Journal of Applied Botany and Food Quality 91, 88 - 95 (2018), DOI:10.5073/JABFQ.2018.091.012

1Applied Plant Ecology, Biocenter Klein Flottbek, University of Hamburg, Germany
2 Department of Forestry and Forest Resources Management, UDS, Ghana

3 Department of Bioscience, Aarhus University, Denmark

Effects of elevated carbon dioxide and climate change on biomass
and nutritive value of Kyasuwa (Cenchrus pedicellatus Trin.)

Damian Tom-Dery1,2*, Franziska Eller3, Kai Jensen1, Christoph Reisdorff 1

(Submitted: January 22, 2018; Accepted: April 15, 2018)

* Corresponding author

Summary
Atmospheric carbon dioxide enrichment enhances plant growth and
development and may alter the nutritive value of grasses. The objec-
tive of this study was to evaluate growth, biomass partitioning and
nutritive value of Kyasuwa under combinations of atmospheric CO2
concentrations, watering and fertilization treatments. Plants were
grown in two greenhouse chambers; with ambient (aCO2; 400 ppm)
and elevated CO2 (eCO2; 950 ppm), two watering and three fertili-
zation regimes. Elevated CO2 reduced stomatal conductance by 40%,
root to shoot ratio by 8%, leaf to stem ratio (L:S) by 3%, protein
content by 14% and Acid Detergent Lignin (ADL) by 23% with no
significant changes in total biomass and C/N ratio however, slight
increases in leaf area (2%) and Acid Detergent Fiber (ADF) by 4%.
Higher fertilization resulted in increased biomass parameters only
in well-watered plants while; a lower C/N ratio was recorded with
higher fertilization. The L:S ratio was decreased with fertilization
while ADL was increased at higher fertilization in well-watered
plants. Interactive effects were recorded for ADF content and shoot
height. Future eCO2 will be unfavorable to Kyasuwa growth making
them less competitive with a reduced nutritive value in drought prone
and infertile soils.

Keywords: carbon dioxide, climate change, Kyasuwa, nutritive value,
plant growth

Introduction
Cenchrus pedicellatus Trin. (formerly called Pennisetum pedicel-
latum Trin., “Kyasuwa”), belongs to the Poaceae family and is a
C4 grass (SCHMELZER, 1996). It is native to West Africa and was
introduced to India from where it has since spread to South East
Asia and Northern Australia (SCHMELZER, 1996) where it is inva-
sive and regarded as an environmental weed (QUDDUS et al., 2014).
Kyasuwa is tolerant of disturbance with broad climatic amplitude,
produces large quantities of seeds with an efficient dispersal mecha-
nism (SCHMELZER, 1996). It occurs along road edges, disturbed and
abandoned lands and thrives on soils of a wide pH scale in rainfall
regimes ranging between 500-1500 mm with severe drought lasting
4-6 months (FAO, 2010; SCHMELZER, 1996). Kyasuwa provides suc-
culent, palatable and nutritious forage over a long growing season
including the dry periods of October-November and contributes to
meeting Cattle fodder requirements in rural areas (SCHMELZER,
1996; FAO 2010). It is also used as a soil stabilizer, mulch and in soil
erosion control (SCHMELZER, 1996; FAO, 2010).
Climate change is one of the most severe challenges of our time, with
predicted increases in global mean temperature, length and severity
of drought events and atmospheric CO2 concentration, due to human
activities (IPCC, 2014). Many plants respond to elevated atmospheric
carbon dioxide (eCO2) concentrations by increased growth, biomass

and productivity, with C3 plants generally benefitting more than
C4 plants (SANTOS et al., 2014; AINSWORTH and ROGERS, 2007).
Moreover, there are also reports of significant changes in the chemical
composition of plants under eCO2 (MYERS et al. 2014). Nonetheless,
plant responses to eCO2 are not easily predictable because they de-
pend on multiple environmental factors which are not necessarily
additive (ACKERLY et al., 1992). Water and nitrogen (N) availability
are two of the most limiting plant resources and have been repor-
ted to interact with eCO2 (ERBS et al., 2015). Several studies have
shown that the photosynthetic capacity of plants grown at eCO2 will
be acclimated and even down-regulated due to feedback repression
of accumulated carbohydrates (PAUL and DRISCOLL, 1997). This has
especially been observed when the plant’s C/N status is high due to
deficient N supply, leading to a decrease in Rubisco activity and thus,
lower photosynthetic rates (LEAKEY et al., 2012). Biomass enhance-
ment of eCO2 may therefore be dependent on sufficient N supply in
some species (DONG et al., 2016). In contrast, some species may show
enhanced C gain per leaf N due to a suppression of photorespiration
under eCO2 (LEAKEY et al., 2012).
There is a general lack of data on responses of tropical and sub-
tropical plants to future climate changes and more research on plant
responses of C3 than C4 species (LEAKEY, 2009). The purpose of
this work was to determine the effects of future elevated atmospheric
CO2 concentration on biomass development which aids invasive po-
tential and nutritive value of Kyasuwa grass. The objectives of the
study were, (1) to assess the effects of eCO2 on Kyasuwa growth,
biomass allocation and nutritive value, (2) to evaluate how different
water and fertilization regimes affect Kyasuwa growth, biomass al-
location and nutritive value and, (3) to estimate interactive effects of
the three resource factors on these traits of Kyasuwa. We hypothesize
future increases in atmospheric CO2 will increase Kyasuwa growth
and biomass thereby enhancing competitiveness. We also hypothe-
size that nutritive value of Kyasuwa will increase with eCO2 espe-
cially at high fertilization regimes.

Materials and methods
Plant preparation and growth conditions
Seeds for the study were harvested in January 2016 from 150 plants
located in Tolon in the Guinea savanna zone of Ghana (09° 25’N,
00° 58’W). Seeds were transported within 3 weeks of picking to the
greenhouse of Universität Hamburg, Germany (53° 30’N, 10° 12’E).
Germination and pre-treatment growth were carried out as follows
in the greenhouse. After a germination period of 3-6 days in germi-
nation trays, two plants were transplanted into a 3 L plastic pots of
15 cm height, filled with a mixture of 4:1:0.5 (v/v) sand (0.13-
0.36 mm), clay and standard organic substrate (TKS1, Floragard
Vertriebs-GmbH, Oldenburg). The bottom of the pots was secured
with a weed mat to prevent loss of substrate. After pre-treatment
growth for 20 days, the pots were randomly assigned to two growth
chambers (each 28.5 m2) of different CO2 concentrations in a green-

Kyasuwa biomass and nutritive value under elevated CO2 89

house with controlled growth conditions (Day/night air tempera-
ture 25 °C/22 °C; 70%-80% relative air humidity; Length of photo-
period same as Hamburg area). We stimulated the atmospheric CO2
concentration of 950 ppm based on the representative concentration
pathway (RCP) 8.5 scenario by 2100 (IPCC, 2014). In the first growth
chamber, the CO2 concentration was the same as in ambient atmo-
spheric air (aCO2; 400 ppm) while in the other chamber, the CO2 was
elevated to 950 ppm (eCO2).
Three fertilization treatments were applied using commercial li-
quid fertilizer with NPK values of 8-8-6 (WUXAL Super, Aglukon
Spezialdünger GmbH, Düsseldorf, Germany). The treatments corres-
ponded to the equivalent of 75 kg N ha-1 (N-1), 100 kg N ha-1 (N-2)
and 125 kg N ha-1 (N-3) and were applied at planting, 20 days after
planting and 40 d after planting. The WUXAL super fertilizer has
the full complement of macro and micro nutrients. During the ex-
perimental period of 68 days two watering regimes of continuous
watering (wet), and a stimulated drought period of no watering (dry)
for 2 weeks were implemented. The complete design of the study
comprised of two levels of CO2, two watering regimes, three fertili-
zation regimes and six replicates, resulting in a total of 72 pots for the
entire experiment. The pots were equally distributed among the two
growth chambers which resulted in 36 pots in each chamber. To avoid
edge and chamber effects, the positions of individual potted plants
were rotated on the tables on a weekly basis but, it was practically
impossible to switch CO2 concentrations between the two chambers
however, the growth conditions were similar. During the experi-
ments, the CO2 level, temperature and air humidity were monitored
by the Computer Climate model CC 600 (RAM co. Measurement
and Control, Germany) every 12 minutes. The light conditions in the
two chambers were measured for a month with quantum sensors (LI-
190R, LI-COR, Lincoln, NE, USA) connected to data loggers (CR
1000, Campbell Scientific, Logan, UT, USA). The grasses were ex-
posed to the experimental treatments for a total of 68 days during the
growth period (May-July 2016).

Plant growth and physiological measurements
To assess the effects of eCO2, watering and fertilization regimes,
the growth characteristics were recorded before harvesting the grass
80-90 days after sowing. Shoot height was measured from ground
level to the base of the top-most, fully developed leaf or to the base
of the panicle depending upon the stage of the particular plant. The
stomatal conductance was measured on two leaves for three repli-
cates within all treatments using a leaf porometer (SC-1, Decagon
Devices, Pullman, WA, USA) on young but fully developed leaves.
At harvest, the area of the individual grass leaves was determined
using an area meter (LI 3100, LI-COR, Lincoln, NE, USA) while the
fresh weight of leaves and stems were measured separately and the
leaf to stem ratio (L:S) estimated from these measurements. Roots
were thoroughly washed from the soil over a sieve with 1 mm mesh
size. The biomass fractions (leaves, stems and roots) were oven dried
for 48 hours at 65°C. Roots were denominated below-ground bio-
mass (BGB). Above-ground biomass (AGB) was estimated by adding
dry weights of leaves and stems, while total biomass (TB) was the
sum of AGB and BGB. The root to shoot ratio (R:S) was calculated
by dividing BGB by AGB.

Forage nutrient analysis
To evaluate the effects of eCO2, watering and fertilization regimes
on nutritive value of Kyasuwa, oven-dried leaves and stems were
ground separately with a micro hammer mill (Culatti AG, Zürich,
Switzerland) fitted with a 1 mm sieve. Neutral detergent fibre (NDF)
and acid detergent fibre (ADF) were measured sequentially with the
ANKOM filter bag method according to the manufacturer, using a
fibre analyzer (ANKOM-200 Fiber Analyzer, ANKOM Technology,

Macedon, NY, USA). Ground leaf and stem material (500 mg) was
placed in ANKOM F57 filter bags and sealed with heat. All 72 sam-
ples were first extracted with neutral detergent, and the residue was
weighed to determine percentage of NDF. The NDF residue was then
extracted with acid detergent solution, followed by extraction with
72% H2SO4 and ashing to determine the percentage of acid deter-
gent lignin (ADL) (VAN SOEST, 1994; RYAN et al., 1990). The fol-
lowing nutrient parameters were estimated: % NDF, % ADF, and %
ADL.
Total nitrogen and carbon concentration of leaves was measured in
aliquots of oven dried samples by an elemental analyzer following
pyrolysis (EURO-EA 3000, Euro Vector, Italy). Mass calibration
was conducted by the use of the certified standard 2,5-bis (5-tert-
butyl-2-benzoxazol-2-yl) thiophene (6.51% N; 72.52% C; HEKAtech,
Germany). The percentage of proteins in leaves was measured
according to BRADFORD (1976): Dried leaves of all treatments
were finely ground using a Retsch mixer mill (MM-400, Fischer
Scientific, Suwanee, USA) and thereafter digested in 0.1 M NaOH
for 30 minutes (JONES et al., 1989). A volume of 100 μl aliquots of
centrifuged supernatant were assayed with 50 μl Bio-Rad Bradford
dye (Coomasie brilliant blue). Absorbance was measured at 595 nm
after 15 minutes using a multi-mode microplate reader (Synergy HT,
BioTek Instruments, Winooski, USA).

Data analysis
All statistical analyses were carried out using the software package
Statistica 13 (Stat-Soft Inc., Tulsa, OK, USA). Growth, biomass and
nutritive value parameters were analyzed by three-way analysis of
variance (ANOVA) to test for significant main effects of the factors
CO2 concentration, fertilization levels and watering regime as well
as all factor interactions, followed by the Tukey HSD post hoc test of
significant differences. For analysis of nutritive value, data of stems
and leaves were averaged after chemical assays. Prior to the 3-way
ANOVA, all data were analyzed for homoscedasticity using Levene’s
test, and data were transformed appropriately where necessary. In
addition, residual plots and normal probability plots were inspected
to ensure that the assumptions of ANOVA were met.

Results
Environmental conditions during the study
Throughout the experiment, the monitored average daily temperature
in both the ambient and elevated CO2 chambers was 26 °C ± 1 °C.
The relative humidity of the ambient CO2 chamber was 79% ± 7%
while that of the elevated CO2 chamber was similarly 73% ± 8%.
The light conditions of the two chambers were similar. Overall, the
environmental conditions in two chambers were stable and simi-
lar throughout the duration of the study except the ambient CO2
(400 ppm) and the elevated CO2 (950 ppm) in the elevated chamber.

Effects of elevated CO2 on Kyasuwa
Kyasuwa grass grew well in the two CO2 chambers and showed no
signs of nutrient deficiencies or pest attacks. Elevated CO2 signifi-
cantly reduced stomatal conductance (p < 0.001), leaf to stem ratio
(P = 0.04) and root to shoot ratio (P = 0.01) by 40%, 3% and 5%, re-
spectively (Tab. 1, Fig. 1). However, leaf area (P = 0.014) was signifi-
cantly increased by 2% with increased carbon dioxide concentration
(Fig. 1, Tab. 1). Above-ground and total biomass was not significantly
affected by atmospheric CO2 concentration (Tab. 1).
Growth under eCO2 significantly increased structural carbohydrates
(ADF) by 2% (P < 0.001). However, ADL and percentage protein
were reduced by 23% (P < 0.001) and 14% (P < 0.001), respectively
(Fig. 1, Tab. 1). The C/N ratio was not significantly affected by CO2
concentration.

90 D. Tom-Dery, F. Eller, K. Jensen, C. Reisdorff

Water and fertilization regime effects on Kyasuwa growth and
nutritive value
The high-watering level significantly increased by 1% (P < 0.001)
the ADF content (Fig. 1). Increased nutrient availability reduced the
leaf to stem ratio by 6% between N-3 and N-1 (P = 0.002), and by 4%
(P = 0. 04) between N-2 to N-1 (Tab. 1, Fig. 1).
We found significant interactions between watering and fertilization
regimes for AGB, BGB and, TB (P = 0.02; 0.01; and 0.005; respec-
tively) (Fig. 2a). Increasing fertilization caused significant increases
in these three biomass parameters but only with concurrently high
water availability. At low water availability, no biomass responses to
fertilization regime could be detected.
For nutritive components, we observed two way interactions of wa-
tering and fertilization regimes for the C/N ratio (P = 0.005) and
ADL (P = 0.046) (Fig. 2b, c). The C/N ratio was lowered with higher
levels of fertilization, but only in drought-exposed plants. The ADL
concentration was higher at the highest than the two lower fertiliza-
tion levels, but only in well-watered plants.

Interactive effects of elevated CO2 with fertilization and/or wa-
tering regimes
There was a two-way interaction between carbon dioxide enrich-
ment and fertilization regime for NDF (P = 0.008; Tab. 1; Fig. 3a).
Here, eCO2 had no effect on NDF at the low fertilization level, while
it resulted in increased NDF at the higher nutrient availability N-3
(Fig. 3a). A significant three-way interaction (P= 0.02) of CO2 con-
centration, water and fertilization regimes was observed for shoot
height (Fig. 3b). Under aCO2, the higher fertilization regimes (N-2
and N-3) increased shoot height in well-watered plants but decreased
shoot height in drought-treated plants. However, under eCO2, high-
er fertilization regimes (N-2) resulted in increased shoot height at
lower water regimes, but well-watered plants had taller shoots than
drought-treated plants though no fertilization effects were recorded.

Discussion
Growth and biomass
Most plants, both C3 and C4, respond to eCO2 by reducing stoma-
tal conductance (AINSWORTH and ROGERS, 2007; AINSWORTH and
LONG, 2005). In the present study, Kyasuwa responded to eCO2 by
reducing its stomatal conductance (gs), which usually translates
into reduced water loss through lower transpiration rates and higher
water use efficiency at the leaf level (EAMUS et al., 2008). A review
of the effect of eCO2 on both C3 and C4 vegetables pointed to down-
regulating of stomatal conductance and lowering of transpiration, the
combination of which leads to higher water use efficiency (BisBis
et al., 2018).
There are several studies showing that both tropical and temperate
C4 grasses benefit from eCO2 by down-regulating their stomatal
conductance and thus, water loss, rather than increasing their above
ground and or total biomass (XU et al., 2014; KAKANI and REDDY,
2007). Except shoot height, we did not find any interaction of wa-
tering regime and atmospheric CO2 concentration on any other inves-
tigated parameters, so it is unlikely that eCO2 had any effects on the
integrated water-use efficiency of Kyasuwa, although gs was lower
under eCO2, which at first sight might be indicative of an increased
instantaneous water-use efficiency. In the case of shoot height there
was a three-way interaction with differences recorded in the low
water treatments of aCO2 and eCO2 in higher fertilization regimes.
Since consequently the net CO2 flux into the leaves might have been
the same at both high and ambient CO2 concentrations, irrespective
of the water availability, the lower stomatal conductivity in eCO2
was the result of an adjustment of the CO2 influx to an apparently
unchanged CO2 demand reflecting a similar assimilation potential
under both atmospheric CO2 conditions (GHANNOUM et al., 2000).
This suggestion is further supported by similar C/N ratios at aCO2
and eCO2.
The effects of eCO2 on Kyasuwa growth and biomass seemed to be
confined to changes in biomass allocation patterns in the present

Tab. 1: F-values of three-way ANOVA of all measured parameters of Kyasuwa grass grown under elevated and ambient CO2, three fertilization and two
watering regimes. N=6

Sources of Variation

Parameters CO2 (C) Water (W) Fertilization C*W C*N W*N C*W*N
(N)

Growth & Biomass
Height (cm)
Leaf area (cm2)
Stomatal conduc-tance (mmol mol-1)
Leaf to stem ratio
Root to shoot ratio
Above ground biomass (g)
Below ground biomass (g)
Total biomass(g)

Nutritive value
C/N ratio
% Protein
% Neutral detergent Fibre
% Acid detergent Fibre
% Acid detergent lignin

10,63**
6,37**
7,30**
4,62*
7,09**
0,00
9,30**
0,71

0,59
30,38***
23,45***
111,34***
14,38***

47,87***
0,03
0,73
2,49
2,41
63,52***
36,69***
54,04***

53,98***
1,17
1,42
16,61***
2,06

0,77
0,44
0,13
7,04**
0,46
9,67***
9,95***
10,10***

11,60***
0,46
2,36
1,79
0,78

0,58
0,00
2,54
0,10
0,09
1,32
3,11
1,26

1,64
0,44
1,03
0,00
3,87

0,07
0,18
0,23
2,06
0,17
2,59
2,12
2,26

0,39
0,44
5,31**
1,44
1,64

1,42
0,64
0,73
0,06
0,23
7,02***
4,90**
5,68**

5,88**
0,72
2,34
0,82
3,23*

4,15*
0,33
1,03
0,60
0,09
2,56
2,81
2,75

1,22
1,31
0,55
0,23
0,30

Statistically significant values in bold; * <0,05, **<0,01, ***<0,001 probability levels

Kyasuwa biomass and nutritive value under elevated CO2 91

study. The decrease in the root to shoot ratio and the general reduc-
tion in below-ground biomass with eCO2 has previously been ob-
served (KAKANI and REDDY, 2007). The effect of eCO2 on R:S is
mechanistically not well understood, but it varies with plant types
and resource supply (ROGERS et al., 1996). Other studies have both
found an increase (ARNONE et al., 2000) and no change in R:S
(KÖRNER et al., 1997). As a general principle of allocation, under
changing resource availability (light, water, nutrients, CO2), plants
tend to enhance organs that can increase the capture of the resource
that is becoming limiting, partly to the detriment of other organs. In
our study, as in others, increased CO2 supply caused lower stomatal
conductivity leading to reduced water demand. Consistent with the
above-mentioned principle, the allocation to root growth has been
reduced, whilst a significant increase in leaf area as a response to
eCO2 has been observed which is also reported in other studies
(ACKERLY et al., 1992). By lowering investment in below-ground
biomass, the plant’s rooted soil volume is reduced and, thus, not
only less nutrients but also less soil water is available to the plant.
Consequently under eCO2 shorter dry spells could be withstood de-
void of severe distress. However, Kyasuwa will be more susceptible
to extended drought because less soil volume will be used for water
uptake and thus less competitive in the drier savanna.

The observed decrease in leaf to stem ratio (L:S) signifies that eCO2
resulted in higher allocation of biomass to stems, which has been
previously reported in other C4 grasses (SANTOS et al., 2014). On one
hand, if the lower L:S ratio is combined with the increased leaf area,
plants grown at eCO2 developed overall broader leaves and denser
shoots than those grown at aCO2, which is beneficial for future
use of the species, as livestock prefer leaves with broader blades
(BATISTOTI et al., 2012). On the other hand, a high leaf to stem ratio
is generally preferred as an important factor in diet selection, quality
and forage intake (SMART et al., 2004). However, L:S was signifi-
cantly reduced with higher fertilization regimes because of higher
accumulation of stems as reported by other studies (SALVADOR et al.,
2016).
Water availability is one of the main biophysical limitations of grass
growth in savannas (DEL GROSSO et al., 2008). Water in general is
one of the most important limiting resources to plant growth, and in
the present study, low soil water content decreased all the biomass
parameters as well as grass shoot height. Water deficiency decreases
transpiration rate via decreased stomatal conductance, which results
in a decline of net photosynthetic rates (FLEXAS and MEDRANO,
2002). Although C4 species have inherently lower stomatal conduc-
tance than C3 and thus have higher water use efficiency, they can still

%
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N-2
N-3

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Fig. 1: Main effects of CO2 concentrations, watering and fertilization regimes on growth, biomass parameters and nutritive values of Kyasuwa. Different
lowercase letters represent significant differences among treatments. LSM ± SE. eCO2, elevated CO2 concentration; aCO2, ambient CO2 concentra-
tion; LW, Low water treatments; HW, High water treatment; N-3, High fertilization regime; N-2, medium fertilization regime; N-1, low fertilization
regime.

92 D. Tom-Dery, F. Eller, K. Jensen, C. Reisdorff

be threatened by drought. In the present study, it is more likely that
the effects of water availability on growth were displayed through
indirect effects for example lower mobility of nutrients in dry soil
rather than a direct effect via stomatal conductance, which did not
respond to watering regime. The water by fertilization regime inter-
actions for Kyasuwa showed that water supply was the main limit-
ing factor for biomass accumulation and height growth. Water avail-
ability is pivotal for increasing nutrient availability for Kyasuwa
growth and biomass production, and elevated CO2 did not help to im-
prove the water use efficiency of this species. However, some plants
do not reduce water loss through stomata closure but, react to water
shortage by investing assimilates into protective substances (i.e. syn-
thesis of LEA-proteins and other osmoprotectants, compatible so-
lutes etc) and consequently they have a lower biomass under drought
treatment (SCHULZE et al., 2005).

Kyasuwa tissue nutritive value
The significant reduction of the protein content under eCO2 has been
attributed more to a dilution-effect due to an overall increase in
total non-structural carbohydrates, rather than an absolute decrease
of the protein content in leaves (DUMONT et al., 2015). This is un-
likely to be the case in the present study, since the C/N ratio was
unaffected by CO2 treatment. Other studies implicate the decrease in
Rubisco concentration (AINSWORTH and LONG, 2005) due probably
to carbohydrate-dependent decrease in expression of photosynthetic
genes (MOORE et al., 1999) and decreased transpiration and stomatal
conductance (DEL POZO et al., 2007). A meta-analysis on proteins
in food crops also indicates a reduction of proteins at elevated CO2
(TAUB et al., 2008). We suggest that a re-allocation of N to resources
other than proteins occurred within Kyasuwa, for example to struc-

tural N-compounds or free amino acids or even to vacuolar nitrates,
leaving a similar C/N ratio in the aCO2 and eCO2 treatment. The
lack of effect of eCO2 on the C/N ratio could be a result of C4 plants
photosynthesis and biomass accumulation being less affected by
eCO2 (WANG et al., 2012). C4 plants are less affected by eCO2 be-
cause of their C-accumulation strategy, which minimizes photores-
piration through anatomical and biochemical specializations that
concentrate CO2 at the active site of Rubisco (SAGE, 2004) and are
virtually CO2 saturated already at aCO2.
Low soil water potential generally impairs plant metabolism and
there are suggestions that water availability plays a key role in nutri-
ent limitations to grasses of semi-arid areas (LÜ et al., 2012). This
is because of the vital role water plays in nutrient transport and
availability for nutrient uptake in plant roots. The interactive effect
of watering and fertilization regimes on C/N ratios showed higher
fertilization regimes reducing C/N ratios when water availability
was limiting. Several studies support the finding of an interaction
between water availability and nitrogen nutrition for C/N ratio (LÜ
et al., 2012). In nature, soil water availability influences soil N avail-
ability via many microbial aided pathways like litter decomposition
(LIU et al., 2006) and N mineralization (WANG et al., 2006). In the
present study, the high water availability probably had a direct effect
on nutrient availability to plant roots by increasing their diffusion
and mass flow in the soil. The lower C/N ratio may therefore be an
expression for a relative decrease in C-assimilation rates under dry
conditions when nutrient supply was high.
NDF concentration was increased at high fertilization regimes only
under eCO2, which is in concert with previous observations of soil
N supply limiting the ability of plants to respond to eCO2 (DONG
et al., 2016). Plants with low N-demand, however, may respond to
eCO2 even in N-poor soils (NORBY et al., 1992). The present study

Fig. 2: Two-way interactions between watering and fertilization regimes on biomass of Kyasuwa (a). Interactive effects of fertilization and watering regimes
on nutritive value parameters of Kyasuwa (b, c). Different lowercase letters represent significant differences among treatments. LSM ± SE.

C
/N

r
at

100
LW
HW

N-1 N-2 N-3

%
A

ci
d

de
te

rg
en

t f
ib

B
io

m
as

s
(g

d
ry

m
as

s)
5

25
AGB
BGB
AGB
BGB

a
b

a
a

a a

a a a
b

a
b

(a)

(b)

(c)

Kyasuwa biomass and nutritive value under elevated CO2 93

showed that the main effects of eCO2 on NDF were significant but
rather small, as reported by other studies (FRITSCHII et al., 1999;
AKIN et al. 1995), and therefore future atmospheric CO2 concentra-
tion can be expected to increase fiber content of Kyasuwa to a mini-
mal extent, and only if a high fertilization load is provided.
ADF concentration was significantly reduced by drought, which
may have been caused by delayed plant maturity due to stressful
arid growth conditions (KÜCHENMEISTER et al., 2013). Plant fibre
contents turn to increase with age since the stem to leaf ratio in-
creases with age, and fiber content of stems is considerably higher
than leaves (BRUINENBERG et al., 2002). The increases in structural
carbohydrates with eCO2 recorded in this study have previously
been observed and seem to be species-dependent (DUMONT et al.,
2015; MILCHUNAS et al., 2005; FRITSCHI et al., 1999; AKIN et al.,
1995). In wheat for instance, AKIN et al. (1995) recorded increased
contents in fibre fractions with eCO2, while FRITSCHI et al. (1999)
reported increases in structural carbohydrates of Arachis glabrata
leaves. MILCHUNAS et al. (2005) likewise recorded increases in com-
bined cellulose and hemicellulose in Bouteloua gracilis with eCO2.
However, a recent meta-analysis of forage quality of Mediterranean
grasslands pinpointed no change in structural carbohydrates with
eCO2 (DUMONT et al., 2015).
ADL concentration was increased at the highest fertilization level,
but only in well-watered plants, which could be the result of low-
er L:S ratio with higher fertilization regimes. Moreover, ADL was
lowered by eCO2, which means that the tissue quality as forage can
be expected to be higher under future eCO2 concentrations. The
reducing effect of eCO2 on lignin components of forages has been
reported in previous studies (MILCHUNAS et al., 2005; AKIN et al.,
1995) which may be caused by lignin being connected chemically
to proteins and carbohydrates in the cell wall to form large macro-

molecules (MOORE and JUNG, 2001). A higher NDF concentration
reduces animal intake while a higher ADF concentration decreases
digestibility (SAHA et al., 2013). However, decreases in ADL increa-
ses digestibility because lignin limits digestion (MOORE and JUNG,
2001). We therefore propose that the overall quality of Kyasuwa as
forage will be lower under eCO2. Vegetables are reported to increase
in sugars, vitamin C, phenols, flavonoids and antioxidant capacity as
a result of eCO2, however the macro and micronutrients are reduced
(BisBis et al., 2018).

Conclusion
Elevated CO2 increased individual leaf area of Kyasuwa which would
make the grass attractive as forage. However, eCO2 resulted in a
change in biomass allocation towards a lower R:S ratio, that ultimate-
ly may be harmful for the species especially under dry conditions
and low nutrient availability making it less competitive. Moreover,
eCO2 will result in changes in the chemical composition of Kyasuwa
with increases in structural carbohydrates (NDF and ADF) and re-
duction in ADL and protein which will reduce the nutritive value of
Kyasuwa overall. Water and fertilization were the two most limiting
resources for Kyasuwa compared to CO2 and did not interact with
CO2 except in shoot height and NDF. As a compromise for future tis-
sue quality, we suggest to avoid over-fertilization of Kyasuwa to avoid
an undesirable increase in fibre content.

Acknowledgement
The authors are grateful to Prof. Dr. Jörg Fromm and the Institute of
Wood Science/vTI Hamburg for allowing us to use their greenhouse.
We also wish to thank Prof. Dr. Jörg Ganzhorn of Animal Ecology

Fig. 3: Two-way interaction of CO2 concentrations and fertilization regimes on neutral detergent fibre of Kyasuwa (a) and three-way interaction between CO2
concentrations, watering and fertilization regimes on Kyasuwa shoot height (b). Different lowercase letters represent significant differences among
treatments. LSM ± SE.

LW HW LW HW

S
ho

ot
h

ei
gh

t (
cm

)

100

140
N-1
N-2
N-3

%
N

eu
tr

al
d

et
er

ge
nt

fi
br
e

70
N-1
N-2
N-3

a b b
a ca

b b

c c

a
abb

c c c

aCO2 eCO2

aCO2 eCO2 (a)

(b)

94 D. Tom-Dery, F. Eller, K. Jensen, C. Reisdorff

and Conservation, University of Hamburg for allowing us to use their
Fiber Analyzer for nutrient analysis.

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Address of the corresponding author:
Damian Tom-Dery, Applied Plant Ecology, Biocenter Klein Flottbek,
University of Hamburg, Ohnhorststr. 18, 22609 Hamburg, Germany
E-mail: tom_dery@yahoo.co.uk, damian.tom-dery@uni-hamburg.de

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