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.