4 | DISCUSSIONS
4.1 |Responses ofSOC
stocks and grain yields to tillage
Results of the 8-yr study significantly enhanced SOC stocks under
no-tillage with wheat straw
stubbles 30–40 cm in height in the long term. This observation is
consistent with the previous studies reporting that long-term no-tillage
increases SOC stocks when crop residue retained in fields (Somasundaram
et al., 2017; Gonçalves et al., 2019; Liu et al., 2020). In general,
reduction of soil disturbance is one of main reason underlying the
enhanced potential of SOC sequestration under adopting conservation
agriculture (Kan et al., 2020; Sun et al., 2020). No-tillage with straw
retention protects aggregate-associated SOC from microbial attack, thus
reducing SOC mineralization and increasing SOC stocks (Kan et al., 2020;
Wang et al., 2020).
The 8-yr results revealed that no-tillage with wheat straw stubbles
30–40 cm in height increased grain yields of both wheat and maize
compared with those in CK and RT. Previous studies have also reported
positive responses of grain yields to no-tillage. For instance, Xu et
al. (2019) and Peng et al. (2020) reported that long-term no-tillage can
achieve significant improvements in grain yield over conventional
tillage. However, numerous studies have reported that grain yields under
no-tillage remained unchanged or even decreased (Pittelkow et al., 2015;
Fiorini et al., 2020). Zhang et
al. (2018) also reported that on average 6.9% decrease was observed in
wheat yield under continuous no-tillage on a sandy loam soil in the
North China Plain. Yield
improvement in the present study resulted mainly from large amounts of
stored SOC in NT. Previous studies have also demonstrated that
no-tillage can enhance grain yields depending on many external factors,
such as soil type, soil fertility, crop rotation, irrigation condition,
and meteorological factors (Boomsma et al., 2010; Sun et al., 2020).
Changes in site-specific characteristics and management as well as
variations in climate might account for variations in grain yields
including the dramatic decline of wheat grain yields in 2013.
4.2 | Responses of SOCstocks and grain yields
to long-term straw
return
Many studies have shown that straw return is an environmentally friendly
and effective strategy to stabilize soil structure, improve soil
fertility and sequester atmospheric CO2 into
agricultural soils (Zhao et al., 2018b; Guan et al., 2020; Xue et al.,
2020). Our results indicate that long-term (8-yr) straw return
significantly enhanced SOC stocks compared with CK-SR0 or even SOC
levels in 2008, where SOC improvement was probably associated with the
amount of plant-derived C inputs,
as suggested previously (Liu et al., 2014; Ghosh et al., 2012; Berhane
et al., 2020). The SOC levels in NT-SR and RT-SR differed significantly
from those in CK-SR, NT-SR0, RT-SR0, and CK-SR0 in 2016, and SOC stocks
increased as more plant-derived C
inputs increased (Li et al., 2016), thereby effectively mitigating SOC
loss from agricultural ecosystems due to intensive cropping and
anthropogenic disturbance. In the present study, the SOC levels could be
maintained without straw retention. This could be due to the higher
conversion efficiency of plant-derived C to SOC from roots than straw
(Ghafoor et al., 2017). Root-derived C inputs had been demonstrated as
one of main C source for SOC maintenance (Zhao et al., 2018a). High crop
productivity implied greater plant-derived C inputs, especially from
belowground roots, thus resulting in additional C sequestration in
agricultural soils (Liu et al., 2014). However, results of Li et al.
(2016) and Zhao et al. (2018b) showed that SOC levels could not be
maintained under straw removals of both wheat and maize in the same
cropping system and region. This discrepancy might be due to
soil depth and experimental
duration. Thus, the effect of roots’ persistence on SOC sequestration
needs further confirmation and investigation to evaluate the efficiency
of C sequestration and on roots’ persistence for their contribution to
SOC than straw under varied soil types, soil depth, experimental
duration and climate conditions (Sukhdev et al., 2011; Liu et al.,
2014).
The results of our field experiment indicate that grain yields of both
wheat and maize increased under straw return compared with CK-SR0 and
showed significant correlations with SOC stocks. Similar results were
reported by Xu et al. (2019) based on an 11-yr field experiment in the
Northern China Plain (NCP). Karami et al. (2012) and Wang et al. (2018)
reported the positive effects of straw return on grain yields due mainly
to improvements in the stability and strength of soil aggregates, the
levels of soil nutrient and moisture, the activities of soil enzyme,
thus providing a favorable physical, chemical and biological soil
environment. However, a significant non-linear relationship was also
reported between relative grain yields and topsoil SOC stocks (Lal,
2009), indicating no additional benefit from SOC sequestration on grain
yields when SOC stocks have reached a critical value (Loveland and Webb,
2003; Krull et al., 2004). The positive correlation between grain yields
and SOC stocks in the present study suggested that SOC stocks have not
yet reached saturation and can be further enhanced by adopting
appropriate field management practices.
4.3 |Responses of grain yield
stability totillage and straw return
During the 8-yr periods, tillage systems significantly affected seasonal
grain yields of both wheat and maize (P < 0.01, Table
1), which significantly correlated with
SOC stocks. The SOC increases in
NT-SR highlighted that the wheat-maize double cropping systemcan
transform agricultural soils from net sources to net sinks of
atmospheric CO2 without reducing crop
yields. Increased SOC stocks has
potential positive feedback effects on soil fertility, water retention,
biological functions and productivity (Zhang et al., 2018; Zhao et al.,
2019a & b; Feng et al., 2020). However, more than 90% of plant-derived
C under straw return could not be sequestrated by soils (Fig. 1; Table
2; Li et al., 2019). Thus, the improvement
in tillage and straw management
practices has improved grain yields in intensive wheat-maize double
cropping system. Inter-annual
variability in temperature and precipitation contributed to the
year-to-year variability in grain yields (Najafi et al., 2019).
Long-term field experiments provide valuable information regarding
whether it is feasible to adopt no-tillage and straw return with
agricultural sustainability in an intensive cropping system and address
the challenges of achieving high grain yields economically and
environmentally sustainable.
Yield variability was used as a negative measure of yield stability
during the studied periods. Increasing evidences from manipulative
experiments (Boomsma et al., 2010; Zhang et al., 2018) and meta-analyses( Huang et al., 2015; Sun et al., 2020) have documented
that the response of grain yields to tillage and straw management
practices is quite variable. Due to the difficulty of characterizing the
response of yield increase and stability to SOC stocks, whether high SOC
stocks can improve yield stability is still in doubt (Zhang et al.,
2016). The 8-yr results revealed that no-tillage with wheat straw
stubbles 30–40 cm in height increased yield stability of both wheat and
maize under maize straw return, likely because both straw return and
no-tillage have increased SOC stocks. Soil organic C sequestration
contributed to increasing and stabilizing grain yields of both wheat and
maize in NT and RT more than CK, and with SR than SR0 over the 8-yr
periods. However, higher SOC stock is not always accompanied by higher
grain yield. Higher SOC stocks did not result in higher wheat grain
yields for all treatments in 2013 in comparison with those in other
years (Fig. 2 & 3). Increasing SOC stocks in agricultural soils with
adaption of NT-SR might nonetheless contribute to improvements of both
wheat and maize grain yields and reduce the uncertainty of grain
production in China, which is without doubt a matter of concern for the
nation with the largest population in the world (Zhang et al., 2016;
Zhao et al., 2018). Higher SOC stocks and yield stability could result
in higher agricultural sustainability as indicated by higher sustainable
yield index under adoption of no-tillage and/or straw return compared
with those under CK-SR0. Therefore, the above findings reinforce the
importance of adopting no-tillage and/or straw return, especially the
combination of no-tillage with wheat straw stubbles height of 30–40 cm
and maize straw return (NT-SR), for increasing SOC stocks and sustaining
grain yield stability.