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Marginal land conversion to perennial energy crops with biomass removal enhances soil carbon sequestration

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Marginal land conversion to perennial energy crops can provide biomass feedstocks and climate change mitigation. However, the effect of perennial energy crop cultivation on soil organic carbon (SOC) sequestration and its underlying mechanism in marginal land still remains incomplete. Here, SOC turnover, stability, and its potential sequestration were evaluated based on 10 years of land use change from C3 grass dominated marginal land to C4 energy crops Miscanthus and switchgrass cultivation. The naturally occurring 13C signature down to 60 cm depth was used to determine the energy crops‐derived C. Compare to reference marginal land, Miscanthus plantation increased the SOC stock at 0‐60 cm depth by 17.8% and 64.3% in bulk and root zone, respectively. Similarly, the SOC stock under switchgrass was also 16.5% and 93.0% higher in bulk and root zone than reference marginal land, respectively. The higher SOC stock in the root zone of switchgrass relative to Miscanthus was supported by the higher contribution of C4‐derived C to SOC (44.5% vs. 32.4%). The mean residence time of old C was higher under switchgrass than Miscanthus in the bulk zone across 0‐60 cm (p < 0.05) but remained the same at 0‐20 cm in the root zone. Specific SOC mineralization and temperature sensitivity were lower in soils under Miscanthus and switchgrass compared to reference marginal land. The partial least squares path model revealed that perennial energy crop cultivation enhances soil C stock via increased C4‐derived C input and reduced mineralization. In conclusion, marginal land conversion to perennial energy crops is a win‐win strategy for C sequestration to mitigate climate change and support the growing bioenergy sector with biomass supply.
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GCB Bioenergy. 2022;00:1–11.
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1
wileyonlinelibrary.com/journal/gcbb
Received: 2 June 2022
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Accepted: 4 July 2022
DOI: 10.1111/gcbb.12990
RESEARCH ARTICLE
Marginal land conversion to perennial energy crops with
biomass removal enhances soil carbon sequestration
YiXu1
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JieZhou1
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WenhaoFeng1
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RongJia1
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ChunyanLiu1
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TongchenFu2
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ShuaiXue2
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ZiliYi2
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ThomasGuillaume3
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YadongYang1
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LeannePeixoto4
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ZhaohaiZeng1
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HuadongZang1
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
© 2022 The Authors. GCB Bioenergy published by John Wiley & Sons Ltd.
1College of Agronomy and
Biotechnology, China Agricultural
University, Beijing, China
2College of Bioscience & Biotechnology,
Hunan Agricultural University,
Changsha, China
3Research Division Plant Production
Systems, Agroscope, Field- Crop
Systems and Plant Nutrition, Nyon,
Switzerland
4Department of Agroecology, Aarhus
University, Aarhus, Denmark
Correspondence
Huadong Zang, College of Agronomy
and Biotechnology, China Agricultural
University, Beijing, China.
Email: zanghuadong@cau.edu.cn
Funding information
the Joint Funds of the National Natural
Science Foundation of China, Grant/
Award Number: U21A20218; the
National Natural Science Foundation
of China, Grant/Award Number:
32101850; the Young Elite Scientists
Sponsorship Program by CAST, Grant/
Award Number: 2020QNRC001
Abstract
Marginal land conversion to perennial energy crops can provide biomass feed-
stocks and climate change mitigation. However, the effect of perennial energy
crop cultivation on soil organic carbon (SOC) sequestration and its underlying
mechanism in marginal land still remains incomplete. Here, SOC turnover, sta-
bility, and its potential sequestration were evaluated based on 10 years of land use
change from C3 grass- dominated marginal land to C4 energy crops Miscanthus
and switchgrass cultivation. The naturally occurring 13C signature down to 60 cm
depth was used to determine the energy crops- derived C. Compared to refer-
ence marginal land, Miscanthus plantation increased the SOC stock at 0– 60 cm
depth by 17.8% and 64.3% in bulk and root zone, respectively. Similarly, the SOC
stock under switchgrass was also 16.5% and 93.0% higher in bulk and root zone
than in reference marginal land, respectively. The higher SOC stock in the root
zone of switchgrass relative to Miscanthus was supported by the higher contribu-
tion of C4- derived C to SOC (44.5% vs. 32.4%). The mean residence time of old C
was higher under switchgrass than Miscanthus in the bulk zone across 0– 60 cm
(p < 0.05) but remained the same at 0– 20 cm in the root zone. Specific SOC min-
eralization and temperature sensitivity were lower in soils under Miscanthus and
switchgrass compared to reference marginal land. The partial least squares path
model revealed that perennial energy crop cultivation enhances soil C stock via
increased C4- derived C input and reduced mineralization. In conclusion, mar-
ginal land conversion to perennial energy crops is a win– win strategy for C se-
questration to mitigate climate change and support the growing bioenergy sector
with biomass supply.
KEYWORDS
13C natural abundance, C3– C4 vegetation change, marginal land, Miscanthus, soil C
sequestration, switchgrass
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XU et al.
1
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INTRODUCTION
Soil is the largest carbon (C) reservoir in the terrestrial
biosphere containing three times as much C to depths of
1m as the atmosphere (van Groenigen et al.,2011). Thus,
even small proportions of soil C loss could induce large
fluctuations in atmospheric CO2 and trigger feedback on
climate change (Bradford et al.,2016; Lal,2004). Arable
land conversion to perennial energy crops has been shown
to increase soil C stocks on both regional and global
scales (Chen, Lærke, & Jørgensen,2022; Chen, Manevski,
et al., 2022; Ledo et al., 2020). Thus, perennial energy
crop cultivation can serve as a potential strategy for cli-
mate change mitigation. However, to avoid land conflict
with food production, perennial energy crops must be
cultivated on marginal land, which is unsuitable for food
crop cultivation due to low fertility or high environmental
stress. It is estimated that the marginal land area available
for energy crop cultivation was 184.9Mha accounting for
19.2% of the total land area in China (Zhang, Hastings,
et al., 2020). Thus, marginal land conversion to peren-
nial energy crops has the potential to provide biomass
feedstocks for renewable energy and contribute to cli-
mate change mitigation. Yet, this strategy fails to consider
the long- term effect on soil organic carbon (SOC) stocks
under energy crop cultivation in marginal land, which
may impede an adequate estimation of the sustainability
of biomass plantations.
Plant- derived C input is a major source contribut-
ing to the C stocks under perennial energy crops (Rees
et al.,2005). Nearly half of plant assimilated C is usually
transferred to soil, either in the form of rhizodeposition
(i.e., low molecular weight compounds) released from
living roots and root litter input after harvest (Pausch &
Kuzyakov, 2018). Given that the aboveground biomass
of energy crops would be removed, it may cause an alter-
ation in perennial energy crops- derived C input and con-
sequently SOC stocks. For example, unchanged or even
increased SOC stocks were found in various environments
despite aboveground biomass removal due to the high be-
lowground C input (Martani et al.,2021; Xu et al.,2021;
Zhuang et al.,2013). As a plant with a C4 photosynthetic
pathway, energy crops produce tissue C and, finally, SOC
with a 13C signature that differs from the one of SOC in
soils with prevailing C3 vegetation. Thus, the C derived
from the original (C3) and C4 energy crops can be distin-
guished based on changes in the δ13C signature (Flessa
et al., 2000; Zang et al., 2018). Though, most previous
studies investigating the contribution of C4- derived C to
SOC were conducted in grassland or agriculture ecosys-
tems (Holder et al.,2019; Leifeld et al.,2021; Poeplau &
Don,2014; Zatta et al.,2014). So far, the accumulation of
C4- derived C in marginal lands with low soil fertility and
high abiotic stress has not yet been studied. In addition,
most studies available on the changes in soil C after peren-
nial energy crops cultivation have not separated SOC into
new and old pools, which may cause a vague estimation of
the effects of land use on soil C dynamics as the old pools
to have a much longer mean residence time (MRT) than
labile pools (Novara et al.,2013; Zang et al.,2018).
In addition to C input, the C loss via mineralization is
also a vital factor affecting SOC stocks (Mary et al.,2020;
Zhou, Wen, et al.,2021). Typically, perennial energy crop
cultivation reduces soil C mineralization by enhancing
physical protection derived from aggregation due to no-
till systems, a large number of root exudates, and a lon-
ger growth period (Austin et al.,2017; Sartori et al.,2006;
Tiemann & Grandy, 2015). Therefore, perennial energy
crop cultivation has the potential to enhance soil C seques-
tration by reducing soil C mineralization. Collectively,
how marginal land conversion to perennial crop cultiva-
tion affects soil C stocks and its controlling mechanisms
remains elusive.
We established a 10 years field study with vegetation
change from C3 grass- dominated marginal land to C4 en-
ergy crop cultivation. Miscanthus and switchgrass were
selected as leading energy crops suitable for marginal land
cultivation. Miscanthus has a coarse and broad root sys-
tem, while switchgrass has a fine and deep root system
(Winkler et al.,2020; Xue et al.,2015; Zheng et al.,2019).
We aimed to (1) evaluate soil C sequestration from the
conversion of marginal land to Miscanthus and switch-
grass; (2) quantify the plant- derived C input and turnover,
as well as soil C mineralization; and (3) identify the con-
trolling factors for soil C sequestration under energy crops
in marginal land.
2
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MATERIALS AND METHODS
2.1
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Experimental setup
The field was located at the Hunan Agricultural University
experimental station, Liuyang, Hunan province, China
(27°51′N, 113°10′E, 11.4 m a.s.l.). The average tempera-
ture and rainfall were 17.4°C and 1529 mm, respectively.
The soil (collected in April 2011 at a depth of 0– 20 cm
before the field trial establishment) is loam soil with a
pH of 5.12 and contained 5.19 g kg1 soil organic matter,
30.63 mg kg1 available nitrogen, 2.69 mg kg1 available
phosphorus, and 90.77 mg kg1 available potassium. This
land can be classified as marginal land since it is unsuit-
able for food crop cultivation due to low soil fertility and
severe acidification (Fu et al.,2022).
Before the establishment of the experiment, the site
was an abandoned land dominated by a mixture of C3
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XU et al.
weeds for more than 20 years. The field experiment was
conducted as a randomized block design with three treat-
ments. Each treatment contained three replicates, with a
37.5m2 (5m × 7.5m) plot size. The three treatments in-
cluded two energy crops: Miscanthus (Triarrhena lutari-
oriparia L., hybrid Xiangzamang NO.1) and switchgrass
(Panicum virgatum L., lowland ecotype, Alamo), as well
as a C3 reference grassland. The reference grassland was
dominated by a C3 weed mixture (Cyperus rotundus L. and
Setaria viridis L.) without human disturbance. The energy
crops were planted with a row space of 1m and a plant
space of 1m. Aboveground biomass was harvested annu-
ally from late November to early December for bioenergy
production. No additional management practices (e.g.,
fertilization, irrigation, weeding, and pest control) were
used.
2.2
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Plant and soil sampling
Soil and plant samples were collected in January 2021,
corresponding to a cultivation period of 10 years. For
energy crop plots, soil cores were taken in two different
positions to account for the different inter- row spacing
for each species due to the tussock forming condition of
Miscanthus and switchgrass (Martani et al.,2021). For the
energy crop plot, five energy crop plants in the “S” pattern
were randomly selected and harvested for further isotopic
analysis. Soil samples from each plot were obtained after
harvesting biomass using a hand- operated soil core (diam-
eter of 4cm) down to a depth of 60 cm according to the
following steps: (1) 10cm from the center of the plant four
soil cores in four locations were collected and mixed to get
a root zone (R) composite soil sample (Figure1); (2) 10cm
from the edge of the plant (i.e., between the plant rows)
four soil cores were collected and mixed to get a bulk zone
(B) composite sample (Figure1); and (3) the 20 cores (5
plants × 4 positions) from the root zone and the bulk zone
was mixed to get a final composite soil sample from each
plot. For C3 reference plots, 10 randomized soil cores
were pooled to form a mixed soil sample in each plot. Soil
cores were divided into four depth intervals (0– 10cm, 10–
20 cm, 20– 40 cm, and 40– 60 cm). For each of these depths,
soil samples were divided into two sub- samples, with one
stored at room temperature to measure soil C and isotopic
signature, and another stored at 4°C to measure enzyme
activity within 1week. Additional undisturbed soil sam-
ples were taken to determine the bulk density and soil ag-
gregate separation. The dry sieving method was used to
separate soil aggregate (Yan et al.,2022).
2.3
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Isotopic analysis
Soil samples were air- dried at room temperature and
sieved (<2 mm) where all visible root and plant residues
were removed, and the soil was milled. Plant samples
(roots and rhizomes) were dried at 60°C and ball- milled.
The organic C and δ13C signature of the plant and SOC
and total nitrogen were measured using an ANCA- IRMS
(PDZE Europa Limited).
2.4
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Soil enzyme activities
Three hydrolytic enzymes related to soil C cycling
(β- glucosidase, BG, β- cellobiohydrolase, CEL, and
β- xylosidase, XYL) were measured using fluorogenic
labeled substrates (Ma et al., 2022; Zhang, Kuzyakov,
et al.,2020; Zhou, Gui, et al.,2021). Briefly, 1 g of fresh
soil was suspended in 50 ml of sterile water. Then, 50 μl al-
iquot of the soil suspension was pipetted into 96- well mi-
croplates, and mixed with 50 μl of buffer and 100 μl of the
corresponding substrates. The microplates were measured
at 60 and 120 min after substrate addition fluorometrically
at an excitation wavelength of 360 nm and an emission
FIGURE  Schematic diagram of soil
sampling strategy for the perennial energy
crops.
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XU et al.
wavelength of 450 nm (Thermo Fisher Scientific). The
enzyme activities were expressed per SOC unit (i.e., spe-
cific activities) as C is an important determinant of below-
ground functioning (Blagodatskaya & Kuzyakov, 2013;
Sinsabaugh et al.,2009). The three enzyme activities were
averaged to represent the C- acquiring enzyme activities
(Jia et al.,2022; Luo et al.,2018).
2.5
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Incubation experiment
Fresh soil samples from all soil depths (0– 20 cm, 20– 40 cm,
and 40– 60 cm) were weighed (equivalent to 10g dry mass)
and placed into 50 ml polypropylene containers with four
replicates for each treatment. The soil was adjusted to
60% water- holding capacity by adding distilled water. The
bottles were sealed and pre- incubated in the dark at 15
and 25°C for 5 days and then incubated for 40 days at the
corresponding temperatures. During the incubation, the
CO2 evolved from the soils was trapped by 1.5ml NaOH
(1) in a small beaker which was exchanged at 1, 3, 5,
12, and 20 days. The air inside the bottles was changed at
each replacing time via aeration for 30 min to avoid the
anaerobic condition. Soil moisture was maintained (not
generally decreased by more than 10%) during the incuba-
tion by weighing and spraying distilled water evenly over
the soil surface. Four bottles at each temperature without
soil samples were treated in the same way and used as
blanks to correct the CO2 trapped in the air. The efflux of
CO2 trapped in the NaOH solution was measured by ti-
tration with 0.01 HCl against phenolphthalein after the
addition of 1 BaCl2 solution (Zang et al.,2016). Finally,
the specific mineralization was calculated as CO2 release
per unit of SOC.
2.6
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Calculation and statistical analysis
The proportional contributions of the C3 (fC3) and the C4
(fC4) sources to total SOC were calculated according to
Amelung et al.(2008):
where δ13Ct is the δ13C value of the soil under Miscanthus or
switchgrass and δ13C3 is the δ13C value of the corresponding
layer in the reference soil under C3 grasses, δ13C4 indicates
the δ13C value of Miscanthus or switchgrass root. The C3- and
C4- derived C were considered as old and new C hereafter.
The MRT was calculated as the reciprocal of the turn-
over rate as follows (Amelung et al., 2008; Gregorich
et al.,1995):
where k means the turnover rate, t indicates the number of
years after vegetation change (10 years in the present study),
and fC4 is the proportional contribution of the C4 (energy
crop- derived) source to the total C pool.
The SOC stock for a specific layer was calculated as
follows:
where Ci is the SOC stock (t ha1) for different soil layers;
BDi represents the soil bulk density in the corresponding
soil layer, and Hi refers to the thickness of the corresponding
soil layer (m).
The temperature sensitivity (Q10) of SOC mineraliza-
tion was determined based on CO2 efflux rates at two
temperatures at the same incubation date [Equation(5)]
(Zang et al.,2020).
where R25 and R15 are the specific SOC mineralization rates
at 25 and 15°C, respectively.
All statistical analyses were performed with  25.0
software (SPSS Inc.). Normality and homogeneity of vari-
ance (Levene's tests) were confirmed before testing for
significant differences. A two- way analysis of variance
(ANOVA) was conducted to evaluate the main effects
of an energy crop and soil depths as well as their inter-
actions on soil properties. For each dependent variable,
additional one- away ANOVAs with Duncan's multiple
range tests were conducted to determine significant dif-
ferences among energy crops or soil depths. The partial
least squares path modeling (PLS- PM) was conducted to
analyze the direct and indirect effects of energy crops-
derived C, SOC mineralization, temperature sensitivity,
and C- acquiring enzyme activity on the SOC stocks using
the SPLS software (version 3.3.5) after 1000 boot-
straps (Barberán et al.,2014).
3
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RESULTS
3.1
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SOC stock
After 10 years of energy crop cultivation, the SOC stock
between 0 and 60 cm was higher under Miscanthus and
(1)
fC4=
𝛿
13
Ct𝛿
13
C3
𝛿13C
4
𝛿13C
3
,
(2)
f
C
3=1f
C
4,
(3)
MRT
=
1
k
=−tln(1fC4)
,
(4)
Ci=SOC ×BDi×Hi,
(5)
Q10 =R25 R15,
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XU et al.
switchgrass than in the C3 reference grassland (p < 0.05;
Figure2a). The highest SOC stock occurred in the switch-
grass (57.0 ± 1.9t ha1) and Miscanthus (48.6 ± 1.2 t ha1)
root zone, which were 93.0% and 64.3% higher than the
C3 reference grassland, respectively (Figures2a and 6).
Here, Miscanthus and switchgrass cultivation of the root
zone increased the SOC stock between 0– 40 cm (p < 0.05,
Figure2). The SOC stock in the bulk zone of Miscanthus
and switchgrass from 0– 60 cm was 28.3%– 39.6% lower
than the root zone, but 16.5%– 17.8% higher in comparison
to the C3 reference grassland (Figure2). In the bulk zone,
Miscanthus increased the SOC stock between 0 and 20 cm
by 35.2%– 37.0% relative to the C3 reference grassland
(p < 0.05), whereas there was no effect between 20– 60 cm
depth (Figure2). The SOC stock did not statistically differ
at all soil depths in the bulk zone of switchgrass as com-
pared to the C3 reference grassland (Figure2).
3.2
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SOC turnover and derived from
energy crops
Energy crop cultivation increased δ13C values at all soil
depths relative to the C3 reference grassland (p < 0.05,
Figure3a). The δ13C value was higher in the root than in
the bulk zone for switchgrass and Miscanthus. The δ13C
value strongly decreased with depth in the root zone both
in switchgrass and Miscanthus from 16.9‰ to 20.3‰
(p < 0.05), where it marginally decreased with depth in
the bulk zone both in switchgrass and Miscanthus from
19.5‰ to 22.8‰ (p > 0.05). Based on the δ13C values,
the contribution of energy crop- derived C in the root
zone was 0.4– 5.5 times higher than in the bulk zone at
all depths. The amount of C4- derived C was 9.6%– 42.7%
higher under switchgrass than under Miscanthus in the
root zone, where it was 25.2%– 61.7% lower under switch-
grass relative to Miscanthus in the bulk zone (p < 0.05,
Figure3b). The contribution of C4- derived C to SOC be-
tween 0 and 60 cm soil profile was around 44.5% and 32.4%
C under switchgrass and Miscanthus in the root zone,
while it was 6.9% and 13.0% in the bulk zone, respectively
(Figure3c). Furthermore, the MRT of old C rapidly in-
creased with soil depth in the bulk zone under Miscanthus
and switchgrass, where this trend was more pronounced
under switchgrass increasing from 28.3 to 143.2 years
(Figure3d). Conversely, the MRT of old C in the root zone
remained stable between 0 and 60 cm, regardless of energy
crop species (p > 0.05).
3.3
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Soil carbon mineralization and
temperature sensitivity
The specific SOC mineralization between 0 and 60 cm was
lower under Miscanthus and switchgrass than in the C3 ref-
erence grassland at both 15 and 25°C (p < 0.05), except for
soil under Miscanthus in the bulk zone at 15°C (Figure4).
The lower specific SOC mineralization of Miscanthus
(336.1mg C kg1 SOC) and switchgrass (288.1mg C kg1
SOC) in the root zone at 15°C were 26.2% and 36.7% lower
than the C3 reference grassland, respectively (Figure4a).
Similar to the specific SOC mineralization, the Q10 of
Miscanthus from the root and bulk zone were 25.0% and
FIGURE  Soil organic carbon (SOC) stock between 0 and
60 cm after 10 years of cultivation of Miscanthus and switchgrass.
SOC stock of the whole profile (a), 0– 10cm (b), 10– 20 cm (c),
20– 40 cm (d), and 40– 60 cm (e). Values are means (±SE) of three
replicates. Lower- case letters indicate significant differences among
treatments (p < 0.05). CK indicates C3 reference grassland; MR
indicates soil under Miscanthus in the root zone; MB indicates
soil under Miscanthus in the bulk zone; SR indicates soil under
switchgrass in the root zone; SB indicates soil under switchgrass in
the bulk zone.
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XU et al.
18.0% lower than the C3 reference grassland, respectively
(p < 0.05).
3.4
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Factors controlling variation in
SOC stocks
The constructed PLS- PM displayed a good fit (GOF=0.62)
and could explain 46.70% of the variation in the SOC stocks
(Figure5). The PLS- PM also revealed a direct positive ef-
fect of C4- derived C on SOC (1.016), and a direct negative
response of SOC to SOC mineralization (0.623), and Q10
(0.146), as well as C- acquiring enzyme activity (0.300;
Figure5). Overall, energy crops- derived C was the most
important regulator for soil C stocks.
4
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DISCUSSION
4.1
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SOC sequestration
Our results revealed that long- term perennial energy crop
cultivation increases soil C stocks despite the complete re-
moval of aboveground biomass (Figures1a and 6). An in-
creased C stock was also found in a broad range of marginal
lands including saline- alkaline soil, semi- arid degraded
land, and abandoned cropland with biomass removal (Mi
et al.,2014; Xu et al.,2021; Zhao et al.,2020). Although both
Miscanthus and switchgrass cultivation lead to C seques-
tration, they differ in sequestration potential and regula-
tory mechanisms. Specifically, the growth of switchgrass
induced higher SOC stocks than Miscanthus, particularly
in deeper soil layers (40– 60 cm; Figure1). This is irrespec-
tive of similar biomass yields (FigureS1). Therefore, the
higher potential for SOC sequestration of switchgrass can
be attributed to the following two reasons. First, a large
proportion of switchgrass fine roots (with a root diameter
less than 2 mm) extended into the deeper soil layers with
a subsequently greater rhizodeposition than Miscanthus,
thereby stimulating C4- derived C contribution to SOC
(Laurent et al.,2015; Powlson et al.,2011). This was con-
firmed by the higher contribution of switchgrass- derived
C to SOC relative to Miscanthus in the root zone (44.5%
vs. 32.4%; Figure3c). Second, the higher potential stabili-
zation of SOC in response to less C mineralization under
switchgrass (Figure4) also contributed to the high C se-
questration potential under switchgrass. The lower SOC
mineralization under switchgrass was confirmed by the
lower C- acquiring enzyme activities (Figure S3). The
more persistent SOC, reflected by longer MRT, under
switchgrass may also contribute to its high SOC seques-
tration (Sprunger & Robertson,2018). Here, the MRT of
old C3- C under switchgrass was significantly higher than
Miscanthus in the bulk zone (Figure3). This indicates a
FIGURE  Soil organic carbon
(SOC) δ13C values (a), the contribution
of energy crops- deprived C to SOC in
different soil layers (b), the contribution
of energy crops- deprived C to SOC in
the whole soil profile (c), and the MRT
of old C (d) in different soil layers after
10 years of Miscanthus and switchgrass
cultivation. Values are means (±SE) of
three replicates. CK indicates C3 reference
grassland; MR indicates soil under
Miscanthus in the root zone; MB indicates
soil under Miscanthus in the bulk zone;
SR indicates soil under switchgrass in
the root zone; SB indicates soil under
switchgrass in the bulk zone.
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7
XU et al.
lower decomposition rate of old C, thereby facilitating C
sequestration (Zang et al.,2018). Collectively, the higher
C sequestration under switchgrass was driven by both
high C4- C accumulation and SOC stability.
4.2
|
Underlying mechanisms of SOC
sequestration
Our results showed that a greater C4- derived C input is
the most important factor for increasing SOC stocks
(Figure5). After 10 years of cultivation, the contribution of
C4 derived- C to SOC between 0 and 60 cm was more than
one- third in the root zone. Consistent with our results,
Zang et al.(2018) found that 27% of SOC was C4- derived
C after 9 years of Miscanthus cultivation. The PLS- PM
model further confirmed that the C4 derived- C displayed
the greatest total effect on the SOC stocks (Figure5). The
increased new C accumulation in the present study can
be attributed to the large belowground biomass produc-
tion and root exudation of perennial energy crops (Clifton
et al.,2007). The root to shoot ratio of perennial energy
crops is typically greater than 1 and increases with the
duration of growth (Xue et al., 2015). Therefore, peren-
nial grasses allocate more C belowground than con-
ventional grasslands (Mi et al., 2014; Zhao et al., 2020).
Furthermore, as Miscanthus has root crowns, there is a
reduction in the proportion of fine roots and consequently
FIGURE  The specific soil organic carbon (SOC) mineralization at 15°C (a) and 25°C (b) and temperature sensitivity (c) between 0
and 60 cm after 10 years of Miscanthus and switchgrass cultivation. Values are means (±SE) of three replicates. CK indicates C3 reference
grassland; MR indicates soil under Miscanthus in the root zone; MB indicates soil under Miscanthus in the bulk zone; SR indicates soil under
switchgrass in the root zone; SB indicates soil under switchgrass in the bulk zone.
8
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XU et al.
reduced C4- derived C compared to switchgrass (Bazrgar
et al.,2020; Zan et al., 2001). This could be the reason why
new C accumulation and subsequent SOC stocks were
lower for Miscanthus compared to switchgrass in this
study.
Perennial energy crop cultivation increased C stocks by
reducing SOC mineralization and temperature sensitivity
(Figures4 and 5). Here these crops produce a large amount
of belowground biomass resulting in enhanced root exu-
dation facilitating the aggregate formation and enhancing
the physical protection against microbial decomposition
(Gioacchini et al., 2016; Tiemann & Grandy, 2015; Yan
et al., 2022). This was supported by a large proportion
of macroaggregates under Miscanthus and switchgrass
compared with the C3 grassland (FigureS4). Specifically,
the reduced mineralization was more pronounced in the
root zone than in the bulk zone under both Miscanthus
and switchgrass, which may be due to the negative prim-
ing effect caused by perennial energy crop cultivation
(Gauder et al.,2016). The lower pH in the root zone under
Miscanthus and switchgrass relative to the bulk zone be-
tween 0 and 60 cm (TableS1) could constrain microbial
functioning, thereby decreasing SOC mineralization
(Malik et al.,2018). Furthermore, the lower temperature
sensitivity of SOC mineralization under energy crop cul-
tivation suggests a more stable and resistant SOC in the
FIGURE  The partial least squares path modeling showing the direct and indirect effects (a) and total effects (b) of energy crops-
derived C, soil organic carbon (SOC) mineralization, temperature sensitivity (Q10), and C- acquiring enzyme activity, and SOC on the SOC
stock. Each box represents an observed (i.e., measured) or latent variable (i.e., constructs). Red and blue arrows indicate positive and
negative flows of causality (p < 0.05), respectively. Numbers on the arrow indicate significant standardized path coefficients. R2 indicates the
variance of the dependent variable explained by the model.
FIGURE  Graphical abstract
illustrating marginal land conversion
to perennial energy crops enhances soil
carbon sequestration and its underlying
mechanisms. Miscanthus and switchgrass
cultivation increased soil C stocks in both
root and bulk zoon compared to reference
marginal land. New C4- C input exceeds
old C3- C losses via mineralization leading
to C sequestration. SOC, soil organic
carbon.
|
9
XU et al.
context of global warming (Kan et al.,2020). This suggests
that the accumulated SOC under perennial energy crops
can be retained in the soil over longer time frames and
thereby increasing the SOC sequestration.
The distinct driving pathway of SOC sequestration by
these perennial energy crops was presented in the root
zone and bulk zone. In the root zone, perennial energy
crops sequestrated SOC via a fast SOC turnover rate where
new C4- derived C replaced the old C3- C at a rate sufficient
to offset losses. In general, the root zone is characterized
as a microbial hotspot where enhanced plant- derived C
inputs are likely easily metabolized by microorganisms,
resulting in an intense turnover of microbial biomass and
a larger accumulation of necromass (Angst et al., 2021;
Kuzyakov & Blagodatskaya, 2015). This was supported
by higher C- acquiring enzyme activities and higher C4-
derived C in the root zone relative to the bulk zone under
switchgrass and Miscanthus (FiguresS1 and S3). In con-
trast to the root zone, the increased C stock in the bulk
zone was mainly ascribed to the increased stability of old
C. The high stability of old C was reflected by the higher
MRT of old C in the bulk zone relative to the root zone
(Figure3 and FigureS2), which indicated that the low de-
composition rate of old C leads to higher persistence in
soil (Rahmati et al., 2020). Consistent with this, the C3-
derived SOC was lower in the root zone relative to the
bulk zone under perennial energy crops (FigureS2). In ad-
dition, the C4- derived C was relatively low (<15%) in the
bulk zone, especially in the subsoil (20– 60 cm), suggesting
less contribution of C4- derived C for SOC sequestration in
the bulk zone. Collectively, the increased stability of old
C was the dominant driving factor for C sequestration in
the bulk soil.
The present study revealed the main controlling fac-
tors for soil C sequestration under energy crops in mar-
ginal land, which advances our knowledge of C dynamics
within bioenergy systems. An improved understanding
of C dynamics within bioenergy systems will also help to
project C sequestration globally. Additionally, this knowl-
edge could support environmental management strategies
associated with biofuel production, while also informing
policy development and financial incentives available to
landowners (e.g., C offsets), which can encourage farm-
ers to convert degraded agricultural lands into more sus-
tainable and environmentally benign biomass production
systems.
5
|
CONCLUSION
Overall, our results showed that switchgrass had a higher
C sequestration potential in deep soil layers within
the root zone, while Miscanthus had a broader effect
on C sequestration, particularly between 0 and 20 cm.
Switchgrass is thereby preferred over Mischanthus as an
energy crop for marginal land cultivation due to compa-
rable biomass yield but much higher C sequestration. The
large proportion of energy crop- derived C was the most
important factor contributing to increased soil C stock.
In addition, the increased C stability and reduced C min-
eralization were also important factors influencing C se-
questration. The pathway of C sequestration within this
system was mainly via the fast replacement of old C with
C4- derived C in the root zone. In conclusion, marginal
land conversion to perennial energy crops has the poten-
tial to provide biomass feedstocks for renewable energy
and contribute to climate change mitigation.
ACKNOWLEDGMENTS
This study was funded by the National Natural Science
Foundation of China (32101850), the Young Elite Scientists
Sponsorship Program by CAST (2020QNRC001), and the
Joint Funds of the National Natural Science Foundation
of China (U21A20218).
CONFLICT OF INTEREST
The authors declare that they have no known competing
financial interests or personal relationships that could
have appeared to influence the work reported in this
paper.
DATA AVAILABILITY STATEMENT
Data openly available in a public repository. The data that
support the findings of this study are openly available in
researchgate (https://www.resea rchga te.net/). The DOI of
the data is https://doi.org/10.13140/ RG.2.2.12463.00163.
ORCID
Yi Xu https://orcid.org/0000-0002-2549-1446
Jie Zhou https://orcid.org/0000-0002-5896-4529
Shuai Xue https://orcid.org/0000-0002-4397-1013
Thomas Guillaume https://orcid.
org/0000-0002-6926-9337
Leanne Peixoto https://orcid.org/0000-0002-8081-0569
Huadong Zang https://orcid.org/0000-0002-2008-143X
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SUPPORTING INFORMATION
Additional supporting information can be found online
in the Supporting Information section at the end of this
article.
How to cite this article: Xu, Y., Zhou, J., Feng,
W., Jia, R., Liu, C., Fu, T., Xue, S., Yi, Z., Guillaume,
T., Yang, Y., Peixoto, L., Zeng, Z., & Zang, H.
(2022). Marginal land conversion to perennial
energy crops with biomass removal enhances soil
carbon sequestration. GCB Bioenergy, 00, 1–11.
https://doi.org/10.1111/gcbb.12990

Supplementary resource (1)

... Enzyme activities belonging to the same functional group were normalized to evaluate the activities of enzymes involved in C (BG, BX, CE), N (NAG and LAP) and P (AP) cycling Luo et al., 2018;Xu et al., 2022). For example, C acquisition was calculated using the followings: ...
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