3.5 | Carbon pool management index
After
ten years, the SOC lability (L) and SOC lability index (LI) of the 0-10
cm and 35-50 cm depths were higher under CF treatment than those under
BF or LF treatment, whereas the C pool index (CPI) of the topsoil (0-10
cm) was lower under CF than that under BF and LF and CMI of the 0-10 cm
and 10-20 cm depths were greater under BF than those under LF and CF
(Table 2). For tillage effect, the L and LI at the 0-10 cm and 20-35 cm
depths under rotational tillage systems (NS, SP, and PN) were higher
than those under PP tillage (p < 0.05). And compared
with PP, rotational tillage systems significantly increased CPI at the
0-10 cm and 10-20 cm depths (p < 0.05). Meanwhile, CMI
was higher under rotational tillage systems (NS, SP and PN) than that
under PP tillage at the 0-10 cm, and 20-35 cm depths. Moreover, NS and
SP also strongly increased CMI in the 35-50 cm soil
layer
(p < 0.05).
3.6 | Crop yields
The
wheat and maize yields from 2008 to 2016 were strongly affected by
fertilization treatments and tillage
systems
(Table S5 and Figure
6).
For fertilization effect, the yield of wheat and maize were both
followed by BF > CF > LF. Compared with LF,
the yields with BF were significantly increased by 11.70% for wheat on
average and 8.31% for maize in average (p < 0.05). For
tillage effect, rotational tillage systems (NS, SP and PN) produced
higher wheat and maize yields than the PP tillage. More specifically,
compared
with PP, wheat yields on average were increased by 12.71%, 8.89%, and
12.83% in NS, SP, and PN, maize yields in average were increased by
14.05%, 8.83%, and 12.59% in NS, SP, and PN, respectively. On the
whole,
the minimum average yield of wheat and maize were found in LF+PP
treatment, the maximum average yield of wheat and maize were found in
BF+NS treatment.
Regression
analysis showed the positive relationships were found between crop
(wheat and maize) yields and SOC and it’s labile
fractions
(Figure 7). The correlation coefficients of crop yields (wheat yield,
maize yield) and SOC were smaller than the correlation coefficients of
crop yields and soil labile C fractions (ROC, MBC, DOC and POC),
indicating that the positive influences of labile C fractions on crop
yields were more significant than those of SOC.
Wheat and maize yields were
positively correlated with CMI.
4 | DISCUSSION
4.1 | Impacts of long-term tillage rotation and
fertilization on SOC content and SOC
stocks
In
this study,
fertilization
and tillage significantly affected SOC content and stock,
whereas their interaction was
slight.
The
SOC stock under BF was markedly greater than that under CF and LF at the
0-50 cm depth. This was primarily since balanced fertilization was
beneficial to crop growth and increased the amount of returned crop
residues. Compared to CF, BF and LF reduced the application amount of
nitrogen and phosphate fertilizer. Lu et al. (2011) indicated that
excessive application of N fertilizer might
reduce
the ratio of C: N in soil, accelerated decomposition of crop straw by
soil microorganisms, result in the lower carbon
sequestration.
In other words, the increase of SOC is more related to the stabilization
rate of input C, which also was confirmed by that the linear correlation
coefficient between SOC storage and the stabilization rate (0.96) was
higher than that between SOC storage and input C (0.55) (Figure 5). In
our study, BF and LF increased in SOC contents at the 0-10 cm and 10-20
cm depth due to the higher stabilization rate of input
C. In addition, N and P nutrients
enriched by long-term
high
rates of fertilizers could accelerate the decomposition of SOC (Luo et
al., 2019), which may be one reason to explain the SOC content in CF
lower than in BF and LF.
At
the end of the ten-year experiment, the higher levels of SOC stocks were
found at the 0-50 cm depth under
rotational
tillage systems (NS, SP, and PN) than under PP.
That
can be related to the higher plant biomass C input under rotational
tillage systems (Figure 1) and the lower mineralization of
SOC.
Previous studies reported that conservational tillage, such as
no
tillage, could reduce soil disturbance, decreases the mineralization
rate of SOC and promote the accumulation and humification of crop
residues returned to the soil (Mazzoncini et al., 2013). Based on the
observed SOC changes in different soil layers, the rotational tillage
systems (NS, SP, and PN) increased the SOC content and SOC stock
accumulation of the 0-10 cm depth compared to PP. The reason for this
difference is that rotational tillage causes less soil disturbance,
which allows the returned crop residues to cover most of the soil
surface (He et al.,
2019).
The rotational tillage systems also increased SOC content in
> 10 cm layers, and the effect of NS at 10-20 cm and 35-50
cm depths was
obvious.
This can be attributed to two factors:
(1)
The
rotational tillage systems increased the incorporation of crop residues
by input at depth where there may be a greater chance of protecting SOC
by the combination with the mineral
matrix
(Hou et al., 2012); (2) subsoil tillage can break the bottom layer of
the plow pan and promote crop root growth, which increases the
production of root residues and secretions
(Leonard et al., 2012).
Poeplau
and Don (2013) suggested that subsoil tillage
can
increase the production of crop root exudates and root litter and
therefore increase the amount of SOM in the subsoil.
4.2 | Impacts of long-term tillage rotation and
fertilization on soil labile C fractions and
CMI
In
our report, the ROC, DOC, and POC
contents
under the BF were higher than those under the LF or CF at the 0-10 cm
depth. This was primarily because of the higher C input in the BF.
Simultaneously, BF also increased DOC at the 10-20 cm and 20-35 cm depth
compared to LF and CF, due to the
migration
of DOC with soil moisture (Kaiser and Kalbitz,
2012).
Increasing the application of nitrogen and phosphorus fertilizers can
stimulate soil microbial activity and therefore increase MBC
concentrations (Ghosh et al., 2018), and we came to a similar conclusion
that the content of MBC in the all layers (0-50 cm) under BF and CF was
higher than that under
LF.
Compared with PP, the rotational tillage systems (NS, SP, and PN)
increased the content of soil labile C fractions (ROC, DOC, MBC and POC)
in soil surface layer (0-10 cm) by creating an environment in which more
crop residues covered on the soil surface.
Crop
residues provide substrates for soil microorganisms and promote the
accumulation of soil labile C (Jharna et al.,
2018).
In contrast, PP tillage weakened
the physical protection of SOC, exposed protected SOC to microbial
decomposition, accelerated mineralization of active organic matter in
newly turned topsoil, thus increasing the loss of soil labile C (Chen et
al.,
2009).
The
rotational tillage systems also increased the content of soil labile C
fractions in deep soil (20-35 cm and 35-50 cm), which may be as a result
of the increased amount of root debris and exudate returning to the soil
(Hou et al.,
2012).
In
our study, fertilizer application had a significant impact on CMI at the
0-10 cm and 10-20 cm depths and tillage had a significant influence on
CMI in all layers (0-50 cm) (Table 2).
The
positive linear correlation between CMI and carbon input was found in
the present study, indicates that
higher
crop residue inputs caused higher CMI. Similar observations were
reported by Chatterjee et al. (2018). The CMI at the 0-10 cm depth was
lager with BF than that with CF; rotational tillage systems (NS, SP, and
PN) increased CMI compared to PP tillage.
These
results suggested that soil fertility can be healthily developed by
balanced fertilization or rotational
tillage.
The value of CMI depends on SOC content and activity (Blair et al.,
1995), that is increasing organic carbon input or reducing SOC
mineralization in BF and rotational tillage could cause higher SOM
content and labile organic C, thereby resulting in higher
CMI.
4.3 | Impacts of long-term tillage rotation and
fertilization on crop
yields
In
this study, BF with appropriate reductions in N and P fertilizer rates
and supplemental K fertilizer improved wheat and maize yields, which was
confirmed by Yang et al., (2006).
This
suggested that optimal yields cannot be obtained in the Loess Plateau by
the fertilization methods of applying high N and P fertilizer and no K
fertilizer. For tillage, the
rotational tillage systems (NS, SP, and PN) increased wheat and maize
yields compared with PP
tillage.
This
is
because the tillage rotation systems can prevent the decline in SOC
caused by long-term intensive tillage and enhance soil fertility
(Bhattacharyya et al., 2012). Water
shortage is another important factor restricting crop growth in the
Loess Plateau.
Tillage
rotation can reduce soil water evaporation by covering a large of crop
residue on soil surface (Yu et al.,
2020).
In
addition, the higher SOC content produced by tillage rotation promotes
water storage and water absorption by crop, thereby increasing crop
yields (Manns and Berg, 2014).
In
our study,
crop
yields were significantly positively correlated with SOC and
soil
labile C fractions (ROC, MBC, DOC, and POC) (Figure 7), indicating that
the increased content and activity of SOC had a positive impact on crop
production (Li et al., 2016). The correlation coefficients of crop yield
and soil labile organic C fractions were greater than those of crop
yield and SOC, suggesting that soil labile organic C contributes more
significantly to increased crop yields.
Therefore, agricultural practices
that increase the active components of SOC are crucial for maintaining
soil fertility and increasing crop yields.
5| CONCLUSIONS
Our
results demonstrate that fertilization and tillage practices affect the
SOC pool in China’s Loess Plateau. Balanced fertilization (BF) and
rotational tillage (NS, SP, and PN) significantly increased SOC stocks,
and NS rotation combined with BF produced the highest SOC stock among
all treatments. SOC stock accumulation was positively correlated with
plant biomass C input and with the stabilization rate (SR) of returned
plant biomass C, indicate that BF and rotational tillage had positive
effects on SOC sequestration by the increases of input-C and it’s
stabilization rate. The SOC, ROC, DOC, and POC contents were greater
under BF than those under CF at the 0-10 cm depth. Meanwhile, rotational
tillage systems increased the soil labile C contents at the 0-10 cm,
20-35 cm and 35-50 cm depths. BF and rotational tillage also
significantly improved the CMI and soil quality due to changes in the
content and activity of SOC. In addition, fertilization and tillage
practices affected SOC content, soil labile C content and CMI, which in
turn affect crop yields. BF and rotational tillage were effective in
increasing the yields of wheat and maize. The highest average yields of
wheat (increased by 30.93%) and maize (increased by 20.39%) were found
in BF+NS treatment. We found that NS tillage with a balanced application
of nitrogen phosphorus and potassium fertilizers could increase SOC
sequestration, improve soil quality, and increase maize and wheat yields
in China’s Loess Plateau.
ACKNOWLEDGEMENTS
This work was funded by
the
Special Fund for Agro-scientific Research in the Public Interest of the
Ministry of Agriculture, China (Grant no.
201503116), and
the
National
Natural Science Foundation of China (51879224).