Editorial Manager(tm) for BioEnergy Research

 Manuscript Draft

Manuscript Number: 

Title: Biomass Production and Nutrient Utilization of Perennial Grasses under No Inputs in South 

Georgia

Article Type: Original Research

Keywords: biomass crops; energycane; low-input agriculture; napiergrass; switchgrass

Corresponding Author: Joseph E. Knoll, Ph.D.

Corresponding Author's Institution: USDA-ARS

First Author: Joseph E. Knoll, Ph.D.

Order of Authors: Joseph E. Knoll, Ph.D.;William F. Anderson, Ph.D.;Timothy C. Strickland, Ph.D.;Robert 

K. Hubbard, Ph.D.;Ravindra Malik, Ph.D.
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Running Head: Perennial Grasses under No Inputs

Article Type: Research Paper

Title: Biomass Production and Nutrient Utilization of Perennial Grasses under No Inputs 

in South Georgia

Authors:

Joseph E. Knoll (corresponding author)

USDA-ARS, Crop Genetics and Breeding Research Unit

P.O. Box 748

Tifton, GA 31793, USA

Tel: (229) 386-3177

FAX: (229) 391-3701

E-mail: joe.knoll@ars.usda.gov

William F. Anderson

USDA-ARS, Crop Genetics and Breeding Research Unit

P.O. Box 748

Tifton, GA 31793, USA

Timothy C. Strickland

USDA-ARS, Southeast Watershed Research Unit

P.O. Box 748

Tifton, GA 31793, USA

Manuscript

Click here to download Manuscript: Biomass and Nutrient Usage of Perennial Grasses-JK-submission.docx
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Robert K. Hubbard

USDA-ARS, Southeast Watershed Research Unit

P.O. Box 748

Tifton, GA 31793, USA

Ravindra Malik

College of Sciences & Health Professions, Department of Natural Sciences

Albany State University

Albany, GA 31705, USA

Abstract

Warm-season perennial grasses have great potential for sustainable biomass production in 

the Southeast, and should be productive on marginal lands. This study was initiated in fall 2005 

at Tifton, GA, to assess the performance of perennial grasses under rainfed conditions and no 

fertilizer inputs. The test included one energycane (Saccharum sp.), two accessions of 

napiergrass (Pennisetum purpureum Schum.), two experimental lines of switchgrass (Panicum 

virgatum L.), and three giant reed (Arundo donax L.) clones. Each winter, dry matter (DM) 

yield, moisture, and biomass nutrient contents were analyzed. Soil samples (15 cm) were 

collected each year, and deep soil cores were sampled in 2009 to assess possible changes in soil 

nutrients. Napiergrass averaged 31.6 Mg DM/ha for the first two years, and energycane 

averaged 25.5 Mg DM/ha for the first three years. Though lower yielding, switchgrass had good 

biomass quality, tending to be lowest in nitrogen, ash, and moisture at harvest. Napiergrass took 

up large quantities of potassium in the first season (704 kg K/ha), but only about half as much 
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the next season with similar DM yields. Energycane was found to utilize potassium more 

efficiently than napiergrass. Some evidence of potassium depletion in the soil profile was found 

for napiergrass, but no evidence of nitrogen depletion was observed in the soil. Biomass 

nitrogen content was, however, correlated with DM yield for the energycane, one napiergrass, 

and one switchgrass, suggesting that this nutrient was limiting in later seasons.

Keywords: biomass crops, energycane, low-input agriculture, napiergrass, switchgrass

Abbreviations:

BNF – Biological nitrogen fixation

DM – Dry matter

Introduction

Increasing energy demand with reduced reserves, irregular fluctuations in fuel prices, and 

concerns about climate change caused by burning of fossil fuels have brought about renewed 

interest in producing energy from renewable sources. Of particular interest is bioenergy, which 

is energy derived from biomass, either through direct combustion or through conversion to liquid 

fuel, such as bioethanol. Wood waste from milling and paper processing, as well as agricultural 

and municipal waste can all be burned to produce energy. In addition, over four billion gallons 

of bioethanol are produced in the U.S. each year, primarily from corn and other grains (Gray et 

al., 2006). However, these sources alone cannot meet the projected demand for renewable 

bioenergy, and the shift of corn and other food/feed sources to fuel production is raising concerns 

about effects on food prices. The EPA released the provisions of the Renewable Fuel Standard 
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in 2008 to include production of 21 billion gallons of ethanol from non-food sources (. 

Additionally, production of fuel from annual grain crops is relatively energy-inefficient. These 

concerns have led to the development of dedicated bioenergy crops. Perennial grasses have 

perhaps the greatest potential for production of biomass in the Southeastern U.S. The Southeast 

has warm summer temperatures and generally receives adequate, though sometimes erratic, 

rainfall, but generally has poorer soils when compared to the Great Plains. Perennial grasses, 

especially those which utilize the C4 photosynthetic pathway, should be able to take advantage of 

the warmer temperatures, and their deeper, more extensive root systems should be able to adapt 

to poor soils and occasional droughts. Though initial establishment of perennial grasses may be 

more expensive and energy-intensive than planting conventional annual crops, a substantial 

reduction in costs and energy inputs over the long-term should be expected with perennials 

(Boehmel et al., 2008; Karp and Shield, 2008). In addition, evidence suggests that perennial 

agriculture would have fewer negative impacts on the environment in terms of water 

consumption and quality, soil loss, and nutrient loss, compared to annual systems (Glover, 2003).

The milder winter temperatures in the Southeastern U.S. allow for the cultivation of 

subtropical perennial bunch grasses, including napiergrass (Pennisetum purpureum Schum.), 

energycanes (Saccharum hyb.), and Erianthus sp., as well as the more temperate C3 grass giant 

reed (Arundo donax L.). These grasses are generally propagated vegetatively from stem cuttings, 

similar to the method used to propagate sugarcane (Saccharum officinarum L.). In the Southeast 

stem cuttings are planted in late summer to early autumn so that growth begins before the onset 

of winter dormancy. Napiergrass, also known as elephantgrass, is native to tropical Africa. In 

tropical and subtropical areas it is an important forage species, where it is capable of producing 

very high yields of biomass (Magalhães et al., 2006; de Queiros Filho et al., 2000), even in poor 
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soils (de Morais et al., 2009). In Brazil, de Queiros Filho et al. (2000) reported dry matter (DM) 

yields of 30.9 Mg DM/ha at 100 days after planting using the cultivar ‘Roxo’. Also in Brazil, 

Magalhães et al. (2006) reported a more conservative yield of 18.9 Mg DM/ha in napiergrass 

harvested only 84 days after planting. These studies were conducted in plots with adequate 

nitrogen fertilizer, but de Morais et al. (2009) achieved similar yields in N-depleted soils at nine 

months after planting. Subsequent harvests also yielded significant, though decreasing, 

quantities of biomass. In Puerto Rico, a yield of 52.8 Mg DM/ha/yr (85-d cutting interval) was

reported for the cultivar ‘Merkeron’ (Sotomayor-Rios et al., 1997). In tests conducted in Florida 

under irrigation and adequate fertilization, first-season yields of PI-300086 and N51 

napiergrasses averaged 45.7 and 47 Mg DM/ha, respectively (Woodard and Prine, 1993). 

‘Merkeron’, which was developed in Tifton, GA, has survived winter temperatures as low as -

18°C (Burton, 1989), so its potential range could be extended to cooler locations (USDA 

hardiness zone 7b, possibly zone 7a; USDA, U.S. National Arboretum, 2002).

Energycanes are derived from crosses between cultivated sugarcane (S. officinarum L.) 

and related wild species (S. spontaneum L., S. robustum Brandes & Jesw ex. Grassl., Erianthus 

sp., etc.), and were developed specifically for the purpose of producing biomass for energy 

production. They generally have much higher fiber content and better cold tolerance than 

sugarcane varieties used for sucrose production (Legendre and Burner, 1995). Energycane L 79-

1002 yielded an average of 49.0 Mg DM/ha in first-year growth in tests conducted near 

Gainesville, FL, by Woodard and Prine (1993). At St. Gabriel, LA, over five seasons this 

cultivar achieved an average annual yield of 66.6 Mg DM/ha (Bischoff et al., 2008). Similarly 

high yields were reported for the cultivar US 72-1153 by Mislevy et al. (1995). Average yields 

of 43 and 52 Mg DM/ha over four years were reported at two different levels of nitrogen 
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fertilization (168.1 and 336.1 kg N/ha/yr), though yields tended to decline significantly in the 

third or fourth season.

Giant reed (Arundo donax L.) is a large C3 grass that is used for various purposes, 

including the production of reeds for musical instruments. Because of its rapid growth and high 

biomass production, it has recently gained attention as a potential bioenergy crop in southern 

Europe (Angelini et al., 2005; Cosentino et al., 2006; Mantineo et al., 2009) and the Southeastern 

U.S. Among 39 clones of giant reed evaluated in southern Italy, an average yield of 10.6 Mg 

DM/ha in the first year and 22.1 Mg DM/ha in the second year were reported by Consentino et 

al. (2006). In the third year after planting giant reed in Sicily, Mantineo et al. (2009) reported a 

yield of 38.8 Mg DM/ha. Giant reed is tolerant of heavy metals (Papazoglou, 2007) and high 

salinity (McWilliams, 2004), so it could potentially be grown on contaminated sites not suitable 

for production of other crops. It is relatively cold-tolerant, growing as far north as southern 

Illinois and Missouri (USDA, NRCS, 2009). One potential drawback of these exotic grasses is 

their potential to become invasive weeds. Giant reed in particular has received considerable 

attention as an invasive pest in aquatic habitats where it had been introduced for erosion control

(Quinn and Holt, 2008). Despite this limitation, giant reed could potentially be a useful 

bioenergy crop in specific locations if it is properly managed and not planted near aquatic 

habitats. Giant reed is limited to vegetative dispersal; though it flowers in temperate 

environments, successful seed set has not been documented in the U.S. (McWilliams, 2004). In 

some warmer locations, such as extreme southern Florida, napiergrass and energycane may 

flower and set seed, and have the potential to become invasive. However, in areas where frost 

halts the growth and flowering of the plant, seed is not produced, thus reducing unintentional 

spread. Because all of these grasses are reproduced easily from stem pieces, care must be taken 
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to eliminate accidental releases during transportation from production fields to the receiving 

bioenergy facilities. More research is also needed to determine the best methods (i.e. herbicides, 

mechanical control, etc.) for eradicating any unwanted releases of exotic bioenergy crops.

Native prairie grasses have been proposed as a low-input, ecologically sustainable source 

of biomass for much of the U.S. Unlike exotic species, native plants are generally considered 

non-invasive, and they provide ideal habitat for wildlife. In addition, natives should require 

fewer inputs as they are already adapted to local conditions and disease pressures. Native 

species considered for use as biomass include big bluestem (Andropogon gerardii Vitman) and 

switchgrass (Panicum virgatum L.), two of the predominant species of the tallgrass prairies of 

the Great Plains. Switchgrass has been extensively evaluated as a native biomass source. It is a 

relatively long-lived perennial grass with a very broad range of adaptability. It is propagated by 

seed and can also be used for forage or hay, and thus it may be readily adopted by growers with 

little to no investment in new equipment. Several high-yielding improved varieties of 

switchgrass are already available. A yield of 34.6 Mg DM/ha has been reported for the lowland 

cultivar ‘Alamo’ (Sladden et al., 1991), but typical yields are much lower. Mooney et al. (2009) 

estimated average yields of 16.82 Mg/ha/yr for ‘Alamo’ switchgrass on good farmland in 

Tennessee, while Lemus et al. (2008) reported more modest yields averaging 6.8 Mg/ha for the 

upland cultivar ‘Cave-in-Rock’ in southern Iowa. Yields are typically lower for the more 

northerly-adapted upland types.

Ideally, biomass crops should be able to grow on marginal lands so that they do not 

compete with other crops for better farmland. Also, because the per-unit value of biomass is low 

compared to food, feed, and fiber crops, production under minimal inputs is desirable for 

growers to profit from producing biomass. Surprisingly, few studies have been conducted to 
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compare the performance of different biomass feedstock species in the same field under lowinput conditions. The purpose of the present study is to evaluate and compare the performance 

of several potential biomass crops for the Southeast under no inputs, with the aim of determining 

which species will be most adaptable to marginal conditions. Dry matter (DM) yield, moisture at 

harvest, and ash content were assessed each season. Uptake of nitrogen, potassium, and other 

nutrients was monitored throughout the study to determine which crops utilize these nutrients 

most effectively. Changes in nutrient content in the soil were also assessed. Lastly, the 

extensive root systems of perennial crops are expected to sequester a significant amount of

carbon in the soil. Thus, the status of soil carbon was also monitored throughout this study, and 

root biomass was measured in soil cores, to determine the extent to which these crops may 

contribute to soil improvement.

Materials and Methods

Plant material and experimental design

This study was initiated 2 Sept. 2005 at Tifton, GA, and was located on a Tifton loamy 

sand (fine-loamy, kaolinitic, thermic Plinthic Kandiudults). In preceding years the site had been 

used for switchgrass plots, and for annual grain crops before that. The plots were laid out in a 

randomized complete block design with four replications. Plots measured 4 m × 7 m. Eight

entries with potential for use in bioenergy production were included in this test: one energycane

(Saccharum hyb.) L 79-1002; two napiergrasses (Pennisetum purpureum Schum.) ‘Merkeron’ 

and N51; two experimental switchgrass (Panicum virgatum L.) lines, designated PV-1 and PV-2

(both are lowland ecotype); and three giant reed (Arundo donax L.) clones, AD-S, AD-E, and 
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AD-F. Energycane and napiergrass were planted as stem cuttings, with two whole stems planted 

in each row, buried horizontally 10 cm deep, with rows spaced 0.6 m apart (four rows/plot). The 

giant reeds were micropropagated transplants, derived from clones collected in the Southeast, 

and were planted 0.6 m apart in rows 0.6 m apart. Switchgrass was seeded in rows 0.3 m apart 

(seven rows/plot) at a seeding rate of 11.2 kg seed/ha. A bare-ground plot was also included in 

each replication as a comparison for soil properties.

Biomass harvest

Total above-ground biomass from the center of each plot (harvested area = 5 m × 1.2 m) 

was mechanically harvested after senescence on Dec. 5, 2006, Dec. 11, 2007, and Dec. 9, 2008. 

The 2009 harvest was delayed until Feb. 3, 2010 due to mechanical problems with harvesting 

equipment and inclement weather. At harvest, a small sample of biomass from each plot was 

weighed fresh, dried completely at 65°C in an oven, and then weighed again to determine 

percentages of moisture and dry matter. Samples of dried biomass were then ground in a Wiley 

mill to pass a 2 mm screen prior to storage and further analyses.

Biomass nutrient analyses

Total carbon and nitrogen percentages in the dried plant samples were measured by dry 

combustion in a Vario EL-III Universal CHNOS Elemental Analyzer (Elementar 

Analysensysteme, Hanau, Germany) using approximately 5 mg dried, ground tissue. The 

percentage of total ash in biomass was determined by combustion in a Muffle furnace at 450°C 

for six hours. Two replicates were also analyzed for percentages of individual mineral elements 
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(K, P, Ca, Mg, Mn, and Zn) at the University of Georgia Agricultural and Environmental 

Services Laboratories (AESL) by inductively-coupled plasma (ICP) spectrometry.

Soil sampling and analysis

Each year after harvest (Jan. 30, 2007; Jan. 15, 2008; and Apr. 2, 2010) surface layer (0-

15 cm) soil samples were taken from each plot. Five to ten samples were taken from each plot 

and were then combined. On Apr. 30, 2009 soil cores to a depth of 107 cm were collected from 

the center of each plot, immediately adjacent to a growing clump using a hydraulic tractormounted soil probe (Giddings Machine Co. Fort Collins, CO). Cores were separated into four 

portions for analysis (0-15 cm, 30-45 cm, 60-76 cm, and 91-107 cm). A subsample of each 

portion was used for estimation of root biomass (g dry roots/kg dry soil). All soil samples were 

oven-dried and ground in a mill prior to storage and analysis. Soil samples were analyzed for C 

and N content by dry combustion. Approximately 50 mg dried, ground soil was sampled for this 

analysis. Roots from the soil cores were also dried, ground, and analyzed for C and N content (5 

mg samples); pieces of rhizome (typically found in giant reeds) were not sampled. Soil mineral 

composition (K, P, Ca, Mg, Mn, and Zn) was determined in the core samples by the Mehlich-I 

extraction procedure followed by ICP analysis at the AESL. Soil pH in the surface samples 

(2007, 2008, and 2010) was measured at the AESL using an AS-3000 pH Analyser (LabFit, 

Perth, Australia) by mixing dry soil with 0.01 M CaCl2 in a 1:1 (w/v) ratio. Equivalent pH in 

water was then calculated by adding 0.6 pH units (Kissel and Sonon, 2008). Soil pH in the core 

sections (2009) was measured in-house by mixing dry soil and deionized water in a 1:1 (w/v) 

ratio and reading the pH with an electrode after standing for 10 min. Due to differences in 
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sampling, the C, N, and pH measurements from the surface layer in the cores were not combined 

with the yearly analyses.

Statistical procedures

All yearly data were analyzed using the GLM procedure in SAS v. 9.2 (SAS Institute, 

Cary, NC) according to the following model:

y = μ + Y + R + E + (Y × R) + (Y × E) + (R × E) + ε

where y is the dependent variable, μ is the overall mean, Y is the year, R is the replication, E is 

the entry, and ε is the error term. Unless otherwise noted, all tests were conducted at a 

probability level of α = 0.05. Data from the 2009 soil cores were analyzed using a similar model, 

except that the variable year (Y) was replaced with the variable soil depth (D). Tests for 

differences between entries within years or soil depths were also conducted using PROC GLM. 

Tukey’s HSD test or the Tukey-Kramer test was used to determine significant differences 

between means and least-squares means. Correlation analyses were conducted using PROC 

CORR.

Results

Total dry matter (DM) yield varied greatly between entries, and was strongly affected by 

year. The entry by year interaction was also significant (Table 1). In the first season, the two 

napiergrass entries (Merkeron and N51) and energycane L 79-1002 had the highest DM yields 

(30.6, 37.1, and 29.2 Mg DM/ha, respectively; Table 2). These three entries maintained similar 

yields in the second season. Yields of switchgrass and giant reed were generally much lower 

than for napiergrass and energycane, especially in the first two seasons. However, in the third 
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season, dry matter yield of napiergrass had declined significantly to levels similar to that of giant 

reed and switchgrass. In contrast, energycane L 79-1002 maintained relatively consistent high 

yields over the first three seasons, averaging 25.5 Mg DM/ha. L 79-1002 had the highest yield in 

the third season (21.1 Mg DM/ha), but this was not statistically higher than the napiergrasses

(Table 2). Yields of giant reeds and switchgrasses were relatively constant over the entire four 

years of the test, with a slight, but not statistically significant, peak in the second season. Both 

switchgrass entries performed similarly in terms of DM yield, and the three giant reed entries 

also showed similar yield performance. Though giant reed clone AD-S tended to yield slightly 

less than the other two clones, the difference was not statistically significant. All eight entries in 

the test had very low yields in the fourth year (2009), and the yield differences between them 

were not significant within that year, except for L 79-1002 (8.2 Mg DM/ha) and AD-S (3.3 Mg 

DM/ha).

Percent moisture of harvested material was significantly different among years, even 

though the first three years were all harvested in the first or second week of December. The first 

season showed the highest moisture content, and the lowest average moisture content was 

observed in the 2009 harvest, which was delayed until February, 2010. There was a small entry 

by year interaction for moisture, but the most significant factor was the entries themselves. In 

2006, 2008, and 2009 the entries fell into three distinct groupings based on moisture. 

Napiergrass and energycane had the highest moisture content, giant reeds were intermediate, and 

switchgrass had the least moisture (Table 2). There was some overlap between these groupings 

in 2007, but the general trend was still observed. The interaction of years and entries was thus 

relatively small, and in general, the moisture contents of napiergrass and energycane were only 

slightly affected by year. Giant reeds were slightly drier in the late harvest in 2009, but the 
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differences between preceding years were only significant for one clone, AD-S. The effect of 

year on moisture content was strongest for switchgrasses, which were significantly drier in 2009, 

primarily due to the later harvest (average 22.3%) than in 2006 or 2007 (average 37.4% and 

37.7%, respectively; Table 2).

The average nitrogen percentage of the combined biomass feedstocks was not 

significantly different between the first two years, but dropped significantly in both the third and 

fourth years. Nitrogen percentage also varied significantly between entries, and the year by entry 

interaction was also significant (Table 1). Giant reeds tended to have the highest nitrogen 

content (0.590 – 0.692 % in 2006, 0.297 – 0.334 % in 2009), while switchgrasses had the lowest 

(average 0.244% in 2006, average 0.177 % in 2009; Table 3). Nitrogen content was not 

significantly different between any entries in 2007. The nitrogen contents of the switchgrasses

were also the lowest in 2008, but were not significantly different than L 79-1002 energycane or 

N51 napiergrass that year. In the final season, the nitrogen content of L 79-1002 energycane and 

both napiergrasses fell to levels similar to that of switchgrass. Nitrogen content of giant reeds 

remained the highest.

Carbon percentage in biomass varied significantly by year, and also by entry. 

Interactions were not significant (Table 1). Average carbon content of the biomass was not 

significantly different from 2006 to 2007, but then increased significantly in 2008 and again in 

2009. Switchgrasses had the highest carbon content, averaging 47.2%. Carbon content did not 

differ significantly among the other entries. The year by entry interaction for ash content was 

not found to be significant (Table 1), but it should be noted that in 2007 no significant 

differences between entries were observed for ash percentage. Overall, mean ash content was 

relatively unchanged for the first two seasons (4.83% and 4.67%), and then dropped significantly 
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the next two years (3.96% in 2008, and 2.95% in 2009). Switchgrasses generally had the lowest 

ash content, averaging 2.42% for PV-1, and 3.13% for PV-2 over four years. Energycane L 79-

1002 also had relatively low mean ash content (3.54%), not differing significantly from PV-2

overall. Napiergrasses and giant reeds had similar ash content (Table 4). Ash and carbon 

percentages in biomass showed a strong negative correlation (R = -0.711). As mineral content 

decreases, the percentage of carbon increases. Nitrogen was positively correlated with ash (R = 

0.694), and negatively correlated with carbon (R = 0.604).

Carbon to nitrogen (C:N) ratio of the harvested biomass varied significantly by years and 

entries. The year by entry interaction was also significant (Table 1). The average C:N ratio was 

not significantly different for the first two years, but then increased in 2008 and again in 2009. 

Overall the total DM yields of the switchgrasses were similar to those of the giant reeds, but 

switchgrass generally had higher C:N ratios. This is apparent in all but the 2007 season, where 

there were no significant differences between entries for nitrogen content and only two entries 

(Merkeron and PV-1) differed for C:N ratio (Table 3). In 2008 switchgrasses had significantly 

higher C:N ratios than any other grasses in the test. However, in the first season, L 79-1002 

energycane and N51 napiergrass also had C:N ratios similar to that of switchgrass. In the fourth 

season both napiergrasses and L 79-1002 energycane showed high C:N ratios, similar to that of 

the switchgrasses, though N51 napiergrass also did not differ significantly from giant reeds 

(Table 3).

Plant nitrogen contents in 2009 were substantially lower than in peak years for all entries 

(40–71% reductions, Table 3). These reductions were associated with decreases in dry matter 

yield for energycane, napiergrasses, and switchgrasses (Table 4, Figure 1). The magnitude of 

this response was substantially greater for energycane (21 Mg/ha) and the napiergrasses (23.5-32 
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Mg/ha) than for the switchgrasses (2.7-3.8 Mg/ha). Dry matter yields for the giant reeds did not 

appear to be responsive to plant nitrogen content across the ranges observed in this study, except 

for AD-E, but this correlation was only marginally significant (α = 0.10; Table 4). The high dry 

matter yields obtained from energycane and the napiergrassess were correlated with plant 

carbon-to-nitrogen ratios, especially for Merkeron (Table 4, Figure 1). The energycane was able 

to sustain high dry matter yield over a C:N ratio range of 107–185 but declined when the ratio 

exceeded 200. The napiergrasses demonstrated a much more narrow range of tolerance (74–102) 

with both entries demonstrating substantial dry matter yield declines as the C:N ratio dropped 

below 100. While the switchgrasses also exhibited a drop in dry matter yield potential as the 

C:N ratio dropped below 200, the reductions in dry matter produced were much less substantial. 

This correlation was more significant for PV-1 than for the other switchgrass PV-2 (Table 4).

Biomass mineral nutrient composition was analyzed in two replications over all four 

years of the test. Potassium content was very high in both napiergrass accessions (average 

2.085% K) in the first growing season, significantly greater than for any other entries. In the 

second season it dropped significantly to an average of 1.178%, still greater than the other 

entries, but not significantly so, partially due to an increase in K content of the switchgrasses. In 

2008 the K content of the switchgrasses was lower. Merkeron napiergrass and L 79-1002 

energycane had significantly greater potassium content than the switchgrasses in 2008, and in 

2009 L 79-1002 and the other napiergrass N51 were significantly greater in potassium. Giant 

reeds tended to have intermediate K content (Table 5). Of the mineral elements analyzed in 

biomass, potassium had by far the strongest year by entry interaction (Table 1). Potassium 

content also showed significant correlations with DM yield for both napiergrasses and for PV-2 

switchgrass (Table 4, Figure 1). Phosphorus content of the biomass varied by entry and peaked 
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in 2007, though the year by entry interaction was not significant. Averaged over all four years, 

napiergrasses had significantly higher P content than switchgrasses, while energycane and giant 

reeds were intermediate (Table 6). Calcium content was significantly lower in 2009 and varied 

by entry. As with P, the year by entry interaction for Ca content was not significant. Giant reeds 

had the highest Ca content, though some of these did not differ from N51 or energycane. 

Magnesium content showed a very similar pattern as calcium, dropping significantly in the 

fourth year. The year by entry interaction was significant, but very small (p = 0.0246; Table 1). 

As with Ca, giant reeds also had the highest Mg content, but energycane and napiergrass had the 

least (Table 6). Switchgrass DM yields showed strong correlations with both calcium content 

and magnesium content (Table 4, Figure 1). Zinc content of the biomass actually increased from 

2006 to 2007, and again from 2007 to 2008, but there were no significant differences between 

entries for biomass Zn content (Table 6). Manganese content also varied between entries and 

years, with a small but significant interaction of these factors (p = 0.0073; Table 1). Overall, 

giant reeds tended to have the highest Mn content, and switchgrass the lowest (Table 6).

The top 15 cm of soil was sampled in each plot, including the bare ground control, each 

spring after biomass harvest, but before new growth began. In 2007, the soil carbon content of 

the various entries and bare ground control did not differ significantly. In succeeding years, 

carbon content of the soil tended to increase only slightly in some planted plots, though not 

significantly. Carbon content of the bare soil control decreased slightly, though also not 

significantly. However, by 2010 the soil carbon contents of PV-1 switchgrass and giant reed 

clone AD-F were significantly greater than the bare ground control (Figure 2). Soil nitrogen 

content showed a similar trend as soil carbon, these two factors being highly correlated (R = 

0.868) across years. More significant differences between entries were noted for soil nitrogen. 
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From 2007 to 2010, all planted plots maintained soil nitrogen percentage, or showed a slight 

increase in nitrogen, though most of the changes were not significant. The bare ground control 

decreased slightly, but not significantly, in nitrogen percentage. However, by 2010, the bare 

ground control soil contained significantly less nitrogen than all planted plots, except for 

energycane L 79-1002 (Figure 2).

In spring 2009, soil cores were taken and analyzed for carbon, nitrogen, and mineral 

nutrient composition. Carbon and nitrogen were generally highest in the surface layer and 

decreased significantly with each depth increment. Few differences were observed between 

entries or bare control for soil C and N content in the core sections. Napiergrass N51 did appear 

to have slightly higher carbon accumulation than the giant reeds at the 60-76 cm depth, but this 

difference is only marginally significant (α = 0.10; data not shown). Some differences were 

noted between soil cores for potassium distribution (Figure 3). In the surface layer, napiergrasses 

had the highest soil K, but the only significant difference was between N51 (70.94 mg K/kg soil) 

and the bare control (22.96 mg K/kg soil). At the 30-45 cm depth switchgrass PV-1 and giant 

reed AD-S had slightly more soil K than energycane L 79-1002, though these differences are 

only significant at the α = 0.10 level. Napiergrasses also had lower soil K at this depth, but were 

not significantly different from the other entries. At the 60-76 cm depth, both napiergrasses had 

the lowest soil K content (average 14.65 mg K/kg soil), significantly less than PV-1 switchgrass 

(30.51 mg K/kg soil) at α = 0.05. The other switchgrass PV-2 also had high soil K (29.17 mg 

K/kg soil) at this depth, also significantly greater than napiergrass, but only at a significance 

level of α = 0.10. Also at α = 0.10 giant reed AD-S had greater soil K than either napiergrass at 

the 60-76 cm depth. No significant differences in soil K were observed in the deepest soil layer 

(91-107 cm). The other minerals (P, Mg, Mn, and Zn) showed typical distribution patterns in the 
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soil cores, being highest in the upper soil layers, and decreasing with depth. Few significant 

differences were observed between entries for these nutrients at the various soil depths. Calcium 

had a more variable soil profile, being greatest in both the top soil layer (0-15 cm) and the 60-76 

cm layer, and lowest in the 30-45 cm and 91-107 cm layers. However, no significant differences 

between entries were observed (data not shown).

Roots were quantified at different depths in the soil cores, and their nitrogen content was 

also measured. No significant differences between entries were found for root nitrogen content. 

However, roots in the upper soil layer generally had slightly more nitrogen, and the difference 

was statistically significant. Roots in the upper soil layer averaged 0.55% nitrogen, while roots in 

the three lower soil layers averaged 0.39, 0.34, and 0.42% nitrogen, by increasing depth. Root 

biomass was estimated on a dry weight basis, based on the amount of roots found in the soil 

cores. Estimates of root biomass for giant reeds were highly variable at all soil depths (standard 

deviations exceeded means). It appears that the roots of giant reed tend to be large and unevenly 

distributed, so that the method of sampling roots with relatively narrow soil cores is probably not 

suitable for this species. Also, the large rhizomes were not sampled, so the amount of 

underground biomass for these entries would be underestimated. Thus, root data for giant reed 

were not included in further analyses. Among the other entries, significantly more roots were 

found in the uppermost soil layer (0-15 cm) than in deeper layers, and there were some

differences found between entries for mean root biomass (Figure 4). Within soil layers, the

differences in root biomass were only significant in the uppermost layer. Switchgrass PV-1 had 

significantly higher mean root biomass (0.8688 g roots/kg soil, dry basis averaged over all 

layers) than any other accession. The other four entries did not differ significantly from each 
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other. Merkeron had more roots than the samples taken in the bare plots, though the difference 

was not statistically significant.

Discussion

Yields reported in other studies of giant reed (Cosentino et al., 2006; Mantineo et al., 

2009) are generally higher than those observed in this study, most likely due to the origin of the 

planting material. The giant reed plants used in this study originated from micropropagation, 

rather than from conventional vegetative material (cane cuttings or rhizomes). Yield reduction in 

tissue culture-derived sugarcane compared to conventionally propagated cane was reported by 

Bailey and Bechet (1989), though the exact tissue culture procedure was not reported. In one 

sugarcane cultivar, out of three tested by Hoy et al. (2003), reduced yield was noted from plants 

generated from leaf-roll callus, but not from apical meristem culture. Thus, it is certainly 

possible that in vitro propagation may decrease productivity in giant reed.

Very low yields were observed for the entire test in 2009, despite the fact that more rain 

was received during this growing season than in the previous three (Table 7). Though some 

differences in DM yield were observed between entries in 2009, most were not statistically 

significant. Most likely this yield reduction is due to depletion of nutrients from the soil, as is 

evidenced by the reductions in plant nitrogen and potassium contents, the widening of plant C:N 

ratios, and their correlations with yield (Table 4). All of these responses would be expected with 

no addition of fertilizer, but detailed studies are needed to determine the optimum fertilization 

requirements for these grasses. A small but significant fraction of yield loss in the 2009 season 

may also be due to the lateness of the harvest (delayed until Feb., 2010), as a significant amount 

of leaves had fallen to the ground in some plots, and this biomass was not harvested. Though it 
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was not quantified it appeared from visual observations that energycane and giant reed retain 

more leaves than switchgrass or napiergrass over the winter. Previous studies (de Morais et al.,

2009) have shown that leaves make up approximately 30% of harvestable biomass in mature tall 

varieties of napiergrass. Despite the potential for yield loss, several studies have reported 

improvements in biomass quality or increased nutrient use efficiency in late harvesting of 

perennial grasses due to drying, nutrient translocation, and nutrient leaching (Landström et al.,

1996).

It is unclear whether the decreased nitrogen contents of the energycane and napiergrasses 

in the 2009 harvest was due to the late harvest time or to other factors such as depletion of 

nutrients in the soil or age of the crop. The nitrogen content of most entries did tend to decrease 

as plant-available nitrogen was presumably depleted from the soil, though no evidence for 

nitrogen depletion was actually observed in the soil. Nitrogen actually appeared to increase 

slightly in the topsoil of planted plots. Because it was strongly correlated with soil carbon (R = 

0.868) this increase is probably mostly from organic nitrogen, derived from leaf and root 

residues. Because no fertilizer was applied in this study, it is assumed that the nitrogen in the 

harvested biomass was derived from residual nitrogen in the soil. However, the possibility that 

some of the nitrogen uptake was derived from biological nitrogen fixation (BNF) should not be 

overlooked. It has been shown that sugarcane in the tropics can derive a significant amount of 

nitrogen from associated nitrogen-fixing (diazotrophic) bacteria (Urquiaga et al., 1992). 

Endophytic diazotrophs have also been isolated from napiergrass grown in Brazil (Kirchhof et 

al., 1997), and it is possible that they are present in cooler locations as well. Nitrogen-fixing 

bacteria have also been isolated from Miscanthus and Spartina pectinata grown in Germany, 

though at lower populations than those found in tropical napiergrass (Kirchhof et al., 1997). 
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Nitrogen-limiting conditions may actually promote the growth of nitrogen-fixing bacteria in 

some grasses (Kirchhof et al., 1997). BNF could partly explain the sustained growth for two or 

even three seasons of the energycane and napiergrasses with no added nitrogen in this study, and 

could account for part of the large amount of nitrogen that was removed with the biomass. In 

this study napiergrasses removed around 385 kg N/ha over the first three years (Figure 5), much 

more than giant reeds or switchgrass, though no depletion of nitrogen was observed in the soil 

cores taken in spring 2009. On two nitrogen-depleted soils in Brazil, de Morais et al. (2009) 

estimated that an average of 36% of the nitrogen taken up by napiergrass was supplied by BNF. 

Despite the possible contributions of BNF, it is still unlikely that all of the nitrogen needs of 

these grasses can be met with no application of nitrogen fertilizer. Based on the nutrient uptake 

data, future fertilization studies of napiergrass should probably include N application rates 

around 200 kg N/ha. Energycane should be able to produce yields with N application rates 

between 100–150 kg N/ha, based on the uptake rates observed in this study.

Both napiergrasses took up an exceptionally large amount of potassium in the first 

growing season (average 704 kg K/ha). In the next season, they only took up about half as much 

(average 345 kg K/ha; Figure 5), but maintained high biomass yields. Napiergrass may be

subject to luxury consumption of potassium, which may have significant implications for 

fertilization and biomass quality. These results suggest that fertilization rate studies for 

napiergrass focus the development of response curves up to a maximum of 300 kg K/ha. The 

soil core data suggested a possible depletion of soil K at the 60-71 cm depth in napiergrass plots 

compared to switchgrass. The energycane L 79-1002 also took up a large quantity of potassium, 

but at a much lower rate than napiergrass. Also, K content in the energycane was less variable, 

suggesting that the energycane is a more efficient user of potassium. Efficient use of available 
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soil K is likely one factor contributing to the relatively sustained yield of the energycane for 

three seasons. Based on the uptake data in this study, fertilization rate studies for energycane 

should probably not exceed application rates of 200 kg K/ha. Studies are currently underway to 

more accurately assess the yield response of these species to fertilization rates. Switchgrass PV1 showed a positive correlation between yield and N uptake, and PV-2 showed a positive 

correlation between yield and uptake of K, suggesting possible yield responses to availability of 

these nutrients. Yields of both switchgrasses were strongly correlated with Ca and Mg uptake as 

well. Thomason et al. (2004) reported a limited response to applied N fertilizer in switchgrass, 

and also noted positive correlations between yield and K and Mg concentrations.

Prior to establishment of this experiment, the test site had been used for switchgrass 

breeding plots, and for annual grain crops before that. All of these plots had been given 

adequate, if not excess, amounts of fertilizer, and most of the biomass was never removed from 

the field. Davis et al. (1996) reported potassium buildup in a Tifton loamy sand after repeated 

yearly applications of potassium fertilizer (48 or 96 kg K/ha/yr) for eleven years under corn 

cultivation, where only the grain and cobs were removed from the field. In an eleven-year study 

of bermudagrass on a newly-cleared sandy soil in North Carolina (Eustis sand), Woodhouse 

(1968) reported that, at moderate application rates (46.5 and 93 kg K/ha/yr), more K was 

removed in forage than was applied (400 – 500 kg/ha over 11 years), and suggested that release 

of native K from non-exchangeable forms was occurring in the soil profile. At a higher 

application rate (186 kg K/ha/yr) some K accumulation was observed in the soil profile. It is 

likely that residual nutrients had built up deep in the soil prior to planting the current study, and 

these nutrients were being utilized by the perennial grasses. It is also possible that some nonexchangeable K was released and made available to the plants (Woodhouse, 1968). Roots were 
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detected at the 91–107 cm depth for all accessions, but due to the difficulty in deeper sampling, it 

is not known just how far down they extend. Extraction of deep nutrient reserves could also 

account for the high rates of potassium and nitrogen removal that were observed in the 

napiergrass and energycane plots. The root profiles observed in this study for napiergrass agree 

with observations reported by Woodard and Prine (1993), who estimated that around 70% of the 

root biomass of napiergrass occurs in the top 25 cm of soil. However, Woodard and Prine 

(1993) also sampled only to a depth of 1 m, so the importance of deeper roots is still unclear. 

Switchgrass roots are known to extend beyond three meters depth in prairie soils (McLaughlin 

and Walsh, 1998), and sugarcane roots have been observed at a maximum depth of 4.70 m only 

332 days after planting in Brazil under rainfed conditions (Laclau and Laclau, 2009). It certainly 

seems probable that the species tested in this study were able to tap deep nutrient reserves. The 

increased potassium levels in the topsoil of some plots, specifically napiergrass and switchgrass, 

also suggest possible redistribution of potassium from deeper soil layers into the topsoil by these 

grasses.

Moisture content of harvested biomass is another important quality factor to consider. 

Transportation of wet biomass is more costly than moving dry biomass, but there are also costs 

associated with post-harvest drying. Wet biomass may be difficult to store and would certainly 

be unfit for direct combustion, but for biochemical conversion to ethanol, moisture may be of 

less concern. Although the delayed harvest of 2009 cannot be directly compared to other years 

in this respect, the switchgrasses were significantly drier in the 2009 harvest than in 2006 and 

2007, suggesting that their stems do dry in the field over winter. Switchgrasses averaged only 

22.3% moisture in the 2009 harvest. Adler et al. (2006) reported that switchgrass harvested in 

the fall averaged 35% moisture, and only 7% moisture in late winter/early spring harvests in 
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Pennsylvania. Similar observations have been reported in reed canarygrass (Phalaris 

arundinacea L.; Landström et al. 1996) and Miscanthus (Lewandowski and Heinz, 2002). 

Though biomass quality in grasses such as switchgrass tends to improve with later harvest, leaf 

drop, lodging, and breakage of brittle stem tops generally leads to some loss of biomass (Adler et 

al., 2006). Unlike switchgrass, energycane and napiergrass did not appear to lose moisture over 

the winter. The moisture content of these grasses was relatively unaffected by years because 

their thick stems do not dry as well in the field, especially since the fall tends to have increased 

rainfall and lower temperatures.. Giant reeds had less moisture at harvest than napiergrass and 

energycane, but later harvesting appears to have only a minimal effect on further reducing the 

moisture content in this species. Studies are currently underway to more precisely determine the 

effects of delayed harvest on biomass quality and nutrient composition in these species.

To maximize biomass yields and quality from perennial grasses, most of the biomass will 

likely be harvested at the end of the growing season and throughout the winter months. 

Switchgrass, for example, can tolerate two harvests per growing season, though the nitrogen 

content of the biomass and the amount of nitrogen removed from the soil will be higher than 

with only one post-senescence harvest (Reynolds et al., 2000). Harvesting certain varieties of 

napiergrass multiple times throughout the growing season is reported to reduce overall yields, 

and can lead to winter stand reductions in temperate areas (Woodard and Prine, 1991). 

Therefore, systems for storing harvested biomass for later use need to be developed. For 

example, it has been shown that napiergrass and energycane biomass can be ensiled, with little 

loss of total biomass (Woodard et al., 1991). Some of the seasonal demand for biomass could 

also be filled using annual crops such as sweet or forage sorghum (Sorghum bicolor (L.) 

Moench; Saballos, 2008), or perennials that tolerate multiple harvests such as bermudagrass 
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(Cynodon dactylon (L.) Pers.; Anderson et al., 2008). Also, different soil types and climatic 

conditions will affect which of these crops perform best in specific locations. In addition, 

different feedstocks have different properties, which may affect their suitability for specific 

applications, such as direct combustion or biochemical conversion to ethanol. Studies are also 

underway to compare the biochemical conversion properties of these various feedstocks. For 

biomass production under low inputs in South Georgia, napiergrass and energycane show the 

greatest yield potential. These species establish quickly and in this study were able to produce 

acceptable yields the first and second years with no inputs, perhaps by tapping deep 

accumulations of soil nutrients. Fertilization in subsequent years is, of course, recommended to 

maintain yields, and studies have been initiated to determine the optimum rates of fertilization 

for biomass production in these species. For marginal areas with the primary purpose of erosion 

control or wildlife refuge, switchgrass would be an ideal crop, as it is not generally considered 

invasive. Though establishment is slower, it is long-lived under good management, and 

produces reasonable yields of good-quality low-nitrogen biomass. In this study switchgrass was 

found to take up much less nitrogen than the other biomass crops. In addition, switchgrass has 

the ability to dry considerably in the field, and tended to have low ash content, making it the 

most suitable crop in this test for direct combustion applications (Lewandowski and Kicherer, 

1997; Adler et al., 2006)

Acknowledgements

The authors would like to acknowledge Freddie Cheek, Tony Howell, and Lorine Lewis 

for technical assistance. The authors would also like to thank Dr. Myron Parker (Prof. Emeritus, 
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University of Georgia) and Dr. C. Corley Holbrook (USDA-ARS) for critical reviews of this

manuscript.
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Tables:

Table 1. Analysis of variance (ANOVA) mean squares for biomass dry matter (DM) yield (Mg DM/ha), moisture percentage, and 

elemental composition of eight perennial grasses grown at Tifton, GA for four years under rainfed conditions with no additional 

inputs.

df DM Yield % Moisture % N % C C/N Ratio % Ash

Year (Y) 3 1045.53 *** 704.78 *** 0.3807 *** 80.93 *** 84891.0 *** 22.97 ***

Replicate (R) 3 16.27 ns 6.17 ns 0.0141 ns 1.00 ns 3679.0 ns 2.13 ns

Entry (E) 7 680.82 *** 2701.00 *** 0.1997 *** 4.90 *** 48206.8 *** 14.20 ***

Y × R 9 10.18 ns 9.22 ns 0.0085 ns 0.49 ns 1366.3 ns 1.39 ns

Y × E 21 132.12 *** 28.96 ** 0.0220 ** 0.57 ns 5280.8 *** 0.90 ns

R × E 21 40.65 *** 8.20 ns 0.0155 ns 0.24 ns 1487.5 ns 0.48 ns

Error 63a13.18 15.80 0.0098 0.42 1336.7 0.82

df % K % P % Ca % Mg Mn (ppm) Zn (ppm)

Year (Y) 3 0.931 *** 0.00130 * 0.0846 *** 0.0142 *** 1547.9 *** 216.17 ***

Replicate (R) 1 0.021 ns 0.00000 ns 0.0043 ns 0.0006 ns 150.3 ns 0.02 ns
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Entry (E) 7 0.908 *** 0.00209 *** 0.0591 *** 0.0080 *** 4932.6 *** 35.84 **

Y × R 3 0.017 ns 0.00042 ns 0.0022 ns 0.0015 ns 176.0 ns 54.76 **

Y × E 21 0.139 *** 0.00032 ns 0.0105 ns 0.0017 * 395.3 ** 17.87 *

R × E 7 0.024 ns 0.00032 ns 0.0163 * 0.0016 ns 290.0 ns 13.82 ns

Error 21 0.033 0.00030 0.0055 0.0007 131.0 8.38

* = significant at α = 0.05, ** = significant at α = 0.01, *** = significant at α = 0.001, ns = not significant

a

Error df = 60 for %N, 59 for %C and C/N Ratio, and 62 for % ash.
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Table 2. Yearly mean dry matter (DM) yields and biomass moisture percentages at harvest for 

eight perennial grasses grown at Tifton, GA for four years (2006–2009) under rainfed conditions 

with no additional inputs.

Yield (Mg DM/ha) % Moisture

Entry 2006 2007 2008 2009 2006 2007 2008 2009

L 79-1002 29.2 26.1 21.1 8.2 64.2 59.1 62.5 58.5

AD-S 5.3 8.8 5.5 3.3 52.6 44.7 49.2 39.6

AD-E 7.0 12.3 6.4 5.6 47.5 49.4 44.5 40.2

AD-F 7.2 12.9 8.2 6.5 48.4 47.8 44.7 41.8

Merkeron 30.6 30.5 11.4 7.1 70.1 67.8 62.7 57.4

N51 37.1 28.1 12.3 5.1 68.6 66.4 59.9 58.0

PV-1 9.2 13.0 8.3 6.5 37.5 36.4 30.8 21.9

PV-2 9.4 13.5 8.8 5.6 37.2 39.0 32.1 22.7

LSD0.05 10.9 13.5 11.3 4.8 6.2 11.1 10.0 7.1

Least Significant Differences (LSD) were determined by Tukey’s HSD test at α = 0.05.
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Table 3. Yearly mean nitrogen percentage and carbon:nitrogen (C:N) ratio in harvested biomass 

(whole shoots) for eight perennial grasses grown at Tifton, GA for four years (2006–2009) under 

rainfed conditions with no additional inputs.

% N C:N Ratio

Entry 2006 2007 2008 2009 2006 2007 2008 2009

L 79-1002 0.320 0.435 0.260 0.170 184.6 107.1 179.7 286.9

AD-S 0.692 0.451 0.434 0.334 65.8 102.1 109.6 143.6

AD-E 0.590 0.559 0.470 0.325 83.4 88.5 102.0 150.1

AD-F 0.673 0.435 0.449 0.297 69.5 114.7 107.2 168.2

Merkeron 0.614 0.546 0.404 0.179 73.6 87.2 123.6 279.7

N51 0.473 0.547 0.304 0.234 102.3 98.8 165.4 209.0

PV-1 0.245 0.295 0.153 0.171 190.8 163.0 308.1 293.4

PV-2 0.243 0.303 0.151 0.182 203.2 153.6 311.9 285.6

LSD0.05 0.373 ns 0.202 0.077 99.6 75.3 94.1 89.9

Least Significant Differences (LSD) were determined by Tukey’s HSD test at α = 0.05. ns = no 

significant differences.
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Table 4. Correlations of biomass nutrient contents with dry matter yield. Values are Pearson 

correlation coefficients (R). All correlations significant at α ≤ 0.05 are plotted in Figure 1.

Entry %N C:N Ratio % K % Ca % Mg

L 79-1002 0.563 ** -0.646 *** 0.539 ns 0.358 ns 0.425 ns

AD-S 0.285 ns -0.407 ns 0.295 ns 0.479 ns 0.485 ns

AD-E 0.456 * -0.452 * 0.646 * 0.521 ns 0.315 ns

AD-F 0.266 ns -0.374 ns 0.377 ns 0.173 ns -0.122 ns

Merkeron 0.730 *** -0.721 *** 0.805 ** 0.148 ns 0.384 ns

N51 0.441 * -0.513 * 0.890 *** -0.116 ns -0.364 ns

PV-1 0.554 ** -0.563 ** 0.525 ns 0.765 ** 0.728 **

PV-2 0.460 * -0.485 * 0.724 ** 0.853 *** 0.766 **

* = significant at α = 0.10, ** = significant at α = 0.05, *** = significant at α = 0.01, ns = not 

significant
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Table 5. Yearly mean ash and potassium (K) percentages in harvested biomass (whole shoots) 

for eight perennial grasses grown at Tifton, GA for four years (2006–2009) under rainfed 

conditions with no additional inputs.

% Ash % K

Entry 2006 2007 2008 2009 2006 2007 2008 2009

L 79-1002 3.62 4.54 3.55 2.44 1.121 0.892 1.037 0.749

AD-S 6.15 5.14 4.88 3.86 0.905 0.737 0.749 0.415

AD-E 5.10 5.05 4.83 3.44 0.690 0.830 0.599 0.415

AD-F 5.12 4.65 4.93 3.59 0.921 0.749 0.680 0.536

Merkeron 6.15 5.71 4.57 3.01 2.134 1.196 1.112 0.599

N51 5.94 4.82 4.26 3.69 2.037 1.159 0.870 0.749

PV-1 2.44 3.28 2.33 1.64 0.345 0.719 0.351 0.190

PV-2 4.09 4.16 2.33 1.92 0.286 0.601 0.323 0.207

LSD0.05 2.65 ns 0.98 1.18 0.820 ns 0.618 0.478

Least Significant Differences (LSD) were determined by Tukey’s HSD test at α = 0.05. ns = no 

significant differences.
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Table 6. Biomass mineral composition (entry means and yearly means) for eight perennial 

grasses grown at Tifton, GA for four years (2006–2009) under rainfed conditions with no 

additional inputs. 

K P Ca Mg Mn Zn

Entry % ppm

L 79-1002 0.950 0.105 0.252 0.072 47.49 15.30

AD-S 0.701 0.089 0.422 0.180 74.81 11.62

AD-E 0.633 0.078 0.375 0.140 94.50 10.55

AD-F 0.721 0.077 0.379 0.145 93.13 11.58

Merkeron 1.260 0.110 0.202 0.109 59.54 14.66

N51 1.204 0.110 0.262 0.110 42.89 15.37

PV-1 0.401 0.076 0.218 0.121 31.46 10.84

PV-2 0.354 0.072 0.227 0.130 36.65 14.78

LSD0.05 0.303 0.029 0.125 0.044 19.19 ns 

Year % ppm

2006 1.055 0.081 0.297 0.150 59.01 8.74

2007 0.860 0.102 0.366 0.143 60.23 11.62

2008 0.715 0.087 0.314 0.126 72.51 17.12

2009 0.482 0.087 0.193 0.084 48.48 14.88

LSD0.05 0.178 0.017 0.073 0.026 11.28 2.85

Least Significant Differences (LSD) were determined by Tukey’s HSD test at α = 0.05. ns = no 

significant differences.
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Table 7. Monthly rainfall (cm) recorded at Tifton, GA during the study period.

2005 2006 2007 2008 2009 2010

Jan - 19.41 8.69 8.69 4.17 16.89

Feb - 10.26 5.69 19.86 5.51 8.53

Mar - 0.74 3.86 8.05 20.90 7.16

Apr - 4.27 1.14 7.67 20.96 -

May - 6.86 0.33 4.65 16.61 -

Jun - 10.59 14.94 5.79 5.00 -

Jul - 7.29 9.09 8.38 19.76 -

Aug - 7.24 17.68 31.29 17.83 -

Sep 0.05 5.13 6.65 0.38 4.32 -

Oct 1.47 2.90 7.70 16.28 10.19 -

Nov 10.49 7.42 2.31 9.86 6.71 -

Dec 12.62 14.27 14.43 7.90 19.25 -
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Figure Captions:

Figure 1. Significant correlations between biomass nutrient content and dry matter yield. For 

clarity, only the yearly means are plotted. Pearson correlation coefficients (R) based on the 

actual raw data are given in Table 4.

Figure 2. Yearly soil carbon and nitrogen percentages, measured in the top 15 cm of soil in 

plots of perennial grasses and bare ground control. C and N percentages are indicated on the yaxes, years on the x-axes. Colors/symbols for individual entries are indicated in the legend at the 

bottom of the figure. Because of variability in sampling methods, data for 2009 are not included.

Figure 3. Soil potassium content (mg K/kg soil, dry basis) at four soil depths taken from cores 

in 2009 before spring regrowth, after three years of growth of eight perennial grasses and bare 

ground control with no fertilizer inputs. Error bars indicate one standard deviation of the mean.

Figure 4. Root biomass (g roots/kg soil, dry basis) sampled from four depths of soil cores taken 

in spring, 2009. Because of extreme variability for this species, giant reeds (AD-S, AD-E, and 

AD-F) are not included. Error bars represent one standard deviation of the mean.

Figure 5. Calculated yearly removal of N, K, and P (kg element/ha) by eight perennial grasses 

grown at Tifton, GA for four years (2006–2009) under rainfed conditions with no additional 

inputs. Nutrients are indicated on the y-axes, years on the x-axes. Colors/symbols for individual 

entries are indicated in the legend at the bottom of the figure.
4

6

8

10

12

14

16

18

0.0 0.1 0.2 0.3 0.4

Yield (Mg DM/ha)

% Ca or % Mg in biomass

PV-1 %Ca PV-2 %Ca

PV-1 %Mg PV-2 %Mg

0

5

10

15

20

25

30

35

40

45

0 1 2 3

Yield (Mg DM/ha)

% K in biomass

Merkeron

N51

PV-2

0

5

10

15

20

25

30

35

0 100 200 300 400

Yield (Mg DM/ha)

C:N Ratio of biomass

L 79-1002

Merkeron

PV-1

0

5

10

15

20

25

30

35

0 0.2 0.4 0.6 0.8

Yield (Mg DM/ha)

% N in biomass

L 79-1002

Merkeron

PV-1

Figure 1

Click here to download Figure: Knoll-Fig1.eps
0.40

0.45

0.50

0.55

0.60

0.65

0.70

2007 2008 2009 2010

% C in Soil 

0.020

0.025

0.030

0.035

0.040

2007 2008 2009 2010

% N in Soil 

L79-1002 AD-S AD-E

AD-F Merkeron N51

PV-1 PV-2 Bare

Figure 2

Click here to download Figure: Knoll-Fig2.eps
Figure 3

Click here to download Figure: Knoll-Fig3.eps
0.0 0.5 1.0 1.5 2.0 2.5 3.0

L79-1002

Merkeron

N51

PV-1

PV-2

Bare

g roots/kg soil

0-15 cm

30-45 cm

60-76 cm

91-107 cm

Figure 4

Click here to download Figure: Knoll-Fig4.eps
0

50

100

150

200

250

2006 2007 2008 2009

Yearly N Removal

(kg N/ha)

0

100

200

300

400

500

600

700

800

2006 2007 2008 2009

Yearly K Removal

(kg K/ha)

0

20

40

60

2006 2007 2008 2009

Yearly P Removal

(kg P/ha)

L79-1002 AD-S AD-E AD-F

Merkeron N51 PV-1 PV-2

Figure 5

Click here to download Figure: Knoll-Fig5.eps