Low-Input Production of Biomass from Perennial Grasses

in the Coastal Plain of Georgia, USA

Joseph E. Knoll & William F. Anderson &

Timothy C. Strickland & Robert K. Hubbard &

Ravindra Malik

# The Author(s) 2011. This article is published with open access at Springerlink.com

Abstract Warm-season perennial grasses are a promising

source of biomass for energy production in Southeast USA,

and low-input production is desirable. With only residual

fertility in the soil and no irrigation, this test compared biomass

yields of eight grasses under low-input production: L 79–1002

energycane (Saccharum hyb.), Merkeron and N51 napiergrass

(Pennisetum purpureum Schum.), three clones of giant reed

(Arundo donax L.), and two switchgrass (Panicum virgatum

L.) lines. For the first 2 years napiergrass maintained dry

matter (DM) yields over 25 Mg DM ha−1 year−1

, and

energycane yielded over 20 Mg DM ha−1

year−1 for 3 years.

Switchgrass yields were lower (8.6 Mg DM ha−1 year−1

average of 4 years), but the biomass contained less moisture at

harvest than the other, larger-stemmed grasses. Switchgrass

biomass also had the lowest concentrations of N, K, and ash.

Average yields of giant reeds were also low (6.4 Mg

DM ha−1 year−1

), while ash and N concentrations were

relatively high compared with switchgrass and energycane. In

4 years of production, energycane and napiergrass removed

between 269 and 386 kg N ha−1 and 830–1,159 kg K ha−1

,

while the other grasses removed significantly less of these

nutrients. Giant reed removed 126 kg N ha−1 and 193 kg

K ha−1

, and switchgrass removed 83 kg N ha−1 and 140 kg

K ha−1

. In Southeast USA, it is possible to produce biomass

from perennial grasses with minimal inputs but the high

nutrient removal rates of some species suggest that it may not

be sustainable for long periods of time.

Keywords Energycane . Giant reed . Napiergrass.

Nitrogen . Switchgrass

Introduction

Warm-season perennial grasses have very good potential for

production of biomass in Southeast USA. This region has

warm summer temperatures and generally receives adequate,

though sometimes erratic, rainfall. 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 allow them to

continue growing during occasional droughts. Though initial

establishment of perennial grasses may be more costly and

energy intensive than planting conventional annual crops, a

substantial reduction in costs and energy inputs over the longterm should be expected with perennials [5, 13]. In addition,

evidence suggests that perennial agriculture might have fewer

negative impacts on the environment in terms of water

consumption and quality, soil loss, and nutrient loss,

compared with annual systems [9].

The milder winter temperatures in Southeast USA allow for

the overwintering of subtropical grass species, including

napiergrass (Pennisetum purpureum Schum.), and energycane

(Saccharum hyb.), as well as the more temperate grass giant

reed (Arundo donax L.). Napiergrass, also known as

elephantgrass, is native to tropical Africa. In tropical and

subtropical areas, it is an important forage species, which is

capable of producing very high yields of biomass, even when

J. E. Knoll (*) : W. F. Anderson

USDA-ARS, Crop Genetics and Breeding Research Unit,

P.O. Box 748, Tifton, GA 31793, USA

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

T. C. Strickland : R. K. Hubbard

USDA-ARS, Southeast Watershed Research Laboratory,

P.O. Box 748, Tifton, GA 31793, USA

R. Malik

Department of Natural Sciences, College of Sciences and Health

Professions, Albany State University,

Albany, GA 31705, USA

Bioenerg. Res.

DOI 10.1007/s12155-011-9122-x
grown on low-fertility soils [7]. Energycanes are derived

from crosses between cultivated sugarcane (Saccharum

officinarum L.) and related wild species (Saccharum spontaneum L.,Saccharum robustum Brandes & Jesw ex. Grassl.,

Erianthus sp., etc.) and were selected 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

[15]. Dry biomass yields exceeding 45 Mg ha−1 year−1 have

been reported for both napiergrass and energycane in Florida

[24]. Giant reed is a large C3 grass that is used for various

purposes, including the production of reeds for musical

instruments. This grass has recently gained attention as a

potential bioenergy crop in southern Europe, where dry

biomass yields over 37 Mg ha−1 year−1 in well-fertilized plots

have been reported [2, 17].

One potential drawback of exotic bioenergy crops is their

potential to become troublesome invasive weeds. Native prairie

grasses have been proposed as a low-input, ecologically

sustainable source of biomass for much of the USA [10, 22].

Unlike exotic species, native plants are generally considered

non-invasive, and they provide ideal habitat for wildlife. In

addition, native grasses should require fewer inputs as they

are already adapted to local conditions and disease pressures.

Switchgrass (Panicum virgatum L.), a broadly adapted and

long-lived perennial forage grass, has been extensively

evaluated as a native biomass source in the USA [20, 22].

Because the per-unit value of biomass is low compared

with food, feed, and fiber crops, production under minimal

inputs is desirable for growers to profit from producing

biomass. Additionally, production of biomass crops on socalled “marginal” lands has been proposed so that they do not

compete with other crops for better farmland. The high yields

reported above for napiergrass and energycane [24] and giant

reed [2, [17] were obtained by applying ample amounts of

fertilizer, irrigation, or both, but less is known about the yield

potential of these grasses under less than ideal conditions,

especially in Southeast USA, where soils are often of low

fertility. Few studies have been conducted to compare the

performance of different biomass feedstock species in the same

field under low-input conditions. The purpose of this study was

to evaluate and compare biomass yields and nutrient removal of

eight perennial grasses under minimal inputs, with the aim of

determining which species will be most productive in the Coastal

Plain region of Georgia under such conditions over 4 years.

Materials and Methods

Plant Material and Experimental Design

The study was conducted at Tifton, GA, on a site comprised

of two soils, an Alapaha loamy sand (Loamy, siliceous,

subactive, thermic Arenic Plinthic Paleaquults) and a

Clarendon loamy sand (fine-loamy siliceous semiactive

thermic Plinthaquic Paleudult). In preceding years the site

had been used for switchgrass breeding plots, and for

annual grain crops before that, during which time adequate

fertilization was applied and most of the crop residues were

retained in the field. The study was initiated 2 Sept. 2005.

Eight entries with potential for use in bioenergy production

were included in this test: one energycane (L 79–1002; [4]),

two napiergrasses (Merkeron and N51; [6]), two experimental lowland switchgrass lines (designated as PV-1 and

PV-2), and three giant reed clones (AD-S, AD-E, and ADF). The plots were laid out in a randomized complete block

design with four replications. Two plots for each entry were

located on soil mapped as Alapaha or Clarendon, with the

following exceptions. Three plots of Merkeron were on

Clarendon soil and one on Alapaha, and two plots (one

AD-F and one PV-2) were mapped as being equally split by

Alapaha and Clarendon. Plots measured 3.7×6 m. Energycane and napiergrass were planted as stem cuttings, with

two whole stems planted in each row, buried horizontally

10 cm deep, with rows spaced at 0.9 m apart (four rows per

plot). The giant reeds were micropropagated transplants,

derived from clones collected in the Southeast, and were

planted 0.9 m apart in rows 0.9 m apart. Switchgrass was

seeded in rows at 0.2 m apart, at a seeding rate of 11.2 kg

seed ha−1

. Irrigation was applied weekly during the first

month of establishment, but was not applied afterwards.

Weeds were controlled by mid-March applications of 2,4-

dichlorophenoxyacetic acid (2,4-D) and Surflan (United

Phosphorus, Inc., Trenton, NJ) and manual cultivation as

needed. No fertilizer was applied during the study.

Biomass Harvest

Total above-ground biomass was mechanically harvested after

the first killing freeze of each year (Table 1) and removed

from each plot. A sample from the center of each plot (5×

1.2 m) was weighed fresh, oven dried at 65°C until constant

weight is reached, and then weighed again to determine

percentages of moisture and DM. 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 N concentration 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. Total ash concentration was determined by combustion in a Muffle furnace at 450°C for 6 h. Potassium

concentration was measured at the University of Georgia

Bioenerg. Res.
Agricultural and Environmental Services Laboratories

(AESL) by inductively coupled plasma (ICP) spectrometry.

Root and Soil Sampling and Analysis

On 30 Apr 2009, a soil core (58-mm inside diameter) to a

depth of 107 cm was collected from the center of each plot,

immediately adjacent to a growing clump of grass using a

hydraulic tractor-mounted soil probe (Giddings Machine

Co. Fort Collins, CO). Three cores were also taken from the

alleys between the plots (two on Clarendon soil and one on

Alapaha) for comparison to bare soil. Four subsamples of

each core (0–15, 30–45, 60–76, and 91–107 cm) were used

to estimate root biomass and rooting depth. Roots were

meticulously separated from soil samples by hand, and

were then dried completely at 65°C in an oven. Pieces of

rhizome (typically found in giant reeds) were not collected.

Dried root samples were ground and analyzed for C and N

concentration as for above-ground biomass. Samples of

oven-dried soil were ground in a roller mill, and were also

analyzed for C and N concentration by dry combustion.

Potassium concentration in soil samples was determined by

Mehlich I extraction followed by ICP spectrometry at the

AESL.

Statistical Procedures

Data were analyzed using the GLIMMIX procedure in SAS

v. 9.2 (SAS Institute, Cary, NC). Because of the perennial

nature of the crops and the expected depletion of nutrient

reserves over time, year was treated as a fixed factor.

Replicates and soil type were treated as random factors. All

tests were conducted at a probability level of α=0.05.

Tukey’s HSD or the Tukey–Kramer procedure was used to

test for significant differences between least-squares means

estimates. Pearson correlations among biomass qualities

were calculated using the CORR procedure.

Results

Large differences in DM yield were observed between

entries, and changes in yields over the 4 years were also

noted (Table 2). Overall, the highest yielding grasses were

energycane L 79–1002 (18.7 Mg DM ha−1

) and the two

napiergrasses (16.4 Mg DM ha−1 for Merkeron and

15.3 Mg DM ha−1 for N51). Switchgrasses also had good

yields, lower than napiergrass in the first year, but not

significantly lower when averaged over 4 years (8.6 Mg

DM ha−1 for both entries). Giant reeds had the lowest

yields, but were not significantly different from switchgrass

(Table 2). The energycane maintained yields above 20 Mg

DM ha−1 for the first three seasons and then declined in the

fourth. Both napiergrasses yielded above 25 Mg DM ha−1

for the first 2 years, then declined in the third and fourth

seasons. By contrast, yields of giant reed and switchgrass

peaked in the second season. Though significant, yield

declines in the third and fourth seasons were much less

pronounced in these species than in napiergrass or

energycane. Yields of all entries were lowest in the 2009

season (Table 2).

Moisture concentration of freshly harvested biomass was

also different among entries, which fell into three distinct

groups (Table 3). Napiergrasses and energycane had the

Precipitation (cm)

Month 2005 2006 2007 2008 2009 2010

Jan – 19.4 8.7 8.7 4.2 16.9

Feb – 10.3 5.7 19.9 5.5 8.5

Mar – 0.7 3.9 8.1 20.9 7.2

Apr – 4.3 1.1 7.7 21.0 –

May – 6.9 0.3 4.6 16.6 –

Jun – 10.6 14.9 5.8 5.0 –

Jul – 7.3 9.1 8.4 19.8 –

Aug – 7.2 17.7 31.3 17.8 –

Sep 0.1 5.1 6.7 0.4 4.3 –

Oct 1.5 2.9 7.7 16.3 10.2 –

Nov 10.5 7.4 2.3 9.9 6.7 –

Dec 12.6 14.3 14.4 7.9 19.3 –

Total (cm) – 96.4 92.5 128.8 151.2 –

GDD – 3,661 3,732 3,424 3,677 –

First freeze 20 Dec 20 Nov 8 Nov 19 Nov 21 Dec –

Harvest – 5 Dec 11 Dec 9 Dec – 3 Feb

Table 1 Harvest dates and

weather data recorded at Tifton,

GA [8] during the study period

Growing degree days (GDD)

were calculated between dates

of last and first freeze with a

base temperature of 15°C.

Date of first killing freeze is

reached when the minimum air

temperature ≤0°C

Bioenerg. Res.
highest moisture concentration, ranging from 574 to 701 g

H2O kg−1 biomass. Giant reeds had intermediate moisture,

ranging between 396 and 526 g H2O kg−1 biomass, while

switchgrass biomass had by far the lowest moisture at

harvest, between 219 and 375 g H2O kg−1 biomass.

Moisture concentration of the energycane did not vary

significantly over years, and relatively minor differences

were noted for napiergrass and giant reed. Switchgrass,

however, was considerably drier in the 2009 harvest than in

previous years, likely from the delay in the date of harvest.

Ash concentration of the biomass, which ideally should

be as low as possible for direct combustion and other

thermal processes, was found to be highest in napiergrass

and giant reed, averaging 46.2 g ash kg−1 DM. Energycane

biomass had lower concentration of ash, averaging 34.4 g

ash kg−1 DM for the 4 years of the test. Switchgrass

biomass had the lowest ash concentration. Ash concentration of PV-2 (28.2 g ash kg−1 DM) was similar to that of

energycane, while PV-1 (23.1 g ash kg−1 DM) was

significantly lower than energycane (Table 4). For the first

2 years, the overall mean ash concentration in harvested

biomass did not change, but it decreased in each of the next

two seasons.

The overall N concentration in harvested biomass

showed a similar trend as ash (Table 5). In fact, these two

factors were highly correlated (R=0.694). Giant reeds

tended to have the highest biomass N concentration, with

a 4-year mean of 4.5 g N kg−1 DM. Napiergrass biomass N

concentration was similar (3.7 g N kg−1 DM). Energycane

biomass averaged 2.7 g N kg−1 DM, similar to switchgrass

and N51 napiergrass. Switchgrass biomass averaged 2.1 g

N kg−1 DM over the 4 years of the test. The K

concentration of harvested biomass also tended to decrease

in the last two seasons (Table 6). Napiergrass appeared to

take up an exceptionally large amount of K in the first

growing season. In 2006 napiergrass biomass averaged 21 g

K kg−1 DM, much greater than any other species. In the

following seasons napiergrass K concentration was more

similar to that of the other grasses, but was still greater than

that of switchgrass. Averaged over all 4 years, napiergrass

and energycane biomass had the highest K concentrations

(10.9 g K kg−1 DM for napiergrass and 9.2 g K kg−1 DM

for energycane). Potassium concentration of giant reed

biomass was intermediate (6.6 g K kg−1 DM), and that of

switchgrass was lowest (3.2 gK kg−1 DM). Biomass N and

K concentrations showed a moderate correlation (R=

0.537), and as expected, K concentration was also

correlated with ash concentration (R=0.606). Net N and K

removal at harvest were highest in the first 2 years

following establishment for all species (Table 7). Annual

Mg DM ha−1

Species Entry 2006 2007 2008 2009 4-year LS mean

Energycane L 79-1002 28.3 a 25.6 a, b 21.0 a 8.1 a 18.7 a

Giant Reed AD-S 5.3 b 8.5 c 4.1 c 2.9 b 4.8 c

Giant Reed AD-E 6.8 b 12.2 b, c 5.9 b, c 5.4 a, b 7.2 c

Giant Reed AD-F 7.0 b 10.1 c 7.1 b, c 5.8 a, b 7.3 c

Napiergrass Merkeron 30.4 a 30.1 a 11.2 a, b 7.1 a 16.4 a, b

Napiergrass N51 36.3 a 27.4 a, b 11.2 a, b 5.0 a, b 15.3 a, b

Switchgrass PV-1 8.5 b 12.9 b, c 7.9 b, c 6.4 a, b 8.6 b, c

Switchgrass PV-2 9.4 b 13.3 a, b, c 7.9 b, c 5.5 a, b 8.6 b, c

Yearly LS means 12.6 A 15.8 B 8.6 C 5.5 D

Table 2 Annual biomass yields

of eight perennial grasses

Within columns, LS means

with the same small letters are

not significantly different.

Yearly LS means with the same

capital letters are not significantly different

g H2O kg−1 biomass

Species Entry 2006 2007 2008 2009 4-year LS mean

Energycane L 79–1002 642 a 591 b 625 a 585 a 611 a

Giant Reed AD-S 526 b 447 c, d 492 b 396 b 465 b

Giant Reed AD-E 475 b 494 c 445 b 402 b 454 b

Giant Reed AD-F 484 b 478 c 447 b 418 b 457 b

Napiergrass Merkeron 701 a 678 a 627 a 574 a 645 a

Napiergrass N51 686 a 664 a, b 599 a 580 a 632 a

Switchgrass PV-1 375 c 364 d 308 c 219 c 317 c

Switchgrass PV-2 372 c 390 d 321 c 227 c 328 c

Yearly LS means 533 A 513 A 483 B 425 C

Table 3 Moisture concentration

of freshly harvested biomass of

eight perennial grasses

Within columns, LS means

with the same small letters are

not significantly different.

Yearly LS means with the same

capital letters are not significantly different

Bioenerg. Res.
N removals ranged from 12 to 186 kg ha−1 for napiergrass,

14–110 kg ha−1 for energycane, 14–47 kg ha−1 for giant

reeds, and 10–39 kg ha−1 for the switchgrass. Similar

patterns were observed in K removal: 37–698 kg ha−1 for

napiergrass, 55–350 kg ha−1 for energycane, 20–83 kg ha−1

for giant reed, and 11–78 kg ha−1 for switchgrass.

In spring 2009, soil cores were taken in order to estimate

root biomass and root distribution of these perennial

grasses, and to determine whether there were speciesspecific effects on soil K content, or on soil C and N

accretion. Roots were sampled at alternating depths in the

soil cores (0–15, 30–45, 60–76, and 91–107 cm). Excluding rhizomes, the roots of giant reed were more evenly

distributed throughout the sampled profile, while 60–78%

of root biomass from energycane, napiergrass, and switchgrass was located in the top 15 cm of soil. Only one third of

giant reed root biomass was located in this layer (Fig. 1).

Based on soil bulk density, the total root biomass in each

soil layer was calculated for each species. Estimates of total

root biomass ranged between 3,684 kg DM ha−1 for

napiergrass to 5,956 kg DM ha−1 for switchgrass, though

the differences between species were not statistically

significant (Table 8). However, all species had greater root

biomass than the bare soil samples taken from the alleys.

While some roots were detected in the alleys, they probably

originated from nearby plots. When root biomass was

compared with harvested above-ground biomass in the year

prior to collection of soil cores, significant differences were

observed in the percentage of total plant biomass comprised

in roots. Energycane roots comprised the lowest percentage

of biomass (17.6%) followed by napiergrass (24.3%),

switchgrass (43.7%), and giant reeds (45.7%; Table 8). A

complete census of roots and rhizomes at all depths would

obviously have resulted in higher percentages of the total

plant biomass. There were no differences between the four

grass species or the bare soil control for soil C or N

concentrations or total C or N pools (data not shown).

However, in planted plots, from 1,536–2,901 kg C ha−1 and

16.4–28.3 kg N ha−1 were contained in the roots (Table 8).

For all species, roots in the upper soil layer had higher

nitrogen concentrations than deeper roots, despite differences in root distribution. Roots in the upper soil layer

averaged 5.5 g N kg−1 DM, while roots in the three lower

soil layers averaged 3.9, 3.4, and 4.2 g N kg−1 DM by

increasing depth. Differences between species in soil profile

K content were observed. The total soil K in napiergrass

and energycane plots was lower than that in switchgrass

and giant reed plots, but was not different from the bare soil

control (Fig. 2). However, the distribution of K in the bare

soil profile was very different from planted plots. Soil K

g ash kg−1 DM

Species Entry 2006 2007 2008 2009 4-year LS mean

Energycane L 79–1002 36.0 b, c 45.3 a, b 35.3 a, b 24.3 b 34.4 b

Giant Reed AD-S 60.9 a 50.9 a 48.6 a 38.2 a 49.0 a

Giant Reed AD-E 50.4 a, b 50.5 a 48.1 a 34.1 a, b 45.2 a

Giant Reed AD-F 50.0 a, b 46.5 a, b 49.0 a 35.8 a, b 44.9 a

Napiergrass Merkeron 60.9 a 54.0 a 45.5 a 29.8 a, b 45.9 a

Napiergrass N51 58.8 a 48.1 a, b 42.5 a 36.5 a, b 45.8 a

Switchgrass PV-1 24.1 c 32.3 b 23.2 b 15.8 c 23.1 c

Switchgrass PV-2 35.0 b, c 40.4 a, b 23.3 b 19.2 c 28.2 b, c

Yearly LS means 45.0 A 45.5 A 37.8 B 28.0 C

Table 4 Ash concentration in

dry biomass of eight perennial

grasses

Within columns, LS means with

the same small letters are not

significantly different. Yearly

LS means with the same

capital letters are not significantly different

g N kg−1 DM

Species Entry 2006 2007 2008 2009 4-year LS mean

Energycane L 79–1002 2.8 b, c 4.3 a, b, c 2.6 b, c 1.7 b 2.7 b, c

Giant Reed AD-S 6.9 a 4.5 a, b, c 4.3 a, b 3.3 a 4.6 a

Giant Reed AD-E 5.7 a 5.3 a 4.6 a 3.2 a 4.6 a

Giant Reed AD-F 6.6 a 4.2 a, b, c 4.4 a, b 2.9 a, b 4.3 a

Napiergrass Merkeron 6.1 a 5.3 a 3.9 a, b 1.8 b 3.8 a

Napiergrass N51 4.5 a, b 5.2 a, b 2.9 a, b 2.3 a, b 3.5 a, b

Switchgrass PV-1 2.4 b, c 2.9 c 1.5 c 1.7 b 2.1 c

Switchgrass PV-2 2.4 c 3.0 b, c 1.5 c 1.8 b 2.1 c

Yearly LS means 4.3 A 4.2 A 2.9 B 2.3 C

Table 5 Nitrogen concentration

in dry biomass of eight perennial grasses

Within columns, LS means with

the same small letters are not

significantly different. Yearly

LS means with the same

capital letters are not significantly different

Bioenerg. Res.
was concentrated in the surface layer for all planted plots,

but was evenly distributed through the profile in the bare

soil control (Fig. 2).

Discussion

The high biomass yields observed for napiergrass and

energycane in the first three seasons in this study are

slightly lower than reports from Florida on well-fertilized

and irrigated plots, where first year DM yields exceeded

45 Mg ha−1 [24]. However, the yields of these grasses are

very consistent with other studies in which fertility was

more limited. For example, on two low-fertility soils in

Brazil, napiergrass averaged 26.3, 20.9, and 9.8 Mg

DM ha−1 in three successive harvests with no application

of N fertilizer [7]. The authors attributed the yield decline to

depletion of N from the soil. On a relatively fertile soil in

Louisiana, L 79–1002 energycane yielded 22.2, 13.2, and

18.1 Mg DM ha−1 for the first 3 years with no fertilization.

It also continued to yield well for four additional years,

though yields were still greater in fertilized treatments [4].

Yields of switchgrass in this study are slightly lower than

those reported in the literature for lowland types on wellfertilized plots [20] but were generally consistent with

yields reported for some unfertilized treatments on sandy

soils [21, 23]. It has also been reported that switchgrass

yields usually do not reach peak production in the first year

[20], which is also consistent with observations in this

study. Like switchgrass, giant reed has also been reported to

establish more slowly, reaching peak yields in two or three

Fig. 1 Relative distributions of roots for four perennial grass species,

sampled at four depths from soil cores taken in spring, 2009. Within

species, means with the same letter are not significantly different

Table 6 Potassium concentration in dry biomass of eight perennial grasses

g K kg−1 DM

Species Entry 2006 2007 2008 2009 4-year LS mean

Energycane L 79–1002 12.4 b 8.9 a, b 9.3 a, b 6.9 a 9.2 a

Giant Reed AD-S 9.8 b 7.9 a, b 6.5 a, b 4.5 a, b 6.9 b

Giant Reed AD-E 7.9 b 8.4 a, b 6.0 b 3.9 b 6.3 b

Giant Reed AD-F 8.5 b 8.1 a, b 6.1 b 4.9 a, b 6.7 b

Napiergrass Merkeron 21.3 a 10.6 a 10.2 a 6.1 a, b 10.9 a

Napiergrass N51 20.7 a 11.2 a 9.4 a, b 6.5 a 10.9 a

Switchgrass PV-1 3.4 c 6.1 b 2.6 c 1.8 c 3.1 c

Switchgrass PV-2 3.5 c 5.8 b 2.7 c 2.1 c 3.3 c

Yearly LS means 9.0 A 8.2 A 5.9 B 4.1 C

Within columns, LS means with the same small letters are not significantly different. Yearly LS means with the same capital letters are not

significantly different

Table 7 Annual removal of N and K by biomass harvest of four

perennial grass species

Species 2006 2007 2008 2009 Total

N removal (kg N ha−1

)

Napiergrass 186 a 150 a 38 a, b 12 a 386 a

Energycane 91 a 110 a 54 a 14 a 269 a

Giant Reed 40 b 47 b 25 b 14 a 126 b

Switchgrass 22 c 39 b 12 c 10 a 83 c

K removal (kg K ha−1

)

Napiergrass 698 a 314 a 110 a 37 a 1,159 a

Energycane 350 a 229 a 196 a 55 a 830 a

Giant Reed 55 b 83 b 35 b 20 a 193 b

Switchgrass 30 c 78 b 21 b 11 b 140 c

Within columns, LS means with the same letters are not significantly

different

Bioenerg. Res.
seasons. However, biomass yields of giant reed observed in

the present study were far lower than those reported in the

literature [2, 17]. The lack of applied fertilizer or other local

growing conditions could partly explain this. Lack of water is

probably not a factor, though, as rainfall was more plentiful

in 2008 and 2009 than in the previous 2 years (Table 1), but

yields declined. Average rainfall for Tifton (1961–1990) is

around 122 cm year−1 [8]. The origin of the planting material

(micropropagation) could also be a factor, or these particular

genotypes may not be suited to the local conditions. Yield

reduction in tissue culture-derived sugarcane compared with

conventionally propagated sugarcane has been reported for

some cultivars and tissue culture procedures [3, 11]; it is

possible that similar effects occur in giant reed.

Net N removal at harvest was highest in the first 2 years

following establishment for all species (Table 7). Sharp

declines in N removal during 2008 preceded similarly sharp

yield declines for energycane and napiergrass in 2009

(Table 2). Although the amounts of harvested N also

declined for the giant reeds and switchgrasses following

2008, a similar large yield drop was not observed for these

two species. Napiergrass removed approximately 374 kg N

ha−1 and 1,121 kg K ha−1 over the first 3 years. Energycane

also took up a large amount of N (255 kg ha−1

) and K

(774 kg ha−1

) over the first 3 years (Table 7). While there

were no apparent differences in the total soil N content in

the cores taken after 3 years of production, evidence of K

uptake by napiergrass and energycane was observed in the

soil cores (Fig. 2). A moderate negative correlation (R=

−0.498, p=0.004) was observed between the amount of K

removed in 3 years of production and the total remaining K

detected in the profile. The distribution of K in the soil

profiles shows evidence of K cycling by the perennial

grasses [12]. Although some segments of the core were not

sampled, and deeper sampling was not possible, these

observations suggest that K is indeed being depleted

primarily by uptake in napiergrass and energycane plots,

and must be leaching from the bare soil control. Over

4 years, only 72 kg N ha−1 and 129 kg K ha−1 were

removed by switchgrass (Table 7). It is possible that the

switchgrass plots in this study could maintain yields near

current levels for several more years with no fertilizer

application. In contrast, the large nutrient removals and

sharp yield declines observed in the larger grasses suggest

that subsequent fertilization will be needed to maintain high

biomass production in these species.

Overall yields tended to decline in the third year (2008),

and 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, and the

number of growing degree days was similar to the first two

seasons (Table 1). Yield declines in the third and fourth

seasons were most pronounced for the high-yielding

energycane and napiergrass. Because these grasses removed

very large quantities of both N and K, the yield reductions

observed in these grasses are most likely due to depletion of

these nutrients from the soil. The declines in DM yield and

in concentrations of N and K in harvested biomass in the

last season might also suggest soil nutrient depletion, but

because harvest was delayed into the winter, deterioration

of biomass, nutrient translocation, and nutrient leaching

from the senescent biomass could also partly explain these

observations [1, 16].

The root profiles observed for napiergrass in this study

agree with observations reported by Woodard and Prine

Fig. 2 Distribution of soil K (Mehlich I extractable) sampled at four

depths in spring, 2009 under four perennial grass species and bare soil

for comparison. Within species, means with the same small letter are

not significantly different. Species totals with the same capital letter

are not significantly different

Species kg roots ha−1 Percent of 2008 DM in roots kg C ha−1 in roots kg N ha−1 in roots

Energycane 4,370 a 17.6 c 1,786 a 20.7 a

Giant Reed 4,557 a 45.7 a 2,271 a 27.7 a

Napiergrass 3,684 a 24.3 bc 1,536 a 16.4 a

Switchgrass 5,956 a 43.7 ab 2,901 a 28.3 a

Bare soil 388 b – 109 b 2.3 b

Table 8 Root data from soil cores

taken after 3 years of biomass

production (sum of 0–15, 30–45,

60–76, and 91–107 cm layers)

Within columns, LS means with

the same letters are not significantly different

Bioenerg. Res.
[24], who estimated that around 70% of the root biomass of

napiergrass occurs in the top 25 cm of soil. Roots were also

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 deeper they extend. Extraction of deep nutrient

reserves could account for some of the high rates of N and

K removal that were observed in the napiergrass and

energycane plots. However, Woodard and Prine [24] also

sampled only to a depth of 1 m, so the importance of deeper

roots is still unknown. Switchgrass roots are known to

extend beyond 3-m depth in prairie soils [19], and

sugarcane roots have been observed past 4.5 m less than a

year after planting in Brazil [14]. It certainly seems possible

that the grasses in this study were able to tap deep nutrient

reserves. Deep rooting also supports our observation that

these grasses, once established, were able to withstand

prolonged dry periods with no irrigation.

Conclusions

For low-input biomass production in Southeast USA,

napiergrass and energycane show the greatest yield potential. These species were able to produce high biomass

yields the first and second years with no application of

fertilizer or irrigation, though fertilization in subsequent

years seems necessary to maintain yields. It appears that

these grasses were able to efficiently tap the nutrients that

had previously accumulated in the soil. The ability to take

up large quantities of nutrients may be advantageous for

certain locations. For example, napiergrass or energycane

could be planted in buffers adjacent to heavily fertilized

fields to control runoff of nutrients into groundwater [18],

in addition to producing a useful biomass product. The

lower nutrient removal rates for switchgrass and giant reed

may suggest they are more suitable for upland production.

In addition to high biomass yields, quick establishment is

another considerable advantage of napiergrass and energycane, whereas giant reed and switchgrass require two or

more seasons to reach peak yields. The higher moisture

content of napiergrass, energycane, and giant reed present

some challenges for efficient use and transportation of the

harvested biomass. More research will be required to

determine whether the higher dry matter production by

napiergrass and energycane can offset the increased

transportation costs and whether economical systems of

drying, storage, or on-site pre-processing can be developed

for this type of biomass. In this respect, switchgrass has a

clear advantage, as it can dry considerably in the field after

senescence. It also has lower N and ash concentrations than

the other feedstocks in this test, except perhaps energycane.

These characteristics make switchgrass preferable for direct

combustion applications [1, 16]. The choice of which

species to grow will obviously be determined by its

intended application, logistical factors such as transportation costs, and external factors such as invasiveness. The

results of this study also show that different perennial grass

feedstock species will require specific production practices

to remain sustainable for a prolonged period of time, and

that conversion plant siting feasibility studies and economic

analyses for long-term profitability should be based on

productivity trials running at least 4 years. Soil C and N

accretion studies for perennial grasses may need to be

conducted over much longer periods of time.

Acknowledgments The authors would like to acknowledge Freddie

Cheek, Tony Howell, Lorine Lewis, Bobby Shiver, and DeeAnne

Webb for technical assistance. The authors would also like to thank

Dr. Myron Parker (Prof. Emeritus, University of Georgia) and Dr. C.

Corley Holbrook (USDA-ARS) for critical reviews of this manuscript.

Open Access This article is distributed under the terms of the

Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any

medium, provided the original author(s) and source are credited.

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