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Elevated atmospheric CO2 reduces CH4 and N2O emissions under two contrasting rice cultivars from a subtropical paddy field in China

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Abstract

Elevated CO2 (eCO2) and rice cultivars can strongly alter CH4 and N2O emissions from paddy fields. However, detailed information on how their interaction affects greenhouse gas fluxes in the field is still lacking. In this study, we investigated CH4 and N2O emissions and rice growth under two contrasting rice cultivars (the strongly and weakly responsive cultivars) in response to eCO2 , 200 µmol mol −1 higher than the ambient CO2 (aCO2), in Chinese subtropical rice systems relying on a multi-year in-situ free-air CO2 enrichment platform from 2016 to 2018. Our findings showed that eCO2 increased rice yield by 7%-31%, while it decreased seasonal cumulative CH4 and N2O emissions by 11%-59% and 33%-70%, respectively, regardless of rice cultivar. Compared to aCO2 , the decrease in CH4 emissions under eCO2 was possibly ascribed to the lower CH4 production potential (MPP) and the higher CH4 oxidation potential (MOP) correlated with the higher soil redox potential (Eh) and O2 concentration ([O2 ]) in the surface soil. The mitigating effect of eCO2 on N2O emissions was likely associated with the reduction of soil soluble N content. The strongly responsive cultivars had lower CH4 and N2O emissions than the weakly responsive cultivars. The main reason might be that the former induced higher soil Eh and [O2 ] in the surface soil, and had larger plant biomass and greater N uptake. The findings indicated that the breeding of strongly responsive cultivars with the potential for greater rice production and lower greenhouse gas emissions is an effective agricultural cultivar practice to ensure food security and environmental sustainability under future climate change scenarios.. 2022. Elevated atmospheric CO2 reduces CH4 and N2O emissions under two contrasting rice cultivars from a subtropical paddy field in China. Pedosphere. 32(?): 1-12.
Pedosphere 32(?): 1–12, 2022
doi: 10.1016/S1002-0160(xx)60xxx-x
ISSN 1002-0160/CN 32-1315/P
©2022 Soil Science Society of China
Published by Elsevier B.V. and Science Press
Elevated atmospheric CO
2
reduces CH
4
and N
2
O emissions under two contrasting
rice cultivars from a subtropical paddy field in China
Haiyang YU1,2, Guangbin ZHANG1, Jing MA1, Tianyu WANG1,2, Kaifu SONG1,2, Qiong HUANG1,2, Chunwu ZHU1,
Qian JIANG1, Jianguo ZHU1and Hua XU1,
1State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008 (China)
2University of Chinese Academy of Sciences, Beijing 100049 (China)
(Received xxx xx, 20xx; revised xxx xx, 20xx; accepted xxx xx, 20xx)
ABSTRACT
Elevated CO
2
(eCO
2
) and rice cultivars can strongly alter CH
4
and N
2
O emissions from paddy fields. However, detailed information on how their
interaction affects greenhouse gas fluxes in the field is still lacking. In this study, we investigated CH
4
and N
2
O emissions and rice growth under two
contrasting rice cultivars (the strongly and weakly responsive cultivars) in response to eCO
2
, 200
µ
mol mol
1
higher than the ambient CO
2
(aCO
2
), in
Chinese subtropical rice systems relying on a multi-year in-situ free-air CO
2
enrichment platform from 2016 to 2018. Our findings showed that eCO
2
increased rice yield by 7%–31%, while it decreased seasonal cumulative CH
4
and N
2
O emissions by 11%–59% and 33%–70%, respectively, regardless of
rice cultivar. Compared to aCO
2
, the decrease in CH
4
emissions under eCO
2
was possibly ascribed to the lower CH
4
production potential (MPP) and the
higher CH
4
oxidation potential (MOP) correlated with the higher soil redox potential (Eh) and O
2
concentration ([O
2
]) in the surface soil. The mitigating
effect of eCO
2
on N
2
O emissions was likely associated with the reduction of soil soluble N content. The strongly responsive cultivars had lower CH
4
and
N
2
O emissions than the weakly responsive cultivars. The main reason might be that the former induced higher soil Eh and [O
2
] in the surface soil, and
had larger plant biomass and greater N uptake. The findings indicated that the breeding of strongly responsive cultivars with the potential for greater rice
production and lower greenhouse gas emissions is an effective agricultural cultivar practice to ensure food security and environmental sustainability under
future climate change scenarios.
Key Words: climate change, free-air CO
2
enrichment, greenhouse gas emission, methane oxidation potential, methane production potential, oxygen, soil
redox potential
Citation: Yu H Y, Zhang G B, Ma J, Wang T Y, Song K F, Huang Q, Zhu C W, Jiang Q, Zhu J G, Xu H. 2022. Elevated atmospheric CO
2
reduces CH
4
and
N2O emissions under two contrasting rice cultivars from a subtropical paddy field in China. Pedosphere.32(?): 1–12.
Corresponding author. E-mail: hxu@issas.ac.cn.
INTRODUCTION
Rice paddies produce staple food for more than 50%
of the global population, and the rice demand is projected
to continually increase by approximately 30% by 2050 due
to population growth and economic development (Alexan-
dratos and Bruinsma, 2012). However, rice paddies are also
important sources of CH
4
and N
2
O emissions, accounting
for 11% and 10% of anthropogenic emissions, respectively
(Saunois et al., 2020; Wang et al., 2020). As the critical
greenhouse gas (GHG), the atmospheric CO
2
concentra-
tion has reached 410
µ
mol mol
1
in 2020, which exceeds
the pre-industrial levels by about 168% (available online at
http://www.esrl.noaa.gov/gmd/ccgg/trends/). This elevated
CO
2
(eCO
2
) concentration could stimulate grain yield (Allen
et al., 2020) as well as alter CH
4
and N
2
O emissions (Zheng
et al., 2006; Wang C et al., 2018a, b; Yao et al., 2021).
For example, a 13% increase in rice production (Ainsworth,
2008; Allen et al., 2020) was followed by 34% and 10%
increases in CH
4
and N
2
O emissions, respectively, under
eCO
2
(Liu et al., 2018). In other words, rice cultivation
practices under eCO
2
are expected to improve rice yields
accompanying by an increase in CH4and N2O emissions.
Breeding high-yielding rice cultivars is a key measure
to meet the increasing global food demand (Jiang et al.,
2017). Some rice cultivars have been found to enhance grain
yield in response to eCO
2
, and breeding such ‘positively
responsive’ rice cultivars has received considerable attention
recently as a strategy to increase rice production in response
to climate change (Hu et al., 2020). For instance, the cultivars
weakly responsive to eCO
2
showed a yield growth rate of
10%–15% (Kim et al., 2003; Yang et al., 2006, 2007), while
the strongly responsive cultivars significantly increased rice
grain yield by more than 30% (Liu et al., 2008; Yang et al.,
2009). Accordingly, the strongly responsive cultivars will
be probably preferred in the future to meet the rising rice
consumption due to the growing population. However, the
breeding strategies, as well as eCO
2
, may influence CH
4
2 H. Y. YU et al.
and N
2
O emissions from fields (Lou et al., 2008; Wang B
et al., 2018). Previous studies have mainly focused on the
response of CH
4
and N
2
O emissions to eCO
2
from the fields
of weakly responsive cultivars (Inubushi et al., 2003; Zheng
et al., 2006; Xie et al., 2012). Whether the effects of eCO
2
on
CH
4
and N
2
O emissions with the strongly responsive cultivar
and the underlying mechanisms differed from those of the
weakly responsive cultivars have not yet been documented.
Most studies indicate that eCO
2
stimulates CH
4
emis-
sions from rice paddies due to enhanced plant growth,
more release of root exudates, and increased abundance of
methanogens for CH
4
production (Inubushi et al., 2003;
Bhattacharyya et al., 2013; Wang C et al., 2018a; Qian et al.,
2020). However, eCO
2
could also lead to the attenuation of
CH
4
production due to more O
2
being delivered to the rice
rhizosphere, which is caused by the increased underground
biomass and porosity (Schrope et al., 1999). The quantity and
quality of root exudates and soil porosity are different from
rice cultivars, which play an important role in the production,
oxidation, and transportation of CH
4
from rice fields (Lin
and You, 1989; Lin, 1993). As such, eCO
2
and rice cultivars
are likely to interact in determining CH
4
emissions from
rice paddies. Moreover, the strongly responsive cultivars
might deliver more O
2
to the rice rhizosphere because of
their higher biomass and porosity. Thus, we hypothesized
that, compared with the weakly responsive cultivars, CH
4
emissions under the strongly responsive cultivars would be
decreased by eCO
2
because of the higher O
2
concentration
([O2]) in the rhizosphere.
The effects of eCO
2
on N
2
O emissions are inconsistent.
For instance, N
2
O emissions have been reported to increase
(Bhattacharyya et al., 2013; Pereira et al., 2013; Wang B
et al., 2018; Wang C et al., 2018b), decrease(Sun et al., 2018;
Yao et al., 2021), or remain unchanged (Xu et al., 2002;
Cheng et al., 2006) in response to eCO
2
. It should be noted
that previous studies on the response of N
2
O emissions to
eCO
2
have primarily focused on weakly responsive cultivars
(Xu et al., 2002; Cheng et al., 2006; Bhattacharyya et al.,
2013; Pereira et al., 2013; Sun et al., 2018; Wang B et al.,
2018; Wang C et al., 2018b), whereas studies on the strongly
responsive cultivars are lacking. Under eCO
2
conditions,
strongly responsive cultivars have higher nitrogen (N) uptake
capacity, net photosynthetic assimilation, and larger plant
biomass than the weakly responsive cultivars (Zhu et al.,
2014). Thus, we hypothesized that eCO
2
might decrease
much more N
2
O emissions under the strongly responsive
cultivar than under the weakly responsive cultivars due to
increased N uptake by plant roots.
Therefore, the goal of this study was to investigate the
effects of eCO
2
on CH
4
and N
2
O emissions from rice paddies
under strongly responsive cultivars based on a free-air CO
2
enrichment (FACE) platform initiated in 2004. We expected
that breeding strongly responsive cultivars would meet rice
yield demands and weaken the contribution of CH
4
and N
2
O
emissions from paddy fields to global climate change.
MATERIALS AND METHODS
Site description
This field experiment was conducted from 2016 to 2018
at the FACE system located in Xiaoji Town (119
42
0
′′
E, 32
35
34
′′
N), Yangzhou City, Jiangsu Province, in a
typical Chinese rice-growing region with a typical northern
subtropical monsoon climate (Zhu et al., 2014). The FACE
platform, which was created in 2004, consists of three
identical 14-m diameter octagonal rings receiving eCO
2
that is 200
µ
mol mol
1
above the ambient CO
2
(aCO
2
)
concentration. The target eCO
2
was achieved by injecting
pure CO
2
into the FACE rings during daylight hours. More
detailed descriptions of the design and operation of the
FACE facility can be found in the following studies: Xie
et al. (2012), Hu et al. (2020), and Li et al. (2020).
Rice cultivation
The four strongly responsive cultivars selected in this
experiment were Yangdao6 (Y6, an indica rice cultivar
planted in 2016 and 2017), YIIyou900 (Y900, a hybrid indica
rice cultivar planted in 2017), LongIIyou1988 (L1988, a hy-
brid indica rice cultivar planted in 2018), and Yongyou1540
(Y1540, a hybrid japonica rice cultivar planted in 2018). The
three weakly responsive cultivars were Wuyungeng23 (W23,
ajaponica rice cultivar planted in 2016 and 2017), Wuyun-
geng27 (W27, a japonica rice cultivar planted in 2017 and
2018), and Huaidao5 (H5, a japonica rice cultivar planted in
2018) (Table SI, see Supplementary Material for Table SI).
All management practices, including water and fertilization
regimes, in the experimental plots were consistent with those
in the local area.
The seeds were grown under aCO
2
conditions until
they were transplanted at a three-leaf stage. According to the
recommended density of rice cultivation, they were manually
transplanted to the aCO
2
and eCO
2
rings at a density of two
seedlings of the weakly responsive cultivars and one seedling
of the strongly responsive cultivars per hill in late June in
each rice season. The spacing of the hills was 16.7 cm
×
25 cm (equivalent to 24 hills m
2
). Nitrogen was supplied
in the form of urea at 22.5 g m
2
. In each season, N was
applied as basal fertilizer one day before rice transplanting
(40% of the total), as a top dressing at the early tillering stage
(30% of the total), and at the panicle initiation stage (30% of
the total). Phosphorus (9 g P
2
O
5
m
2
) and potassium (9 g
K2O m2) were applied as basal fertilizers.
Biomass (aboveground and underground biomass) and
grain yield were measured at crop harvest. To ensure rep-
resentative sampling, 16 hills were randomly selected from
each subplot. Thereafter, 6 hills were counted for effective
tiller numbers. For each sampling, the biomass samples were
oven-dried at 80
C for 72 h to a constant weight, and grain
yields were determined by subtracting a moisture content of
0.14 g H2O g1fresh weight (Liu et al., 2008).
ELEVATED CO2REDUCES PADDY CH4AND N2O EMISSIONS 3
Gas and soil sampling measurements
The CH
4
and N
2
O fluxes were measured using a static
chamber-gas chromatography (GC) method (Cai et al., 2009).
The chambers were cuboid, 35 cm in length and width, 60
or 120 cm in height (according to plant height), and with a
plastic base (35 cm
×
35 cm) installed before the initiation
of the experiment. Gas was sampled at 4–5 d intervals during
drainage and re-flooding, and at 7–10 d intervals for the
rest of the sampling period. Four gas samples from each
chamber were injected into the prepared 21-mL vacuum
vials at an interval of 12 min between 8:30 and 11:30 a.m.
on every sampling day. The samples were then taken to the
laboratory to measure the concentrations of CH
4
and N
2
O
using GC (Agilent 7890B, Agilent Technologies, Santa Clara,
USA). The fluxes of CH
4
and N
2
O were calculated from the
change in gas concentrations in the enclosed chamber over
time. Seasonal cumulative CH
4
and N
2
O emissions were
calculated directly from the measured fluxes.
When gas fluxes were monitored, soil redox potential
(Eh) of the rice soils was simultaneously measured using a
portable oxidation-reduction potential meter (Hirose Rika
Co. Ltd., Tokyo, Japan). Soil samples (0–15 cm) were
collected during the main rice growth stage in each plot
to analyze soil mineral N (NH
+
4
-N and NO
3
-N), dissolved
organic C (DOC), CH
4
production potential (MPP), and
CH4oxidation potential (MOP).
Soil NH
+
4
-N and NO
3
-N were extracted with 2 mol L
1
KCl solution (soil:solution
=
1:5, weight:weight) by shaking
for 60 min at 250 r min
1
and were quantified colorimetri-
cally using a continuous flow autoanalyzer (San
++
System,
Skalar Analytical BV, Breda, the Netherlands). The DOC was
extracted with 0.5 mol L
1
K
2
SO
4
extracts (soil:solution
=
1:4, weight:weight) by shaking for 30 min at 250 r min
1
,
centrifuged for 15 min at 4 000 r min
1
, and then filtered
through a 0.45-
µ
m polyethersulfone membrane filter. The
extracts were determined using a TOC analyzer (Vario TOC
Cube, Elementar, Hanau, Germany).
Determinations of MPP and MOP
The MPP of fresh soil samples was determined anaer-
obically and calculated using the linear regression of CH
4
increasing with the incubation time. Approximately 25 g
(dry weight) of soil was quickly transferred into 150-mL
flasks. Sterile water flushed by N
2
was added into each
flask to prepare the soil slurry with a soil/water ratio of
1:1 (weight:weight). All flasks were sealed with rubber
plugs with a silicone septum, which allowed the sampling
of headspace gas. The flasks used for MPP were flushed
with N
2
six times to purge the remaining CH
4
and O
2
. They
were then incubated at 25
C for 50 h in dark. Gas samples
were collected twice with a pressure lock syringe at 1 and
50 h after the flasks were heavily shaken by hand, and then
analyzed for CH4.
The MOP was determined aerobically using the same
devices as described above but with headspace air in the
flasks. About 1 mL of pure CH
4
was injected into each flask
to obtain a high CH
4
concentration inside (approximately
10 000
µ
L L
1
). The flasks were then incubated in dark
at 25
C and shaken at 120 r min
1
. The CH
4
depletion
was measured by sampling the headspace gas in the flask
after vigorous shaking for subsequent GC-flame ionization
detection analysis. The first sample was collected 30 min
after pure CH
4
was injected and homogeneously distributed
inside the flask. Samples were then taken at 2 h intervals
during the first 8 h of the experiment. The flasks were left
overnight and sampled in the next day at 2 h intervals. CH
4
oxidation was calculated using linear regression of CH
4
depletion with incubation time.
O2concentration
The [O
2
] at the soil-water interface was measured in
situ using a Unisense field microprofiling system (Unisense,
Aarhus, Denmark) on July 18, July 28, and August 10 in 2018.
The O
2
microelectrode is a miniaturized Clark-type sensor
connected to a field microsensor multimeter (Li and Wang,
2013; Huang et al., 2020). Then, the O
2
microelectrode was
placed in the sense holder and inserted into the soil near
the rice plant approximately 20 mm from the field motor.
The stepwise sequence was set up in advance using a field
microsensor multimeter. In our sequence, the path size was
set to 2 mm and the end depth was 20 mm below the soil-
water interface, while the periods for “wait before measure”
and “measure” were both set to 3 s.
Statistical analysis
All statistical analyses were performed using SPSS soft-
ware version 25.0 (SPSS Inc., Chicago, USA). Statistical
significance was determined at 0.05 probability level, and
the differences between means under eCO
2
and aCO
2
of the
same rice cultivar were examined with Tukey’s honestly sig-
nificant difference test. A two-way analysis of variance (CO
2
and rice cultivar) was used to identify factors associated with
CH4and N2O emissions.
RESULTS
Rice biomass and tiller number
Relative to aCO
2
, eCO
2
increased the grain yield by
19%–31% (
P <
0.01) of strongly responsive cultivars and
by 7%–14% (
P >
0.05) of weakly responsive cultivars
(Fig. 1a, b). The eCO
2
increased aboveground and under-
ground biomass by 20% and 18%, respectively, of strongly
4 H. Y. YU et al.
Fig. 1 Effect of elevated CO
2
(eCO
2
) on grain yield, aboveground biomass, underground biomass, and effective tiller numbers of strongly (a, c, e, and g)
and weakly (b, d, f, and h) responsive rice cultivars planted in 2016–2018. Strongly responsive cultivars selected in this experiment were Yangdao6 (Y6),
YIIyou900 (Y900), LongIIyou1988 (L1988), and Yongyou1540 (Y1540), and weakly responsive cultivars were Wuyungeng23 (W23), Wuyungeng27 (W27),
and Huaidao5 (H5). Vertical bars indicate standard deviations of the means (
n=
3). Significant differences between eCO
2
and ambient CO
2
(aCO
2
) levels
were determined by Tukey’s honestly significant difference test. The asterisks * and ** indicate significant differences at
P <
0.05 and
P <
0.01, respectively.
responsive cultivars, and by 14% and 16% of weakly respon-
sive cultivars (Fig. 1c, f). On average, grain yield, above-
ground biomass, and underground biomass of the strongly
responsive cultivars under eCO
2
were 15% (
P <
0.05),
6% (
P >
0.05), and 2% (
P >
0.05), respectively, higher
than those of the weakly responsive cultivars. The eCO
2
increased the tiller numbers (
P >
0.05), irrespective of the
rice cultivar (Figs. 1 g, h, and S1, see the Supplementary
Material for Fig. S1).
CH4and N2O emissions
Compared with the weakly responsive cultivars, CH
4
fluxes under the strongly responsive cultivars showed similar
temporal patterns but varying amplitudes (Fig. 2). They
increased rapidly from the time of rice transplanting until
the end of July, when it reached the maximum, and sharply
declined during the mid-season drainage stage. Thereafter,
CH
4
fluxes reached the second peak during the intermittent
irrigation stage and then declined to zero during the harvest
stage. For all the strongly responsive cultivars, the highest
CH
4
fluxes under eCO
2
conditions (17.9 to 24.0 mg CH
4
m
2
h
1
) were generally lower than those under aCO
2
conditions (20.5 to 31.0 mg CH4m2h1).
Cumulative CH
4
emissions under the strongly and
ELEVATED CO2REDUCES PADDY CH4AND N2O EMISSIONS 5
Fig. 2 CH
4
fluxes from rice paddies of strongly (a, c, and e) and weakly (b, d, and f) responsive cultivars under ambient CO
2
(aCO
2
) and elevated CO
2
(eCO
2
) conditions in 2016 (a and b), 2017 (c and d), and 2018 (e and f). Strongly responsive cultivars selected in this experiment were Yangdao6 (Y6),
YIIyou900 (Y900), LongIIyou1988 (L1988), and Yongyou1540 (Y1540), and weakly responsive cultivars were Wuyungeng23 (W23), Wuyungeng27 (W27),
and Huaidao5 (H5). Vertical bars indicate standard deviations of the means (n=3).
weakly responsive cultivars varied from 115 to 197 kg
CH
4
ha
1
and from 93 to 312 kg CH
4
ha
1
, respectively,
under eCO
2
conditions and from 224 to 280 kg CH
4
ha
1
and from 144 to 351 kg CH
4
ha
1
, respectively, under aCO
2
conditions (Table I). On average, in the 3-year experiment,
CH
4
emission decreased more under eCO
2
from rice paddies
of strongly responsive cultivars (
39%) than under eCO
2
from rice paddies of weakly responsive cultivars (
30%)
(Table I). Cumulative CH
4
emissions decreased correlatively
with increasing biomass and tiller numbers, induced by eCO
2
and affected by rice cultivar (Table SII, see Supplementary
Material for Table SII).
Very low N
2
O fluxes were measured throughout most of
the rice-growing season, except for a flux peak at the midsea-
son drainage stage (Fig. 3). Compared with the weakly re-
sponsive cultivars, the highest N
2
O fluxes under the strongly
responsive cultivars were observed under aCO
2
conditions,
ranging from 314 to 597
µ
g N
2
O-N m
2
h
1
. The fluxes of
N
2
O for the strongly and weakly responsive cultivars under
eCO
2
conditions were lower than those under aCO
2
condi-
tions (Fig. 3). The cumulative N
2
O emissions under eCO
2
conditions ranged from 0.25 to 0.41 kg N
2
O-N ha
1
for
the strongly responsive cultivars, and from 0.25 to 0.70 kg
N
2
O-N ha
1
for the weakly responsive cultivars. These
values were lower than those observed for the strongly (0.63–
0.92 kg N
2
O-N ha
1
) and weakly (0.42–1.05 kg N
2
O-N
ha
1
) responsive cultivars under aCO
2
conditions (Table I).
On average, eCO
2
significantly decreased cumulative N
2
O
emissions from rice paddies of the strongly and weakly
responsive cultivars by 60% and 43%, respectively (Table I).
The cumulative N
2
O emissions were significantly correlated
with rice biomass and tiller numbers, which were affected
by CO2concentration and rice cultivar (Table SII).
Soil parameters
The seasonal mean soil Eh under eCO
2
for strongly
(
114 to
84 mV) and weakly (
125 to
84 mV) respon-
sive cultivars was higher than that under aCO
2
for strongly
(
150 to
90 mV) and weakly (
164 to
93 mV) respon-
sive cultivars (Fig. 4a, b). The soil Eh on average increased
by 7%–9% for strongly responsive cultivars compared with
that for weakly responsive cultivars. Generally, regardless of
6 H. Y. YU et al.
TABLE I
Accumulative CH4and N2O emissions from rice paddies of strongly and weakly responsive cultivars affected by elevated CO2(eCO2) relative to ambient
CO2(aCO2) in 2016–2018
Rice cultivar Year Genotype Accumulative CH4emissions Accumulative N2O emissions
aCO2eCO2Changea) aCO2eCO2Change
kg CH4ha1% kg N2O-N ha1%
Strongly responsive
cultivar
2016 Yangdao6 280 ±18b) bAc) 115 ±18cB 59 NDd) ND
2017 Yangdao6 233 ±37bcdA 185 ±47bA 21 0.92 ±0.04aA 0.28 ±0.09cB 70
YIIyou900 236 ±56bcdA 197 ±12bA 16 0.63 ±0.03cA 0.32 ±0.06cB 50
2018 LongIIyou1988 224 ±65bcdA 126 ±2cB 44 0.91 ±0.1aA 0.41 ±0.11bcB 55
Yongyou1540 253 ±29bcA 129 ±25cB 49 0.73 ±0.11bA 0.25 ±0.05cB 66
Weakly responsive
cultivar
2016 Wuyungeng23 228 ±16bcdA 106 ±8cB 54 ND ND
2017 Wuyungeng23 351 ±30aA 312 ±5aA 11 0.42 ±0.03dA 0.25 ±0.06cB 40
Wuyungeng27 144 ±30dA 128 ±30cA 11 1.02 ±0.05aA 0.57 ±0.11abB 44
2018 Wuyungeng27 165 ±17cdA 93 ±11cB 44 1.05 ±0.01aA 0.70 ±0.15aB 33
Huaidao5 215 ±39bcdA 129 ±13cB 40 0.94 ±0.04aA 0.43 ±0.07bcB 54
a)Change =(data under eCO2/data under aCO21) ×100%.
b)Means ±standard deviations of the means (n=3).
c)
Means followed by the different lowercase letters within each column are significantly different at
P <
0.05 among rice cultivars, and those followed by the
different uppercase letters within each row are significantly different at P < 0.05 between aCO2and eCO2levels for CH4or N2O emissions.
d)No data.
Fig. 3 N
2
O fluxes from rice paddies of strongly (a and c) and weakly (b and d) responsive cultivars under ambient CO
2
(aCO
2
) and elevated CO
2
(eCO
2
) conditions in 2017 (a and b) and 2018 (c and d). Strongly responsive cultivars selected in this experiment were Yangdao6 (Y6), YIIyou900 (Y900),
LongIIyou1988 (L1988), and Yongyou1540 (Y1540), and weakly responsive cultivars were Wuyungeng23 (W23), Wuyungeng27 (W27), and Huaidao5 (H5).
Vertical bars indicate standard deviations of the means (n=3).
eCO
2
or aCO
2
, CH
4
fluxes in the rice paddies of strongly
and weakly responsive cultivars were negatively associated
with the dynamics of soil Eh (Table SII).
The eCO
2
increased soil DOC in rice paddies of the
strongly and weakly responsive cultivars by
2% to 14%
and by 1% to 32%, respectively (Fig. 4c, d). There was
no correlation between cumulative CH
4
emissions and the
average concentration of DOC, but a significant negative
correlation was observed between N
2
O emissions and DOC
across all rice cultivars during the rice-growing season from
2017 to 2018 (Table SII).
Compared to aCO
2
, eCO
2
reduced the average content
of soil NH
+
4
-N in rice paddies of the strongly and weakly
responsive cultivars by 1% to 44% and by 23% to 33%,
respectively (Fig. 4e, f). Generally, the content of NO
3
-N
was one order of magnitude lower than that of NH
+
4
-N,
and there was a significant difference in NO
3
-N content
between eCO
2
and aCO
2
for the strongly responsive cultivars
L1988 and Y1540 in 2018 (Fig. 4 g, h). N
2
O emissions were
positively correlated with NH
+
4
-N content (
P <
0.01; Table
SII).
ELEVATED CO2REDUCES PADDY CH4AND N2O EMISSIONS 7
Fig. 4 Effect of elevated CO
2
(eCO
2
) on mean soil redox potential (Eh), dissolved organic carbon (DOC), NH
+
4
-N, and NO
3
-N in rice paddies of strongly
(a, c, e, and g) and weakly (b, d, f, and h) responsive cultivars. Strongly responsive cultivars selected in this experiment were Yangdao6 (Y6), YIIyou900
(Y900), LongIIyou1988 (L1988), and Yongyou1540 (Y1540), and weakly responsive cultivars were Wuyungeng23 (W23), Wuyungeng27 (W27), and
Huaidao5 (H5). Vertical bars indicate standard deviations of the means (
n=
3). Significant differences between eCO
2
and ambient CO
2
(aCO
2
) levels were
determined by Tukey’s honestly significant difference test. The asterisks * and ** indicate significant differences at P < 0.05 and P < 0.01, respectively.
MPP and MOP
The mean MPP in rice paddies of strongly responsive
cultivars decreased from 0.15–0.39
µ
g CH
4
g
1
d
1
under
aCO
2
to 0.06–0.28
µ
g CH
4
g
1
d
1
under eCO
2
, and for
weakly responsive cultivar, it decreased from 0.23–0.63 µg
CH
4
g
1
d
1
under aCO
2
to 0.06–0.25
µ
g CH
4
g
1
d
1
under eCO
2
in 2016–2018 (Table II). Compared to aCO
2
,
eCO
2
decreased the MPP in rice paddies of strongly and
weakly responsive cultivars by 17%–81% and 54%–86%,
respectively. The MPP was significantly positively correlated
with CH
4
emissions (Table SII). Contrary to changes in MPP,
eCO
2
tended to increase the mean MOP in rice paddies of
strongly and weakly responsive cultivars by 37% and 22%,
respectively (Table II). Although MOP was not significantly
affected by eCO
2
or cultivar, it was significantly negatively
correlated with CH4emissions (Table SII).
Concentration of O2at soil-water interface
The eCO
2
significantly increased (
P <
0.01) the mean
[O
2
] in 0–20 mm soil in rice paddies of the strongly and
weakly responsive cultivars by 24%–37% and 22%–29%,
8 H. Y. YU et al.
TABLE II
Mean CH
4
production potential (MPP) and CH
4
oxidation potential (MOP) in rice paddies of strongly and weakly responsive cultivars affected by elevated
CO2(eCO2) relative to ambient CO2(aCO2) in 2016–2018
Rice cultivar Year Genotype MPP MOP
aCO2eCO2Changea) aCO2eCO2Change
µg CH4g1d1%µg CH4g1d1%
Strongly responsive
cultivar
2016 Yangdao6 0.33 ±0.03b) abAc) 0.23 ±0.02abB 32 9.16 ±1.48aA 10.71 ±1.80aB 17
2017 Yangdao6 0.17 ±0.05bA 0.13 ±0.08abA 23 9.34 ±3.16aA 9.92 ±0.31aA 6
YIIyou900 0.34 ±0.30abA 0.28 ±0.18aA 17 10.30 ±2.38aA 11.38 ±0.87aA 11
2018 LongIIyou1988 0.15 ±0.04bA 0.06 ±0.03bB 59 8.69 ±3.07aA 12.84 ±0.27aA 48
Yongyou1540 0.39 ±0.26abA 0.07 ±0.03bA 81 9.64 ±3.56aA 12.99 ±1.23aA 35
Weakly responsive
cultivar
2016 Wuyungeng23 0.63 ±0.03aA 0.25 ±0.06abB 60 9.47 ±0.13aA 12.73 ±0.02aB 34
2017 Wuyungeng23 0.54 ±0.19abA 0.24 ±0.02abB 55 9.54 ±2.38aA 11.89 ±1.24aA 25
Wuyungeng27 0.25 ±0.05abA 0.11 ±0.01abB 54 10.13 ±1.69aA 10.33 ±2.75aA 2
2018 Wuyungeng27 0.60 ±0.14aA 0.09 ±0.01bB 86 9.94 ±1.93aA 11.63 ±1.11aA 17
Huaidao5 0.23 ±0.10abA 0.06 ±0.04bB 75 9.53 ±2.75aA 10.78 ±1.56aA 13
a)Change =(data under eCO2/data under aCO21) ×100%.
b)Means ±standard deviations of the means (n=3).
c)
Means followed by the different lowercase letters within each column are significantly different at
P <
0.05 among rice cultivars, and those followed by the
different uppercase letters within each row are significantly different at P < 0.05 between aCO2and eCO2levels for MPP or MOP.
Fig. 5 Effect of elevated CO
2
(eCO
2
) on mean O
2
concentration ([O
2
])
in rice paddies of tstrongly (LongIIyou1988, L1988; Yongyou1540, Y1540)
and weakly (Wuyungeng27, W27; Huaidao5, H5) responsive cultivars.
Vertical bars indicate standard deviations of the means (
n=
3). Significant
differences between eCO
2
and ambient CO
2
(aCO
2
) levels were determined
by Tukey’s honestly significant difference test. The asterisk ** indicates
significant difference at P < 0.01.
respectively (Fig. 5). Compared with weakly responsive
cultivar, the [O
2
] under strongly responsive cultivar increased
(
P <
0.01) by 19% under aCO
2
and 23% under eCO
2
. In
the surface water of the rice field, the [O
2
] was relatively
stable (Fig. S2a, b, see Supplementary Material for Fig. S2).
However, it rapidly decreased with soil depth, reaching near
0
µ
mol L
1
at approximately 10 mm below the soil-water
interface (Fig. S2a, b).
DISCUSSION
Effects of strongly and weakly responsive cultivars on CH
4
emissions under eCO2
In this first-ever study on the effects of eCO
2
on GHG
emissions from rice fields with strongly responsive cultivars
using FACE technology, eCO
2
reduced more CH
4
emissions
from rice paddies of strongly responsive cultivars (16%–
59%) than of weakly responsive cultivars (11%–54%) (Fig. 2,
Table I). However, regardless of rice cultivar, this decline
in CH
4
emissions under eCO
2
conditions was in contrast
with previous studies (Inubushi et al., 2003; Zheng et al.,
2006; Bhattacharyya et al., 2013; Pereira et al., 2013; Wang
Bet al., 2018; Wang C et al., 2018a), and even differed from
a non-significant increase (15%) of the seasonal total CH
4
emissions from the same experimental fields reported by
Xie et al. (2012). The reduction in CH
4
emissions induced
by eCO
2
from rice paddy soil might be attributed to higher
soil Eh and greater O
2
transport into the soil-water interface
due to eCO
2
-induced production of more tillers and rice
biomass (Figs. 4, 5, S1, and S3, see Supplementary Material
for Fig. S3). These responses were significantly correlated
with the lower MPP and higher MOP under eCO
2
relative
to those under aCO
2
(Table SII). Although there was only
an upward trend for MOP in all treatments under eCO
2
, the
significant positive relationship between the lower MPP and
reduced CH
4
emissions further suggests that decreased CH
4
production is the key factor for decreased CH
4
emissions.
Meanwhile, CH
4
emissions were negatively correlated with
MOP, although there was only an upward trend for MOP
in all treatments under eCO
2
(Table II). This significant
correlation of CH
4
emission with MPP and MOP might
support the hypothesis that eCO
2
reduces CH
4
emission
from paddy fields. In addition, the effect of eCO
2
on CH
4
emissions may be altered by different soil types. The present
study was conducted in sandy loam paddies, which might
result in higher hydraulic conductivity and soil Eh (Xie
et al., 2012). These related soil properties can inhibit MPP
and stimulate MOP. It has been reported that MPP can be
accurately calculated from soil Eh development (Mitra et al.,
2002), whereas MOP is influenced by soil moisture content,
ELEVATED CO2REDUCES PADDY CH4AND N2O EMISSIONS 9
the plant-mediated diffusion rate of CH
4
, and O
2
availability
(Zhang et al., 2012). Therefore, the eCO
2
-induced decrease
in CH
4
emissions is likely caused by the changes in MPP
and MOP and by many other factors due to the direct and
indirect effects of eCO2(Malyan et al., 2016).
Rice cultivar breeding is a win-win strategy, as it simul-
taneously increases grain yield and decreases CH
4
emissions
(Jia et al., 2006). It is well recognized that high-yielding rice
cultivars strongly decrease CH
4
emissions from paddy soils
with high organic C content (Jiang et al., 2017). In this study,
eCO
2
increased aboveground and underground biomass and
tiller number of the strongly and weakly responsive cultivars
(Fig. 1), which is consistent with previous studies (Inubushi
et al., 2003; Wang B et al., 2018). Meanwhile, the negative
linear relationship between seasonal cumulative CH
4
emis-
sions and aboveground and underground biomass (Table
SII) likely indicates that CH
4
emissions decreased with an
increase in rice biomass. Indeed, regardless of rice cultivar,
higher biomass under eCO
2
(Fig. 1) was accompanied with
higher [O
2
] in the soil-water interface (Fig. 5), which favored
CH
4
oxidation (Ma et al., 2010; Jiang et al., 2017, 2019).
CH
4
emissions from rice paddies of strongly responsive
cultivar were much lower than those of weakly responsive
cultivar, regardless of the CO
2
level (Table I). This likely
depended on the more rice biomass, higher soil Eh, and
more [O
2
] at the surface soil (Figs. 4, 5, and S2, Table SII),
suggesting that CH
4
production is lower and CH
4
oxidation
is higher in rice paddies of strongly responsive cultivar.
These results were demonstrated by the decreased MPP
and increased MOP in rice paddies of strongly responsive
cultivars relative to weakly responsive cultivars under eCO
2
conditions (Table II).
Compared to aCO
2
, soil DOC content in rice paddies
of strongly and weakly responsive cultivars under eCO
2
conditions did not significantly increase, while CH
4
emission
decreased (Fig.4, Table I). This result indicates that the
difference in CH
4
emissions may not depend on soil DOC
content. Jiang et al. (2017) reported that high-yielding rice
cultivars facilitated CH
4
oxidation by increasing O
2
transport
and promoting methanotrophic organisms when DOC was
high. Therefore, compared with other factors, soil DOC
content may not be the main factor affecting CH
4
emissions.
In our FACE system, previous studies have shown that
soil organic C (SOC) sequestration increased with high N
input through a dissolved organic matter (DOM)-microbial
pathway in 2015 under 11-year eCO
2
conditions (Hu et al.,
2020). The eCO
2
induced higher microbial C use efficiency
and lower effectiveness of the plant to prime DOM, thus
increasing SOC sequestration and reducing soil C loss (Hu
et al., 2020). In addition, these processes probably led to
more methanogenic substrates but less CH
4
production and
emissions under eCO
2
conditions (Tables I and II, Fig. 2).
Moreover, the cropping system in our FACE was changed
from rice-wheat to rice-fellow in 2010, which may increase
SOC sequestration and thereby mitigate the effects of high-
yielding rice cultivars on CH
4
emissions (Jiang et al., 2017).
Consequently, consistent with our hypothesis, we pro-
pose a new inference that, regardless of rice cultivar, eCO
2
may reduce CH
4
emissions from rice paddy fields in the
context of continuous climate change and global warming.
First, soil conditions and microbial communities may change
under eCO
2
. In this case, the increase in rice biomass under
eCO
2
promotes root aerenchyma tissue for transporting O
2
from air to paddy soil through the increased number of tillers
(Inubushi et al., 2003; Kim et al., 2003). This enhances
the transmission capacity of O
2
, resulting in higher soil
Eh, which inhibits MPP and stimulates MOP, and thereby
decreasing CH
4
emissions (Table SII, Figs. 4a, b, and 5).
Second, eCO
2
inhibits the abundance of methanogens due
to the utilization balance of CH
4
substances for rice plant
growth and soil microbial activity, although it increases CH
4
substrate availability with the contribution of root exudates
by photosynthesis (Tokida et al., 2011). Additionally, the
stimulation of rice growth requires more irrigated water,
which changes soil conditions and indirectly affects CH
4
emission from paddy fields. This change in hydrological
conditions may be exacerbated by CO2.
Effects of strongly and weakly responsive cultivars on N
2
O
emissions as affected by eCO2
The increased N
2
O emissions from rice soil as affected
by eCO
2
are primarily regulated by plant-mediated N and C
availability induced by eCO
2
(Carnol et al., 2002; Cai et al.,
2009; Bhattacharyya et al., 2013; Wang C et al., 2018b).
Contrary to those previous studies, our multi-year FACE
results clearly showed that N
2
O emissions from rice paddies
of strongly and weakly responsive cultivars were lower at
eCO
2
conditions than at aCO
2
conditions (Table I). These
results suggest that, regardless of rice cultivar, N
2
O fluxes
mainly occurred at the midseason drainage stage and the
decrease in total N
2
O emissions induced by eCO
2
was due
to a sharp reduction of N
2
O emissions at this stage (Fig. 3),
which is consistent with the results of previous studies (Wang
Cet al., 2018b; Yao et al., 2021). In the present study, the
reduction of N
2
O emissions under eCO
2
was significantly
correlated with a reduction in soil NH
+
4
-N content and an
increase in rice biomass (Table SII). Thus, the inhibitory
effect of eCO
2
on N
2
O emissions from rice systems was
due to the lower soil N availability for plant uptake (Sun
et al., 2018; Yao et al., 2021). Meanwhile, the eCO
2
caused
substantial losses of NH
+
4
-N (Fig. 3), which resulted from the
anaerobic oxidation of ammonium coupled to the reduction
of iron (Xu et al., 2020). Thus, the high demand for available
N for rice plant growth and the eCO
2
-induced losses of
10 H. Y. YU et al.
NH
+
4
-N would inhibit N
2
O production and emissions from
rice soil under eCO2conditions.
In this study, the lower N
2
O emissions from rice paddies
of strongly responsive cultivars compared to those of weakly
responsive cultivars under eCO
2
conditions (Table I) were
mostly attributed to more NH
+
4
-N content for plant growth
and reduced N availability for N
2
O production in rice pad-
dies of strongly responsive cultivars (Jiang et al., 2020). In
addition, the enhanced soil C sequestration under strongly
responsive cultivars suggested that this type of cultivar may
provide little C source for denitrification, thus curbing N
2
O
emissions (Yao et al., 2021). Meanwhile, the significant de-
crease in the ratio of NO
2
-reducing bacteria (nirS/nirK-type
denitrifiers) abundance to N
2
O-reducing bacteria (nosZ-type
denitrifiers) abundance under eCO
2
conditions suggested that
eCO
2
reduced N
2
O emission by facilitating the consumption
of N
2
O and favoring N
2
production through denitrification
(Sun et al., 2018).
Therefore, consistent with our hypothesis, the results of
this study verified that eCO
2
significantly reduced N
2
O emis-
sions from the rice field, and the strongly responsive cultivars
induced lower N
2
O emissions than the weakly responsive
cultivars. This finding might help to better understand the
different negative effects of eCO
2
and rice cultivars on N
2
O
emissions from paddy fields (Yao et al., 2021). In addition to
eCO
2
, the reduction of N
2
O emissions is also closely related
to rice cultivar and the interaction between rice cultivar and
eCO2.
CONCLUSIONS
The CH
4
and N
2
O emissions varied among the strongly
and weakly responsive cultivars, but they decreased signifi-
cantly under eCO
2
conditions and were positively correlated
with the stimulated rice biomass. The decrease in CH
4
emis-
sions under eCO
2
conditions was strongly linked to the
decreased MPP and the increased MOP associated with soil
Eh and [O
2
] in the water-soil interface. The reduction in
N
2
O emissions under eCO
2
conditions was mainly related to
the decrease in soil available N and the increase in N uptake
caused by rice biomass production. The greater reduction
of CH
4
and N
2
O emissions in rice paddies of strongly re-
sponsive cultivar compared with weakly responsive cultivar
was due to the higher soil Eh and [O
2
], lower MPP and
higher MOP, and greater N uptake. However, considering
the limitations of microbial testing of C and N cycle, the lim-
itations of irrigation water variation, and the limited synergy
of elevated temperature with eCO
2
on the emissions of CH
4
and N
2
O, further research is needed to better understand the
long-term mechanisms involved.
ACKNOWLEDGEMENTS
This work was financially supported by the National
Key Research and Development Program of China (No.
2017YFD0300105), the National Natural Science Founda-
tion of China (No. 41877325), and the Youth Innovation
Promotion Association of the Chinese Academy of Sci-
ences (No. 2018349). We thank Prof. Lianxin YANG from
Yangzhou University, China for providing the rice seeds of
LongIIyou1988, Yongyou1540, and Wuyungeng27 cultivars.
We would like to express our appreciation to the editor and
anonymous reviewers for their useful comments and insight-
ful suggestions on the manuscript. We are also grateful to
Ms. Qing Lü, Mr. Rong ZHU, and Mr. Guoxin ZHU for their
help in sampling and measurements. The authors declare no
competing financial interests.
SUPPLEMENTARY MATERIAL
Supplementary material for this article can be found in
the online version.
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