Optimization
of tillage rotation and fertilization increases the soil organic carbon
pools and crop yields in a wheat-maize cropping system on China’s Loess
Plateau
Xia Zhang1|Sixu
Lu1|Chenguang
Wang1|Afeng
Zhang1|Xudong Wang1,2
1College
of Natural Resources and Environment,
Northwest
A&F University, Yangling, Shaanxi 712100, PR China
2 Key Laboratory of Plant Nutrition and
Agri-environment in Northwest China, Ministry of Agriculture, Yangling,
Shaanxi 712100, PR China
Correspondence : Xudong Wang and Afeng Zhang; College of Natural
Resources and Environment, Northwest A&F University, No. 3, Taicheng
Road, Yangling 712100, Shaanxi, PR China.
E-mail:
wangxudong01@126.com (X.
Wang);
zhangafeng@nwsuaf.edu.cn
(A. Zhang).
ABSTRACT: Long-term
application of high nitrogen and phosphorus fertilizer and mono-tillage
practices can adversely affect soil health, carbon sequestration and
crop
growth.
A 10-year field experiment was conducted in a wheat-maize cropping
system on the Loess Plateau in China to explore fertilization and
tillage methods that improve SOC sequestration and crop yields. We
evaluated the effects of (1) fertilization (balanced fertilization (BF),
low fertilization (LF), and conventional fertilization (CF)) and (2)
alternating years of different tillage (no-tillage and subsoiling (NS),
subsoiling and ploughing (SP), ploughing and no-tillage (PN)) or
continuous ploughing tillage (PP) on input-C, SOC pool, and crop yields.
BF
and rotational tillage (NS, SP, and PN) increased the amount and
stabilization rate of input-C, thereby increased SOC storage, and the
highest effect was found in BF+NS treatment.
Simultaneously,
BF produced higher contents of SOC, readily oxidizable carbon (ROC),
particulate organic carbon (POC) and dissolved organic carbon (DOC) and
C pool management index (CMI) at 0-10 cm depth.
For
tillage, rotational tillage increased labile C contents and CMI at 0-10
cm, 20-35 cm and 35-50 cm depths, which improved soil quality. Crop
yields showed an increase tendency with the increases of SOC content,
labile C fraction contents, and CMI. Therefore, the higher yields of
wheat and maize were found in BF and rotational tillage; the highest
were in BF+NS treatment. Our finding suggested that NS combined with BF
may be the best management to increase SOC storage, improve soil quality
and productivity on China’s Loess
Plateau.
KEYWORDS: rotational
tillage; fertilization; soil organic carbon sequestration;
organic
carbon fractions; crop yields.
1. INTRODUCTION
Soil
organic carbon (SOC) is not only a major indicator to evaluate soil
fertility, but also a primary pool of global carbon storage (about 1580
Gt C)
(Schimel,
1995). Efforts to increase SOC content are thought to significantly
reduce atmospheric CO2 and enhance soil fertility and
crop productivity (Sauerbeck, 2001).
Soil
tillage is one of the main human
factors to cause the changes in the
SOC pool.
Long-term
conventional tillage (CT) decreased labile C content (Cheng et al.,
2012) and caused 25-75% SOC mineralization loss (Nicoloso et al.,
2018). Meanwhile,
conservation
tillage
reportedly
increase the labile C contents and SOC sequestration (Somasundaram et
al., 2016; Badagliacca et al., 2018).
The
influence of tillage is dependent on tillage depth.
Carbonell
et al. (2015) observed that the positive impacts of NT on SOC and labile
C were limited to the topsoil (0-10 cm). The influence of tillage method
on the SOC pool also is dependent on soil type and environmental
factors. Also, different tillage practices significantly influence crop
yields.
Previous researchers found that NT
and minimum tillage increase SOC sequestration and crop
yields
(Sun et al., 2018a; Márcio et al., 2018) and that subsoiling tillage
(ST) could enhance root growth and improve crop yields (Sun
et
al., 2018b).
However,
other
studies suggested that long-term mono-conservation tillage was
noneffective to improve crop productivity (Stewart et al., 2017; Li et
al., 2018) and some even found that NT negatively affected crop yields
compared with CT (Zhao et al., 2017; Bogunovic et al.,
2018).
Therefore, the influences of single continuous conservational tillage on
crop yields are unclear. Recent studies have reported that rotational
tillage effectively prevented the disadvantages of mono-tillage, while
increasing SOC content and crop productivity (Chu et al., 2016; Wang et
al.,
2018).
Nevertheless,
the data about the impacts of tillage rotation systems on SOC
sequestration, labile C content, soil quality, and crop productivity on
the China’s Loess Plateau is lacking.
Fertilization
is an indispensable agricultural practice used to achieve high yields,
and it also changes SOC content.
Previous
studies have indicated that inorganic
fertilizer
application significantly improved crop productivity and increased crop
residues amount returned to the field and therefore increased SOC and
labile C contents (Nayak et al., 2012; Ghimire et al., 2017).
A study reported by Chaudhary et
al. (2017) indicated that soils fertilized over the long term contained
higher levels of SOC, readily oxidizable C (ROC), particulate organic C
(POC), and dissolved organic C (DOC) than unfertilized soils. Some
studies have indicated that the positive influences of fertilization on
crop yields only occur with balanced fertilization and not with
unbalanced fertilization (Kukal et al., 2009; Shahid et al., 2017).
Whereas, Liang et al. (2012) observed that inorganic fertilizer
application has no significant positive effects on SOC, labile C, and
crop yields. These inconsistent results
may
be related to climate, soil type, crop system, experimental duration,
and other factors (Samuel et al., 2018).
Therefore, studies of the impacts
of fertilization on SOC sequestration and crop growth should be
accordance with local conditions.
The
Loess Plateau is located in Northwest China and is a major production
region for wheat and maize (Ren et al.,
2016).
Long-term
intensive conventional tillage system has caused
a series of troubles, such as
increased soil erosion, loss of SOC due to mineralization, and
degradation in soil quality (Chen et al., 2009). Therefore,
conservational and rotational tillage practices with crop straw return
have been viewed as potential options.
Loess soil fertility is low because
of low contents of SOM, total nitrogen (N), and available phosphorus
(P), whereas total potassium (K) content is high due to the rich illite
minerals in
soil
parent material (De et al., 2011).
Thus,
to pursue short-term high crop yields, farmers have always adopted a
fertilizing mode of high N and P in this region, which is not conducive
to sustainable soil production.
Recently,
some studies have shown that the
balanced application of N, P, and K fertilization can strongly improve
crop productivity (Lu et al., 2017). On the whole,
to find the optimal mode of
fertilization and tillage for the Northwest China region, it is
essential to assess the impacts of fertilization and rotational tillage
on SOC sequestration, soil quality and crop productivity in this region.
In
this study, our objectives were to study the changes of SOC
sequestration, soil labile C fractions, C pool management index (CMI),
and crop yields under different fertilization and tillage methods. Thus,
a 10-year (2007 to 2016) field experiment with different fertilization
and tillage modes in a wheat-maize crop system in the Loess Plateau was
established to test the following hypotheses: (i)
balanced
application of N, P, K fertilization with the rotation of different
tillage practices would increase wheat and maize yields and increase the
SOC stock by increasing the amount of crop residues returned to the
field and
(ii)
annual rotation of no-tillage and subsoiling would increase the SOC
content and soil labile C fractions, thus improving the CMI and crop
productivity in the China’s Loess Plateau.
2 | MATERIALS AND
METHODS
2.1 | Site
This
work was conducted over ten years (2007 to 2016)
in the Dryland Agricultural
Research Station of Northwest A & F University, located in Heyang
County (104°04′E, 35°19′N altitude 877 m), Shaanxi Province of Northwest
China.
The
climate at the research station is temperate semi-arid continental
monsoon. In this region, the mean annual frost-free period is 210 days.
The annual mean precipitation, evaporation and temperature were 536.6
mm, 1,833 mm and 11.5 °C, respectively.
The soil in this region is
classified as a Chromic Cambisol (sand 34%, silt 39%, clay 27%) using
the FAO/UNESCO Soil Classification (1993). The main soil characteristics
(0-50 cm) at the beginning of study are listed in Table S1.
2.2 | Experimental design
We
adopted a plit-plot design for this experiment. Fertilization treatment
was kept in primary plot treatment and included:
balanced
fertilization (BF), low fertilization (LF), and conventional
fertilization (CF). Tillage method was the sub-plot treatment and
included: no-tillage rotated with
subsoiling in alternating years (NS), subsoiling rotated with ploughing
in alternating years (SP),
ploughing
rotated with no-tillage in alternating years (PN), and ploughing applied
every year
(PP).
Five complete cycles of rotations were implemented during the ten years
(2007 to 2016). The two factors were combined into 12 treatments.
Fertilization treatment: BF,
N:
150 kg ha-1, P2O5: 120
kg ha-1, and K2O: 90 kg
ha-1; LF, N: 75 kg ha-1,
P2O5: 60 kg ha-1, and
K2O: 45 kg ha-1; CF, N: 225 kg
ha-1, P2O5: 180 kg
ha-1, and no potassium fertilizer
applied.
Wheat and maize had the same amount of fertilizer application under the
same fertilization treatment.
The
N, P2O5, and K2O were
from urea
(N:
46.4%), diammonium phosphate (N: 18%,
P2O5: 44%), and potassium chloride
(K2O: 60%), respectively.
The
full rates of P and K plus 50% of N was applied on the sowing date for
maize, the remaining 50% of N was used at the 12th leaf stage (the
middle of June each year). The full
rates of N, P, and K fertilizers were used once on the planting date for
wheat.
Tillage
methods:
In
no-tillage (N),
crop
residue was chopped and covered on the topsoil, the soil was left
undisturbed until sowing. In subsoiling (S),
sub-soiling,
the
crop residue was chopped and covered on the topsoil.
The
topsoil remained undisturbed while the subsoil was subsoiled (30-35 cm
depth, 60 cm width) using a subsoiler. In ploughing (P),
the crop residue was chopped, and
mixed with the soil (20-25 cm depth) using a moldboard
plow. Fields were tilled according
to the described methods after crop harvest each year.
Planting:
The
winter wheat/spring maize rotation system was carried out in this study.
For
winter wheat (cultivars: Jinmai 47 before 2014, Chang 6359 after 2014),
the time of planting and harvesting were in the end of September and
early June of the following year, respectively. The seeding rate and row
spacing were 150 kg ha-1 and 20 cm, respectively. For
spring maize (cultivars: Yuyu 22 before 2014, Zhengdan 958 after 2014),
the time of planting and harvesting were in April and the end of
September, respectively. The
seeding rate and row spacing were 37.5 kg ha-1 and 60
cm, respectively. Table S2 describes the crop rotation schedule from
2007 to 2016.
2.3 | Soil sampling and
processing
Soils
were randomly sampled in 0-10 cm, 10-20 cm, 20-35 cm and 35-50 cm layers
with three
replications
under different treatments in June 2016 after winter wheat harvest.
After air-drying, samples were
ground and sieved to 0.25-mm for
analyzing
the soil C fractions. SOC was obtained with the
K2CrO7-H2SO4digestion method (Walkley and Black, 1934). Readily oxidizable C (ROC)
was
measured
using the 333 mmol L-1 KMnO4 oxidation
method described by Blair et al. (1995). Dissolved organic C (DOC) was
determined as
described
by Jiang et al. (2006). The CHCl3 fumigation-extraction
method was used to measure Microbial biomass C (MBC) (Vance et al.,
1987).
Particulate
organic C (POC) was measured according to the description of Cambardella
and Elliott (1992).
Total SOC stock was calculated as
\(\text{\ SOC}_{\text{stock}}=0.1\times\text{SOC}_{c}\times D\times T\)(1)
Where \(\text{SOC}_{\text{stock}}\) (Mg ha-1) is
the
stock of SOC, \(\text{SOC}_{c}\) (g/kg) is SOC content, D (g
cm-3) and T (cm) represent soil bulk
density
and soil thickness, respectively.
\(\text{\ ΔSOC}_{\text{stock}}=\text{\ SOC}_{\text{stock}}-\text{\ SOC}_{stock0}\)(2)
Where\(\text{ΔSOC}_{\text{stock}}\) (Mg/ha) is the SOC stock accumulation
and\(\text{\ SOC}_{stock0}\)(g
kg-1) is the SOC stock before the experiment.
CMI was calculated according to the description of Blair et al. (1995).
The
CF+PP treatment is the reference in the equation:
\(L=\text{ROC}_{c}/\text{SOC}_{c}\) (3)
Where L is the lability of SOC, \(\text{ROC}_{c}\) is the content
of readily oxidized carbon, and \(\text{SOC}_{c}\) (g
kg-1) is SOC content.
\(\text{CPI}=\text{SOC}_{\text{cs}}/\text{SOC}_{\text{cr}}\) (4)
WhereCPIstands for carbon pool index, \(\text{SOC}_{\text{cs}}\) and\(\text{SOC}_{\text{cr}}\) (g/kg) are the soil organic carbon content of
the sample and the reference (CF+PP treatment), respectively.
\(\text{LI}=L_{C}/L_{R}\) (5)
Where LI is the C lability index, \(L_{C}\) and \(L_{R}\) stand
for the C lability of the sample and the reference (CF+PP treatment).
\(\text{CMI}=\text{CPI}\times\text{LI}\times 100\) (6)
2.4 | Estimation of plant biomass carbon
inputs
In
each treatment, wheat and maize samples were randomly collected from
three 3 m2 and three 9 m2 areas at
the harvest period. After air-drying, the samples were manually threshed
and weighed to determine the yields and straw amount.
The input-C from straw, \(C_{\text{straw}}\), was calculated as:
\(C_{\text{straw}}=B_{\text{straw}}\times 0.40\) (7)
Where \(B_{\text{straw}}\) is crop straw biomass,
plant
biomass C input was calculated by assuming that C content in plant
tissue is 40% according to the study of Johnson et al. (2006).
The input-C from crop stubble, \(C_{\text{stubble}}\), was calculated
as:
\(C_{\text{stubble}}=B_{\text{straw}}\times r\times 0.40\) (8)
Where r is the stubble to straw ratio. For Maize, stubble
represented 10% of straw biomass. For wheat, stubble represented 20%
of straw biomass (Li et al., 2016).
The input-C from roots, \(C_{\text{root}}\), was calculated as:
\(C_{\text{root}}=B_{\text{straw}}\times r\times 0.40\) (9)
Wherer is the root to straw ratio.
Root
represented 23% (maize) and 22% (wheat) of straw biomass according to
the description of Kong et al. (2005).
The
input-C from rhizodeposition (\(C_{\text{rhizodep}}\)) was calculated as
follows (Maillard et al., 2018):
\(C_{\text{rhizodep}}=C_{\text{root}}\times 0.65\) (10)
The stabilization rate (SR, %) of plant biomass carbon into SOC at the
0-50 cm depth was assessed as follows (Srinivasarao et al., 2012):
\(SR=\text{\ ΔSOC}_{\text{stock}}/C_{\text{input}}\) (11)
Where\(\mathbf{C}_{\mathbf{\text{input}}}\)is accumulative plant biomass C
input.
2.5 | Statistical
analysis
SAS
9.0 (SAS Systems, Cary, NC, USA)
was
used for data analysis. Origin 2016 was used for graphing. The
Split-plot analysis of variance (ANOVA) was applied to assess the
impacts of fertilization and tillage on input-C, SOC, soil labile C
fractions (including ROC, DOC, MBC and POC), CMI and crop yields.
The
multiple comparisons were conducted on the basis of the Duncan Multiple
Range Test (at < 0.05 level).
And
the relationships among the indexes were evaluated by regression
equations.
3 RESULTS
3.1 | Accumulative plant biomass C
input
The
cumulative C input from crop residue (including crop straw, crop roots
and rhizodeposition) in the 10-year experimental period (2007-2016) are
shown in Table S3 and Figure 1.
The
fertilization treatments caused significant differences
in the accumulative plant biomass C
input, and the total C input under BF higher than that under LF and CF.
Simultaneously, rotational tillage systems (SP, PN, and NS)
significantly increased the C inputs from straw, root residue, stubble,
and rhizodeposition. Compared with the
PP,
total C input were 18.10%, 11.04% and 14.72% higher in the SP, PN,
and NS tillage than that in PP, respectively.
3.2 | SOC
The
effects of the fertilization on SOC content at 0-20 cm depth were
significant after ten years, but were not significant in the deeper
layers (Figure 2, p < 0.05). Compared with the CF, the
BF and LF significant increased the SOC content and stocks at the 0-10
cm and 10-20 cm depth.
At
the 0-50 depth, SOC stock under different fertilization treatments was
followed by BF > CF > LF, and the statistical
differences were found among three fertilization treatments (Table S4;
Figure 3; p < 0.05). Significant effects on SOC content
and stocks due to tillage system were detected at
the
0-50 cm depth (p < 0.05). SOC stock was
markedly
higher in the rotational tillage (SP, PN, and NS) than that in PP
tillage. More specifically, compared with PP tillage, the NS, SP, and PN
tillage primarily increased SOC content and stocks at the 0-10 cm depth.
Similarly, the rotational tillage systems also increased SOC content in
> 10 cm soil layers. The difference was significant between
NS and PP in10-20 cm and 35-50 cm layers, was significant between SP and
PP in the 20-35 cm and 35-50cm layers, and was significant between PN
and PP at the 20-35 cm depth (p < 0.05).
The
SOC stocks of all treatments increased after the ten-year experiment,
and the maximum increment by 6.83 Mg ha-1 was in the
treatment of BF+NS, the minimum increment by 2.54 Mg
ha-1 was in the treatment of CF+PP (Figure 3). The SOC
stock accumulation at the 0-50 depth was higher under BF and LF than
that under CF, and there was a statistical difference between BF and CF
(p < 0.05). Changes in SOC stock accumulation under
different tillage systems decreased in the order of NS > PN
> SP> PP, and the statistical differences were
found between rotational tillage and PP tillage (p <
0.05).
3.3 | Stabilization
rate
The
proportion of SOC stock accumulation to total C input is the
stabilization rate (Figure 4). The stabilization rate in the treatment
of BF+NS was 17.08%, which was the highest among all treatments. The
lowest stabilization rate was recorded (7.66%) in the CF+PP treatment.
For fertilization effect, the stabilization rate were increased by
39.00% in BF and by 33.79% in LF compared with CF. Meanwhile, the
effect of tillage on the stabilization rate was significant. Compared
with the PP, the stabilization rate were significantly increased by
61.54%, 20.83%, and 26.92% in the NS, SP, and PN, respectively
(p <0.05).
Figure
5 showed that SOC stock accumulation was positively correlated with
total C input, and the stabilization rate of plant biomass C
(p <0.05).
3.4 | Soil labile C
fractions
Table
1 showed the differences of soil labile C fractions (ROC, DOC, MBC, and
POC) among treatments (p <
0.05).
For
fertilization effect, the order of ROC content under different
fertilization treatments was BF > CF > LF in
the topsoil (0-10 cm), and there were statistical differences between BF
and LF (p < 0.05). However,
the
differences in ROC content among the three fertilization treatments were
nonsignificant at > 10 cm depth. Tillage had significant
effects on ROC in all soil layers (0-50 cm). Compared with PP tillage,
NS and SP significantly increased ROC content at the 0-10 cm, 20-35 cm,
and 35-50 cm depth; and PN significantly increased ROC content at the
0-10 cm and 20-35 cm depth (p < 0.05).
Significant
effects of the fertilization treatments or tillage systems on DOC
content were detected in all soil layers (0-50 cm) (Table 1). DOC
content with BF was higher than that with CF at the 0-10 cm, 10-20 cm,
and 20-35 cm depth, and DOC content was lower with LF than that with CF
at the 0-10 cm and 35-50 cm depth (p < 0.05). For
tillage effect, DOC content was significantly higher in rotational
tillage systems (NS, SP, PN) than that in PP tillage in all soil layers
(0-50
cm)(P <0.05).
More specifically, compared with PP, DOC content increased by 40.92%,
10.89%, 15.64%, 30.95% under NS, increased by 13.56%, 5.03%,
8.00%, 20.48% under SP and increased by 24.94%, 12.29%, 10.55%,
13.33% under PN at 0-10 cm, 10-20 cm, 20-35 cm and 35-50 cm
depth, respectively.
The
content of MBC was followed by CF > BF > LF in
all layers (0-50 cm), and there were statistical differences between LF
and CF (Table 1). Tillage method affected MBC content in all layers
(0-50 cm). Compared with PP tillage, rotational tillage systems (NS, SP
and PN) significantly increased MBC content at the 0-10 cm, 20-35 cm,
and 35-50 cm depth. Meanwhile, NS, SP and PN decreased MBC content
compared to PP tillage at 10-20 cm depth (p < 0.05).
Fertilization
treatment markedly affected POC content at the 0-10 cm and 10-20 cm
depth, and POC content was higher with the BF treatment than that with
the CF and LF treatments (Table 1). Tillage significantly affected POC
content. Compared with PP, POC content increased by 58.79%, 15.73%,
67.39% under NS, increased by 26.06%, 41.57%, 36.96% under SP and
increased by 42.42%, 28.09%, 30.43% under PN at 0-10 cm, 10-20 cm and
35-50 cm depths, respectively.