Effect of Potassium and Nitrogen Fertilizer on Switchgrass Productivity and Nutrient Removal Rates under Two Harvest Systems on a Low Potassium Soil

Biomass demand for energy will lead to utilization of marginal, low fertility soil. Application of fertilizer to such soil may increase switchgrass (Panicum virgatum L.) biomass production. In this three-way factorial field experiment, biomass yield response to potassium (K) fertilizer (0 and 68 kg?K?ha^?1) on nitrogen (N)-sufficient and N-deficient switchgrass (0 and 135 kg?N?ha^?1) was evaluated under two harvest systems. Harvest system included harvesting once per year after frost (December) and twice per year in summer (July) at boot stage and subsequent regrowth after frost. Under the one-cut system, there was no response to N or K only (13.4 Mg?ha^?1) compared to no fertilizer (12.4 Mg?ha^?1). Switchgrass receiving both N and K (14.6 Mg?ha^?1) produced 18 % greater dry matter (DM) yield compared to no fertilizer check. Under the two-cut harvest system, N only (16.0 Mg?ha^?1) or K only (14.1 Mg?ha^?1) fertilizer produced similar DM to no fertilizer (15.1 Mg?ha^?1). Switchgrass receiving both N and K in the two-cut system (19.2 Mg?ha^?1) produced the greatest (P?<?0.05) DM yield, which was 32 % greater than switchgrass receiving both N and K in the one-cut system. Nutrient removal (biomass?×?nutrient concentration) was greatest in plots receiving both N and K, and the two-cut system had greater nutrient removal than the one-cut system. Based on these results, harvesting only once during winter months reduces nutrient removal in harvested biomass and requires less inorganic fertilizer for sustained yields from year to year compared to two-cut system.     

Yield and Nutrient Removal by Bioenergy Grasses on Swine Effluent Spray Fields in the Coastal Plain Region of North Carolina

Bioenergy grasses such as giant miscanthus (Miscanthus × giganteus) and switchgrass (Panicum virgatum L.) are promising alternatives to the traditional coastal bermudagrass [Cynodon dactylon (L.) Pers.] at spray fields in Eastern North Carolina. The objective of this study was to determine the impact of different harvest practices on yield and nutrient removal of miscanthus and switchgrass in a swine (Sus scrofa domesticus) lagoon effluent spray field environment. Field trials of grasses under six single-cut and double-cut harvest practices (May/October, June/October, July/October, Aug/October, October only, and December only) were established at three commercial swine farms in Eastern North Carolina in either 2011 or 2012. Throughout the 4-year experimental period (2012–2015), both miscanthus and switchgrass produced significantly higher biomass yield than coastal bermudagrass. Two-cut harvest systems significantly improved the yields of miscanthus and switchgrass relative to a single harvest in December at spray fields. The maximum yields were 24 Mg ha^?1 year^?1 for miscanthus and 18 Mg ha^?1 year^?1 for switchgrass. Bioenergy grasses removed more nutrients under two-cut systems than under a single harvest. The significantly greater nutrient removals under two-cut harvest systems would result in lower requirements for receiver crop acreage and are more desirable from a spray field nutrient management perspective.     

Impact of Harvest Time and Switchgrass Cultivar on Sugar Release Through Enzymatic Hydrolysis

Switchgrass (Panicum virgatum L.) is a native North American prairie grass being developed for bioenergy production in the central and eastern USA. The objective of this study was to identify the impacts of harvest time and switchgrass cultivar had on sugar release variables determined through enzymatic hydrolysis. Previously, we reported that delaying harvest of switchgrass until after frost and until after winter resulted in decreased yields of switchgrass but it reduced the amount of ash and nutrients in the biomass. The current study used near-infrared reflectance spectroscopy (NIRS) to broaden an existing set of calibration equations designed to predict composition and sugar release variables of switchgrass. These updated calibrations were then applied to the full set of samples from a multi-year and multi-location switchgrass harvest-management study. Composition and processor sugar yields were significantly affected by location, year, cultivar, and harvest time, of which the time of harvest was the most important. Delaying the time of harvest until after frost or post-winter increased the concentration of structural carbohydrates from 500 to over 570 g kg^?1 in the biomass and lignin content from 160 to over 200 g kg^?1. Conversely, delaying harvest time lowered the amounts of ash and soluble sugars. The later harvest times also yielded more sugars following processing with yields increasing over 20% from the first harvest. Increased sugar yields are attributable to both increased concentration of sugars in the biomass upon harvest and reduced biomass recalcitrance. Based upon processed sugar yields, it is estimated that a biorefinery producing 76 million liters of ethanol per year would require 229–373 km² of land cultivated with switchgrass.     

Yield and Nitrogen Removal of Bioenergy Grasses as Influenced by Nitrogen Rate and Harvest Management in the Coastal Plain Region of North Carolina

The agronomic performances of giant miscanthus (Miscanthus x giganteus) and switchgrass (Panicum virgatum L.) grown as bioenergy grasses are still unclear in North Carolina, due to a relatively short period of introduction. The objectives of the study were to compare the biomass yield and annual N removal of perennial bioenergy grasses and the commonly grown coastal bermudagrass [Cynodon dactylon (L.) Pers.], and to determine the optimum N rates and harvest practices for switchgrass and miscanthus. A 4-year field trial of the grasses under five annual harvest frequencies (May/Oct, June/Oct, July/Oct, Aug/Oct, and October only) and five annual N rates (0, 67,134, 202, and 268 kg N ha^?1) was established at a research farm in Eastern North Carolina in 2011. Across harvest treatments and N rates, greatest biomass was achieved in the second growth year for both miscanthus (19.0 Mg ha^?1) and switchgrass (15.9 Mg ha^?1). Grasses demonstrated no N response until the second or the third year after crop establishment. Miscanthus reached a yield plateau with a N rate of 134 kg ha^?1 since achieving plant maturity in 2013, whereas switchgrass demonstrated an increasing fertilizer N response from 134 kg N ha^?1 in the third growth year (2014) to 268 kg N ha^?1 in the fourth growth year (2015). The two-cut harvest system is not recommended for bioenergy biomass production in this region because it does not improve biomass yield and increased N removal leads to additional costs.     

Management of warm-season grass mixtures for biomass production in South Dakota USA

Switchgrass (Panicum virgatum L.), big bluestem (Andropogon gerardii Vitman), and indiangrass (Sorghastrum nutans (L.) Nash) are native warm-season grasses commonly used for pasture, hay, and conservation. More recently switchgrass has also been identified as a potential biomass energy crop, but management of mixtures of these species for biomass is not well documented. Therefore, the objectives of our study were to: (1) determine the effects of harvest timing and N rate on yield and biomass characteristics of established warm-season grass stands containing a mixture of switchgrass, big bluestem, and indiangrass, and (2) evaluate the impact of harvest management on species composition. Five N rates (0, 56, 112, and 224 kg ha(-1) applied annually in spring and 224 kg ha(-1) evenly split between spring and fall) and two harvest timings (anthesis and killing frost) were applied to plots at two South Dakota USA locations from 2001 to 2003. Harvesting once a year shortly after a killing frost produced the greatest yields with high concentrations of neutral detergent fiber (NDF), acid detergent fiber (ADF), and acid detergent lignin (ADL) along with lower concentrations of total nitrogen (TN) and ash. This harvest timing also allowed for the greatest percentage of desirable species while maintaining low grass weed percentages. While N rates of 56 and 112 kg ha(-1) tended to increase total biomass without promoting severe invasion of grass and broadleaf weed species, N application did not always result in significant increases in biomass production. Based on these results, mixtures of switchgrass and big bluestem were well suited for sustainable biomass energy production. Furthermore, N requirements of these mixtures were relatively low thus reducing production input costs.     

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.     

Impact of Harvest Time and Cultivar on Conversion of Switchgrass to Bio-oils Via Fast Pyrolysis

The study of the effects of harvest time on switchgrass (Panicum virgatum L.) biomass and bioenergy production reported herein encompasses a large study evaluating the harvest of six switchgrass cultivars grown at three northern US locations over 3 years, harvested at upland peak crop (anthesis), post-frost, and post-winter. Delaying harvest of switchgrass until after frost and until after winter has resulted in decreased yields of switchgrass and reduced amounts of minerals in the biomass. This report examines how changes in biomass composition as a result of varying harvest time and other factors affect the distribution of products formed via fast pyrolysis. A subset (50) of the population (n = 864) was analyzed for fast pyrolysis and catalytic pyrolysis (zeolite catalyst) product yields using a pyrolysis-GC/MS system. The subset was used to build calibrations that were successful in predicting the pyrolysis product yield using near-infrared reflectance spectroscopy (NIRS), and partial least squares predictive models were applied to the entire sample set. The pyrolysis product yield was significantly affected by the field trial location, year of harvest, cultivar, and harvest time. Delaying harvest time of the switchgrass crop led to greater production of deoxygenated aromatics improving the efficiency of the catalytic fast pyrolysis and bio-oil quality. The changes in the pyrolysis product yield were related to biomass compositional changes, and key relationships between cell wall polymers, potassium concentration in the biomass, and pyrolysis products were identified. The findings show that the loss of minerals in the biomass as harvest time is delayed combined with the greater proportion in cellulose and lignin in the biomass has significant positive influences on conversion through fast pyrolysis.     

Nutrient Uptake by ‘Alamo’ Switchgrass Used as an Energy Crop

Field studies were conducted in 2000 and 2001 to examine yields and nutrient removal by Alamo switchgrass (Panicum virgatum L.) grown at eight locations within five states in the upper southeastern USA. Plots, which had been established for >5 years as part of a larger study, were cut either once (late fall) or twice (midsummer and late fall). Plots cut once received 50 kg N per hectare per year, while twice-cut plots received 100 kg N per hectare per year. Nutrient concentrations of and nutrient removal by harvested biomass were determined. Partitioning of nutrients into leaf and stem fractions was determined at the time of the midsummer harvest in 2000. Biomass production during 2000 and 2001 averaged 15.9 Mg/ha per year across all sites and was as high as 21.7 Mg/ha per year at one site. Two cuttings plus the additional 50 kg N per hectare did not generally increase seasonal yields; and, in one quite productive location, that management caused a yield reduction. Nitrogen removal with two cuts was much higher than with a single cut due largely to the higher N content in the midsummer harvest. Over the 2 years, twice as much N was removed with the two annual cuts as with one cut. Nitrogen removal exceeded the amounts of N applied in both managements, suggesting N was being supplied via mineralization or other processes. Phosphorus removal also increased significantly with the two-cut management. Seasonal K and Ca removals were more similar between the two managements. Nitrogen and P concentrations generally declined basipetally in tillers, with older leaves and internodes having lower concentrations of both nutrients. Potassium was more uniformly distributed than N throughout the tiller components (leaf and stem). Calcium was higher in older leaf blades. Levels of soil P, K, and Ca at most locations appeared not to be limiting biomass production and were adequate for long-term productivity.     

Switchgrass Biomass and Nitrogen Yield with Over-Seeded Cool-season Forages in the Southern Great Plains

In dry climates with long, hot summers and freezing winters, such as that of the southern Great Plains of North America, switchgrass (Panicum virgatum L.) has proven potential as a cellulosic bioenergy feedstock. This trial looked at dry matter (DM) and N yield dynamics of switchgrass overseeded with cool-season legumes and rye (Secale cereale L.), compared to switchgrass fertilized with 0, 56 and 112 kg N ha^-1 yr^-1 at an infertile and a fertile location. Optimal N fertilizer rate on switchgrass was 56 kg N ha^-1 at the infertile location. Legume yield was greater in the first season after planting, compared to subsequent years where annual legumes were allowed to reseed and alfalfa (Medicago sativa L.) was allowed to grow. This suggests that the reseeding model for annual legumes will not work in switchgrass swards grown for biomass unless soil seed banks are built up for more than one year, and that overseeding with alfalfa may have to be repeated in subsequent years to build up plant populations. Overseeding rye and legumes generally did not suppress or enhance switchgrass biomass production compared to unfertilized switchgrass. However, cumulative spring and fall biomass yields were generally greater due to winter and spring legume production, which could be beneficial for grazing or soil conservation systems, but not necessarily for once-yearly late autumn harvest biofuel production systems.     

Biomass and seed yields of big bluestem, switchgrass, and intermediate wheatgrass in response to manure and harvest timing at two topographic positions

A principle attribute of perennial grasses for biomass energy is the potential for high yields on marginal lands. Objectives of this study were to compare biomass and seed production of intermediate wheatgrass ( Thinopyrum intermedium [Host] Barkworth and D.R. Dewey), big bluestem ( Andropogon gerardii Vitman), and switchgrass ( Panicum virgatum L.) as affected by harvest timing and manure application on two topographic positions (footslope and backslope). Footslope is the hillslope position that forms the inclined surface at the base of a slope and backslope forms the steepest, middle position of the hillslope. Grasses were harvested for biomass at anthesis (summer), after a killing frost (autumn), or the following spring after overwintering in the field. Seed was harvested at maturity during 2003 and 2004. Two rates of beef cattle ( Bos taurus L.) manure (target rates of 0 and 150 kg total-N ha⁻¹) were surface applied annually. Maximum annual biomass yield ranged from 4.4 to 5.2, 2.7 to 4.2, and 3.7 to 5.6 Mg ha⁻¹ for intermediate wheatgrass, big bluestem, and switchgrass, respectively. Biomass yields were not different between fall and spring harvest treatments. Biomass yields of big bluestem and switchgrass at the backslope position were 86% and 96% of biomass yields at the footslope position with normal precipitation, respectively. Manure application increased biomass yield approximately 30% during the second year on both topographic positions. The highest seed yield was obtained from intermediate wheatgrass, followed by switchgrass and big bluestem. Utilizing these management practices in our environment, it appears that switchgrass and big bluestem could be allowed to overwinter in the field without suffering appreciable loss of biomass.     

Seasonal Changes in Shoot and Root Nitrogen Distribution in Switchgrass (Panicum virgatum)

Switchgrass is a promising bioenergy source that is perennial, productive, native to a broad geographic region, and can grow on marginal, nitrogen (N)-poor soils. Understanding N dynamics in switchgrass is critical to predicting productivity, conserving N, and optimizing the timing of harvest. We examined seasonal changes in N distribution in above- and belowground tissues in switchgrass to quantify N retranslocation rates. Above- and belowground biomass from three sites (two in PA and one in NE) were collected and analyzed for biomass growth and N concentrations at 30-day intervals from June through October. Total living plant mass ranged from 10.3?±?2.4 standard error (SE) to 14.9?±?2.5 SE Mg ha^?1. Belowground mass comprised 52–57 % of total mass. Blades had the highest N concentration during summer, ranging from 6 to 22 g kg^?1 N. Aboveground N concentrations decreased from September until autumn senescence, whereas belowground N concentration increased from August until senescence. Across the sites, total N retranslocated from aboveground to belowground components between September and October averaged 16.5?±?7.1 (SE)?kg ha^?1 N representing 26.7 % of the average maximum N content of aboveground biomass. Based on N fertilizer costs, delayed harvest would conserve some N and provide financial savings on fertilizer ($9 ha^?1) if harvest occurs after senescence but before overwinter biomass loss. However, biomass losses of even 10 % will negate potential economic savings accrued from N retention. To maximize environmental and economic savings from N retranslocation and to simultaneously minimize harvest losses, it would be optimal to harvest switchgrass as soon as possible after complete senescence.     

Soil phosphorus drawdown by perennial bioenergy cropping systems in the Midwestern US

Without fertilization, harvest of perennial bioenergy cropping systems diminishes soil nutrient stocks, yet the time course of nutrient drawdown has not often been investigated. We analyzed phosphorus (P) inputs (fertilization and atmospheric deposition) and outputs (harvest and leaching losses) over 7 years in three representative biomass crops—switchgrass (Panicum virgatum L.), miscanthus (Miscanthus × giganteus) and hybrid poplar trees (Populus nigra × P. maximowiczii)—as well as in no-till corn (maize; Zea mays L.) for comparison, all planted on former cropland in SW Michigan, USA. Only corn received P fertilizer. Corn (grain and stover), switchgrass, and miscanthus were harvested annually, while poplar was harvested after 6 years. Soil test P (STP; Bray-1 method) was measured in the upper 25 cm of soil annually. Harvest P removal was calculated from tissue P concentration and harvest yield (or annual woody biomass accrual in poplar). Leaching was estimated as total dissolved P concentration in soil solutions sampled beneath the rooting depth (1.25 m), combined with hydrological modeling. Fertilization and harvest were by far the dominant P budget terms for corn, and harvest P removal dominated the P budgets in switchgrass, miscanthus, and poplar, while atmospheric deposition and leaching losses were comparatively insignificant. Because of significant P removal by harvest, the P balances of switchgrass, miscanthus, and poplar were negative and corresponded with decreasing STP, whereas P fertilization compensated for the harvest P removal in corn, resulting in a positive P balance. Results indicate that perennial crop harvest without P fertilization removed legacy P from soils, and continued harvest will soon draw P down to limiting levels, even in soils once heavily P-fertilized. Widespread cultivation of bioenergy crops may, therefore, alter P balances in agricultural landscapes, eventually requiring P fertilization, which could be supplied by P recovery from harvested biomass.     

Agricultural Management of Switchgrass for Fuel Quality and Thermal Energy Yield on Highly Erodible Land in the Driftless Area of Southwest Wisconsin

Converting row crop production to a perennial grass crop on highly erodible land has numerous benefits. Switchgrass, grown as a biofuel crop, can provide soil conservation benefits as a perennial crop and also provide economic value to the grower. However, little information exists regarding switchgrass management and production on these lands. The objectives of this study were to determine the effect of two management practices, nitrogen (N) fertilizer rate (0, 56, 112, 168, and 224 kg ha^?1) and harvest timing (mid-fall, late-fall, and spring), on: (1) dry matter (DM) yield, (2) switchgrass quality components (moisture, ash, and chloride (Cl^?) concentrations), and (3) combustion energy content and yield. The study was conducted in 2009 and 2010 on highly erodible lands in the Driftless Area of southwest Wisconsin. Results showed a positive response of switchgrass DM to N fertilizer, with no yield gain above 112 kg ha^?1 of N, although application of N increased Cl^? concentrations. Harvest timing also affected switchgrass yield, with decreases in yield observed with progressively later harvest timings; this yield decrease was slightly greater compared with previous studies. Progressively later harvest timings led to a decrease in moisture, ash concentration, and Cl^? concentration in both years. Energy content of switchgrass was not significantly affected by management. Energy yields, similar to DM yields, were maximized with 112 kg ha^?1 of N with a mid-fall harvest. The similarities between this study and other research indicate there is a universal response of switchgrass to N in the northern USA and yields determined in this study indicate that highly erodible lands in the Driftless Area can be used to produce switchgrass at regionally expected yields.     

Seasonal variability in net ecosystem carbon dioxide exchange over a young Switchgrass stand

Understanding carbon dynamics of switchgrass ecosystems is crucial as switchgrass (Panicum virgatum L.) acreage is expanding for cellulosic biofuels. We used eddy covariance system and examined seasonal changes in net ecosystem CO₂ exchange (NEE) and its components – gross ecosystem photosynthesis (GEP) and ecosystem respiration (ER) – in response to controlling factors during the second (2011) and third (2012) years of stand establishment in the southern Great Plains of the United States (Chickasha, OK). Larger vapor pressure deficit (VPD > 3 kPa) limited photosynthesis and caused asymmetrical diurnal NEE cycles (substantially higher NEE in the morning hours than in the afternoon at equal light levels). Consequently, rectangular hyperbolic light–response curve (NEE partitioning algorithm) consistently failed to provide good fits at high VPD. Modified rectangular hyperbolic light–VPD response model accounted for the limitation of VPD on photosynthesis and improved the model performance significantly. The maximum monthly average NEE reached up to ?33.02 ± 1.96 μmol CO₂ m^?2 s^?1 and the highest daily integrated NEE was ?35.89 g CO₂ m^?2 during peak growth. Although large differences in cumulative seasonal GEP and ER were observed between two seasons, total seasonal ER accounted for about 75% of GEP regardless of the growing season lengths and differences in aboveground biomass production. It suggests that net ecosystem carbon uptake increases with increasing GEP. The ecosystem was a net sink of CO₂ during 5–6 months and total seasonal uptakes were ?1128 ± 130 and ?1796 ± 217 g CO₂ m^?2 in 2011 and 2012, respectively. In conclusion, our findings suggest that the annual carbon status of a switchgrass ecosystem can be a small sink to small source in this region if carbon loss from biomass harvesting is considered. However, year‐round measurements over several years are required to assess a long‐term source‐sink status of the ecosystem.     

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.     

Crop Management Impacts Biofuel Quality: Influence of Switchgrass Harvest Time on Yield, Nitrogen and Ash of Fast Pyrolysis Products

Although upgrading bio-oil from fast pyrolysis of biomass is an attractive pathway for biofuel production, nitrogen (N) and mineral matter carried over from the feedstock to the bio-oil represents a serious contaminant in the process. Reducing the N and ash content of biomass feedstocks would improve process reliability and reduce production costs of pyrolytic biofuels. This study investigated: (1) How does switchgrass harvest date influence the yield, N concentration ([N]), and ash concentration of biomass and fast pyrolysis products? and (2) Is there a predictive relationship between [N] of switchgrass biomass and [N] of fast pyrolysis products? Switchgrass from five harvest dates and varying [N] from central Iowa were pyrolyzed using a free-fall reactor. Harvestable biomass peaked in August (8.6 Mg ha^?1), dropping significantly by November (6.7 Mg ha^?1, P?=?0.0027). Production of bio-oil per unit area mirrored that of harvested biomass at each harvest date; however, bio-oil yield per unit dry biomass increased from 46.6 % to 56.7 % during the season (P?=?0.0018). Allowing switchgrass to senesce lowered biomass [N] dramatically, by as much as 68 % from June to November (P?<?0.0001). Concurrently, bio-oil [N] declined from 0.51 % in June to 0.17 % by November (P?<?0.0001). Significant reductions in ash concentration were also observed in biomass and char. Finally, we show for the first time that the [N] of switchgrass biomass is a strong predictor of the [N] of bio-oil, char, and non-condensable gas with R ² values of 0.89, 0.94, and 0.88, respectively.     

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.     

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.     

Switchgrass, Big Bluestem, and Indiangrass Monocultures and Their Two- and Three-Way Mixtures for Bioenergy in the Northern Great Plains

High yielding, native warm-season grasses could be used as renewable bioenergy feedstocks. The objectives of this study were to determine the effect of warm season grass monocultures and mixtures on yield and chemical characteristics of harvested biomass and to evaluate the effect of initial seeding mixture on botanical composition over time. Switchgrass (Panicum virgatum L.), indiangrass [Sorghastrum nutans (L.) Nash], and big bluestem (Andropogon gerardii Vitman) were planted as monocultures and in all possible two- and three-way mixtures at three USA locations (Brookings and Pierre, SD and Morris, MN) during May 2002. Biomass at each location was harvested after a killing frost once annually from 2003 to 2005. Total biomass yield significantly increased with year at all locations. Switchgrass monocultures or mixtures containing switchgrass generally out-yielded big bluestem or indiangrass in monocultures or the binary mixture. Cellulose and hemicellulose concentrations were higher in 2004 and 2005 compared with 2003. Switchgrass or mixtures containing switchgrass tended to have less cellulose than either big bluestem or indiangrass. Results were more variable for total N, lignin, and ash. Switchgrass was the dominant component of all mixtures in which it was present while big bluestem was dominant when mixed with indiangrass. Indiangrass was maintained only in monocultures and declined over years when grown in mixtures at all locations. Our results indicated if biomass yield in the northern Great Plains is a primary objective, switchgrass should be a component of binary or tertiary mixtures that also contain big bluestem and/or indiangrass.     

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.