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Manure acts as a better fertilizer for increasing crop yields than synthetic fertilizer does by improving soil fertility

Authors:
  • Institute of Environment and Sustainable Development in Agriculture
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Soil & Tillage Research
journal homepage: www.elsevier.com/locate/still
Manure acts as a better fertilizer for increasing crop yields than synthetic
fertilizer does by improving soil fertility
Andong Cai
a,b
, Minggang Xu
a,
, Boren Wang
a
, Wenju Zhang
a
, Guopeng Liang
b
, Enqing Hou
c
,
Yiqi Luo
b,d
a
National Engineering Laboratory for Improving Quality of Arable Land, Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural
Sciences, Beijing, 100081, China
b
Center for Ecosystem Science and Society, Department of Biological Sciences, Northern Arizona University, Flagsta, AZ, 86011, USA
c
Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, 510650,
China
d
Department of Earth System Science, Tsinghua University, Beijing, China
ARTICLE INFO
Keywords:
Manure
Synthetic fertilizer
Crop yields
Soil organic carbon
Soil nutrients
Soil pH
ABSTRACT
Fertilization is an important management strategy for crop yields by mediating soil fertility. However, rare
studies quantitatively assessed the interactions among fertilization, crop yields, and soil fertility. Here, data from
a 25-year fertilization experiment in the humid subtropical region of Southern China were used to evaluate and
quantify the eect of fertilization on crop yields via soil fertility. Seven treatments were chosen: CK (non-
fertilizer); N (synthetic nitrogen); NP (synthetic N and phosphorus); NPK (synthetic N, P and potassium); NPKM
1
(synthetic NPK with manure); 1.5NPKM
1
(1.5 times of NPKM
1
); and M
2
(manure alone). Overall, the crop yields
of wheat and maize under manure (1.361.58 and 3.85-5.82 Mg ha
1
) were higher than those under CK (0.34
and 0.25 Mg ha
1
) and synthetic fertilized treatments (0.270.97 and 0.482.65 Mg ha
1
), as the averaged of
19912015. Higher SOC stocks were found under the NPKM
1
, 1.5NPKM
1
, and M
2
treatments with a pronounced
increase in SOC over the rst 10 years and stable over the last 15 years. By the boosted regression trees, manure,
synthetic fertilizer and soil properties (SOC storage, soil pH, and soil nutrients) accounted for 39%, 21%, and
40% of the variation of the relative yield, respectively. Path analysis identied a network of inter-relations of
manure, synthetic fertilizer, and soil properties in the relative yields. Compared to synthetic fertilized treat-
ments, manure application strongly and positively aected the relative yield by increasing SOC storage, soil
nutrients, and soil pH (path coecients: 0.90, 0.88, and 0.76). These factors explained 72% of the crop yields'
variance. These results suggest that manure application is a viable strategy for regulating crop yields due to its
improvement in soil fertility.
1. Introduction
Promoting global crop productivity to feed the ever-increasing po-
pulation and high living standards has become a great challenge
(Fischer et al., 2014). The most debating question of recent times is
How to increase the crop yields?(Foley et al., 2011). Technological
progress in eld management has contributed to large increases in crop
yields (Deryng et al., 2011). Among all the management strategies,
fertilization has been suggested as a promising strategy to increase crop
yields. Based on 153 eld experiments in China, Chen et al. (2014)
observed an increase in crop yields of approximately 8.5-14.2 Mg ha
1
following fertilization with manure without any increase in nitrogen
(N) fertilizer. Using data from 20 eld experiments in Europe, Hijbeek
et al. (2016) reported that crop yields increased by 2.0 Mg ha
1
due to
synthetic fertilization and had negligible change (an increase of 1.4%)
in response to manure. It is obvious that the eect of dierent fertilizer
types on crop yields is inconsistent via dierent mechanisms. Therefore,
to achieve high crop productivity, it is important to understand the
impact of dierent fertilizer inputs on crop yields.
Exogenous fertilization inuences the crop yields by improving soil
fertility, such as soil carbon, nutrients, and pH. Soil carbon content is
the essential index for dierent yields (Tian et al., 2016). Soil organic
carbon (SOC) sequestration can be enhanced by fertilization such as
incorporation of crop residues or the direct application of manure,
which implies by high carbon inputs (Cai et al., 2016). Synthetic fer-
tilization can also change SOC by the return of crop residues. For
https://doi.org/10.1016/j.still.2018.12.022
Received 11 February 2018; Received in revised form 17 December 2018; Accepted 26 December 2018
Corresponding author.
E-mail address: xuminggang@caas.cn (M. Xu).
Soil & Tillage Research 189 (2019) 168–175
0167-1987/ © 2018 Elsevier B.V. All rights reserved.
T
instance, Zhang et al. (2012) showed that carbon inputs were 2.5-
5.0 Mg ha
1
year
1
by synthetic fertilization in southern China. The
greater amount of carbon inputs by green manure improved the SOC
stock by 1424% and reduced synthetic fertilization of 2551% com-
pared with fallow (Yao et al., 2017). In their meta-analysis, McDaniel
et al. (2014) reported that the rate of SOC sequestration under cover
crop is signicantly higher than SOC sequestration rate under no cover
crop. The application of farmyard manure, rice straw, and fertilizer
nitrogen could maintain SOC almost at the same level as for the un-
cultivated soil for rice-wheat cropping systems in the Indo-Gangetic
plains (Benbi et al., 2012). However, the mechanisms by which dif-
ferent fertilizer inputs aect crop yields by improving SOC remain
unclear.
Soil nutrients are the major yield-limiting factors. Therefore, to
achieve an ecient and protable crop production relies on the large
inputs of synthetic fertilizers. However, less than half of the total nu-
trients provided by synthetic fertilizers is eectively utilized and left-
over having a range of negative ecological eects (Galloway et al.,
2008). The application of manure could provide not only carbon but
also dierent nutrients for crop uptakes. The residual eect of manure
application was visible after many years, leading to higher nutrient
availability for crop growth (Cai et al., 2018;Demelash et al., 2014).
Soil acidication has received considerable attention in intensive
agricultural systems due to its negative impacts on agricultural pro-
duction and soil fertility (Cai et al., 2014;Guo et al., 2010). The ex-
cessive application of synthetic fertilizer is the main reason for soil
acidication (Zhu et al., 2018). Although China has attained great
achievements in crop yields, the major croplands have still been suf-
fering from signicant acidication since the 1980s (Guo et al., 2010).
In general, the eect of manure on soil pH is concerned with the ash
alkalinity of manure (Rukshana et al., 2013). The alkalinity of manure
is one of the reasons for the increased pH following the manure ap-
plication to soil, although the nitrogen nitrication can generate pro-
tons for decreased pH (Xu et al., 2006). Therefore, it is of interest to
explore the mechanism how dierent fertilizers aect crop yields by
soil nutrients and soil pH.
The area of subtropical arable land in China is approximately
446,890 km
2
and 4% of the worlds subtropical arable land surface,
which could support 23% of China's population. Soil acidication and
available nutrients restrict crop growth. Therefore, we tried to answer:
Which is the driving factor of manure, synthetic fertilizer, and soil
property controls over crop yields? How do fertilizer applications aect
overall crop yields in Southern China?
2. Materials and methods
2.1. Study site
The experiment was conducted at the experimental station of the
Chinese Academy of Agricultural Sciences (26°45N, 111°52E),
Southern China. The area receives an average temperature of 18.1 °C,
and the active eective accumulated temperature of 4947 °C. The area
receives an average annual precipitation of 1431 mm. The soil type is
Eutric Cambisol and ferrosols soil based on Chinese soil classication
system. The initial topsoil (020 cm) properties were as follows: soil
organic matter of 13.6 g kg
1
; total N (TN) of 1.07 g kg
1
; total P (TP)
of 0.45 g kg
1
; total K (TK) of 13.7 g kg
1
; soil bulk density (BD) of
1.19 kg m
-3
; soil pH of 5.70; soil available N, P and K of 79, 14, and
104 mg kg
1
.
2.2. Experimental design
This experiment was randomly designed, and seven treatments were
selected for this research (Table 1): (1) CK (no fertilizer); (2) N (syn-
thetic nitrogen); (3) NP (synthetic N and phosphorus); (4) NPK (syn-
thetic N, P and potassium); (5) NPKM
1
(synthetic NPK and manure); (6)
1.5NPKM
1
(1.5 times NPKM
1
); and (7) manure (M
2
). Each plot was
replicated twice (20 × 9.8 m) and isolated by 1 m cement bae plates.
The synthetic fertilizers were applied as urea, calcium superphosphate,
and potassium chloride. Manure was pure pig manure (solid manure)
and composed of approximately 75% water with an average content
(during the experiment) of 413, 20.1, 12.9, and 12.5 g kg
1
for carbon,
nitrogen, phosphorus, and potassium (dry weight). The average content
(during the experiment) of nitrogen and phosphorus was 6.1 and 0.81 g
kg
1
for wheat residues and 9.5 and 1.3 g kg
1
for maize residues.
Crop yields and straw were removed and crop residues remained.
Therefore, the amount of nitrogen input was the same under all ferti-
lizer treatments except the 1.5NPKM
1
treatment. All of the fertilizers
were applied before the sowing, 30% and 70% of fertilization were
assigned to wheat and maize, respectively. Specic fertilization
amounts are shown in Table 1.
2.3. Crop management
The experimental eld was cultivated under a wheat-maize rotation
system. Three years were taken to dispose of the experimental eld to
ensure the same soil physical and chemical property. Annual winter
wheat variety Xiangmai was sown in early November with the rate of
about 160 seeds per m
2
(63 kg ha
1
) and harvested in early May of next
year. Summer maize of variety Yedan 13 was sown in early April at a
planting density of 60,000 seeds ha
-1
and was harvested in late July.
No-irrigation was applied for winter wheat and summer maize due to
large amounts of annual precipitation. Omethoate and carbofuran
pesticides were applied to control the wheat aphid population during
the postulation period and maize borers. Glyphosate herbicide was
applied to control grassy weeds after maize harvest. The crop was
harvested manually, the stubble was approximately 6 cm in height, and
the roots were left in the soil. The collected straw and grains were air-
dried and weighed separately for each species.
2.4. Soil sampling and analysis
Soil samples were collected from the cultivated horizon (020 cm)
in September. Each treatment was randomly sampled for ve to ten
cores, which was 0.05 m in diameter. Then, the soil samples were
thoroughly mixed and then stored for later analysis. To measure soil
nutrients, the air-dried soil samples were crushed to pass through a
0.25-mm sieve. The SOC content was measured using the oxidation
method by vitriol acid potassium dichromate oxidation (Page et al.,
1982). Total soil nitrogen, phosphorus, and potassium were measured
with the methods of Black (1965);Murphy and Riley (1962), and
Knudsen et al. (1982), respectively. Available N was measured in ac-
cordance with the methods of Lu (2000), and available P (Olsen-P) and
available K were determined in accordance with the Olsen-P method
(Olsen, 1954) and the methods of Page et al. (1982), respectively. Soil
BD was measured with cutting ring (inner diameter, 50.46 mm; sam-
pling depth, 50 mm; volume, 100cm
3
) method and three repetitions
(Lu, 2000).
2.5. Calculations
The SOC content was converted to SOC density by the equation (Lal
and Bruce, 1999):
××SOC SOC d BD 10
density content (1)
where SOC
density
is soil organic carbon density (Mg ha
1
); SOC
content
is
soil organic carbon content (g kg
1
); d and BD are the depth of the soil
layer (0.20 m) and soil dry BD (kg m
-3
).
The amounts of C input include plant residues plus returned
manure. The annual C input (C
input
,tha
1
) was calculated from be-
lowground biomass C (C
root
,Mgha
1
) and stubble (C
stubble
,Mgha
1
),
which was incorporated into the topsoil (as Eqs. (3) and (4)), as well as
A. Cai, et al. Soil & Tillage Research 189 (2019) 168–175
169
the amount of manure (C
manure
,Mgha
1
). The method of carbon inputs
was the following:
=++CC C C
input belowground stubbles manure
(2)
CRC
belowground bg biomass (3)
CRC
stubbles stubble s biomass (4)
where R
bg
is the ratio of annual underground carbon from crops to
above-ground biomass carbon, which is estimated as 30% from Kundu
et al. (2007).R
stubbles
is the ratio of stubble incorporated into the soil to
aboveground biomass.
The relative yields (YR) were used to allow the datasets from the
individual treatments to be more comparable. The relative yield was
calculated as follows:
=−YR Y Y
treatment control
(5)
where Y
treatment
is the actual yield under the fertilization treatments
(Mg ha
1
) in a given year and Y
control
is the yield of the no-fertilization
treatment (Mg ha
1
) in the same year.
2.6. Statistical analyses
To assess the dierent fertilization treatment eects on the relative
crop yields; three periods were analyzed (19912000, 20012005 and
20062015) by one-way ANOVA as implemented with the SPSS 19.0
software package. We also explored the trends of SOC under the dif-
ferent fertilization treatments. Dierent equations were selected and
performed by SigmaPlot 10.0. Boosted regression tree (BRT) was con-
structed using the recommended parameter values (Elith et al., 2008).
The procedure of boosted regression tree was applied using the gbm
package in R version 3.3.3. Structural equation model (SEM) was used
to quantify the relationships among relative yields, soil fertility, and
dierent fertilizations as conducted with the Amos package. All of the
graphs were prepared with SigmaPlot 10.0 software.
3. Results
3.1. Crop yields
Crop yields in each treatment exhibited similar changes among
years, which increased over time in NPKM
1
, 1.5NPKM
1,
and M
2
and
decreased in CK, N, NP, and NPK despite some uctuations in some
years (Fig. 1). There was no crop yield in the N treatment after 12-year
fertilization. The crop yields varied from 0.34 (CK) to 1.58 Mg ha
1
(NPKM
1
) and from 0.25 (CK) to 5.82 Mg ha
1
(1.5NPKM
1
), respec-
tively, as averaged over 19912015 (Table 2). Signicantly higher
yields were observed in the NPKM
1
, 1.5NPKM
1
, and M
2
treatments
during the period 20012005. Compared with the period of
19912000, CK caused 10% to 41% decrease of wheat yield while
maize yield showed 45% to 56% during the period of 20012005. The
largest decrease (26100%) of crop yields was found in N, NP, and NPK
treatments. However, 265% increases in crop yields were found in
NPKM
1
and M
2
treatments during 20062015, whereas 1.5NPKM
1
de-
creased wheat and maize yields by 1223% during 20062015.
3.2. Dynamic changes of SOC and responses to fertilization treatment
The average annual carbon inputs in the CK treatment were 0.17
(wheat) and 0.13 Mg ha
1
(maize), respectively, each year from crop
residue (Fig. 2). The applications of NP and NPK signicantly increased
carbon biomass, with 0.36-0.61 Mg ha
1
year
1
. Carbon inputs under
NPKM
1
and 1.5NPKM
1
treatments were 1.721.87 times for the wheat
and 1.982.31 times for the maize under the NPK treatment. SOC was
unchanged under the CK treatment and slightly decreased under the N
treatment (Fig. 3). Higher SOC values were observed under the NPKM
1
,
1.5NPKM
1
, and M
2
treatments, depicting a pronounced increase over
the rst 10 years and then a high stable level over the last 15 years. The
relationship between annual SOC sequestered and duration was de-
scribed with y = 0.80e
-0.08x
and y = 3.16e
-0.05x
under the synthetic
fertilizer and manure treatments (Appendix 4a and b). A signicant and
positive relationship was found between carbon inputs and annual SOC
sequestered (Fig. 4). The estimated decomposition rate of SOC was
0.17 Mg ha
1
each year (when carbon inputs were zero), and the
minimum rate of carbon inputs necessary to maintain SOC content was
0.36 Mg ha
-1
each year (when soil carbon sequestration rate was zero).
3.3. Soil nutrients changes
Soil nutrients, pH, and BD were displayed in Table 3. Soil TN,
available N, and K showed no signicant dierences among CK, N, and
NP during 19912000, 20012005 and 20062015 periods. Compared
with CK, NPK treatment signicantly increased soil TN and available K
during both 20012005 and 20052015 periods but did not sig-
nicantly aect soil TN, available N, and available K during 19912000
periods. Soil TP and available P did not signicantly dier between the
CK and N treatments but were signicantly increased in NP and NPK
treatments relative to CK treatment in all three fertilization periods. Soil
TK did not dier among the fertilizer treatments. Compared with CK,
the synthetic fertilizers (N, NP, and NPK) signicantly decreased soil
pH. Compared with their initial values, soil TN, available K, pH, and BD
decreased on average by 9, 9, 19, and 1%; available N, TP, available P
and TK increased on average by 20, 50, 81, and 1%, respectively, under
the synthetic fertilizer treatments. Soil TN, AN, TP, AP, TK, AK, pH, and
BD increased by 24, 54, 208, 815, 3, 180, 8, and 13%, respectively,
under manure treatments.
3.4. Mechanisms of relative yield
The result of BRT suggested that manure was the most inuential
trigger on relative yields among the studied 11 variables (39%, Fig. 5a).
Table 1
Annual rate (kg ha
1
) of synthetic nitrogen (Sy-N), phosphorus (Sy-P), and potassium (Sy-K) fertilizer addition and organic nitrogen (Or-N), phosphorus (Or-P), and
potassium (Or-K) fertilizer applied in the various fertilization treatments. Notes: CK, no fertilizer; N, synthetic nitrogen; NP, synthetic N, and phosphorus; NPK,
synthetic N, P, and potassium; NPKM
1
, synthetic NPK, and manure; 1.5NPKM
1
, 1.5 times NPKM
1
; and M
2
manure. Synthetic N fertilizer is urea; P added as calcium
superphosphate; K added as KCl.
Fertilizations Wheat Maize
Sy-N Sy-P Sy-K Or-N Or-P Or-K Sy-N Sy-P Sy-K Or-N Or-P Or-K
CK 000000000000
N 90 0 0 0 0 0 210 0 0 0 0 0
NP 90 16 0 0 0 0 210 37 0 0 0 0
NPK 90 16 30 0 0 0 210 37 70 0 0 0
NPKM
1
27 16 30 63 40 39 63 37 70 147 94 91
1.5NPKM
1
40 24 45 94 61 59 95 55 105 220 141 137
M
2
0 0 0 90 58 56 0 0 0 210 135 131
A. Cai, et al. Soil & Tillage Research 189 (2019) 168–175
170
The synthetic N, P, and K accounted for 21% of the relative yields. The
relative individual inuence of soil properties was small. The BRT
model driven by these variables explained 98% of the relative yields
(Fig. 5b). In the SEM analysis, dierent pathways were constructed to
examine the eect of the dierent variables on the variation of crop
yields (Fig. 6). These variables were grouped to two latent variables
(synthetic fertilizers and soil nutrients) and three factors (manure, soil
pH, and SOC storage) in the path analysis. The loading scores suggested
that synthetic phosphorus application and soil available phosphorus
were more powerful indicators of synthetic fertilizer and soil nutrients,
respectively, than were synthetic N application and soil available N.
Synthetic fertilizer was signicantly and positively associated with re-
lative soil nutrients (0.27), while its associations with relative soil pH
(-0.36) and SOC storage (-0.12) were signicantly negatively corre-
lated. Manure application strongly and positively aected the soil nu-
trients, soil pH, and SOC storage (path coecients: 0.90, 0.76, and
0.88). The soil nutrients, soil pH and SOC storage directly inuenced
the relative yields (path coecients: 0.23, 0.44, and 0.25). The path
analysis explained 72% of the variance of relative yields (R
2
= 0.72).
4. Discussion
Crop yields increased over time in the NPKM
1
, 1.5NPKM
1,
and M
2
and decreased over time in CK, N, NP, and NPK, although there were
some uctuations in some years (Fig. 1 and Table 2). These results
could be attributed to the residual eect of manure application (Shen
et al., 2007). The residual eect could maintain crop yields for several
years after manure application ceases (Demelash et al., 2014). A posi-
tive residual eect on crop yields is observed with improvements in
plant dry matter production. Another reason is the indiscriminate use of
synthetic fertilizer, which has caused soil acidication, particularly in
the south of China (Cai et al., 2014;Guo et al., 2010). The extent and
intensity of soil acidication are the main factors that contribute to the
dramatic yield reduction (Zhu et al., 2018). Accordingly, there was no
yield in the N treatment after 12 years with the lower soil pH (Fig. 1 and
Table 3). N fertilization acidies the soil by the oxidation of dry-de-
posited compounds, loss of basic cations through ion exchange, and
plant uptake and nitrication of ammonium. The lower pH not only
Fig. 1. Annual wheat (a) and maize (b) grain
yields under various fertilization treatments of
a long-term experiment in a wheat-maize
system. Notes: CK, no fertilizer; N, synthetic
nitrogen; NP, synthetic N, and phosphorus;
NPK, synthetic N, P, and potassium; NPKM
1
,
synthetic NPK, and manure; 1.5NPKM
1
, 1.5
times NPKM1; and M
2
manure.
Table 2
Wheat and maize grain yields for the periods 19912015, 19912000,
20012005 and 20062015 under various fertilization treatments in a long-
term experiment in a wheat-maize system. Notes: CK, no fertilizer; N, synthetic
nitrogen; NP, synthetic N, and phosphorus; NPK, synthetic N, P, and potassium;
NPKM
1
, synthetic NPK, and manure; 1.5NPKM
1
, 1.5 times NPKM
1
; and M
2
manure. Dierent letters in the same column indicate signicant dierences
(P< 0.05) among treatments.
Crops Treatments 1991-
2015
1991-
2000
2001-
2005
2006-
2015
2001-
2005
2006-2015
Mg ha
1
Change (%)
Wheat CK 0.34 d 0.42 c 0.38 d 0.25 d 10 41
N 0.27 d 0.64 c 0.06 e 0.00 e 91 100
NP 0.79 c 1.32 ab 0.68 cd 0.31 cd 49 76
NPK 0.97 bc 1.46 ab 1.08 b 0.42 c 26 71
NPKM
1
1.58 a 1.58 a 1.85 a 1.44 a 17 9
1.5NPKM
1
1.57 a 1.71 a 1.80 a 1.32 ab 5 23
M
2
1.36 ab 1.16 b 1.90 a 1.25 b 65 8
Maize CK 0.25 e 0.36 d 0.20 e 0.16 d 45 56
N 0.48 e 1.15 d 0.11 e 0.00 d 91 100
NP 1.66 d 3.02 c 1.12 c 0.57 d 63 81
NPK 2.65 c 4.02 bc 2.72 b 1.24 c 32 69
NPKM
1
5.19 a 4.61 b 6.11 a 5.30 a 33 15
1.5NPKM
1
5.82 a 5.91 a 6.86 a 5.21 a 16 12
M
2
3.89 b 3.70 bc 3.79 b 4.08 b 2 10
Fig. 2. Average annual carbon input from crop
and manure under various fertilization treat-
ments in wheat (a) and maize (b) system.
Notes: Dierent lower-case letters (annual
carbon input from crops) and upper-case let-
ters (total annual carbon input) indicate sig-
nicant dierences at the P< 0.05 level for
each treatment. CK, no fertilizer; N, synthetic
nitrogen; NP, synthetic N, and phosphorus;
NPK, synthetic N, P, and potassium; NPKM
1
,
synthetic NPK and manure; 1.5NPKM
1
, 1.5
times NPKM1; and M
2
manure.
A. Cai, et al. Soil & Tillage Research 189 (2019) 168–175
171
increases the availability of potentially toxic heavy metals but also
contribute to the severe reduction of the microbial community that
promotes the root functions (Stevens et al., 2009). A combination of
manure and synthetic fertilizers could improve nutrient availability for
plant uptake with a positive eect on crop yields (Diacono and
Montemurro, 2010). The results of our study showed that wheat yield
was signicantly higher under manure than NPK treatment (Table 2).
The application of manure alone has a vibrant increasing eect on
maize yield but a weaker increasing eect than NPK on wheat yield.
One potential reason for this nding was that the manure was applied
before the wheat was sown. Another potential reason is that the higher
soil temperatures and precipitation in summer increase nutrient mi-
neralization (Agehara and Warncke, 2005). Meanwhile, the eect of
manure on crop yields is crucial by improving soil pH (Fig. 6c). Overall,
the crop yields were signicantly higher under manure than synthetic
fertilizer treatments.
Apparently, manure treatments can increase SOC and soil nutrients
over long-term cropping. Manure not only directly increases carbon
inputs into the soil but also inuences crop residues, which determine
the benets of agricultural SOC sequestration and nutrient release
(Fig. 2 and Appendix 2) (Kuzyakov and Blagodatskaya, 2015;Lal,
2008). The observed non-linear relationship between SOC sequestration
and carbon inputs indicated that SOC was approaching an equilibrium
level. This result agrees with many previous studies (Cong et al., 2012;
Stewart et al., 2007;Zhang et al., 2012), but diers from other re-
searches (Kong et al., 2005;Majumder et al., 2007) that reported linear
relationships. The contrasting ndings may be partially due to dier-
ences in the ranges of carbon inputs and carbon stabilization. Our
carbon inputs rate showed a much wider range than observed in pre-
vious studies, being 0.30-9.36 Mg ha
1
y
1
. Another long-term ex-
periment is being performed to examine the relationship between SOC
sequestration and carbon inputs (Fig. 4 and Appendix 5). In contrast,
Majumder et al. (2007) reported a carbon input of only 1.96-4.10 Mg
ha
1
each year. We estimated that the minimum rate of carbon inputs
required maintaining SOC content was 0.36 Mg ha
1
each year. The
value is slightly higher than N treatment (0.26 Mg ha
1
each year),
resulting in a signicantly decreasing trend. The minimum carbon in-
puts under NPK treatment (1.06 Mg ha
1
each year) were signicantly
higher than the minimum rate of carbon inputs (0.26 Mg ha
1
each
year), contributing to the increasing trend in SOC. These results de-
monstrated that balanced fertilization (NPK) could maintain or even
improve SOC via the return of crop residues.
Exogenous application of synthetic fertilizer accelerated soil acid-
ication, whereas manure or interactive application of manure with
synthetic fertilizer prevented this process (Table 3). Plants generally
extrude net excess of H
+
; conversely, they extrude net excess of OH
/
HCO
3
or consume H
+
when anion uptake exceeds cation uptake
(Tang et al., 2010). NH
4+
-fed plants are characterized by a high cation/
anion uptake ratio, while NO
3
-fed plants have a low cation/anion
uptake ratio (Tang et al., 2010). Synthetic N application signicantly
reduced the exchangeable base cations in soils, which lead to declined
soil pH. Additionally, synthetic N application has shifted soils into the
Al
3+
buering stage. Al is released into solution at a pH below 5 by the
hydrolysis of both Al-hydroxides and silicates on clay mineral surfaces.
A number of other heavy metals behave in a manner similar to Al. A
decline in base saturation is symptomatic of soil acidication (Stevens
et al., 2009). Accordingly, many studies reported that synthetic ferti-
lizer application could signicantly decrease soil pH (Cai et al., 2014;
Zhu et al., 2018). In general, the ash alkalinity of manure is associated
with soil acidication with protons to neutralize soil acidity (Rukshana
et al., 2013). The alkalinity of organic materials following the dec-
arboxylation of organic anions and the ammonication of organic N are
the major causes of increases in soil pH, although nitrication of mi-
neralized N can generate protons for the decrease of soil pH to some
degree (Xu et al., 2006). Following long-term manure application, soil
Fig. 3. Trends in soil organic carbon under various fertilization treatments in a long-term experiment in a wheat and maize system. Notes: CK, no fertilizer; N,
synthetic nitrogen; NP, synthetic N, and phosphorus; NPK, synthetic N, P, and potassium; NPKM
1
, synthetic NPK, and manure; 1.5NPKM
1
, 1.5 times NPKM1; and M
2
manure.
Fig. 4. Correlation between soil organic carbon storage and annual carbon
input in a long-term experiment in a wheat-maize system. Notes: Experiment
points come from our study site and test points come from Cai et al. (2018)
reported site.
A. Cai, et al. Soil & Tillage Research 189 (2019) 168–175
172
pH increased due to manure residues.
As expected, manure signicantly aected crop yields, compared to
other variables including synthetic fertilizer and soil fertility (Fig. 5).
Manure mainly plays an important role in regulating plant growth,
potential nutrient input, and microbial decomposition activity. This
role can largely mediate the soil nutrient and soil micro-environment,
which have a strong impact on crop growth. In addition, manure could
also result in increased microbial biomass and changes in community
structure, which provide a better environment for the growth of the
crop (Peacock et al., 2001). Manure application to cropland can aect
soil properties, but the eects may not be apparent over a short time
period. We identied a network of correlations among synthetic ferti-
lizer, manure, and soil fertility in determining crop yields (Fig. 6c). The
application of manure strongly and positively aected crop yields by
increasing SOC storage, soil nutrients, and soil pH. Synthetic fertilizer
aected crop yields by weakly increasing soil nutrients and decreasing
SOC storage and soil pH. SOC, soil nutrients, and soil pH directly in-
uenced crop yields, and soil pH played a more important role in in-
creasing crop yields than did soil nutrients and SOC in this experimental
eld in the south of China. Increased soil acidication can reduce the
availability of soil nutrients to plants in the soil and it thereby reduced
crop yields (Wright, 1989). Soil pH-induced changes in soil enzyme
activity and microbial composition might be important mechanisms for
alleviating acid stress on crop yields by various ameliorants. In
addition, the total eect of soil pH, the SOC and soil nutrients on crop
yield were also identied. Cai et al. (2018) reported that manure in-
uenced crop yields via aecting soil TN and available N and P (soil
pH) based on an 8-year eld experiment. Our results showed further
evidence that the interplay of dierent fertilization, soil pH, SOC and
soil nutrients and their interaction jointly inuenced crop yields
(Fig. 6). These results suggest that manure is a better fertilizer than
synthetic fertilizer for regulating crop yields by improving soil fertility
for Chinese subtropical arable soils.
5. Conclusions
Signicant dierences in soil fertility and crop yields among dif-
ferent fertilization treatments were found in this study. The manure or
combined with synthetic fertilizer signicantly increased crop yields,
SOC, soil nutrients and soil pH compared with CK. The crop yields in-
creased with increasing amount of added manure. Manure inputs ac-
counted for 39% of the relative inuence on relative yield, followed by
synthetic fertilizer of 21% and soil fertility of 40%. Synthetic fertilizers
indirectly aected crop yields by weakly increasing soil nutrients and
decreasing SOC storage and soil pH. Manure indirectly aected crop
yields by strongly and positively increasing soil nutrients, SOC storage
and soil pH. Our results suggest that manure acts as a better fertilizer
than synthetic fertilizer in increasing crop yields by improving soil
Table 3
Contents of soil nutrients of dry soil for the period of 19912015, 19912000, 20012005 and 20062015 under various fertilizations of long-term experiments in
wheat-maize systems. Notes: CK, no fertilizer; N, synthetic nitrogen; NP, synthetic N, and phosphorus; NPK, synthetic N, P, and potassium; NPKM
1
, synthetic NPK and
manure; 1.5NPKM
1
, 1.5 times NPKM
1
; and M
2
manure. Dierent letters in the same column during year indicate that there are signicant dierences (P< 0.05)
among the dierent treatments.
Treatments Treatments TN AN TP AP TK AK pH BD
Initial year 1.07 79 0.45 14 13.7 104 5.7 1.19
1991-2000 CK 0.80 e 96 c 0.46 d 5 d 14.63 a 82 c 5.75 b 1.15 ab
N 0.94 de 120 bc 0.45 d 4 d 15.76 a 70 c 4.98 c 1.11 b
NP 0.95 de 101 bc 0.69 c 20 c 14.61 a 89 c 5.05 c 1.12 b
NPK 1.06 cd 112 bc 0.77 c 21 c 15.32 a 128 c 4.90 c 1.22 ab
NPKM
1
1.23 ab 114 bc 1.00 b 49 b 14.54 a 198 b 6.04 ab 1.32 a
1.5NPKM
1
1.31 a 130 bc 1.30 a 74 a 16.88 a 288 a 6.05 ab 1.23 ab
M
2
1.13 bc 142 ab 0.82 c 37 b 12.76 a 200 b 6.48 a 1.30 a
2001-2005 CK 0.81 c 64 c 0.44 e 5 e 13.64 a 56 d 5.63 a 1.17 d
N 0.90 c 90 bc 0.41 e 4 e 11.66 a 47 d 4.30 b 1.15 d
NP 0.87 c 81 c 0.70 d 37 d 12.92 a 51 d 4.53 b 1.17 d
NPK 1.03 b 85 c 0.79 d 43 d 12.13 a 136 c 4.59 b 1.30 c
NPKM
1
1.36 a 119 ab 1.35 b 148 b 12.85 a 283 b 5.77 a 1.48 a
1.5NPKM
1
1.44 a 133 a 1.74 a 226 a 12.74 a 404 a 5.68 a 1.42 b
M
2
1.22 a 91 bc 1.05 c 100 c 12.11 a 190 c 6.58 a 1.40 b
2006-2015 CK 0.86 d 60 c 0.45 e 4 d 14.72 a 59 e 5.75 c 1.23 b
N 0.86 d 86 c 0.45 e 4 d 13.54 a 50 e 4.04 e 1.16 c
NP 0.96 cd 85 c 0.86 d 51 c 13.72 a 64 e 4.30 d 1.19 c
NPK 1.05 c 86 c 0.98 d 48 c 14.79 a 201 d 4.35 d 1.18 c
NPKM
1
1.34 b 115 b 1.69 b 170 b 14.62 a 350 b 5.88 bc 1.36 a
1.5NPKM
1
1.55 a 138 a 2.32 a 245 a 14.43 a 484 a 5.94 b 1.33 a
M
2
1.49 a 119 b 1.50 c 147 b 14.22 a 253 c 6.66 a 1.39 a
Fig. 5. The relative contributions (%) of pre-
dictor variables for the boosted regression tree
model of relative yield (a). Observed and pre-
dicted relative crop yield by the boosted re-
gression tree model using predictors shown in
Fig. 5b. The dashed line shows the 1:1 line.
Notes: SOC, soil organic carbon; Manure,
amounts of manure input; Synthetic N, P, and
K, amounts of synthetic N, P, and K input.
A. Cai, et al. Soil & Tillage Research 189 (2019) 168–175
173
fertility for Chinese subtropical arable soils.
Acknowledgments
Financial support from the National Natural Science Foundation of
China (41620104006) and National Key Research and Development
Program of China (2016YFE0112700) is gratefully acknowledged. The
authors are very grateful for revisions to the manuscript by Muhammad
Nadeem Malik. We are thankful to all of our colleagues who were in-
volved in these long-term trials for their unremitting eorts to maintain
these unique experiments. We are also grateful to the anonymous re-
viewers for their comments.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.still.2018.12.022.
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