Journal of Environmental Protection, 2010, 1, 73-84
doi:10.4236/jep.2010.12010 Published Online June 2010 (http://www.SciRP.org/journal/jep)
Growing Cover Crops to Improve Biomass
Accumulation and Carbon Sequestration: A
Phytotron Study
Qingren Wang1,2*, Yuncong Li1,2, Ashok Alva3
1Tropical Research and Education Center, Homestead, USA; 2Department of Soil and Water Science, University of Florida, Gainesville,
USA; 3USDA, ARS, Vegetable and Forage Crops Research, Prosser, USA.
Received March 10th, 2010; revised April 13th, 2010; accepted April 14th, 2010.
ABSTRACT
Cover crop system has shown a potential approach to improving carbon sequestration and environmental quality. Six of
each winter and summer cover crops were subsequently grown in two soils, Krome gravelly loam soil (KGL), and
Quincy fine sandy soil (QFS), in phytotrons at 3 temperatures (10/20, 15/25, 25/30oC for winter/summer cover crops) to
investigate their contributions for carbon (C) sequestration. Among winter cover crops, the highest and the lowest
amounts of C accumulated were by bellbean (Vicia faba L.), 597 g/m2 and white clover (Trifolium repens), 149 g/m2,
respectively, in the QFS soil. Among summer cover crops, sunn hemp (Crotalaria juncea L.) accumulated the largest
quantity of C (481 g/m2), while that by castorbean (Ricinus communis) was 102 g/m2 at 30oC in the KGL soil. The mean
net C remained in the residues following the 127 d decomposition were 187 g/m2 of C (73% of the total) and 91 g/m2 (52%
of the total) for the winter and summer cover crops, respectively. Following a whole cycle of winter and summer cover
crops grown, the mean soil organic C (SOC) increased by 13.8 and 39.1% in the KGL and QFS soil, respectively,
compared to the respective soils before. The results suggest that triticale, ryegrass, and bellbean are the promising
winter cover crops in the QFS soil, while sunn hemp, velvetbean (Mucuna pruriens), and sorghum sudangrass (Sor-
ghum bicolor × S. bicolor) are recommended summer cover crops for both soils under favorable temperatures.
Keywords: Carbon to Nitrogen Ratio (C:N), Greenhouse Gas (GHG), Krome Gravelly Loam (KGL), Quincy Fine Sand
(QFS), Soil Organic Carbon (SOC)
1. Introduction
Soil carbon (C) sequestration by terrestrial vegetation, as
one of the main approaches for greenhouse gas (GHG)
mitigation, has long been identified by the Intergovern-
mental Panel on Climate Change [1]. Terrestrial ecosys-
tems associated with land use and soil management play
an important role in the global C budget [2]. For example,
the current global terrestrial sink for C is estimated to
hold 550-700 Pg of C in vegetation and 1200-1600 Pg in
soil organic matter [3]. Soil is the largest terrestrial C
pool, constitutes at 2500 Pg of total C (organic and inor-
ganic) within one meter depth [4]. This soil C pool is
approximately two-thirds of the total C in ecosystems [5].
The former is about 3.3-fold greater than the atmospheric
C pool (760 Pg), and 4.5-fold greater than the biotic C
pool (560 Pg) [6]. In addition, soil organic carbon (SOC)
pools have the slowest turnover rates in general terres-
trial ecosystems [7], therefore C sequestrated in soils has
a great potential to mitigate CO2 emission to the atmos-
phere [3].
Increased fixation of atmospheric CO2 with terrestrial
vegetation, and in turn, contributing to enhanced SOC
leads to a reduction in GHS emissions and related nega-
tive effects on the environment. However, the efficiency
of C sequestration by various vegetations differs largely
as influenced by differences in their physiological char-
acteristics, growth rates, biomass accumulation rates, etc.,
and by many environmental factors, such as the soil type,
temperature, etc.
Maximizing biomass production by optimizing input
use is a major goal in agro-ecosystems. Conversion of
plant sequestered C to SOC is important since the latter
is very stable with long residence time, i.e., hundreds and
even thousands of years [8]. Agricultural soils under ap-

Growing Cover Crops to Improve Biomass Accumulation and Carbon Sequestration: A Phytotron Study
74
propriate management can contain substantial amounts of
soil C in the forms of soil organic matter (SOM). Ex-
cluding carbonated rocks, soils constitute the largest sur-
face C pool, approximately 1500 Gt, which is equivalent
to almost three fold greater than the quantity stored in the
terrestrial biomass and twice the amount stored in the
atmosphere [9].
Cover crops provide an effective practice to enhance
SOC [10-13] in addition to their role in improving soil
and water conservation [14]. Cover crops can also en-
hance soil fertility and productivity for a sustainable ag-
ricultural production [15,16]. A linear relationship has
been reported between the amount of C sequestered in
the soil and C input as plant biomass or residues by a
number of researchers [17-19]. Removal of cover crop
tops from the soil greatly decreased soil C [20]. The
growth rates and amounts of cover crop biomass as well
as sequestered CO2 C differed among different cover
crop species and environmental factors [3].
In a wide range of agroclimatic regions, winter cover
crops are generally grown during fall through spring.
Summer cover crops facilitate conserving soil and wa-
ter during the rainy summer season, improving soil
fertility, leading to increased yields and quality of the
subsequent cash crops [14-16,21]. In Canadian prairies,
growing summer cover crops instead of fallow seques-
tered approximately 1.5 Tg CO2 per year from the at-
mosphere [22].
To improve C sequestration efficiency, plant seques-
tered C in organic forms need to be transferred to stable
forms, such as recalcitrant SOC via humification or car-
bonization processes. The stability of organic C in plant
residues or in soil depends on the cover crop species used
in the production system in addition to effects of envi-
ronmental factors, including soil type, temperature, and
moisture. Organic C in plants comprises active and inac-
tive components, which also refer to as labile and recalci-
trant pools [23]. The active organic C consists of four
fractions; i.e., decomposable organic C, resistant organic
C, microbial biomass organic C and humified organic C
[24]. The relative distribution of organic C in the above
fractions depends on the physiological and chemical
characteristics of plant residues, i.e., C:N ratio and lignin
content. Most of the previous studies on the role of cover
crops on C sequestration or SOC accumulation are ex-
clusively related to agricultural practices: such as tillage,
cropping systems, crop rotation, land use or shifting cul-
tivation, fertilization, etc. [9,12,13,19,20,25,26]. Infor-
mation is lacking on the influence of different cover crop
species on the efficacy of soil C sequestration. In addi-
tion, it is hard if it is not impossible to evaluate both
winter and summer cover crops under filed conditions.
Therefore, the objective of the current research was, un-
der the controlled environment, to elucidate effects of
soil and temperature conditions on the quantities of C
accumulation, biomass production, mineralization rates
and efficiency of C sequestration among different winter
and summer cover crops typically grown in the temperate
and subtropical regions.
2. Materials and Methods
2.1 Soils and Cover Crops
A Krome gravelly loam (KGL) soil (loamy-skeletal,
carbonatic, hyperthermic Lithic Udorthents) from Miami
Dade County, FL, and a Quincy fine sandy soil (mixed,
mesic Xeric Torripsamments) from Benton County, WA,
were used in this study. Some characteristics of the
above soils are shown in Table 1. The above soils sam-
pled at 0-15 cm depth were sieved to remove large rocks
and plant residues. Plastic pots, 25 cm diameter, 23 cm
high, 11.3 liters in volume with a capacity of 8 kg soil
per pot, were used. Six each winter and summer cover
crops, including three each legumes and nonlegumes
were evaluated.
The winter cover crops used in the experiment were in
the order of: triticale (Triticale hexaploide Lart.), rye-
grass (Lolium perenne ssp. Multiflorum), mustard (Bras-
sica juncea, ssp. Indian gold), bell beans (Vicia faba L.),
purple vetch (Vicia benghalensis L.), and white clover
(Trifolium repens). The summer cover crops included in
the order of: sorghum sudangrass [Sorghum bicolor × S.
bicolor var. sudanense (Piper) Stapf.], okra (Abelmo-
schus esculentus L.), castor bean (Ricinus communis),
sunn hemp (Crotalaria juncea L. cv. Tropic Sun), vel-
vetbean [Mucuna pruriens var. utilis (Wall. ex Wright)
Baker ex Burck], and cowpea (Vigna unguiculata L., cv.
Iron Clay).
Table 1. Selected characteristics of the two soils used in this study
KCl extractable
pH†
(water)
EC‡
(µS/cm)
Organic C
(g/kg)
CaCO3 equivalent
(g/kg)
Total N
(g/kg)
Total P
(g/kg) NH4-N
(mg/kg)
NO3-N
(mg/kg)
KGL soil± 7.8 148.0 14.2 571.7 0.69 0.26 26.3 8.1
QFS soil 7.6 78.8 2.1 ND≠ 0.01 0.12 31.7 6.6
†Soil pH was measured in a ratio of 1:2.5 (soil: water).
‡Soil electrical conductivity (EC) was measured in a ratio of 1:2 (soil: water).
±KGL = Krome gravelly loam, and QFS = Quincy fine sand.
≠
ND: not detected.

Growing Cover Crops to Improve Biomass Accumulation and Carbon Sequestration: A Phytotron Study 75
2.2 Experimental Design, Phytotron Setup and
Management
A nested factorial design was adopted with temperature
as a main factor, soil types and 6 cover crop species (for
either winter or summer cover crops) were nested as sub-
factors. The experiment was conducted in three individ-
ual phytotrons (Conviron 8601, Conviron Products
Company, Winnipeg, Canada) to simulate different tem-
peratures but kept other parameters, e.g., light intensity,
relative humidity and day length, etc. the same to repro-
duce the growth conditions for the winter and summer
cover crops with a respective fallow. For winter cover
crops (November 20th, 2007 through March 7th, 2008),
temperatures were: 10/8, 15/10, and 20/15oC (day/night,
d/n), following the seed germination at 20oC across all 3
treatments above. Light intensity was gradually increased
or decreased 40% per hour with ramp procedure with 10 h
day length. The light intensity was 0.294 × 103 µmol s-1 m-2,
calibrated by a light intensity sensor (LI-COR, Quantum
with LI-1000 datalogger), and relative humidity of
50/75% (d/n).
The temperatures for summer cover crops (April 9th
through June 25th, 2008) were 20/15, 25/20, and 30/25oC
(d/n), the day length of 14 h. Light intensity was same as
that used for the winter cover crops, with relative humid-
ity of 75/85% (d/n). Plant density was 10 per pot for okra,
bellbean, purple vetch, sorghum sudangrass and sunn
hemp; 30 for white clover, ryegrass and triticale; while 3
-5 per pot for castor bean, velvetbean, mustard and cow-
pea. Three gram of fertilizer (10 N-4.3 P-8.3 K) was ap-
plied per pot. No inoculation was applied to legumes. All
plants were irrigated through drip line to adjust a flow
rate of 2 L/h, the frequency and duration were deter-
mined based on plant growth stages for quantities of wa-
ter required.
With a separate experiment, cover crop decomposition
rates were preliminarily evaluated in an extra phytotron
with the single (KGL) soil individually for both winter
and summer cover crops. In this experiment, the same
cover crop species as in the previous study were grown
and harvested at the same temperature in the same (KGL)
soil, the aboveground biomass was cut into approxi-
mately 1-cm pieces, the fresh weight and moisture con-
tent were determined with subsamples and those sub-
samples were also used for chemical analysis. The cer-
tain amount of fresh cover crop residues was evenly dis-
tributed on the soil surface of the same pot used for re-
spective cover crop growth. Residue surface application
instead of soil incorporation was to extend the residue
residence time in a similar field approach of no tillage
practice. Water content of the soil was maintained at
75% of the field capacity for the respective soil by
weighing the pots once a week. Temperatures maintained
at 15oC for winter and 25oC for summer cover crops, and
the same lighting conditions that described above for the
winter and summer cover crop growth studies were
adapted. At the end of 127 d mineralization study, the
remaining plant residues were carefully removed from
each pot, dried and weighed, and subsamples were taken
again for chemical analysis. The mass weight loss of
plant residues due to decomposition was calculated based
on the difference between the amount of residues applied
and the amount remained after decomposition. The total
amount of C decomposed was calculated based on the
residue weight loss and the residue C concentrations de-
termined in the subsamples. The concentrations of SOC
and total N were determined before and after the de-
composition experiment.
2.3 Sampling and Chemical Analysis
Soil samples were collected from the center of each pot
at 0-10 cm depth prior to and one month after cover crop
growth. Soil samples were air dried and ground to pass
through a < 1 mm mesh sieve for chemical analysis. The
experiment was terminated at the time when any one of
these cover crops was flowering, which was about 90
days for both winter and summer cover crops under the
experimental conditions. The aboveground plant biomass
was harvested, and fresh and dry weights (at 75oC for 7 d)
of biomass were recorded. A subsample of the above-
ground biomass was ground to pass through a < 0.5 mm
mesh sieve for chemical analysis.
Total C and nitrogen (N) contents in the soil and cover
crops were analyzed using CNS Auto-analyzer (Vario
Max Elementar, Hanau, Germany). Soil inorganic C was
determined via pressure calcimeter method and the or-
ganic C was calculated by subtracting the inorganic C
from the total C [27].
2.4 Statistical Analysis of the Data
The data were subjected to analysis of variance (ANOVA)
using SAS [28] with nested design and a general linear
model. For the ANOVA with the nested design in the
experiment, three different error structures were con-
ducted to compare mean squares of different sources of
variation. The main factor was tested against the main
factor error, where the sub-treatment was tested against
the interaction of replicates (or block) × sub-treatment
and the interaction between the main factor and the
sub-factor were tested against the sub-factor error [29].
Further analysis was conducted for each individual factor
to separate means with the single factor or combination
of the factors for the interaction effects using Duncan test
at p = 0.05, as needed.
3. Results and Discussion
3.1 Cover Crop Biomass, and C and N Contents
Total C and N as well as biomass production of both
winter and summer cover crops were significantly influ-

Growing Cover Crops to Improve Biomass Accumulation and Carbon Sequestration: A Phytotron Study
76
enced by cover crop species and growth temperatures
(Table 2). The soil type had no significant influence on
the above parameters for the summer cover crops, while
in the case of winter cover crops, it significantly influ-
enced the biomass production and the total N. Also, sig-
nificant interaction effects were found for temp × crop
for both winter and summer cover crops on total C and N,
and soil × crop for winter cover crops for all three
evaluation parameters.
Result implies that soil types have strong influence on
the biomass production and total quantity of N accumu-
lation for the winter cover crops rather than the summer
cover crops. The growth temperatures rather than soil
types influence the biomass production and the C:N ratio
for the summer cover crops rather than the winter cover
crops and a significant interaction effect (temp × crop)
occurred.
3.2 Total C in Aboveground Biomass
Winter cover crops: Total C in the aboveground bio-
mass varied among the winter cover crops (Table 3). In
the QFS soil, total biomass C (mean across all tempera-
tures) was greater in bellbean as compared to that in the
remaining species, except triticale. In the KGL soil, total
biomass C decreased in the order: triticale > ryegrass >
bellbean = mustard = purple vetch = white clover. Over-
all biomass total C was significantly greater in the QFS
soil as compared to that in the KGL soil for all cover
Table 2. Analysis of variance (ANOVA) for biomass production and total biomass C and N for winter and summer cover crops
F-value
df Carbon Nitrogen Biomass C:N ratio
Winter cover crop
Temperature (Temp) 2 14.08* 8.83* 38.91** 38.91**
Soil 1 3.24NS 217.16** 50.94* 50.94*
Cover crop (Crop) 5 41.10*** 49.76*** 23.19*** 23.19***
Temp × Soil 2 2.90NS 3.31NS 4.72NS 4.72NS
Temp × Crop 10 16.95*** 5.34*** 2.07NS 2.07NS
Soil × Crop 5 6.26** 113.77*** 14.42** 14.42***
Temp × Soil × Crop 10 1.35NS 6.11*** 4.46** 4.46**
Summer cover crop
Temperature (Temp) 2 9.31* 10.90* 10.45* 0.89NS
Soil 1 2.06NS 0.72NS 2.10NS 521.69**
Cover crop (Crop) 5 17.45*** 17.96*** 16.89*** 90.94***
Temp × Soil 2 2.49NS 0.90NS 2.60NS 1.03NS
Temp × Crop 10 5.28*** 3.40** 5.27*** 4.60**
Soil × Crop 5 0.75NS 2.21NS 0.79NS 6.75**
Temp × Soil × Crop 10 2.12NS 2.89* 2.02NS 5.41**
*Significant at p ≤ 0.05; ** significant at p ≤ 0.01; *** significant at p ≤ 0.001; and NS: no significant difference at p ≤ 0.05.
Table 3. Total C (g/m2) in aboveground biomass of winter cover crops
Soil† Temp Triticale Ryegrass Bellbean Mustard Purple vetch White clover
QFS soil 25oC 369b* 342b 597a 247bc 378b 149c
15oC 411a 393a 418a 390a 282ab 139b
10oC 384a 297b 291b 261bc 194c 80d
KGL soil 25oC 266a 228ab 161b 160b 144b 232ab
15oC 317a 260ab 233bc 179cd 132d 139d
10oC 303a 267a 169b 168b 183b 80c
Means
QFS soil 388Aab 344Abc 435Aa 299Ac 285Ac 122Ad
KGL soil 295Ba 252Bb 188Bc 169Bc 153Bc 150Ac
25oC 317Aab 285Aab 379Aa 204Ab 261Aab 191Ab
15oC 364Aa 326Aa 326ABa 284Aab 207Abc 139Bc
10oC 344Aa 282Ab 230Bc 214Ac 189Ac 80Cd
†KGL= Krome gravelly loam; and QFS = Quincy fine sand.
*Values followed by different letter (s), lower case within the same row, and upper case within the same column of a subset (either the soil or tem-
perature), represent significant difference at p ≤ 0.05.

Growing Cover Crops to Improve Biomass Accumulation and Carbon Sequestration: A Phytotron Study77
crops except white clover. Temperature effect was sig-
nificant only in bell bean and white clover species. The
greatest amount of biomass C (597 g/m2) was obtained in
bellbean grown in the QFS soil at 25oC. At 10oC in the
QFS soil, biomass C was greater by triticale than that by
the other cover crops. In the KGL soil at 25oC, the bio-
mass C was greater in triticale than that in either purple
vetch, mustard or bellbean. At 15 and 10oC, the biomass
C in triticale and ryegrass were greater than that in the
remaining cover crops.
Summer cover crops: In the QFS soil at 30oC, bio-
mass C in sunn hemp was significantly greater that by
castorbean, okra, and cowpea (Table 4). At 25oC, sunn
hemp biomass C was significantly greater than that by
the remaining five cover crops. At 20oC, cowpea biomass
C was significantly lower than that by the other cover
crops. Similarly, in the KGL soil, the relative ranking of
biomass C response among the six cover crop species
was different at different temperatures. Soil type influ-
ence (mean across all temperatures) was significant on
biomass C only in sorghum sudangrass. Biomass total C
was significantly greater at 30oC as compared to that at
either 20 or 25oC for sunn hemp, velvetbean, and sor-
ghum sudangrass. In general, biomass C accumulation
was greater in the winter than in the summer cover crops
under the experiment conditions (Tables 3 and 4).
3.3 Aboveground Biomass and Total N in
Various Cover Crops
The aboveground biomass and total N response followed
somewhat similar pattern as total C accumulation among
various cover crops (Table 5). The mean biomass pro-
duction and total N of winter cover crops were generally
greater in the QFS than those in the KGL soil. No sig-
nificant difference was observed between these two soils
for summer cover crop biomass and/or total N but these
summer cover crops accumulated more biomass and N at
high temperature than at low temperature. This result has
confirmed that the soil type had greater influence on the
growth and accumulation by the winter cover crops.
Temperature appeared to have a dominant influence on
the above response variables of the summer cover crops.
Among the summer cover crops, sunn hemp produced
the greatest amount of biomass (Table 5), equivalent to
11 Mg ha-1 at 30oC. This agrees with results of our paral-
lel field studies [21,30], which showed 15-20 Mg ha-1 of
aboveground biomass by sunn hemp.
Nitrogen under some growth conditions is a dominant
factor to limit the biomass production and C accumula-
tion. Therefore, legume cover crops, by virtue of their
ability to fix atmospheric N, can overcome this limitation,
thus are able to produce greater amount of biomass as
compared to that by nonlegume cover crops [12,30].
Some winter cover crops, such as bellbean and purple
vetch, accumulated 30-35 g/m2 of N under the optimal
conditions (25oC) in the QFS soil (Table 5). All winter
cover crops, except white clover, accumulated greater
amount of N while grown on the QFS soil than that on
the KGL soil. A similar trend was also observed with
respect to accumulation of organic C (Table 3).
Table 4. Total C (g/m2) in aboveground biomass of summer cover crops
Soil† Temp Sunn hemp Velvetbean Castorbean Sorghum sudangrass Cowpea Okra
QFS soil 30oC 480a* 264ab 164b 347ab 146b 157b
25oC 365a 170b 102b 137b 75b 133b
20oC 178a 114ab 137ab 183a 44b 117ab
KGL soil 30oC 481a 376a 102b 164b 121b 126b
25oC 157a 147a 43b 113ab 81ab 90ab
20oC 183a 160a 45b 130a 33b 198a
Means
QFS soil 338Aa 183Abc 134Abc 222Ab 89Ac 136Abc
KGL soil 274Aa 228Aa 63Ab 136Bb 78Ab 138Ab
30oC 481Aa 320Ab 133Ac 255Abc 134Ac 142Ac
25oC 257Ba 159Bb 72Ac 125Bbc 78ABbc 112Abc
20oC 181Ba 137Bab 91Abc 157Bab 39Bc 157Aab
†KGL= Krome gravelly loam; and QFS = Quincy fine sand.
*Values followed by different letter (s), lower case within the same row, and upper case within the same column of a subset (either the soil or tem-
perature), represent significant difference at p ≤ 0.05.

Growing Cover Crops to Improve Biomass Accumulation and Carbon Sequestration: A Phytotron Study
78
Table 5. Total aboveground biomass and N (g/m2) in winter and summer cover crops
25oC/30oC† 15oC/25oC 10oC/20oC
QFS≠ KGL QFS KGL QFS KGL
Winter cover crops
Biomass TC± 917b‡ 640a 982a 736a 907a 709a
(g/m2) RG 880b 571ab 980a 577ab 7287b 640b
BB 1392a 402bc 974a 546b 705b 414b
MT 607bc 424abc 970a 442bc 630bc 422b
PV 903b 326cd 691ab 326cd 495cd 428b
WC 485c 587ab 387b 359c 347d 220c
N (g/m2) TC 9b 4
b 7 c 5 b 9 a 9 a
RG 15b 5
b 8 bc 5 b 9 a 6 a
BB 35a 6
b 24a 6 ab 10a 8 a
MT 7b 5
b 10bc 5 b 8 a 6 a
PV 30a 5
b 18ab 3 b 12a 8 a
WC 10b 17a 10bc 9
a 6 a 6 a
Summer cover crops
Biomass SH 1131a 1111a 834a 368a 427a 432ab
(g/m2) VB 615ab 885a 397b 335a 274ab 369ab
CB 391b 249b 247b 106b 334ab 110c
SS 802ab 395b 331b 267ab 431a 307b
CP 353b 302b 180b 191ab 108b 77c
OK 401b 331b 338b 227ab 286ab 502a
N (g/m2) SH 17a 21a 12a 8
ab 5 a 10a
VB 11ab 21a 9
ab 12a 4 ab 8 a
CB 4b 2
b 2 c 1 b 3 ab 2 b
SS 3b 4
b 3 bc 2 b 3 ab 4 b
CP 6b 5
b 4 bc 5 b 1 b 2 b
OK 5b 3
b 3 bc 2 b 2 b 4 b
†The first temperature was for winter and the second for summer cover crops.
±TC: triticale, RG: ryegrass, BB: bellbean, MT: mustard, PV: purple vetch, WC: white clover, SH: sunn hemp, VB: velvetbean, CB: castorbean, SS:
sorghum sudangrass, CP: cowpea, and OK: okra.
‡Values followed by different letters within the same column of a subset (by winter or summer cover s and by each response parameter) represent
significant difference at P ≤ 0.05.
3.4 Concentrations of C and N in Cover Crop
Biomass
Among the winter cover crops, C concentration was sig-
nificantly lower in white clover (36%) as compared to
that in the remaining five winter cover crops (40-43%)
(Figure 1). Biomass N concentrations were in the range
of 1.8-2.5% for legumes, which was significantly greater
than the range of 0.9-1.1% for the nonlegumes. The C:N
ratio varied from 13.9 (white clover) to 52.4 (triticale)
among these six winter cover crops. The mean C concen-
tration as well as C:N ratio across all winter cover crops
were greater for the KGL than those for the QFS soil,
while the converse was observed for N concentrations.
The temperature effect was not significant on concentra-
tions of C and N across all winter cover crops and soil
types but the C:N ratio was significantly lower at 10oC
than that at other temperatures (Figure 1).
The C concentrations in the summer cover crop bio-
mass were significantly greater for sunn hemp, vel-
vetbean, sorghum sudangrass and cowpea followed by
castorbean and okra (Figure 2). The N concentration was
the greatest for velvetbean, while the lowest for sorghum
sudangrass. As a result, the C:N ratio followed the pat-
tern: sorghum sudangrass > castorbean = okra > sunn
hemp = velvetbean = cowpea. Overall, there was no sig-
nificant difference found between soil types for the C
concentration but plants grown in KGL soil had a greater
N concentration than those in the QFS soil, which re-
sulted in a greater C:N ratio of plant biomass in the latter
than that in the former soil. The temperature effect was
non-significant on the biomass C and N contents as well
as C:N ratio. This result agrees well with the previous
studies. For instance, the C concentrations in various
winter cover crops remained constant but the N concen-
trations differed greatly between legume and nolegume
cover corps, which resulted in a large variation in C:N
ratio in different cover crop biomass [11,32].
3.5 Residue Decomposition and C Sequestered
The rate of decomposition of crop residues over 127-d
under similar conditions as those adapted during the re-
spective cover crop growth period varied among both
winter and summer cover crops. The total C remained in
the soil following 127 d of decomposition varied from 53
to 79% and 18-58% of total C accumulated in the respec-
tive winter and summer cover corps (Table 6). This agrees
with the reports of Dossa et al. [33], i.e. 59-81% of the C
dded as shrub residue was mineralized in 118 d. a

Growing Cover Crops to Improve Biomass Accumulation and Carbon Sequestration: A Phytotron Study79
a
aaaa
b
10
15
20
25
30
35
40
45
TriticaleRyegrassBellbean
Figure 1. Concentrations of C, N and C:N ratio in winter cover crop biomass
aa
b
a
a
c
37
38
39
40
41
42
43
Sunnhemp Velvetbean CastorbeanSorghum
suda ngras s
Cowpea Okra
C%
b
a
c
d
ab
cd
0
0.5
1
1.5
2
2.5
Sunnhemp VelvetbeanCastorbeanSorghum
sudangrass
Cowpea Okra
N%
c
c
b
a
c
b
0
10
20
30
40
50
60
70
Sunnhemp Velvetbean CastorbeanSorghum
sudangrass
Cowpea Okra
C:Nratio
Covercrop
a
a
41
41.2
41.4
41.6
41.8
42
Sandysoil Gravellysoil
C%
b
a
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Sandysoil Gravellysoil
N%
a
b
30
32
34
36
38
40
42
Sandysoil Gravellysoil
C:Nratio
Soil
a
a
a
41
41.2
41.4
41.6
41.8
42
30 25 20
C%
a
a
a
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
30 25 20
N%
a
a
a
30
32
34
36
38
40
42
30 2520
C:Nratio
Temperature(oC)
(℃)
Figure 2. Concentrations of C, N and C:N ratio in summer cover crop biomass
MustardPurplevetchWhiteclover
C%
b
a
38
38.5
39
39.5
40
40.5
41
Sandysoil Gravellysoil
C%
a
a
a
38
38.5
39
39.5
40
40.5
41
25 1510
C%
cc
b
c
b
a
0
0.5
1
1.5
2
2.5
3
TriticaleRyegrass Bellbean MustardPurple
vetch
White
clover
N%
a
b
1.3
1.4
1.5
1.6
1.7
1.8
Sandysoil Gravellysoil
N%
a
a
a
1.3
1.4
1.5
1.6
1.7
1.8
25 15 10
N%
a
b
c
b
c
d
0
10
20
30
40
50
60
TriticaleRyegrass Bellbean MustardPurple
vetch
White
clover
C:Nratio
Wintercovercrop
b
a
0
5
10
15
20
25
30
35
40
Sandysoil Gravellysoil
C:Nratio
Soil
a
a
b
0
5
10
15
20
25
30
35
40
25 15 10
C:Nratio
Temperature(oC)
MustardPurpleWhite
Vetchclover
(℃)

Growing Cover Crops to Improve Biomass Accumulation and Carbon Sequestration: A Phytotron Study
80
Table 6. Carbon accumulation in cover crop aboveground biomass and that retained in the soil following 127-d decomposition
Cover crop Total C input
(g/m2)
Total C decomposed
(g/m2)
Amount of C left in
residues (g/m2)
% of C retained in
residues
Winter cover crops
Triticale 341.6a* 75.4a 266.2a 78a
Ryegrass 292.2a 62.6ab 229.7a 79a
Bellbean 311.4a 79.0a 232.5a 75a
Mustard 234.2ab 55.8b 178.4b 76a
Purple vetch 218.9b 74.8a 144.1bc 66b
White clover 136.1c 64.3ab 71.8c 53c
Mean 255.7 68.7 187.1 73
Summer cover crops
Sunn hemp 305.9a 127.4a 178.5a 58ab
Velvetbean 205.2ab 98.1ab 107.1b 52b
Castorbean 98.8c 28.1c 70.7c 72a
Sorghum sudangrass 179.1ab 78.5b 100.6b 56ab
Cowpea 83.4c 68.4b 15.0d 18c
Okra 137.0bc 61.7b 75.3bc 55b
Mean 168.2 76.6 91.2 52
*Values followed by different letter (s) within the same column of a subset (winter or summer cover crops) represent significant difference at p ≤ 0.05.
Among the winter cover crops, total C input as well as the
amount of C left in the residues after decomposition were
greater for triticale, ryegrass and bellbean than that by the
remaining three cover crop species (Table 6). White clover
ranked the lowest for both parameters although its amount
of leftover was not significantly different from purple vetch.
Among the summer cover crops, sunn hemp ranked the
highest and cowpea the lowest for the total C left in the
residues. The percent of C retained in the residues was sig-
nificantly greater by castorbean, sunn hemp and sorghum
sudangrass than that by cowpea. Percent of C retained in the
soil was greater for castorbean (72%) as compared to that
for velvetbean, okra and cowpea (Table 6).
The N concentration associated with C:N ratio is often
an important factor to determine the biomass quality
[34-36] and biomass decomposition [11,13,33,37-39],
which is closely related to the C sequestration efficiency.
The current study showed that the decomposition rate
(Table 6) is related to the N concentration or the C:N
ratio, as in the cases of sunn hemp and velvetbean com-
pared to castorbean or of bellbean and purple vetch
compared to mustard with various C:N ratios (Figure 1
and 2). The quantities of C decomposed following 127 d
were quite high for both triticale and sorghum sudangrass
(Table 6), which had high C:N ratios, 52 and 67, respec-
tively. Therefore, the biomass C:N ratio is an important
trait of biomass quality that influences decomposition
rate. In addition, some biochemical properties, such as
lignin, polyphenolic and tannin contents also influence
residue decomposition rate [38].
3.6 Soil Organic Carbon Changes with Cover
Crops and Temperatures
After winter cover crops grown, concentrations of SOC
in either the QFS or the KGL soil showed no significant
difference regardless of different cover crops grown in
comparison with fallow (data not shown). After summer
cover crops grown, compared to fallow, no any signifi-
cant change in SOC was observed in the QFS soil at all
temperatures, and it seemed hardly to observe such a
change in the KGL soil due to a considerable fluctuation
even the concentration of SOC in one treatment was sig-
nificantly greater than that in the other (Table 7). Short
duration of cover crop growth approved to have very
little influence on the SOC changes in the soil. However,
since the winter cover crop residues were returned to the
soil (soil surface applied) and subsequently the summer
cover crops were grown, the SOC at the termination of
summer cover crop growth showed some changes among
the cover crop species though such changes were fluctu-
ated and not significant. Therefore, a long term trial is
needed to monitor SOC changes with cover crops in the
agricultural system. The fluctuation changes in SOC
have been observed by other researchers [13]. Our results
agree with that of Lal [2] who concluded that the use of
cover crops as a short-term green manure may not nec-
essarily enhance the SOC pool.
However, increases of SOC occurred after cover crops
grown as compared to the respective soils before the ex-
periment. For instance, the SOC content (mean across all
winter cover crops and temperatures) increased by 0.9
and 4.8% in the KGL and QFS soils, respectively, as
compared to the respective soils prior to the experiment.
The increases in the SOC content following the growth
of the summer cover crops vs. winter cover crops were
13.7 and 25.9% for the respective soils. The correspond-
ing increases after the summer cover crops (including
winter cover crop residues returned to the soil) compared
to the SOC prior to the experiment were 13.8 and 31.9%
(Table 8). The SOC content of the QFS soil was signifi-

Growing Cover Crops to Improve Biomass Accumulation and Carbon Sequestration: A Phytotron Study81
Table 7. Concentrations of SOC (g/kg) after summer cover crops grown in different soils at various temperatures
Soil† Temp Sunn hemp Velvetbean Castorbean Sorghum sudangrassCowpea Okra Fallow
QFS soil 30oC 3.1a* 3.3a 3.3a 2.7a 3.4a 2.2a 2.3a
25oC 2.4a 2.2a 2.9a 2.7a 2.9a 2.6a 2.0a
20oC 2.9a 3.1a 1.9a 4.2a 2.7a 2.8a 2.7a
KGL soil 30oC 21.9a 12.6c 12.8c 15.2bc 23.0a 13.5c 21.2ab
25oC 13.4b 12.7b 12.2b 17.1ab 14.5ab 10.3b 22.0a
20oC 14.4cd 11.1d 17.2abc 20.0a 19.3ab 19.4ab 15.6bc
Means
QFS soil 2.8Ba 2.8Ba 2.7Ba 3.2Ba 3.0Ba 2.5Ba 2.3Ba
KGL soil 16.6Aab 12.1Ac 14.0Abc 17.4Aab 18.9Aa 14.4Abc 19.6Aa
30oC 12.5Aab 7.9Ac 8.1Ac 8.9Bbc 13.2Aa 7.9ABc 11.7Aabc
25oC 7.9Aab 7.5Aab 7.5Aab 9.9ABab 8.7Aab 6.5Bb 12.0Aa
20oC 8.7Abc 7.1Ac 9.5Aabc 12.1Aa 11.0Aab 11.1Aab 9.1Babc
†KGL = Krome gravelly loam; and QFS = Quincy fine sand.
*Values followed by different letter (s), lower case within the same row, and upper case within the same column of a subset (either the soil or tem-
perature), represent significant difference at p ≤ 0.05.
Table 8. The overall changes of SOC (g/kg) before and after cover crops grown in different soils
Soil† Prior to growing
cover crops
After winter
cover crops grown
After summer cover
crops grown
Net change (winter
cover crops vs. prior)
Net change (summer
vs. winter cover crops)
Net change (winter
+ summer cover
crops vs. prior)
QFS soil 2.10 2.20 2.77 4.8% 25.9% 31.9%
KGL soil 14.21 14.34 16.16 0.9% 13.7% 13.8%
†KGL = Krome gravelly loam; and QFS = Quincy fine sand.
cantly lower than that of the KGL soil regardless of
cover crop species or fallow soil, and the former soil
rather than the latter soil had more SOC increase after
cover crops grown as compared to the respective soils
before the cover crop growth. This result agrees with a
previous report that soil with low SOC usually has more
potential to regain organic C than soil with high SOC [2].
Accumulation of SOC is a slow and long-term process.
Sainju [19] reported that to observe differences in SOC
under field conditions even with substantial C inputs by
cover crops requires more than two years. Our previous
study [16] with two years of cover crops grown in the field
(KGL soil), showed no SOC increase as compared to that
of the fallow soil. Increased cropping intensity in crop
rotations by reducing the frequency of bare fallow can
increase crop production and C inputs to the soil [24]. Kuo
and Jellum [11] also indicated that concentrations of C and
N in the surface soil (0-15 cm) increased with increasing
total C input from cover crops because sol accumulation of
C and N is a function of the total input of organic C [40].
Bordovsky et al. [41] found that the surface (0-5 cm)
SOC concentration increased with time in their 11-year
field experiment following continuous cultivation of
grain sorghum [Sorghum bicolor (L.) Moench] and
wheat (Triticum aestivum L.) in the Miles fine sandy
loam soil in Texas. In Georgia cotton (Gossypium hirsu-
tum L.) production region with winter cover cropping
system, SOC in the Dothan sandy loam at 0-10 cm in-
creased by 6-8% over a period of 3 years with winter
cover crops, such as rye (Secale cereal L.) and hairy
vetch (Vicia villosa Roth). The rate of SOC sequestration
was 233-300 kg C ha-1 yr-1 while the rate of loss was 167
kg C ha-1yr-1 in the soil without these cover corps [13].
Conversely, changing crop-fallow to continuous mono-
culture or rotation cropping, or increasing the number of
crops in a rotation system was less effective in seques-
tering SOC as compared to shifting to no till practice
[26].
Integrated agricultural practices, including cover crops
with no tillage or at least with conservation tillage, are
needed to improve soil C sequestration. Tillage or incor-
poration of plant residues into the soil increases SOC
mineralization [2,42]. In strip- and chisel-tilled plots, the
SOC decreased by 3-17% and 4-17% in 0-10 cm and
10-30 cm depth, respectively, but in no till treatments,
SOC increased by 6-8% with winter cover crops at 0-10
cm and by 0.4% with rye and 3% with biculture of vetch
and rye at 0-30 cm [13].
The beneficial effects of growing cover crops in en-
hancing SOC pool have been reported from around the
world [43-45]. Furthermore, the enhancement of C se-
questration by growing cover crops associated with con-
servation tillage has been reported by a number of re-
searchers [12,16,43,46,47]. For example, Sainju et al. [32]

Growing Cover Crops to Improve Biomass Accumulation and Carbon Sequestration: A Phytotron Study
82
observed that hairy vetch under no till can improve SOC,
and cover cropping associated with N fertilization can
have effects in storing SOC in no tilled soils due to the
reduction in mineralization rates of crop residues and soil
organic matter. Metay et al. [46] found that no till with
cover crop (Crotalaria) increased the storage of C in the
topsoil layer (0-10 cm) compared to disc tillage, with the
latter only less than 10% of cover crop residues returned
to the soil.
No tillage or conservation tillage can conserve crop
residues but cannot increase the soil C or biomass input.
The SOC accumulation or C sequestration requires an
increase in organic matter or crop residue inputs along
with a decrease in decomposition rate of soil organic
matter [2,3,48]. Paustian et al. [49] observed that SOC
increases linearly with increased addition of crop resi-
dues. Cover crops or cover cropping systems not only
serve a large sink to remove the atmospheric CO2 but
also increases the biomass input into the soil. Therefore,
cover cropping system combined with the conservation
tillage has shown a great advantage in improving C se-
questration and sustainable development in agriculture.
The contribution of cover crops to SOC or carbon se-
questration via assimilating atmospheric CO2 into SOC
has great potential in reducing the CO2 concentration
from the atmosphere. This potential may last at least a
few decades because the SOC has been depleted over the
world in arable land and may reach a new equilibrium in
50-100 years [3,26].
4. Conclusions
The total amounts of C accumulated by aboveground
biomass varied greatly among both winter and summer
cover crops. Therefore, choice of cover crops species is
important for increased efficiency in biomass production
and carbon sequestration. Soil and temperature influ-
enced the biomass production and C accumulation under
some circumstances. Biomass production and C accu-
mulation of most winter cover crops, except white clover,
were greater in the QFS soil than those in the KGL soil.
Such difference was not evident with respect to the
summer cover crops. When the aboveground cover crop
biomass was returned to the soil for decomposition with
over 127 days, about 73 and 52% of the aboveground
biomass C was retained from the winter and summer
cover crop residues, respectively. After a year rotation
summer cover crops following winter cover crops, SOC
increased by 13.8 and 31.9% in the KGL and QFS soil,
respectively, compared to the respective soils prior to the
experiment. This study has demonstrated improved SOC
accumulation by sequestration of atmospheric C follow-
ing the growth of cover crops.
5. Acknowledgements
The research was founded by USDA-ARS with a col-
laboration research program. We sincerely thank Dr.
Thomas Davenport, Mr. Robert Stubblefield, Jorge Ver-
gel, Jacob Hall, Ms. Guiqin Yu, and Laura Rosado at
TREC, University of Florida for their supports with phy-
totron facilities and/or help in sample collections and
chemical analyses.
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