sustainability
Article
Increasing Soil Organic Matter Enhances Inherent
Soil Productivity while Offsetting Fertilization
Effect under a Rice Cropping System
Ya-Nan Zhao 1 , Xin-Hua He 1,2 , Xing-Cheng Huang 1 , Yue-Qiang Zhang 1,3 and Xiao-Jun Shi 1,3, *
1
2
3
*
College of Resources and Environment, Southwest University, Chongqing 400715, China;
[email protected] (Y.-N.Z.); [email protected] (X.-H.H.); [email protected] (X.-C.H.);
[email protected] (Y.-Q.Z.)
School of Plant Biology, University of Western Australia, Crawley, WA 6009, Australia
National Monitoring Station of Soil Fertility and Fertilizer Efficiency on Purple Soils,
Chongqing 400715, China
Correspondence: [email protected]; Tel.: +86-23-6825-0146
Academic Editor: Iain Gordon
Received: 17 June 2016; Accepted: 29 August 2016; Published: 1 September 2016
Abstract: Understanding the role of soil organic matter (SOM) in soil quality and subsequent crop
yield and input requirements is useful for agricultural sustainability. SOM is widely considered
to affect a wide range of soil properties, however, great uncertainty still remains in identifying the
relationships between SOM and crop yield due to the difficulty in separating the effect of SOM from
other yield-limiting factors. Based on 543 on-farm experiments, where paired treatments with and
without NPK fertilizer were conducted during 2005–2009, we quantified the inherent soil productivity,
fertilization effect, and their contribution to rice yield and further evaluated their relationships
with SOM contents under a rice cropping system in the Sichuan Basin of China. The inherent soil
productivity assessed by rice grain yield under no fertilization (Y-CK) was 5.8 t/ha, on average,
and contributed 70% to the 8.3 t/ha of rice yield under NPK fertilization (Y-NPK) while the other 30%
was from the fertilization effect (FE). No significant correlation between SOM content and Y-NPK
was observed, however, SOM content positively related to Y-CK and its contribution to Y-NPK
but negatively to FE and its contribution to Y-NPK, indicating an increased soil contribution but a
decreased fertilizer contribution to rice yield with increasing SOM. There were significantly positive
relationships between SOM and soil available N, P, and K, indicating the potential contribution of
SOM to inherent soil productivity by supplying nutrients from mineralization. As a result, approaches
for SOM accumulation are practical to improve the inherent soil productivity and thereafter
maintain a high crop productivity with less dependence on chemical fertilizers, while fertilization
recommendations need to be adjusted with the temporal and spatial SOM variation.
Keywords: soil organic carbon; fertilizer; crop productivity; rice yield; paddy soil; soil fertility
1. Introduction
The whole world, particularly developing countries including China, currently faces huge
challenges to achieve agricultural sustainability while ensuring food security, environmental health,
and greenhouse gas emission mitigation [1]. In recent decades, SOM accumulation, and hence soil
organic carbon (SOC) sequestration, has been given much attention as a climate change mitigation
option on global and regional scales to address the rapidly increasing CO2 in the atmosphere [2].
On the other hand, SOM accumulation is an important option, not only to mitigate climate change,
but also to improve soil quality because of its extensive impacts on soil physical, chemical, and
biological properties [3,4]. In previous studies, changes of SOM in croplands have been quantified
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using different scales in China [5,6], however, the potential influence of the SOM change on crop
yields and input requirements in the farmland systems is unclear because great uncertainty about the
relationship between SOM and crop yield still remains.
There were a number of studies about the effects of SOM on crop yield, however, results were
inconsistent. Studies on individual sites demonstrated a positive effect of increasing SOM content
on crop yields for a variety of crops and locations [7–9]. In reviews of studies, Lal [10–12] stressed
the double benefits from SOM accumulation in crop yield increase and organic carbon sequestration,
thereby enhancing global food security and mitigating climate change, particularly in instances where
SOM was depleted. Based on a statistic dataset, Pan et al. [13] found that SOM was positively correlated
with crop productivity but negatively correlated with yield variability at a province level in China,
although their analysis did not account for other variables that might explain yield. The long-term
experiment at Rothamsted and Woburn showed that yields for a rotation of potatoes, winter wheat,
sugar beet, and spring barley were always larger on soils holding more organic matter, despite equal
levels of nitrogen (N) application [3]. However, Alvarez and Grigera [14] found that growing season
precipitation was the factor more closely associated with wheat and corn yield, while organic matter
had no detectable influence on wheat and corn yield in the semi-arid Argentine Pampas. In temperate
regions there is little quantitative evidence to indicate that a reduction in SOM would have a marked
effect on crop yield [15]. Furthermore, based on the analysis of a large dataset, Oelofse et al. [16]
challenged the importance of SOM in contributing to crop yield in context with similar soils and
climates, and proposed that further studies were thus required to elucidate the effect of SOM on
crop yields.
SOM can contribute to soil quality and subsequent crop yield in a number of ways, for instance,
nutrient cycling and supplying during its decomposition, aggregate stability and soil porosity,
water-holding capacity especially available water, and cation exchange capacity [3,4]. On the other
hand, the crop yield is a consequence of interactions among intrinsic soil properties, external climatic
conditions, and management strategies including fertilization, irrigation, and tillage. Therefore, it is
difficult to predict the overall effect of SOM on crop productivity and to identify and quantify which
attributes of SOM contribute to this effect. Among many attributes, the contribution of SOM to the
supply of indigenous nutrients for plant growth by mineralization is one of the important aspects that
affects crop yields [17,18], but this effect might be confused with applied mineral nutrients. The crop
yield under no-fertilization, defined as inherent soil productivity, can reflect the primary capacity
of soil itself for food production [19]. As a result, partitioning inherent soil productivity from the
fertilization effect and then evaluating their response to SOM would promote our understanding of
the relationship between SOM and crop yield.
Based on a total of 543 on-farm experiments across a range of SOM contents with similar climates
and soils under rice cropping systems in the Sichuan Basin of Southwest China, the purpose of this
paper is (1) to quantify the inherent soil productivity, fertilization effect, and their contributions
to crop yield; and (2) to investigate the relationship between SOM and inherent soil productivity,
the fertilization effect, and their contributions to rice yield.
2. Materials and Methods
2.1. Study Site
Multi-field experiments were conducted over 2.6 × 105 km2 of the Sichuan Basin, located in
the east part of Sichuan Province (97◦ 210 E–108◦ 310 E, 26◦ 030 N–34◦ 190 N) and most of the Chongqing
Municipal City (105◦ 110 E–110◦ 110 E, 28◦ 100 N–32◦ 130 N), China. With ~5 million ha of intensive
agricultural farmlands, this basin accounts for 11% of both the cultivation and production of rice
(Oryza sativa L.) for the whole country. The Sichuan Basin has a subtropical monsoon climate
with 14–19 ◦ C mean annual temperature, 920–1570 mm mean annual evaporation, 1000–1200 mm
mean annual rainfall and 270–363 mean annual frost-free days. The landscape in this basin is mostly
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characterized as low hills and alluvial plains between 200 m and 1500 m above the sea level. The major
soil type
in this 2016,
basin
is classified as Purpli-Udic Cambosols with a typical purplish color. 3 of 12
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2.2. Experimental
Design
2.2. Experimental
Design
During
the 2005–2009
croppingseason,
season, on-farm
on-farm experiments
were
conducted
at 543atsites
During
the 2005–2009
ricerice
cropping
experiments
were
conducted
543 sites
(11
in
2005,
115
in
2006,
171
in
2007,
210
in
2008
and
36
in
2009)
located
in
87
counties
or districts
(11 in 2005, 115 in 2006, 171 in 2007, 210 in 2008 and 36 in 2009) located in 87 counties
or districts
(counties hereafter), which represent a range of climate and soil variation in the Sichuan Basin as
(counties hereafter), which represent a range of climate and soil variation in the Sichuan Basin as
above mentioned (Figure 1). The cropping system consisted of a single crop of rice per year (a range
above of
mentioned
(Figure 1). The cropping system consisted of a single crop of rice per year (a range of
varieties or cultivars with similar yield potential) transplanted in April and harvested in August,
varieties
cultivars
withwas
similar
in from
AprilAugust
and harvested
inthe
August,
andorthe
paddy field
then yield
underpotential)
fallow withtransplanted
flooding water
to April of
next and
the paddy
year. field was then under fallow with flooding water from August to April of the next year.
1. Geographic locations of 543 on-farm experimental sites during the 2005–2009 rice seasons
FigureFigure
1. Geographic
locations of 543 on-farm experimental sites during the 2005–2009 rice seasons
in 76 counties (the grey areas) of the Sichuan Basin, China.
in 76 counties (the grey areas) of the Sichuan Basin, China.
In each field site, all data were collected from two treatments with or without chemical nitrogen
phosphorus
(K) fertilizers.
Annual
chemicalwith
fertilization
rates chemical
between 2005
In(N),
each
field site,(P),
all and
datapotassium
were collected
from two
treatments
or without
nitrogen
and 2009 were
based
local farm (K)
practice
and averaged
150chemical
kg N, 80 kg
P2O5, and 84rates
kg K2between
O per ha 2005
(N), phosphorus
(P),
andon
potassium
fertilizers.
Annual
fertilization
andwere
ranged
from on
45–278
kgfarm
N, 27–150
kg Pand
2O5 and 15–210 kg K2O per ha. The fertilizers applied were
and 2009
based
local
practice
averaged 150 kg N, 80 kg P2 O5 , and 84 kg K2 O per ha
urea, calcium superphosphate, and potassium chloride. Half of the N and all of the P and K
and ranged from 45–278 kg N, 27–150 kg P2 O5 and 15–210 kg K2 O per ha. The fertilizers applied
fertilizers were applied to paddy soils during plowing prior to transplanting, while the other half of
were urea, calcium superphosphate, and potassium chloride. Half of the N and all of the P 2and K
the N was top-dressed at the rice tiller or panicle initiation stage. The plot sizes were at least 20 m
fertilizers
applied
to paddybut
soils
plowing
to transplanting,
while
the other
withwere
at least
three replicates
the during
replicates
were notprior
necessarily
the same size,
according
to thehalf of
2
the N experimental
was top-dressed
at
the
rice
tiller
or
panicle
initiation
stage.
The
plot
sizes
were
at
least
field. To avoid nutrients flowing with water between plots, each plot was isolated all20 m
with atyear
least
three
butbaffle
the replicates
werewith
notwaterproof
necessarily
the sheeting.
same size, according to the
round
by replicates
40 cm tall soil
plates wrapped
plastic
experimental field. To avoid nutrients flowing with water between plots, each plot was isolated all
2.3. Soil
Measurement
year round
byand
40Plant
cm tall
soil baffle plates wrapped with waterproof plastic sheeting.
Soil samples at 0–20 cm depth were collected with 5 cm diameter angers before rice planting. To
2.3. Soil
and Plant
Measurement
ensure
homogeneity,
five cores as one composite sample were randomly taken from each site. While
still moist, the soil was gently broken into small clods by hand in the laboratory. After debris
Soil samples at 0–20 cm depth were collected with 5 cm diameter angers before rice planting.
removal, soils were air-dried and then sieved through 0.25 mm mesh for the analyses of SOM
To ensure
homogeneity,
coresmethod
as one with
composite
were randomly taken from each site.
according
to the wetfive
digestion
H2SO4-Ksample
2Cr2O7 [20]. The chemical properties for the
While bulk
still soil
moist,
soilsites
was(Mean
gently
broken
into
smallpH
clods
in the
fromthe
the 543
± SD)
were as
follows:
6.3 ± by
1.1, hand
SOM 26.2
± 8.0laboratory.
g/kg, total N After
debris1.45
removal,
air-dried andNthen
0.25 mm
mesh
formg/kg,
the analyses
of
± 0.54 soils
g/kg, were
Alkali-hydrolyzable
135.3sieved
± 43.9through
mg/kg, Olsen-P
12.4
± 12.1
and
NH4OAc-Kto
86.7
32.7 mg/kg.
Drymethod
weight of
grain
was
calculated
from
the
crops
harvested
SOM according
the± wet
digestion
with
Hyields
SO
-K
Cr
O
[20].
The
chemical
properties
for
2
4 2
2 7
from
a 5from
m2 section
of the
middle
area±
ofSD)
eachwere
plot with
a 14% moisture
adjustment.
the bulk
soil
the 543
sites
(Mean
as follows:
pH 6.3
± 1.1, SOM 26.2 ± 8.0 g/kg,
total N 1.45 ± 0.54 g/kg, Alkali-hydrolyzable N 135.3 ± 43.9 mg/kg, Olsen-P 12.4 ± 12.1 mg/kg,
and NH4 OAc-K 86.7 ± 32.7 mg/kg. Dry weight of grain yields was calculated from the crops harvested
from a 5 m2 section of the middle area of each plot with a 14% moisture adjustment.
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2.4. Data Calculations and Statistical Analyses
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The plant-based agronomic approach was used to assess inherent soil productivity in this
DataIn
Calculations
and Statistical
Analyses
study2.4.
[19].
this approach,
the yield
under no-fertilization was defined as the inherent soil
productivity
while
the fertilizer
effect
the difference
between
Y-CKinand
rice yield
The(Y-CK)
plant-based
agronomic
approach
was(FE)
usedwas
to assess
inherent soil
productivity
this study
under[19].
the combination
of
chemicals
N,
P,
and
K
fertilization
(Y-NPK).
For
each
experiment,
the
relative
In this approach, the yield under no-fertilization was defined as the inherent soil productivity
contribution
of
soil
(CS)
and
contribution
of
fertilizer
(CF)
to
crop
yield
was
defined
as
a
percentage
(Y-CK) while the fertilizer effect (FE) was the difference between Y-CK and rice yield under the of
chemicals
N, P, and mentioned
K fertilization
(Y-NPK).
For each experiment,
Y-CK combination
or FE to riceof
yield.
All parameters
above
were calculated
as follows: the relative
contribution of soil (CS) and contribution of fertilizer (CF) to crop yield was defined as a percentage
of Y-CK or FE to rice yield. All parameters
above
FE (t/ha) mentioned
= Y − NPK
− Ywere
− CKcalculated as follows:
(1)
FE (t/ha) = Y-NPK − Y-CK
(1)
CS (%) = Y-CK/Y-NPK × 100%
(2)
CF (%) = FE/Y-NPK × 100%
(3)
CS (%) = Y − CK/Y − NPK × 100%
CF (%) = FE/Y − NPK × 100%
(2)
(3)
The Pearson correlation analysis was used to assess correlations among SOM and these parameters.
The Pearson
correlation
waswith
used SigmaPlot
to assess correlations
among
SOM Inc.,
and San
theseJose,
All statistical
calculations
were analysis
performed
12.0 (Systat
Software
parameters.
All
statistical
calculations
were
performed
with
SigmaPlot
12.0
(Systat
Software
Inc.,
CA, USA).
San Jose, CA, USA).
3. Results
3. Results
3.1. Y-NPK, Y-CK and FE
3.1. Y-NPK, Y-CK and FE
Although a great variation among yields was observed, the rice yield under NPK fertilization
Although a great variation among yields was observed, the rice yield under NPK fertilization
(Y-NPK)
ranged from 5.6 t/ha to 11.5 t/ha and averaged 8.3 t/ha in the Sichuan Basin (Figure 2a).
(Y-NPK) ranged from 5.6 t/ha to 11.5 t/ha and averaged 8.3 t/ha in the Sichuan Basin (Figure 2a). The
The average
assessedby
byrice
riceyield
yieldunder
under
no-fertilization
(Y-CK)
5.8 t/ha
averageinherent
inherent soil
soil productivity
productivity assessed
no-fertilization
(Y-CK)
waswas
5.8 t/ha
and accounted
for 70%
of the
average
NPK fertilization
fertilization
(Y-NPK)
(Figure
and accounted
for 70%
of the
averagerice
riceyield
yield under
under NPK
(Y-NPK)
(Figure
2b,d).2b,d).
Correspondingly,
the the
fertilization
chemicalNPK
NPK
averaged
2.5 t/ha
and contributed
Correspondingly,
fertilizationeffect
effect(FE)
(FE) of
of chemical
averaged
2.5 t/ha
and contributed
to
30%
of Y-NPK
(Figure2c,e).
2c,e).
to 30%
of Y-NPK
(Figure
25
Mean=8.3
SD=1.2
Median=8.2
Min=5.6
Max=11.5
25%=7.5
75%=9.1
a
Percentage (%)
20
15
10
5
0
6
6.5
7
7.5
8
8.5
9
9.5
10 10.5 11
Y-NPK (t/ha)
25
25
Mean=5.8
SD=1.2
Median=5.8
Min=2.4
Max=9.1
25%=4.9
75%=6.6
b
Percentage (%)
20
15
15
10
10
5
5
0
Mean=2.5
SD=1.0
Median=2.5
Min=0.0
Max=5.9
25%=1.9
75%=3.2
c
20
0
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
0.5
1
1.5
2
Y-CK (t/ha)
3
3.5
4
4.5
5
5.5
25
Mean=69.5
SD=10.9
Median=69.9
Min=40.1
Max=99.3
25%=62.2
75%=76.9
d
20
Percentage (%)
2.5
FE (t/ha)
25
15
15
10
10
5
5
0
Mean=30.5
SD=10.8
Median=30.0
Min=0.7
Max=59.9
25%=23.0
75%=37.7
e
20
0
45
50
55
60
65
70
CS (%)
75
80
85
90
95
5
10
15
20
25
30
35
40
45
50
55
CF (%)
2. Frequency distribution of rice yield with (a) NPK fertilizers (Y-NPK) and without (b) NPK
FigureFigure
2. Frequency
distribution of rice yield with (a) NPK fertilizers (Y-NPK) and without (b) NPK
fertilizers (Y-CK), (c) fertilization effect (FE), and (d) contribution of Y-CK (CS) and (e) contribution
fertilizers (Y-CK), (c) fertilization effect (FE), and (d) contribution of Y-CK (CS) and (e) contribution of
of fertilizer (CF) to Y-NPK.
fertilizer (CF) to Y-NPK.
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shown
in Figure
3, Y-CK
was positively
correlated
to Y-NPK
(r p= <0.61,
p but
< 0.05),
but
AsAs
shown
in Figure
3, Y-CK
was positively
correlated
to Y-NPK
(r = 0.61,
0.05),
negatively
As shown in Figure 3, Y-CK was positively correlated to Y-NPK (r = 0.61, p < 0.05), but
negatively
to FE
(r
= −0.44,
p < 0.05),
indicating
that high
rice
yield under
fertilizer
application
but
to FE
(r
=
−
0.44,
p
<
0.05),
indicating
that
high
rice
yield
under
fertilizer
application
but
low
fertilization
negatively to FE (r = −0.44, p < 0.05), indicating that high rice yield under fertilizer application but
low
fertilization
effect
were
likely
to
be
obtained
in
fields
with
high
inherent
soil
productivity.
effect
likely toeffect
be obtained
in fields
with high
soil
productivity.
lowwere
fertilization
were likely
to be obtained
ininherent
fields with
high
inherent soil productivity.
12000
12000
Y-NPK
Y-NPKand
andFE
FE(kg/ha)
(kg/ha)
10000
10000
● Y-CK vs. Y-NPK
●
vs. Y-NPK
y =Y-CK
0.6042x
+ 4828
yr == 0.6139,
0.6042xP+ <4828
0.05
r = 0.6139, P < 0.05
8000
8000
6000
6000
4000
4000
2000
2000
0
0
○ Y-CK vs. FE
○
vs. FE
y =Y-CK
-0.3745x
+ 4702
yr == 0.4442,
-0.3745xP+<4702
0.05
r = 0.4442, P < 0.05
3000
4000
3000
4000
5000
5000
6000
6000
7000
7000
8000
8000
9000
9000
Y-CK (kg/ha)
Y-CK (kg/ha)
Figure3.3.Relationships
Relationships between
between Y-CK
Figure
Y-CK and
andY-NPK
Y-NPKor
orFE.
FE.
Figure 3. Relationships between Y-CK and Y-NPK or FE.
3.2.
Relationships
between SOMand
and Y-NPK,Y-CK
Y-CK orFE
FE
3.2.3.2.
Relationships
between
Relationships
betweenSOM
SOM andY-NPK,
Y-NPK, Y-CKor
or FE
The SOM content varied from 5.0 g/kg to 50.6 g/kg and averaged 26.2 g/kg among all sites (Figure
The
SOM
from5.0
5.0g/kg
g/kg
to 50.6
and averaged
26.2
g/kgall
among
all sites
The
SOMcontent
content varied
varied from
to 50.6
g/kgg/kg
and averaged
26.2 g/kg
among
sites (Figure
4). No significant correlation between SOM content and rice yield under NPK fertilization (Y-NPK)
4). No4).
significant
correlation
betweenbetween
SOM content
rice yield
under
NPK
fertilization
(Y-NPK)
(Figure
No significant
correlation
SOMand
content
and rice
yield
under
NPK fertilization
was observed (Figure 5a). However, after partitioning the fertilization effect and inherent soil
was observed
(Figure(Figure
5a). However,
after partitioning
the fertilization
effect and
inherent
soil
(Y-NPK)
was observed
5a). However,
after partitioning
the fertilization
effect
and inherent
productivity, SOM contents positively related to Y-CK (r = 0.21, p < 0.05), but negatively to FE
productivity,
SOM
contents
positively
related
to
Y-CK
(r
=
0.21,
p
<
0.05),
but
negatively
to
FEFE
soil productivity, SOM contents positively related to Y-CK (r = 0.21, p < 0.05), but negatively to
(r = −0.23, p < 0.05) (Figure 5b). Similarly, SOM contents positively related to the relative contribution
= 0.23,
−0.23,pp<<0.05)
0.05)(Figure
(Figure 5b).
5b). Similarly,
SOM contents
positively
related totothe
relative
contribution
(r =(r
Similarly,
contents
positively
the
relative
contribution
of−inherent
soil productivity
(CS,
r = 0.26, SOM
p < 0.05),
but negatively
torelated
fertilization
effect
to Y-NPK
(CF,
of
inherent
soil
productivity
(CS,
r
=
0.26,
p
<
0.05),
but
negatively
to
fertilization
effect
to
Y-NPK
(CF,
of inherent
soil
productivity
(CS,
r
=
0.26,
p
<
0.05),
but
negatively
to
fertilization
effect
to
Y-NPK
(CF,
r = −0.26, p < 0.05) (Figure 5c).
= 0.26,
−0.26,pp<<0.05)
0.05)(Figure
(Figure 5c).
5c).
r =r−
6
6
Mean=26.2
Mean=26.2
SD=8.0
SD=8.0
Median=25.8
Median=25.8
Min=5.0
Min=5.0
Max=50.6
Max=50.6
25%=20.8
25%=20.8
75%=31.5
75%=31.5
Percentage
Percentage(%)
(%)
5
5
4
4
3
3
2
2
1
1
0
0 0
0
5
5
10
10
15
15
20
20
25
25
30
30
35
35
SOM content (g/kg)
SOM content (g/kg)
40
40
45
45
50
50
55
55
Figure
4.
Frequency distribution of
of soil
soil organic
matter
(SOM)
content
in plough
layer
(0–20
cm) of
Figure
4. 4.
Frequency
organicmatter
matter(SOM)
(SOM)
content
in plough
layer
(0–20
cm) of
Figure
Frequencydistribution
distribution of soil
organic
content
in plough
layer
(0–20
cm) of
experimental
fields.
experimental
fields.
experimental fields.
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879
66 of
of 12
12
12000
a
Y-NPK (kg/ha)
11000
10000
9000
8000
7000
6000
5000
● SOM vs. Y-NPK NS
9000
10 Y-CK15
●5SOM vs.
Y-CK and FE (t/ha)
8000
y = 31.24x + 4983
r = 0.2116, P < 0.05
20
25
30
35
40
45
50
40
45
50
40
45
50
SOM content (g/kg)
b
7000
6000
5000
4000
3000
2000
1000
0
100.0
90.0
○ SOM vs. FE
y = -28.46x + 3270
r = 0.2310, P < 0.05
10 CS 15
●5SOM vs.
y = 0.3608x + 60.25
r = 0.2605, P < 0.05
20
25
30
35
c
SOM content (g/kg)
CS and CF (%)
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
○ SOM vs. CF
y = -0.3489x + 39.34
r = 0.2592, P < 0.05
5
10
15
20
25
30
35
SOM content (g/kg)
Figure
between
SOMSOM
content
and Y-NPK,
Y-CK, fertilization
effect (FE), contribution
Figure 5.5.Relationships
Relationships
between
content
and Y-NPK,
Y-CK, fertilization
effect (FE),
of
Y-CK (CS) of
or Y-CK
FE (CF)
to Y-NPK.
(a) to
Y-NPK;
(b)(a)
Y-CK
and fertilization
effect
(FE) and effect
(c) CS(FE)
andand
CF.
contribution
(CS)
or FE (CF)
Y-NPK.
Y-NPK;
(b) Y-CK and
fertilization
(c) CS and CF.
No significant correlations between SOM content and N, P, or K fertilizer rates were detected
No6),
significant
content
and N,confounding
P, or K fertilizer
rates were
detected
(Figure
excludingcorrelations
the potentialbetween
variableSOM
of fertilizer
amount
the negative
relationships
(Figure
6),
excluding
the
potential
variable
of
fertilizer
amount
confounding
the
negative
relationships
between SOM and FE.
between SOM and FE.
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7 of 12
300
a
Fertilizer N rate (kg/ha)
250
200
150
100
NS
50
0
5
Fertilizer P2O5 rate (kg/ha)
b
15
20
25
30
35
40
45
50
SOM content (g/kg)
150
120
90
60
30
0
Fertilizer K2O rate (kg/ha)
10
NS
c5
10
15
20
25
30
35
40
45
50
SOM content (g/kg)
200
160
120
80
40
0
NS
5
10
15
20
25
30
35
40
45
50
SOM content (g/kg)
Figure 6. Relationships between SOM content and nitrogen (N), phosphorus (P), and potassium (K)
Figure 6. Relationships between SOM content and nitrogen (N), phosphorus (P), and potassium (K)
fertilizer rates. (a) Fertilizer N rate (kg/ha); (b) Fertilizer P2O5 rate (kg/ha) and (c) Fertilizer K2O rate
fertilizer rates. (a) Fertilizer N rate (kg/ha); (b) Fertilizer P2 O5 rate (kg/ha) and (c) Fertilizer K2 O
(kg/ha).
rate (kg/ha).
3.3. Relationship between SOM and Soil Nutrients
3.3. Relationship between SOM and Soil Nutrients
As showed in Figure 7, there was a significantly positive correlation between SOM contents and
As showed in Figure 7, there was a significantly positive correlation between SOM contents and
soil available nutrients (p < 0.05), indicating that supplying nutrients from mineralization is one of
soil available nutrients (p < 0.05), indicating that supplying nutrients from mineralization is one of
the important contributions of SOM to crop yield. Due to the majority of soil N existing in an organic
the important contributions of SOM to crop yield. Due to the majority of soil N existing in an organic
form, the soil alkali-hydrolyzable N was strongly correlated with SOM content (r = 0.57). Albeit
weak, the relationship between SOM content and Olsen-P (r = 0.12) and NH4Ac-K (r = 0.17) was
significant.
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form, the soil alkali-hydrolyzable N was strongly correlated with SOM content (r = 0.57). Albeit weak,
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the relationship between SOM content and Olsen-P (r = 0.12) and NH4 Ac-K (r = 0.17) was significant.
Soil alkali-hydrolyzable N (mg/kg)
350
300
250
200
150
100
50
0
80
70
Soil Olsen-P (mg/kg)
a
y = 3.15x + 52.49
r = 0.5710, P < 0.05
5
10
15
20
25
30
35
40
45
50
5
10
15
20
25
30
35
40
45
50
5
10
15
20
25
30
35
40
45
50
y = 0.18x + 7.37
r = 0.12, P < 0.05 SOM content (g/kg)
b
60
50
40
30
20
10
0
240
Soil NH4OAc-K (mg/kg)
200
y = 0.71x + 68.27
r = 0.17, P < 0.05 SOM content (g/kg)
c
160
120
80
40
0
SOM content (g/kg)
Figure
OAc-K.
Figure 7.
7. Relationships
Relationships between
between SOM
SOM content
content and
and soil
soil alkali-hydrolyzable
alkali-hydrolyzable N,
N, Olsen-P
Olsen-P or
or NH
NH44OAc-K.
(a)
Soil
alkali-hydrolyzable
N
(mg/kg);
(b)
Soil
Olsen-P
(mg/kg)
and
(c)
Soil
NH
OAc-K
(mg/kg).
(a) Soil alkali-hydrolyzable N (mg/kg); (b) Soil Olsen-P (mg/kg) and (c) Soil NH44OAc-K (mg/kg).
4. Discussion
The 8.3 t/ha of average rice yield under fertilization in the Sichuan Basin (Figure 2) was 30%
higher than that in the whole of China at 6.4 t/ha and 90% higher than that from around the world at
4.4 t/ha in the same period from 2005–2009 based on statistical data [21]. The fertilization effect was
similar with results from the same rice cropping system in the Hubei province, which is nearby the
Sichuan Basin, from 2006–2008 [22]. However, the inherent soil productivity (5.8 t/ha) was higher than
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4. Discussion
The 8.3 t/ha of average rice yield under fertilization in the Sichuan Basin (Figure 2) was 30%
higher than that in the whole of China at 6.4 t/ha and 90% higher than that from around the world
at 4.4 t/ha in the same period from 2005–2009 based on statistical data [21]. The fertilization effect was
similar with results from the same rice cropping system in the Hubei province, which is nearby the
Sichuan Basin, from 2006–2008 [22]. However, the inherent soil productivity (5.8 t/ha) was higher
than that in five tropical Asian nations (3.3–4.9 t/ha) [23] while the contribution of chemical NPK
fertilization to rice yields in this study (30%) was at the bottom of the range of a 30%–60% contribution
for major crops including corn, wheat, rice, soybean, and cowpea in the USA and England [24]. These
results indicated the importance of inherent soil productivity in rice yield, which is demonstrated by
the positive correlation of Y-NPK and Y-CK (Figure 3). However, the fertilization effect was negative
to inherent soil productivity, which was mainly attributable to high soil nutrient supply in soils with
high inherent productivity. For instance, Zeng et al. [25] observed a higher contribution of soil N but
lower contribution of N fertilization to rice yield in high compared to low fertility fields in the Jianghan
Plain of China. From a historical perspective, the improvement of inherent soil productivity averagely
contributed to a 31% yield increase for China’s major cereal crops such as rice, wheat, and maize
since 1980 in the Chinese farmlands [19]. As a result, it could be feasible to implement agricultural
management to improve inherent soil productivity and thus crop yield particularly in the low-yield
farmlands, which at present occupy one third of the total farmlands in China [26].
Eighty-five percent of the experiment sites had a SOM content below 35 g/kg (ca. 2% SOC)
(Figure 4), a widely believed critical level below which a potentially serious decline in soil quality
would occur [15]. However, no significant correlation between SOM and Y-NPK was observed,
which differed from previous reports that SOM significantly positively related to crop yield based on
individual sites [12] and provincial level statistic data [13]. In the investigations from individual sites,
the effect of SOM on crop yield was not separated from the improving effect of fertilizer application,
while the conjunct change of SOM and crop productivity among provinces might be the concurrent
results of climate, cropping system, and soil type rather than the causality between SOM and crop
yield. However, the field experiments across comparatively larger multiple sites in the current study
might still allow for a variety of confounding variables (e.g., abiotic micro-climates, soil heterogenetic
properties, and agricultural managements) to mask the role of SOM in crop yield. For instance,
Körschens et al. [27] analyzed the results from 13 European long-term experiments and found no
positive yield response neither by a higher SOM level in soil nor by farmyard manure application
when mineral fertilization was optimized, indicating the role of SOM in crop yield might be covered
by mineral fertilization.
One of the hypotheses was the effect of SOM might be expected to be more pronounced under
nutrient limiting conditions, because mineralization from a larger SOM pool should be able to supply
more nutrients. After partitioning the fertilization effect and inherent soil productivity, the hypothesis
was supported by an increasing trend of both the inherent soil productivity and its contribution to
crop yield with the increase of SOM contents, but a decreasing trend of fertilization effect and its
contribution to crop yield under the combined application of NPK (Figure 5). Fertilizer rate was the
potential variable confounding the negative relationships between SOM and fertilization effect, but
it was excluded by non-significant correlations between SOM content and N, P, or K fertilizer rates
(Figure 6). The opposite trend of inherent soil productivity and fertilization together led to the lack of
relationships between SOM and rice yield under NPK fertilization.
Based on historical data consisting of 560 winter wheat and 309 spring barley field trials in
Denmark, Oelofse et al. [16] recently found no or slight relationships between SOC and the potential
grain yield or the yield with no fertilizer N application, and thus speculated that in situations where
nutrient limitation did not occur, SOC levels above 1% (about 17 g/kg SOM) might be sufficient to
sustain yields. In this study, most the of sites (87%) contained SOM content above 17 g/kg, however,
SOM content significantly related to inherent soil productivity (i.e., unfertilized yield), although it did
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not relate to rice yield under fertilizer application. The possible reasons for the difference were that
the continuous cropping history and favorable irrigation condition of the paddy fields used in this
study excluded the confounded variables, as discussed, including farm type (dairy farm vs. arable
farm) and cropping history, especially the presence of grass leys and soil drainage. Furthermore,
precipitation could play a more important role in crop yield for the upland system than paddy system.
By pooling the published data, Alvarez et al. [28] found that SOC was the variable more correlated with
unfertilized wheat yields under a wide range of soils and management conditions in the Humid Pampa
of Argentina and ascribed this to its ability to act as source of nutrients; however, other variables
including rainfall also correlated positively to yield. In the semiarid Argentine Pampas, data collected
from 134 production fields indicated that wheat yields without fertilization were related to both soil
water retention and SOC contents in 0–20 cm soil layer in years with low moisture, while nutrient
availability was the limiting factor in the absence of water deficit [29].
There was a strong correlation between SOM and soil available N (Figure 7), because
mineralization of SOM by which organic N is converted to mineral N is a major process for the
provision of N. Studies have used SOM contents along with other indexes to evaluate the soil N
supplying capacity and N fertilization rate. For instance, Cui et al. [17] estimated that soil N supply was
increased by 5.37 kg/ha for each g/kg SOM during summer maize season and by 3.68 and 9.76 kg/ha
during winter wheat season for low and high yielding fields, respectively, in the North China plain.
Espe et al. [18] estimated a linear effect of 1.44 kg/ha for every g/kg increase in SOC for rice on organic
soils. However, Cassman et al. [30] observed a poor correlation between indigenous N supply and
SOC in the rice systems, and attributed the N content to the N inputs from sources other than SOM
mineralization, degree of congruence between soil N supply and crop demand, and differences in
SOM quality. Obviously, the N supply from SOM mineralization would vary substantially, and it
should be incorporated into other fertilization recommendation strategies such as site-specific nutrient
management in a given region having similar soil properties, climate, and crop management [18].
Albeit weak, there was significant correlation between SOM and soil available P and K content
(Figure 7). Tiessen et al. [31] reported a larger loss of organic N and P reserves in prairie and forest
soil when the natural rates of SOM mineralization was accelerated during agricultural use without
supplementary fertilization in three different climate zones. They argued that the labile P was reduced,
arising from the mineralization of organic P and the subsequent transformation of surplus inorganic P
to unavailable forms associated with calcium (Ca) in temperate soils or iron (Fe) and aluminum (Al) in
tropical soils. SOM could prevent P being fixed into unavailable forms, keeping it in the form which
remained in the available pool for plants, even if this organic P needed to be mineralized before it
became immediately available to plants [4]. In addition to the effect on N and P, SOM contains both
anion and cation exchange sites that might be able to hold readily available K for crops [3].
In general, a certain amount of nutrients is enough for a crop to attain its potential yield under
specific climate and farming management. When crop demand for nutrients remains constant, an
increase in soil nutrient supply from SOM mineralization would decrease the yield increase from
fertilization, while reduced amounts of SOM mineralization would cause a decrease of indigenous soil
nutrient supply and necessitate fertilization for optimal crop production. For example, data from 13
European long-term experiments showed that 43 kg/ha more mineral N fertilizer was necessary to
reach the highest yield, but optimal N declined from 147 kg/ha at low SOM to 108 kg/ha at high SOM
in the treatments without farmyard manure while the optimal mineral N ranged from 96 kg/ha at low
SOM to 67 kg/ha at high SOM in the farmyard manure plots [27]. Therefore, soils with higher SOM
could supply more nutrients to support high inherent soil productivity but could rely less on external
fertilizer input compared with soils with lower SOM, and over application of fertilizer inputs would
not significantly increase the nutrient uptake by crops for both growth and yield but increase the
potential of environmental risks. So, agricultural practices for SOM accumulation should be considered
as an effective tool to improve the inherent soil productivity and decrease the consumption of chemical
fertilizers while maintaining or increasing crop production; correspondingly, nutrient input should be
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adjusted in accordance with the temporal and spatial changes in SOM levels in order to optimize the
fertilization recommendation [32,33].
5. Conclusions
Our results demonstrated that the inherent soil productivity under no-fertilization and the
combined NPK fertilization contributed 70% and 30% of the total rice yield under fertilization
in the rice cropping system in the Sichuan Basin, China, respectively. An increase of SOM
significantly improved the inherent soil productivity, but also significantly decreased the fertilizer
effect, indicating an increased soil contribution but a decreased fertilizer contribution to the total
crop yield. The significantly positive correlation between SOM contents and soil available N, P, and
K indicate that supplying nutrients from mineralization is a potential contributor of SOM to crop
yield among the wide range of effects in the rice cropping system. Therefore, practices for SOM
accumulation should be implemented to improve the inherent soil productivity, while nutrients input
or fertilization management should be adjusted in accordance with the temporal and spatial changes
of SOM in recent decades to optimize the fertilizer application.
Acknowledgments: This study was supported by the National Key Technology Research and Development
Program (2015BAD06B04).
Author Contributions: Xiao-Jun Shi and Yue-Qiang Zhang designed the research; Xin-Cheng Huang collected
and analyzed the data; Ya-Nan Zhao and Xin-Hua He interpreted results and wrote the paper; all authors read
and approved the final manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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