by increased soil emissions of nitrous oxide?

A field trial was carried out on a 15 year old Miscanthus stand, subject to nitrogen fertilizer treatments of 0, 63 and 125 kg‐N ha^?1, measuring N₂O emissions, as well as annual crop yield over a full year. N₂O emission intensity (N₂O emissions calculated as a function of above‐ground biomass) was significantly affected by fertilizer application, with values of 52.2 and 59.4 g N₂O‐N t^?1 observed at 63 and 125 kg‐N ha^?1, respectively, compared to 31.3 g N₂O‐N t^?1 in the zero fertilizer control. A life cycle analyses approach was applied to calculate the increase in yield required to offset N₂O emissions from Miscanthus through fossil fuel substitution in the fuel chain. For the conditions observed during the field trial yield increases of 0.33 and 0.39 t ha^?1 were found to be required to offset N₂O emissions from the 63 kg‐N ha^?1 treatment, when replacing peat and coal, respectively, while increases of 0.71 and 0.83 t ha^?1 were required for the 125 kg‐N ha^?1 treatment, for each fuel. These values are considerably less than the mean above‐ground biomass yield increases observed here of 1.57 and 2.79 t ha^?1 at fertilization rates 63 and 125 kg‐N ha^?1 respectively. Extending this analysis to include a range of fertilizer application rates and N₂O emission factors found increases in yield necessary to offset soil N₂O emissions ranging from 0.26 to 2.54 t ha^?1. These relatively low yield increase requirements indicate that where nitrogen fertilizer application improves yield, the benefits of such a response will not be offset by soil N₂O emissions.     

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.     

Establishment of reed canary grass with perennial legumes or barley and different fertilization treatments: effects on yield,botanical composition and nitrogen fixation

In two field experiments in northern Sweden, we investigated if intercropping reed canary grass (RCG; Phalaris arundinacea L.) with nitrogen‐fixing perennial legumes could reduce N‐fertilizer requirements and also if RCG ash or sewage sludge could be used as a supplement for mineral P and K. We compared biomass production, N uptake and N‐fixation of RCG in monoculture and mixtures of RCG with alsike clover (Trifolium hybridum L.), red clover (Trifolium pratense L.), goat's rue (Galega orientalis Lam.) and kura clover (Trifolium ambiguum M. Bieb.). In one experiment, RCG was also undersown in barley (Hordeum vulgare L.). Three fertilization treatments were applied: 100 kg N ha^?1, 50 kg N ha^?1 and 50 kg N ha^?1 + RCG ash/sewage sludge. We used a delayed harvest method: cutting the biomass in late autumn, leaving it on the field during the winter and harvesting in spring. The legume biomass of the mixtures at the inland experimental site was small and did not affect RCG growth negatively. At the coastal site, competition from higher amount of clover biomass affected RCG growth and spring yield negatively. N‐fixation in red clover and alsike clover mixtures in the first production year approximately covered half of recommended N‐fertilization rate. Goat's rue and kura clover did not establish well at the costal site, but at the inland site goat's rue formed a small but vital undergrowth. RCG undersown in barley gave lower yield, both in autumn and spring, than the other treatments. The high N treatment gave a higher spring yield at the inland site than the low N treatments, but there were no differences due to fertilization treatments at the coastal site. For spring harvest, there were no yield benefits of RCG/legume intercropping compared with RCG monoculture. However, intercropping might be more beneficial in a two‐harvest system.     

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.     

Switchgrass nitrogen response and estimated production costs on diverse sites

Switchgrass (Panicum virgatum L.) has been the principal perennial herbaceous crop investigated for bioenergy production in North America given its high production potential, relatively low input requirements, and potential suitability for use on marginal lands. Few large trials have determined switchgrass yields at field scale on marginal lands, including analysis of production costs. Thus, a field‐scale study was conducted to develop realistic yield and cost estimates for diverse regions of the USA. Objectives included measuring switchgrass response to fertility treatments (0, 56, and 112 kg N ha^?1) and generating corresponding estimates of production costs for sites with diverse soil and climatic conditions. Trials occurred in Iowa, New York, Oklahoma, South Dakota, and Virginia, USA. Cultivars and management practices were site specific, and field‐scale equipment was used for all management practices. Input costs were estimated using final harvest‐year (2015) prices, and equipment operation costs were estimated with the MachData model ($2015). Switchgrass yields generally were below those reported elsewhere, averaging 6.3 Mg ha^?1 across sites and treatments. Establishment stand percent ranged from 28% to 76% and was linked to initial year production. No response to N was observed at any site in the first production year. In subsequent seasons, N generally increased yields on well‐drained soils; however, responses to N were nil or negative on less well‐drained soils. Greatest percent increases in response to 112 kg N ha^?1 were 57% and 76% on well‐drained South Dakota and Virginia sites, where breakeven prices to justify N applications were over $70 and $63 Mg^?1, respectively. For some sites, typically promoted N application rates may be economically unjustified; it remains unknown whether a bioenergy industry can support the breakeven prices estimated for sites where N inputs had positive effects on switchgrass yield.     

A quantitative review comparing the yield of switchgrass in monocultures and mixtures in relation to climate and management factors

Switchgrass (Panicum virgatum L.), a US Department of Energy model species, is widely considered for US biomass energy production. While previous studies have demonstrated the effect of climate and management factors on biomass yield and chemical characteristics of switchgrass monocultures, information is lacking on the yield of switchgrass grown in combination with other species for biomass energy. Therefore, the objective of this quantitative review is to compare the effect of climate and management factors on the yield of switchgrass monocultures, as well as on mixtures of switchgrass, and other species. We examined all peer‐reviewed articles describing productivity of switchgrass and extracted dry matter yields, stand age, nitrogen fertilization (N), temperature (growing degree days), and precipitation/irrigation. Switchgrass yield was greater when grown in monocultures (10.9 t ha^?1, n=324) than when grown in mixtures (4.4 t ha^?1, n=85); yield in monocultures was also greater than the total yield of all species in the mixtures (6.9 t ha^?1, n=90). The presence of legume species in mixtures increased switchgrass yield from 3.1 t ha^?1 (n=65) to 8.9 t ha^?1 (n=20). Total yield of switchgrass‐dominated mixtures with legumes reached 9.9 t ha^?1 (n=25), which was not significantly different from the monoculture yield. The results demonstrated the potential of switchgrass for use as a biomass energy crop in both monocultures and mixtures across a wide geographic range. Monocultures, but not mixtures, showed a significant positive response to N and precipitation. The response to N for monocultures was consistent for newly established (stand age <3 years) and mature stands (stand age ≥3 years) and for lowland and upland ecotypes. In conclusion, these results suggest that fertilization with N will increase yield in monocultures, but not mixtures. For monocultures, N treatment need not be changed based on ecotype and stand age; and for mixtures, legumes should be included as an alternative N source.     

Efficient Methods of Estimating Switchgrass Biomass Supplies

Switchgrass (Panicum virgatum L.) is being developed as a biofuel feedstock for the United States. Efficient and accurate methods to estimate switchgrass biomass feedstock supply within a production area will be required by biorefineries. Our main objective was to determine the effectiveness of indirect methods for estimating biomass yields and composition of switchgrass fields. Indirect measurements were conducted in eastern Nebraska from 2003 to 2007 in which switchgrass biomass yields were manipulated using three nitrogen rates (0 kg N ha^-1, 60 kg N ha^-1, and 120 kg N ha^-1) and two harvest periods (August and post-killing frost). A modified Robel pole was used to determine visual obstruction, elongated leaf height, and canopy height measurements. Prediction models from the study showed that elongated leaf height, visual obstruction, and canopy height measurements accounted for >?91%, >?90%, and >?82% of the variation in switchgrass biomass, respectively. Regression slopes were similar by cultivar (“Cave-in-Rock” and “Trailblazer”), harvest period, and across years indicating that a single model is applicable for determining biomass feedstock supply within a region, assuming similar harvesting methods. Sample numbers required to receive the same level of precision were as follows: elongated leaf height<canopy height<visual obstruction. Twenty to 30 elongated leaf height measurements in a field could predict switchgrass biomass yield within 10% of the mean with 95% confidence. Visual obstruction is recommended on switchgrass fields with low to variable stand densities while elongated leaf height measurements would be recommended on switchgrass fields with high, uniform stand densities. Incorporating an ocular device with a Robel pole provided reasonable frequency estimates of switchgrass, broadleaf weeds, and grassy weeds at the field scale.     

Perennial Grass Bioenergy Cropping on Wet Marginal Land: Impacts on Soil Properties,Soil Organic Carbon,and Biomass During Initial Establishment

The control of soil moisture, vegetation type, and prior land use on soil health parameters of perennial grass cropping systems on marginal lands is not well known. A fallow wetness-prone marginal site in New York (USA) was converted to perennial grass bioenergy feedstock production. Quadruplicate treatments were fallow control, reed canarygrass (Phalaris arundinaceae L. Bellevue) with nitrogen (N) fertilizer (75 kg N ha^?1), switchgrass (Panicum virgatum L. Shawnee), and switchgrass with N fertilizer (75 kg N ha^?1). Based on periodic soil water measurements, permanent sampling locations were assigned to various wetness groups. Surface (0–15 cm) soil organic carbon (SOC), active carbon, wet aggregate stability, pH, total nitrogen (TN), root biomass, and harvested aboveground biomass were measured annually (2011–2014). Multi-year decreases in SOC, wet aggregate stability, and pH followed plowing in 2011. For all years, wettest soils had the greatest SOC and active carbon, while driest soils had the greatest wet aggregate stability and lowest pH. In 2014, wettest soils had significantly (p?<?0.0001) greater SOC and TN than drier soils, and fallow soils had 14 to 20% greater SOC than soils of reed canarygrass + N, switchgrass, and switchgrass + N. Crop type and N fertilization did not result in significant differences in SOC, active carbon, or wet aggregate stability. Cumulative 3-year aboveground biomass yields of driest switchgrass + N soils (18.8 Mg ha^?1) were 121% greater than the three wettest switchgrass (no N) treatments. Overall, soil moisture status must be accounted for when assessing soil dynamics during feedstock establishment.     

High retention of 15N‐labeled nitrogen deposition in a nitrogen saturated old‐growth tropical forest

The effects of increased reactive nitrogen (N) deposition in forests depend largely on its fate in the ecosystems. However, our knowledge on the fates of deposited N in tropical forest ecosystems and its retention mechanisms is limited. Here, we report the results from the first whole ecosystem ¹⁵N labeling experiment performed in a N‐rich old‐growth tropical forest in southern China. We added ¹⁵N tracer monthly as ¹⁵NH₄¹⁵NO₃ for 1 year to control plots and to N‐fertilized plots (N‐plots, receiving additions of 50 kg N ha^?1 yr^?1 for 10 years). Tracer recoveries in major ecosystem compartments were quantified 4 months after the last addition. Tracer recoveries in soil solution were monitored monthly to quantify leaching losses. Total tracer recovery in plant and soil (N retention) in the control plots was 72% and similar to those observed in temperate forests. The retention decreased to 52% in the N‐plots. Soil was the dominant sink, retaining 37% and 28% of the labeled N input in the control and N‐plots, respectively. Leaching below 20 cm was 50 kg N ha^?1 yr^?1 in the control plots and was close to the N input (51 kg N ha^?1 yr^?1), indicating N saturation of the top soil. Nitrogen addition increased N leaching to 73 kg N ha^?1 yr^?1. However, of these only 7 and 23 kg N ha^?1 yr^?1 in the control and N‐plots, respectively, originated from the labeled N input. Our findings indicate that deposited N, like in temperate forests, is largely incorporated into plant and soil pools in the short term, although the forest is N‐saturated, but high cycling rates may later release the N for leaching and/or gaseous loss. Thus, N cycling rates rather than short‐term N retention represent the main difference between temperate forests and the studied tropical forest.     

Nitrogen and harvest date affect developmental morphology and biomass yield of warm‐season grasses

Information on the growth and development of warm‐season grasses in response to management is required to use them successfully as a biomass crop. Our objectives were to determine optimum harvest periods and effect of N fertilization rates on the biomass production of four warm‐season grasses, and to investigate if traits of canopy structure can explain observed yields with varying harvest dates and N rates. A field study was conducted at Sorenson Research Farm near Ames, IA, during 2006 and 2007. The experimental design was split‐split plot arranged in a randomized complete block with four replications. Big bluestem (Andropogon gerardii Vitman), eastern gamagrass (Tripsacum dactyloides L.), indiangrass (Sorghastrum nutrans L. Nash), and switchgrass (Panicum virgatum L.) were main plots. Three N application rates (0, 65, and 140 kg ha^?1) were subplots, and 10 harvest dates were sub‐sub plots. Biomass of warm‐season grasses increased with advanced maturity, but differently among species. The maximum yield of eastern gamagrass occurred at the highest MSC (1.6 and 2.2) when the largest seed ripening tillers were present. Big bluestem, switchgrass, and indiangrass obtained the maximum yields at MSC 3.5, 3.9, and 2.9, respectively when the largest reproductive tillers were present. In terms of a biomass supply strategy, eastern gamagrass may be used during early summer, while big bluestem and switchgrass may be best used between mid‐ and late‐ summer, and indiangrass in early fall. Nitrogen fertilization increased yield by increasing tiller development. Optimum biomass yields were obtained later in the season when they were fertilized with 140 kg ha^?1.     

Harvest and nitrogen effects on bioenergy feedstock quality of grass-legume mixtures on Conservation Reserve Program grasslands

Perennial grass mixtures established on Conservation Reserve Program (CRP) lands can be an important source of feedstock for bioenergy production. This study aimed to evaluate management practices for optimizing the quality of bioenergy feedstock and stand persistence of grass-legume mixtures under diverse environments. A 5-year field study (2008–2012) was conducted to assess the effects of two harvest timings (at anthesis vs after complete senescence) and three nitrogen (N) rates (0, 56, 112 kg N ha⁻¹) on biomass chemical compositions (i.e., cell wall components, ash, volatiles, total carbon, and N contents) and the feedstock energy potential, examined by the theoretical ethanol yield (TEY) and the total TEY (i.e., the product of biomass yield and TEY, L ha⁻¹), of cool-season mixtures in Georgia and Missouri and a warm-season mixture in Kansas. The canonical correlation analysis (CCA) was used to investigate the effect of vegetative species transitions on feedstock quality. Although environmental variations (mainly precipitation) greatly influenced the management effect on chemical compositions, the delayed harvest after senescence generally improved feedstock quality. In particular, the overall cell wall concentrations and TEY of the warm-season mixtures increased by approximately 7%. Additional N supplies improved the total TEY of both mixtures by ~1.6–4.2 L ha⁻¹ per 1.0 kg N ha⁻¹ input but likely lowered the feedstock quality, particularly for the cool-season mixture. The cell wall concentrations of cool-season mixture reduced by approximately 3%–6%. The CCA results indicated that the increased legume compositions (under low N input) likely enhanced lignin but reduced ash concentrations. This field research demonstrated that with proper management, grass-legume mixtures on CRP lands can provide high-quality feedstock for bioenergy productions.     

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.