management effects on bioenergy sorghum growth, yield and nutrient uptake
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Management effects on bioenergy sorghum growth, yield andnutrient uptake
J.P. Wight a,*, F.M. Hons a, J.O. Storlien a, T.L. Provin a, H. Shahandeh a, R.P. Wiedenfeld b
aDepartment of Soil and Crop Sciences, Texas A & M University, 2474 TAMU, College Station, TX 77843, USAbTexas AgriLife Research and Extension Center, Weslaco, TX 78596, USA
a r t i c l e i n f o
Article history:
Received 1 February 2012
Received in revised form
18 June 2012
Accepted 30 June 2012
Available online 31 July 2012
Keywords:
Biomass
Sorghum
Cropping systems
Fertilization
Nitrogen
Rotation
Nutrient uptake
* Corresponding author. Tel.: þ1 865 567 506E-mail addresses: [email protected] (J
ag.tamu.edu (T.L. Provin), [email protected]/$ e see front matter ª 2012 Elsevhttp://dx.doi.org/10.1016/j.biombioe.2012.06.
a b s t r a c t
Bioenergy sorghum (Sorghum bicolor L. Moench.) productivity and nutrient uptake may be
affected by cropping sequence complexity, fertilization, and residue removal. The goal of
this research was to optimize the efficiency of crop management in high biomass (bio-
energy) sorghum systems. The two-year field study was conducted in two diverse locations
near College Station and Weslaco, Texas. The study utilized a complete factorial design
with four replications of the following factors: Rotation: continuous biomass sorghum vs.
biannual rotation with corn (Zea mays L.); Stover Return: 0, 25, 50% of the sorghum biomass
and all corn stover; and N Rate: 0 vs. non-limiting N. The bioenergy sorghum used was
a high-yield photoperiod-sensitive hybrid. Other inputs and practices were those
commonly used in the respective areas. Sorghum was harvested for yield, and C, N, P and
K, were determined. Rotation, fertilization, and residue return affected yields, plant
growth, and nutrient uptake ( p < 0.05). Total yields, C, N, P, and K uptake in sorghum were
significantly increased by rotation and N fertilization both years, while 25% residue return
increased sorghum yield and N uptake at College Station the first year. Uptake of C, N, P
and K were increased by N fertilization. Sorghum tissue concentrations of N, P and K
declined from the first to the second year although mean yield increased, possibly indi-
cating decreased soil nutrient availability after only two years. A regression equation was
developed relating biomass yield and site, rotation, nitrogen rate, and plant physical traits
(R2 ¼ 0.67).
ª 2012 Elsevier Ltd. All rights reserved.
1. Introduction Sorghum bicolor L. Moench.) may be oneway of helping achieve
The Energy Independence and Security Act of 2007 mandated
production of renewable fuels: 34 hm3 currently, increasing to
136 hm3 by 2022. To meet this goal, the U.S. Department of
Energy, estimates that over 254,952 km2 of dedicated energy
crops will be needed [1]. Lignocellulosic conversion of high
biomass photoperiod-sensitive sorghum (bioenergy sorghum,
6; fax: þ1 979 845 0456..P. Wight), [email protected] (H. Shahandeh), B
ier Ltd. All rights reserved036
this goal.
Bioenergy sorghum is also of interest due to its relatively
low input requirements, drought tolerance, and ability to
maintain high yields under a wide range of environmental
conditions [2e4]. Though relatively little work has been done
to improve bioenergy sorghum, numerous genes affecting
feedstock quality are known. Genes have been isolated that
u.edu (F.M. Hons), [email protected] (J.O. Storlien), TProvin@[email protected] (R.P. Wiedenfeld)..
b i om a s s an d b i o e n e r g y 4 6 ( 2 0 1 2 ) 5 9 3e6 0 4594
allow lignin reduction [5]. This crop is also suited to biofuel
production because it contains a significant quantity of
fermentable carbohydrates and does not directly compete
with food crops. Biomass sorghum has wide environmental
adaptability [6], and dual-use corn (Zea mays) and biomass
sorghum systems, similar to the one in our study, could have
production ranges throughout much of the northern, central,
and southern US.
Although bioenergy sorghum has many traits that make it
ideal for biofuel production, environmental and management
conditions can affect the productivity and sustainability of
these systems. Ethanol yield from grain sorghum can be
affected by soil type and growing location [7]. Management
factors such as location [7], nutrient input [4], crop rotation [8],
and residue return [9] can impact yields, and may be linked to
soil quality. Removing residue inputs from a system can
decrease nutrient cycling, soil organic carbon (SOC), water
holding capacity, aggregation, yields over time and may
increase soil erosion.
Soil is the fundamental resource in crop production
systems, and maintaining soil quality is central in ensuring
sustainable productivity. Management practices that enhance
soil organic matter content can affect nutrient cycling
throughout the soil profile by reducing erosion, improving
structure and water infiltration, increasing cation exchange
capacity (CEC), and releasing plant nutrients upon breakdown
[10,11]. Dou et al. [8] found that after 20 years of grain sorghum
production in south-central Texas, no-till increased SOC,
dissolved organic C, and soil total N compared to conven-
tionally tilled systems. However, it remains unclear how
specific cropping sequences affect soil quality and yield under
bioenergy sorghum production with biomass removal.
Soil organic C dynamics may be affected by crop sequence
complexity, residue rate, and residue quality. In a summary of
soil C studies, West and Post [12] found that when farmland is
converted from conventional till to no-till, increasing rotation
complexity enhanced SOC sequestration. Similar results were
found in Texas sorghumproduction systems,with a sorghum-
wheat (Triticum aestivum, L.)-soybean (Glycine max, L. Merr.)
rotation having 18% higher dissolved organic C than contin-
uous sorghum systems [13], and higher yield and nutrient use
efficiency [14]. Crop rotation often increases yield and SOC
sequestration and, therefore, affects many other soil charac-
teristics. One reason for increased sequestrationwith rotation
is differing quantity and quality of residues with different
crops [14,15].
Studies have found increasing crop residue input often
directly correlates with SOC [16,17]. These authors concluded
that the mulch rate of winter covers was directly related to
gains in SOC. As overall system productivity increases,
quantities of residues returned to soil should also increase.
However, this relationship may be confounded by residue
characteristics. Drinkwater et al. [18] found that residue
quality can affect C and N cycling, and suggested high C-to-N
organic residues increased C retention in soil. Residue return
can also improve soil microbial properties including nutrient
cycling and diversity [19,20]. Powell andHons [9] observed that
removal of more than 50% of aboveground high energy
sorghum biomass for three years resulted in lower yields,
reduced N fertilizer uptake efficiency and reduced SOC
concentrations. Results were more pronounced when all
biomass was removed. With complete biomass removal, N
uptake efficiencies decreased from 68% in the first year of the
study to only 22% in the fourth year. Improving soil nutrient
dynamics can also increase yield. These factors should be
considered when determining biofuel production strategies.
Potential drawbacks of the use of biofuel systems include
the diversion of food crops into fuel, added greenhouse gas
emissions, low profitability, loss of soil quality, drawdown of
water resources, spread of invasive species and increased
non-point source pollution [21]. A key factor for maintaining
the economic viability of the biofuel sector is sustainable
biomass yield, allowing guaranteed feedstock supply while
preserving food resources. Our study quantified how yields of
photoperiod-sensitive bioenergy sorghum were affected by
location, rotation, fertilization, and residue removal.
2. Materials and methods
2.1. Site description
Field studies were established at two sites in 2008. The first
site was at the Texas AgriLife Research Farm near College
Station (ARECCS), TX (30.32�N, 94.26�W, ca. 60 m msl). This
region has a mean annual temperature of 20 �C and averages
97.8 cm of annual precipitation. Soil at the site is classified as
a Weswood silty clay loam (Fine, mixed, thermic Udifluventic
Ustochrept). The second site was at the Texas AgriLife
Research and Extension Center near Weslaco, TX (ARECW)
(26.22�N, 98.23�W ca. 5 m msl). Unlike ARECCS, mean annual
precipitation is lower at 59 cm, but mean annual temperature
is higher at 23.3 �C. The soil type is a Hidalgo sandy clay loam
(Fine-loamy, mixed, active, hyperthermic Typic Calciustoll).
Prior to the study, the field near College Station was in cotton
production in 2007, and rotated annually with corn. Corn was
grown at the Weslaco site. Both fields were under conven-
tional disk tillage.
2.2. Experiment design
The experiment used a randomized complete block design
with three treatment factors: crop rotation, nitrogen rate and
residue return. Crop rotations were either continuous bio-
energy sorghum or biannual rotation with corn. Sorghumwas
planted in the continuous sorghum treatments in both 2008
and 2009, while corn was planted in the rotation treatment in
2008 followed by sorghum in 2009. Nitrogen was applied to
sorghum at either 0 or a non-limiting rate: 336 kg ha�1 in 2008
and 280 kg ha�1 in 2009. Nitrogen application to corn was
either 0 or 168 kg ha�1. Nitrogen as urea was sidedress applied
15-cm deep approximately 6 weeks after planting at the 4-to-
5-leaf stage for sorghum and the 6-to 8-leaf stage for corn.
Residue return rates were 0, 25 or 50% of sorghum biomass
yield at harvest. All corn stover was returned. Each possible
combination of residue return, fertilization, and crop rotation
was replicated four times across both locations.
At both College Station and Weslaco, an additional treat-
ment was included with 0% residue return, but where nutri-
ents removed in aerial sorghum biomass (P, K, Ca, Mg, Mn, Zn,
b i om a s s a n d b i o e n e r g y 4 6 ( 2 0 1 2 ) 5 9 3e6 0 4 595
Fe, andCu)were returned inmineral formprior to planting the
subsequent crop. These plots also received the non-limiting N
rate. At Weslaco, an extra treatment was implemented due to
the importance of sugarcane (Saccharum sp.) production in the
region. This treatment received the equivalent of 100% of dry
biomass removed as sugarcane “mill mud”. Mill mud is
a byproduct of sugarcane milling, consisting of processed
sugarcane leaf and stalk material, soil, and lime added during
the clarification process. The material is a source of N, P, and
Ca. This treatment also received non-limiting N fertilization.
These additional treatments were replicated four times at
their corresponding locations, resulting in a total of 56 plots at
College Station and 64 plots atWeslaco. Fields were blocked to
help control variation, with treatments being randomly
assigned within each block.
Plots were 9.14-m long, four rows wide and planted on 102-
cm row centers, giving a total plot width of 4.08 m. The bio-
energy sorghum used in this study was “4-Ever Green”,
a modern photoperiod-sensitive, one-cross hybrid with high
biomass yield and low lodging potentials (Walter Moss Seed
Co, Waco, TX, U.S.A.). Planting dates ranged from late March
to late April (Table 1), with a seeding rate of 160,000 ha�1.
Bioenergy sorghum and corn were managed under conven-
tional disk tillage. After the final harvest each year, plots were
disked three times to a depth of 15e20 cm, and bedded (see
Table 1 for a list of field operations). Furrow irrigation was
minimally performed as needed and dates and amounts are
also given in Table 1.
2.3. Soil methodology
Composite soil samples for separate depths from each
experimental unit were formed from three soil cores taken at
depth increments of 0e5, 5e15, 15e30, 30e60, and 60e100 cm.
Initial samples were taken in March 2008 prior to the begin-
ning of the study. Samples were oven dried at 60 �C for 7 d,
ground using a flail grinder and sieved to pass through
Table 1 e Field operations and dates performed at College Stat
Operation
2008
College Station W
Soil sampling 24th March 16th
Pre-plant disking 25th March 21st
Interrow cultivation 24th April 12th
Bedding 15th October 4th
Irrigation 10th June 9 cm 25th
10th July 9 cm 2nd
e 3rd
e e
e e
e e
Pre-plant herbicide application 3rd March n/a
Planting 26th March 23rd
Post-plant herbicide application n/a 24th
Fertilization 1st May 29th
Harvest 5th August 14th
14th October 2nd
1.75 mm mesh. Soil pH was determined in 2:1 (water:soil)
suspensions. Residual soil inorganic nitrate-N was extracted
(10:1 solution:soil) with a KCl solution made by dissolving
74.54 g KCl per L of H2O, reduced to nitrite using a cadmium
column, and analyzed using spectrophotometric methods
[22]. Mehlich III extractable nutrients including P, K, Ca, and
Mg were extracted using a method adapted from Mehlich [23]
and measured by inductively coupled spectroscopy (ICP). A
subsample of each composite sample was finely ground with
a puck-and-ring grinder and analyzed for organic and inor-
ganic C and N by combustion [24e26] using an Elementar
Americas Inc, VarioMAX CN analyzer (Mt. Laurel, NJ, U.S.A.).
Differential heating was used to separate inorganic and
organic C. For organic carbon, the primary furnace was set at
650 �C and a 2 L min�1 O2 flow rate. Total N and C were
analyzed with the same instrument at 900 �C [27,28].
2.4. Plant methodology
Biomass sorghum plots were harvested using hand imple-
ments and cut to leave a short stubble (ca. 10 cm) in August
and October 2008 and November 2009 at ARECCS, and in
August and December 2008 and January 2010 at ARECW. For
determination of yield, the two inner rows of each plot were
hand-harvested for yield in 2008 and harvested with
amechanical harvester in 2009. Harvests in 2009 were delayed
at both locations by excessively wet conditions. Residue cor-
responding to 25 and 50% of harvested biomass were returned
to appropriate plots after each harvest.Water content of aerial
plant biomass from each plot was determined by randomly
selecting 5 plants, chopping in a commercial chipper/
shredder, and taking an approximately 600-g subsample.
Subsamples were weighed, oven dried at 60 �C for 7 d, and
then re-weighed to determine moisture content.
Dried plant samples were initially ground to pass a 1-mm
sieve and then powdered in a ring and puck mill prior to total
elemental analysis for C and N by combustion [24] using an
ion and Weslaco, Texas.
Year
2009
eslaco College station Weslaco
April 6th April 26th January
April 6th April 3rd March
May 5th June 27th April
December 6th November 17th Februrary
April 8 cm 16th July 6 cm 12th April 4 cm
June 5 cm 24th August 6 cm 18th May 4 cm
July 5 cm e 9th July 5 cm
e 21st July 3 cm
e 10th August 3 cm
e 5th September 3 cm
9th March 10th April
April 7th April 13th April
April n/a 1st May
May 20th June 19th May
August 5th November 6th January
December n/a n/a
b i om a s s an d b i o e n e r g y 4 6 ( 2 0 1 2 ) 5 9 3e6 0 4596
Elementar Americas, Inc, CNSAnalyzer (Mt. Laurel, NJ). Total P
and K were determined after nitric acid digestion [29] using
a Spectro Arcos ICP (Kleve, Deutschland). Total accumulation
of each element in aerial biomass was calculated as dry
yield � total elemental concentration.
Leaf chlorophyll readings were made using a Minolta 502
SPAD meter (Spectrum Technol., East Plainfield, IL, U.S.A.).
Readings were taken about one third of the distance from the
tip of the leaf to the base and halfway between themidrib and
the leaf edge. The youngest collared leaf was chosen for each
reading. Height and SPAD measurements were taken period-
ically during the growing season at College Station in 2008 and
approximately biweekly at both locations in 2009. At each
measurement, fifteen SPAD observations and five height
measurements were taken from randomly chosen plants in
each plot with one reading taken per plant. Readings were
averaged into a single measurement for each plot.
2.5. Statistical analysis
Plant characteristics, nutrient uptake, and yield changes due
to crop rotation, residue return, nitrogen fertilization, location
and their interactions over the course of the experiment were
analyzed using Mixed Model procedures [30]. Analysis of
variance (ANOVA, PROC Mixed) was performed to identify
differences caused by rotation, fertilization, residue return,
and their interactions. Means separationwas performed using
SAS macro “pdmix800” [31] with a Fisher’s Least Significant
Difference test type one error rate of 0.05% (a ¼ 0.05) after
overall treatment significance was established using ANOVA
(P < 0.05).
Multiple linear least square regression models were
developed to determine which factors and combinations of
factors were related to yield. Models excluded non-significant
factors and included those that explained a significant portion
of biomass sorghum yield variation. Models were developed
using McHenry’s algorithm, a variable selection technique
which seeks the best subset through an iterative process
based on maximizing the coefficient of determination (R2) of
each variable combination. This variable selection technique
was chosen because the regression utilized management
factors as categorical variables and McHenry’s algorithm can
Table 2 e Soil information for bioenergy sorghum research sit
Property
College station
0e5 5e15 15e30 30e60 60e
TC g kg�1a 16.6 15.3 14.5 14.0 1
OC g kg�1 9 7.4 5.8 4.0
TN g kg�1 1.0 0.9 0.8 0.6
NO3 mg kg�1 6 6 6 5
P mg kg�1 24 26 17 6
K mg kg�1 244 236 182 127 10
Ca mg kg�1 753 654 800 1320 126
Mg mg kg�1 243 238 230 228 24
S mg kg�1 22 22 21 27 2
a TC, OC, and TN represent total C, organic C, and total N, respectively.
consider groupings of categorical variables together for
inclusion in or deletion from the model, as a single unit [32].
Potential models were then evaluated through multiple
regression to estimate each variable’s significance and coef-
ficient, ultimate model performance and interpretation. The
“Multivariate Variable Selection and Multiple Regression”
analyses of NCSS 2004 were used [33]. Variables with proba-
bility levels lower than 0.05 were designated significant. The
resultant model quantified yield components as a function of
management and observable sorghum physical traits.
3. Results and discussion
3.1. Site characteristics
TheWeswood soil near College Station is calcareous and near
surface samples rated very low for extractable NO3eN,
moderate for P, and high for K, Ca and Mg (Table 2). The Hil-
dalgo soil at Weslaco was calcareous below 30 cm and near
surface samples rated low for extractable NO3eN, moderate
for P and Ca, very high for K, and high Mg. Soil organic C and
total N were slightly higher in the Weswood soil, especially at
depths above 30 cm. Although soils ranked very low or low in
extractable NO3 in surface samples, significant residual NO3
was observed with depth, especially in the Hildalgo soil.
Air temperatures at College Station were higher than
historic averages during both the 2008 and 2009 growing
seasons (Fig. 1). In August 2008, Hurricane Ike passed near
CollegeStation,depositingover17cmofprecipitation just after
the first sorghum harvest. Air temperatures at Weslaco were
below average for the 2008 growing season and near normal in
2009 (Fig. 2). Hurricane Dolly passed nearWeslaco in July 2008,
resulting in severe lodging and depositing 30.4 cm of rainfall
over twodays.Bothsiteshadhigher thanaverage rainfall in the
fall of 2009, which may have improved late-season growth.
3.2. Height
Plant height has been reported to be a useful indicator of dry
matter yield in photoperiod sensitive sorghum [34]. Biomass
sorghum in this study displayed increasing height throughout
es at College Station and Weslaco, Texas in 2008.
Depth cm
Weslaco
100 0e5 5e15 15e30 30e60 60e100
4.7 5.8 5.9 5.6 6.0 12.4
3.5 5.9 5.7 5.5 4.6 4.8
0.6 0.8 0.8 0.8 0.7 0.7
4 13 17 14 13 14
4 40 39 32 17 19
6 489 474 381 281 217
8 467 514 476 561 1115
7 356 354 357 396 431
8 22 27 28 33 46
Jan.
Feb.
Mar. Apr. May
Jun. Ju
l.Aug
.Sep
.Oct.
Nov.
Dec.
Mo
nth
ly P
recip
itatio
n cm
0
10
20
30
40
Mo
nth
ly T
em
peratu
re o
C
0
10
20
30
40
20082009Long-term Avg.20082009Long-term Avg.
Fig. 1 e Summary of weather data at the Texas A&M
University AgriLife Research and Extension Center at
College Station, Texas for 2008 and 2009 along with
30-year site averages. Lines refer to monthly average
temperature, while bars are monthly precipitation.
b i om a s s a n d b i o e n e r g y 4 6 ( 2 0 1 2 ) 5 9 3e6 0 4 597
the growing season in both years due to the photoperiod-
sensitive characteristic of delayed flowering. Sorghum
height was significantly increased (P < 0.01) by N application
at College Station (Fig. 3A) in 2008. Significant differences
(P < 0.01) in sorghum height were observed due to location,
rotation, and N fertilization in 2009 (Fig. 4). As might be ex-
pected, sorghum height trends were similar to those observed
with dry matter yield, with fertilized and rotated sorghum
being taller than unfertilized continuous sorghum. Heights at
Weslaco were greater than those at College Station (P < 0.01)
and rotated sorghum was significantly taller than continuous
sorghum (P < 0.05). Sorghum fertilized with N also was
significantly taller than unfertilized controls throughout most
of the observed period (P < 0.01).
3.3. SPAD measurements
Increasing SPAD readings have previously been positively
correlated with N rate [35] and chlorophyll content [36] in
Jan.
Feb.
Mar. Apr. May
Jun. Ju
l.Aug
.Sep
.Oct.
Nov.
Dec.
Mo
nth
ly P
recip
itatio
n cm
0
10
20
30
40
Mo
nth
ly T
em
peratu
re o
C
0
10
20
30
40
20082009Long-term Avg.20082009Long-term Avg.
Fig. 2 e Summary of weather data at the Texas A&M
University AgriLife Research and Extension Center at
Weslaco, Texas for 2008 and 2009 along with 30-year site
averages. Lines refer to monthly average temperature,
while bars are monthly precipitation.
sorghum. Readings in our study varied with sampling date
during each growing season. Others also reported SPAD
reading variation through the growing season in stay-green
sorghum [35] and corn [37]. Fertilization significantly
increased (P < 0.01) SPAD readings throughout the growing
season at College Station in 2008 and 2009 (Figs. 3B and 5A).
Both fertilization and rotation affected SPAD readings
throughout the 2009 growing season (P < 0.01), with differ-
ences generally following the same trends as yield and height
(Fig. 5A). SorghumatWeslaco generally exhibited higher SPAD
readings than at College Station (Fig. 5B). Continuous, unfer-
tilized sorghum showed significantly lower SPAD values than
all other treatments throughout most of the growing season,
especially at College Station (P < 0.05). Rotation generally had
a stronger influence than fertilization on readings at both
locations. This effect was more pronounced at Weslaco,
possibly because of greater residual soil NO3with depth at this
location (Table 2). SPAD values tended to decrease for all
treatments after approximately the 8th of June at College
Station. Readings at Weslaco decreased slightly from early in
the season to the 4th of June and then increased significantly
for all treatments. Plots at Weslaco were irrigated on 9 and 21
of July 2009 (Table 1) and this may partially account for
increased SPAD values after these dates. Plots at College
Station received irrigation on 16 July and 24 August 2009, but
did not affect SPAD values. A higher photosynthetic capacity
with increasing SPAD reading may be partly responsible for
the increased yields observed in these treatments [38].
3.4. Bioenergy sorghum yield
Total annual yield at both locations was significantly affected
by application of N fertilizer (P < 0.1) in 2008, but did not differ
across locations. Total yield of fertilized sorghum in 2008 was
23.11 � 2.02 t ha�1 at College Station and 22.53 � 2.4 t ha�1 at
Weslaco (Fig. 6). Yields from the first harvest were greater
than the second at both locations, but the portion of total yield
from the first harvest was greater at College Station. Returning
25 or 50% of the biomass produced during the first harvest in
2008 resulted in significantly greater yield in the second
harvest at College Station compared to complete removal of
biomass (Fig. 7). Total yield, was also affected by biomass
return at College Station, being highest with 25% return.
However, neither the first or second harvest nor total yields
were influenced by this factor at Weslaco in 2008. For the first
harvest, no effect of biomass return was expected since
residue return treatments were implemented beginning after
the first cutting in August 2008. Soil quality parameters related
to soil organicmatter and residue return typically take time to
become evident. For example, Powell and Hons [9] observed
that 50% removal of energy sorghum aboveground biomass
resulted in reduced soil quality parameters and yields after
three years. However, in the second harvest of 2008 at the
College Station site, residue return increased yield (P < 0.05).
This site only received natural precipitation after the first
harvest and 2008 was drier than normal (Fig. 1). Increased
yield with residue return may have been due to reduced soil
evaporation provided by the additional residue cover. This
hypothesis is supported by the fact that no significant yield
differences due to residue returnwere observed for the second
Heig
ht, cm
150
200
250
300
350
400
450
+ N- N
30
35
40
45
50
+ N- N
A B
Fig. 3 e College Station sorghum plant height (A) and SPAD readings (B) In 2008 with or without added N. Bars denote the
standard error.
b i om a s s an d b i o e n e r g y 4 6 ( 2 0 1 2 ) 5 9 3e6 0 4598
harvest at Weslaco, which received two flood irrigations of
approximately 15 cm ha�1 each along with approximately
18 cm ha�1 (Fig. 2) of precipitation between the 1st and second
harvests of 2008.
Of the two cuttings that occurred in 2008, the first harvest
tended to highlight effects of N fertilization at College Station
(Fig. 6), while the effect of residue return was observed on the
second cutting (Fig. 7). Individual yields of the first and second
harvests were not affected by N fertilization at Weslaco, but
total yield was increased by N addition. Nitrogen addition
increased total biomass dry matter yield by about 14% at
College Station and by about 10% at Weslaco in 2008.
First harvest yields at Weslaco in 2008 were slightly lower
than at College Station (Figs. 6 and 7), possibly due to lodging
or stress caused by Hurricane Dolly. Hand harvesting sorghum
in 2008 likely reduced potential yield losses associated with
the severe lodging. The second harvest yields at this location
were greater than at College Station, while total dry matter
yields were similar at both locations.
Total dry biomass yield in 2009 was significantly affected
by site, rotation, fertilization, and the interaction of
rotation � fertilization (Figs. 8 and 9). Mean yield at Weslaco
was 26.1 � 1.2 t ha�1, significantly higher (P < 0.001) than the
mean yield of 20.2 � 1.2 t ha�1 at College Station (Fig. 8). This
difference may have been due to a longer growing season at
Weslaco, given the later harvest date, higher temperatures,
He
ig
ht c
m
0
50
100
150
200
250
300CS + NCS - NSS + NSS - N
A
Fig. 4 e Plant height at College Station (A) and Weslaco (B) In 2
standard error; CS is rotated and SS is continuous sorghum.
and lack of senescence of the photoperiod-sensitive sorghum
used. This site also received significantly more irrigation than
the study at College Station. Fertilization significantly
(P < 0.001) increased yield across location and rotation in 2009
from 20.7 � 1.2 up to 25.1 � 1.2 t ha�1, a gain of approximately
21%. Rotation with corn also significantly (P< 0.001) increased
yield in 2009 across both locations, with continuous sorghum
and rotated sorghum yielding 19.4 and 26.8 t ha�1, an increase
of 38%. Others have reported that rotation of grain sorghum
increased grain yield [14], but less is known about high
biomass sorghums. In Iowa, Buxton et al. [4] found mono-
culture sweet and forage sorghum yields were similar to those
that were rotated.
The interaction of rotation � N fertilization was also
significant (P< 0.001) for total dry biomass yield in 2009 (Fig. 8).
Continuous sorghum responded more to fertilization,
although rotated sorghum yields were higher. Absolute
responses to N fertilization were 3.4 and 6.9 t ha�1 for rotated
and continuous sorghum, respectively. Across both locations,
yield was highest in the fertilized rotated treatment, followed
by unfertilized rotated, fertilized continuous sorghum, and
unfertilized continuous sorghum. Of the continuous sorghum
plots, those with 25% biomass returned in 2008 (Fig. 7)
exhibited generally higher mean yields in 2009 (Fig. 9)
compared with 0 or 50% biomass return, though differences
were not statistically significant. Typically, crop residue is
Heig
ht cm
0
50
100
150
200
250
300CS + NCS - NSS + NSS - N
B
009 as affected by added N and rotation. Bars denote the
SP
AD
R
ead
in
g
25
30
35
40
45
50 CS + NCS - NSS + NSS - N
SP
AD
R
ead
in
g
25
30
35
40
45
50 CS + NCS - NSS + NSS - N
A B
Fig. 5 e SPAD readings at College Station (A) and Weslaco (B) during 2009 as affected by added N and rotation. Bars denote
the standard error; CS is rotated and SS is continuous sorghum.
b i om a s s a n d b i o e n e r g y 4 6 ( 2 0 1 2 ) 5 9 3e6 0 4 599
needed to maintain soil organic matter, which is critical to
nutrient cycling and in soil physical, chemical, and microbial
quality. Powell and Hons [9] noted significant impacts on
sorghum yield due to complete residue removal within three
years of treatment imposition. Although the current study did
not observe overall yield impacts due to residue return, the
experiment is in its’ initial stages and will be ongoing to
determine the long-term effects of biomass removal.
Biomass yields in the mineral replacement þ N fertilizer
treatment were similar to other treatments that also received
N fertilization in 2009 at both locations (Fig. 9). The mill
mudþN treatment at Weslaco also resulted in biomass yields
similar to other treatments receiving N. This supports the
conclusion that mineral depletion due to biomass removal
was not a significantly limiting factor to yield at this time.
3.5. Plant nutrient concentration and uptake and carbonfixation
Site, N fertilization, and rotation affected plant C fixed and
uptake of N, P, and K, while biomass return had no effect
(Tables 3 and 4). Overall at both study sites, sorghum biomass
contained approximately 9.5 t ha�1 of C, and 254, 20, and
375 kg ha�1 of N, P, and K, respectively in 2008. In 2009, total
aboveground biomass contained approximately 9.75 t ha�1 of
C, and 173, 27, and 310 kg ha�1 of N, P, and K. When averaged
across sites, biomass yield was greater in 2009 than 2008,
while N, P, and K removals were lower, possibly indicating
lower nutrient availability after only one year of cropping.
Site affected total nutrient uptake both years (P < 0.01),
with Weslaco having higher nutrient uptake and plant tissue
nutrient concentrations in both study years (Tables 3 and 4).
Besides higher extractable soil nutrients at this site as already
discussed, the longer growing season and higher tempera-
tures at this site may also have had an effect.
Site, N fertilization, and rotation influenced C, N, P, and K
concentrations in sorghum biomass, while biomass return
had no effect on concentrations (Table 3). Sorghum biomass
had greater C concentrations at College Station each year, but
lower concentrations of N, P, and K. Soil at Weslaco had
greater residual NO3eN and greater extractable P and K
throughout the profile compared to College Station (Table 2).
Adding N fertilizer increased the C concentration of biomass
in 2008 and increased N concentrations both years, but had
minimal effects on P and K concentrations. Biomass of rotated
sorghum showed increased C concentration compared to
continuous, but lower N, P, and K concentrations. The latter
effect was likely due to nutrient dilution in the greater yield of
rotated sorghum.
Nitrogen fertilization affected total C fixation, andN, K, and
P uptake at both locations in 2008 and 2009 (Tables 3 and 4).
Tissue concentrations of N were significantly increased by N
fertilization (P < 0.01) both years (Table 3). Nitrogen fertiliza-
tion significantly increased (P < 0.01) total C fixed via photo-
synthesis at both locations during both years. In 2008, total
aboveground C fixed was 9.4 � 0.3 t ha�1 in fertilized plots,
while unfertilized plots fixed 8.3� 0.3 t ha�1 (P< 0.01). In 2009,
differences in C fixation due to fertilization were greater
(P < 0.001), with unfertilized sorghum fixing approximately
9� 0.2 t ha�1 and fertilized sorghumfixing 11� 0.2 t ha�1. Crop
nitrogen uptake was increased both years by N fertilization
due to higher plant N concentrations and biomass yields.
Greater P and K uptake was associated with higher yield with
N application.
Rotation also affected nutrient uptake and C content in
2009 (P< 0.01). Rotated sorghum exhibited significantly higher
fixed C (11 � 0.2 t ha�1), and N (185 � 7.9), P (28 � 1.0), and K
(346� 9.7 kg ha�1) uptake comparedwith continuous sorghum
that contained 8 � 0.2 t ha�1 of C, and 161 � 7.9, 26 � 1.0, and
274 � 9.7 kg ha�1 of N, P, and K, respectively. Continuous
sorghum showed significantly higher (P < 0.01) tissue
concentrations of P (1.3 � 0.05) and K (13.9 � 0.05) g kg�1 than
rotated sorghum (1.0 � 0.05) and K (12.7 � 0.05) g kg�1.
Concentrations of these nutrients were likely lower in rotated
sorghum because of dilution associated with greater yield.
3.6. Regression analysis of biomass sorghum yieldcomponents
To better understand the relationships betweenmanagement
and yield, components explaining variation in yield were
studied through regression analysis. The original parameter
set encompassed many potential variables and their interac-
tions. To identify the smallest subset of properties for
Location
Yie
ld t
ha
-1
0
5
10
15
20
25
30 - N+ N
Location
Yie
ld t
ha
-1
0
5
10
15
20
25
30 - N+ N
Location
College Station Weslaco
Yie
ld t
ha
-1
0
5
10
15
20
25
30 - N+ N
A
B
C
Fig. 6 e Bioenergy sorghum dry matter yields in 2008 near
College Station and Weslaco, Texas for the first cutting (A),
second cutting (B) and total annual yield (C) As affected by
nitrogen fertilization. Error bars denote the standard error.
Yie
ld t
ha
-1
0
5
10
15
20
25
30 0% Return25% Return 50% Return
Yie
ld t
ha
-1
0
5
10
15
20
25
30 0% Return25% Return50% Return
Location
College Station Weslaco
Yie
ld t
ha
-1
0
5
10
15
20
25
30 0% Return25% Return50% Return
A
B
C
Fig. 7 e Bioenergy sorghum yields in 2008 near College
Station and Weslaco, Texas for the first cutting (A), second
cutting (B) and total annual yield (C) as affected by biomass
return. Error bars denote the standard error.
b i om a s s an d b i o e n e r g y 4 6 ( 2 0 1 2 ) 5 9 3e6 0 4600
meaningful prediction of yield, multivariate variable selection
was performed using McHenry’s algorithm. Ultimately, the
regression used site and management treatments as binary
categorical variables and N rate, sorghum height and SPAD
reading as continuous variables. These variables were chosen
over other potential variables due to their direct interpret-
ability and significance in explaining yield variation (Table 5).
All regression coefficients were significant at least to P < 0.01.
The multiple regression equation to predict biomass sorghum
yield was highly significant (P < 0.001, R2 ¼ 0.67);
Y ¼ 2130:50þ 76:09ðNrateÞ � 0:22ðNrateÞ2þ61:55ðHeightÞþ 181:73ðSPADÞ þ 4651:71ðSequence ¼ CSÞþ 5787:58ðSite ¼ WeslacoÞ;
where Y is yield (kg ha�1), Nrate is added N (kg ha�1), height is
average plant height (cm), SPAD (unitless), sequence ¼ CS is
the rotation of sorghum following corn, and site ¼ Weslaco
indicates location where yield corresponds to biomass
measured at the Weslaco experiment station.
Though height and SPAD measurements were taken
throughout the growing season, those taken in early June of
each year exhibited the strongest relationships with yield, and
College Station Weslaco
Yield
t h
a -
1
0
5
10
15
20
25
30
35CS + N CS - N SS + N SS - N
Fig. 8 e Total sorghum biomass dry yields at College
Station and Weslaco, Texas sites in 2009 with or without
added N. Error bars denote the standard error. CS indicates
corn-sorghum rotation, while SS indicates continuous
sorghum.
College Station Weslaco
Yield
t h
a-1
0
5
10
15
20
25
30
350% + N 25% + N 50% + N 0% - N 25% - N 50% - N
Fig. 9 e Total sorghum biomass dry yields of rotated and
continuous sorghum at College Station and Weslaco sites
in 2009 as affected by N fertilization and residue return.
Error bars denote the standard error.
b i om a s s a n d b i o e n e r g y 4 6 ( 2 0 1 2 ) 5 9 3e6 0 4 601
were therefore used in the regression model. Addy et al. [35]
also found SPAD varied through the growing season and
that early season SPAD measurements were useful in yield
prediction of “stay-green” and common grain sorghum.
Results from ANOVA analysis supported the regression
analysis. In the regression, N fertilizationwas significant, with
each kg ha�1 of N fertilizer added, increasing biomass yield
approximately 76 kg ha�1, with a decreasing response rate of
�0.22 kg ha�1 yield per kg2 ha�1 of N fertilizer. This differs
Table 3 e Biomass sorghum tissue nutrient concentrations. CS
Treatment First harvest 2008 Seco
Yield C N P K Yield C
Site t ha�1 g kg�1 g kg�1 g kg�1 g kg�1 t ha�1 g kg�
Weslaco 12.4 409.6 13.0 1.4 22 9.1 399.6
College Station 15.8 414.8 9.9 0.6 17 5.7 415.9
LSD(0.05) 1.9d 3.6c 0.7d 0.1d 1.2d 1.9d 3.6
Fertilization
�N 13.2 410.7 10.7 1.0 20.1 7.0 406.2
þN 15.0 413.8 12.2 1.0 19.1 7.8 409.3
LSD(0.05) 1.9 3.6c 0.7d 0.1 1.2a 1.9 3.6
Rotation
CS e e e e e e e
SS e e e e e e e
LSD(0.05) e e e e e e e
Biomass return
0% e e e e e 6.9 405.5
25% e e e e e 7.8 411.6
50% e e e e e 7.6 406.3
LSD(0.05) e e e e e 2.5 12.2
a Significant at the 0.1 probability level.
b Significant at the 0.05 probability level.
c Significant at the 0.01 probability level.
d Significant at the 0.001 probability level.
from results of Wortmann et al. [39], who showed little
response to nitrogen fertilization in sweet sorghum. However,
the quadratic relationship is common in similar sorghum
crops including sorghum � sudangrass [9,40] and
photoperiod-sensitive sorghum [41]. Plant height measured in
June was directly related to yield, with each additional cm
increasing predicted yield by 61 kg ha�1. Each unit increase of
SPAD resulted in a predicted yield increase of 182 kg ha�1.
Others have linked higher SPAD to increased chlorophyll
concentration in sorghum [36], which can result in increased
indicates rotated while SS represents continuous sorghum.
nd harvest 2008 2009
N P K Yield C N P K
1 g kg�1 g kg�1 g kg�1 t ha�1 g kg�1 g kg�1 g kg�1 g kg�1
8.9 1.1 13 26.1 419.9 7.1 1.4 14.9
10.0 0.8 12 20.2 424.4 8.1 0.9 11.2c 0.7c 0.1d 1.2a ‘ 5.1c 0.6b 0.1d 0.6d
9.0 0.9 12 20.7 423.0 7.0 1.18 13.3
9.7 1.0 12 25.1 424.4 8.2 1.18 13.7
0.7a 0.1 1.2 1.6d 5.1 0.6d 0.1 0.6
e e e 26.8 426.2 7.1 1.0 12.7
e e e 19.4 421.1 8.1 1.3 13.9
e e e 1.6d 5.1b 0.6c 0.1d 0.6d
9.3 1.0 12.2 23.3 422.5 7.6 1.2 12.9
9.7 0.9 12.2 23.5 424.3 7.6 1.1 13.5
9.1 1.0 12.2 22.4 424.2 7.6 1.2 13.6
1.9 0.3 1.3 2.1 6.2 0.8 0.1 0.8
Table 4 e Total biomass sorghum nutrient uptake per unit land area. CS indicates rotated while SS represents continuoussorghum.
Treatment First harvest 2008 Second harvest 2008 2009
C N P K C N P K C N P K
Site t ha�1 kg ha�1 kg ha�1 kg ha�1 t ha�1 kg ha�1 kg ha�1 kg ha�1 t ha�1 kg ha�1 kg ha�1 kg ha�1
Weslaco 5.1 160 18 275 3.7 82 10 116 11.0 182 36 385
College Station 6.7 158 10 266 2.4 56 5 66 8.6 163 18 235
LSD(0.05) 0.7 19.1 1.7d 27.5 0.7a 19.1d 1.7d 27.5d 0.7d 18.7b 3.0d 27.5d
Fertilization
�N 5.5 139 13 261 2.9 64 7 88 8.8 143 24 276
þN 6.2 180 15 280 3.2 75 8 94 10.7 203 30 344
LSD(0.05) 0.7c 19.1d 1.7c 27.5 0.7 19.1 1.7 27.5 0.7d 18.76d 3.0d 27.5d
Rotation
CS e e e e e e e e 11.4 185 28 346
SS e e e e e e e e 8.2 161 26 274
LSD(0.05) e e e e e e e e 0.7d 18.7c 3.0d 27.5d
Biomass return
0% e e e e 2.8 64 7 85 9.9 173 27 306
25% e e e e 3.2 75 8 95 10.0 176 27 317
50% e e e e 3.1 69 8 93 9.5 169 28 308
LSD(0.05) e e e e 0.9 25.6 3.6 28.0 0.9 24.5 3.4 33.6
a Significant at the 0.1 probability level.
b Significant at the 0.05 probability level.
c Significant at the 0.01 probability level.
d Significant at the 0.001 probability level.
y = 7.81+0.65*x (r2=0.67)95 % regression confidence interval95% prediction interval
0
5
10
15
20
25
30
Weslaco RSWeslaco SSCollege Station RSCollege Station SS
Weslaco CSWeslaco SSCollege Station CSCollege Station SS
b i om a s s an d b i o e n e r g y 4 6 ( 2 0 1 2 ) 5 9 3e6 0 4602
yield. Rotation increased predicted biomass sorghum yield by
approximately 4.6 t ha�1. Few have documented the rotation
effect on biomass sorghum yields, but rotation has been
shown to improve grain sorghum yield [42], but may lower
apparent N use efficiency [43]. The location at Weslaco had
soil with higher extractable nutrients and a longer growing
season, which may explain the increased yield prediction of
5.8 t ha�1 when this termwas included in themodel. The deep
rooting pattern and photoperiod sensitivity of biomass
sorghum may allow it to capitalize on both of these factors
[44]. Linear regression of model predicted vs. observed yield
confirmed a linear relationship, normal distribution, and
goodness of fit r2 ¼ 0.67 (Fig. 10).
Table 5 e Parameter estimates for regression of biomasssorghum yield (kg haL1) in 2008 and 2009 onmanagement factors of sequence (CS [ corn-sorghum,SS [ continuous sorghum), site (College Station orWeslaco), nitrogen fertilization rate (Nrate, kg haL1),plant height (height, cm), and chlorophyll meter readings(SPAD, unitless). Parameter estimates are kg haL1.
Yield component Parameterestimate
F prob. Model R2
Partial Total
Intercept 2130.5 � 3115.5 0.496 0.00 0.00
Site ¼ Weslaco 5788.6 � 764.1 0.000 0.26 0.27
Sequence ¼ CS 4651.7 � 721.5 0.000 0.14 0.4
Nrate 76.1 � 25.9 0.004 0.12 0.53
Nrate2 �0.2 � 0.1 0.013 0.09 0.62
Height 61.6 � 10.6 0.000 0.027 0.65
SPAD 181.8 � 70.5 0.011 0.021 0.67
0 5 10 15 20 25 30 35
Fig. 10 e Relationship of actual yield to that predicted from
an optimized multiple regression analysis in studies
conducted at Texas AgriLife Research and Extension
Centers near Weslaco and College Station, Texas in 2008
and 2009. CS indicated corn-sorghum rotation, while SS
indicated continuous sorghum.
4. Conclusions
Biomass sorghum dry yields exceeded 25 t ha y�1 in optimized
systems within this study. Nitrogen fertilization increased
yields of rotated and continuous sorghum at both study sites.
A quadratic response curve indicated diminishing return on N
input, and the optimumN rate should be based on production
conditions. Although continuous sorghum responded to
fertilization, N addition could not fully compensate for
b i om a s s a n d b i o e n e r g y 4 6 ( 2 0 1 2 ) 5 9 3e6 0 4 603
differences in yields due to rotation. Rotation had a large
effect on sorghum biomass yield, and will likely be important
in maintaining long-term productivity. Over two years,
sorghum yields were insensitive to biomass return, indicating
that even with complete biomass removal, short term
productivity may remain high in systems with higher soil
fertility. However, uptake of N, P, and K declined from the first
to the second year even though average yield increased,
possibly indicating a decrease in soil nutrient availability after
only two years.
Variation in yield and nutrient uptake due to experiment
site illustrated the importance of designing biomass produc-
tion strategies tailored to specific regions. SPAD readings and
height were directly related to yields. Reduced early season
SPAD readings and plant height were indicative of lower yield,
indicating a potential for assessment of in-season N needs.
Research is ongoing to determine the effect of biomass
removal and management variables on longer-term yield and
soil quality.
Acknowledgments
We thank Landon Crotwell, Martha Wight, Jim Florey,
Jeff Morris, Justin Ng, Katie Rothlisberger, Vince Saladino, and
Eric Smith for their assistance. This research was funded by
the Texas AgriLife Research Cropping Systems Program and
USDA/NIFA Agriculture and Food Research Initiative (grant
#2011-67009-30050). Seed was supplied by the Walter Moss
Seed Company.
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