1 | INTRODUCTION
Soil degradationis a major impediment to global crop productionandimproper management in arable land is the main driver of land degradation (Ye&Van Ranst, 2011;Sampietro et al., 2018; Lorenz et al., 2019). With drier climates and increasing pressure on resources, land degradation has become a major issue that continues to negatively affect human and natural ecosystems in some parts of the world (Guillaume et al., 2016; Djuma et al., 2020).Continuing degradation in arable soils results indeclines insoil organic carbon(SOC) stocks, crop productivity, and ecosystem service values of increasing agricultural systems (Ma et al., 2020;Raimondo et al., 2020). TheSOCdepletion is also acknowledged as a major source of greenhouse gas emissions, resulting further in losses of soil fertility and ecosystem stability (Amundson et al., 2015; Rüegget al., 2019).Even small changes inthe SOC caused by anthropogenic disturbance in terrestrial ecosystems can profoundly affect the atmospheric C dioxide(CO2)and thus get feedback to climate changes (Sayer et al., 2011). In recent years, increasingattentionshave been paid to SOC sequestration by optimizing appropriate crop rotation, integrated soil fertility management, and conservation tillage including crop residue management in agricultural ecosystems (Xia et al., 2018; Gonçalves et al., 2019; Berhane et al., 2020;Ryan et al., 2020; Sun et al., 2020).Therefore, understanding the SOC dynamics and the role that the soil plays in sequestering more atmospheric CO2into soils are of incredible concern due to their great potential for both climate change mitigation and agricultural sustainability (Zhao et al., 2018a; Feng et al., 2020).
With a straw yield of 703.3 million tons (Mt) in 2014, and an average annual straw increase of 4% over the decades, China has become one of the most abundant countries in terms of straw resources (Hong et al., 2016; Yin et al., 2018). However, the agricultural soils in China have a relatively low SOC level due to the long cultivation history, highly intensive field management, and lower straw-return rates caused by large amounts of straw used as fuel and animal feeds or were burnt in fields (Zhao et al., 2018a;Hou et al., 2019; Berhane et al., 2020). Since crop residue contains large amounts of nitrogen (N), phosphorus (P), and potassium (K), adopting straw-return management practices in farmland facilitates the maintenance of soil nutrient balance and soil quality, and boosts the sustainability of agricultural production (Yin et al., 2018; Guan et al., 2020). As the largest producer and consumer of mineral fertilizers, China has consumed approximately 35% of the global mineral fertilizers for agricultural production (Sun et al., 2012; Yin et al., 2018). The application of mineral fertilizers with low levels of straw-return generally enhances grain yields in the short term, but exacerbates soil organic matter (SOM) depletion, deteriorates soil nutrient availability, resulting in the decline of agricultural sustainability in the long term (Gaffney et al., 2019). Conversely, straw return significantly enhanced SOC contents (14.9%) and crop yields (5.1%) with the same amounts of mineral-N applied in comparison with straw removal in global agro-ecosystems (Xia et al., 2018). Likewise, Zhao et al. (2018a) have reported an increase of 140 kg C ha−1yr−1 SOC stock from 1980 to 2011 in China’s croplands (0−20 cm), and that approximately 40% of the incremental increase could be accounted for by straw return. However, it remains uncertainty that how agricultural soils respond to straw return (Liu et al., 2014; Berhane et al., 2020). For instance, Liu et al. (2014) reported that SOC sequestration was enhanced under straw return during the first 3 years, but afterwards that effect gradually decreased over time. Some studies have also reported that the effects of straw return may be negligible or negative (García-Orenes et al., 2009; Wang et al., 2011). The distinctions might be due to various ecological conditions, soil conditions, methods of disposing straw, straw-returned rate, and experimental duration (Liu et al., 2014).
Straw incorporation through conventional tillage, such as plowing tillage and rotary tillage, is a common and effective field management practice with a strong influence on soil fertility, soil physical and biological properties and crop yields (Yu et al., 2017; Liu et al., 2021). On the one hand, frequent adoption of conventional tillage destroys soil structural stability, decreases the water holding capacity of soils, worsens the biological characteristics, and nutrient reserve and supply (Wang et al., 2011; Xu et al., 2019; Nouri et al., 2019). However, increasing evidences from manipulative experiments (Zhao et al., 2019a; Li et al., 2019) and meta-analyses (Zhao et al., 2019b; Jat et al., 2020) have showed that adopting conservation agriculture (CA) could improve soil quality and agricultural sustainability. The CA mainly consists of various reduced and even no-tillage techniques with more than 30% of crop residue retained on soil surface, providing effective strategies for combating soil degradation (Liu et al., 2020). It has been reported that SOC contents of topsoil increased with years under CA for its superiority on the protection of soil structure and reduced rates of soil degradation (Somasundaram et al., 2017; Reeves et al., 2019). The effects of CA on grain yields are highly variable, varying from increase (Liu et al., 2018; Peng et al., 2020), neutral (Zhang et al., 2018) to decrease (Pittelkow et al., 2015; Fiorini et al., 2020).
The intensive wheat (Triticum aestivum L.)-maize (Zea mays L.) double cropping systemis the major agricultural practice in the southern Loess Plateau of northwest China (Deng et al., 2006; Liu et al., 2017). The growth periods for wheat from mid-October to early June of next year, and for maize were from mid-June to early October. The high intensity with short time interval between wheat and maize growth periods in this agricultural system have been widely reported (Li et al., 2019). To achieve high grain yields, the excessive uses of mineral fertilizers and tillage, especially the plowing and rotary tillage, are indispensable (Yang et al., 2017; Xu et al., 2019). Under favorable conditions of water and fertilizer, the intensive field managements stimulate the degradation of plant residues and SOM, and thus result in SOC loss (Liu et al., 2014; Guan et al., 2020). For a long time, straw burning was adopted by farmers after crop harvest, which did not only lead to a waste of biomass resources, but also gave rise to an excessive emissions of greenhouse gases, and even left these regions in a thick haze (Hou et al., 2019; Li et al., 2019). As such, adopting sustainable, cleaner field management practices (i.e., conservation tillage and straw return) is essential to promote SOC sequestration and grain yields, thus attaining economic and environmental sustainability in grain production (Zhao et al., 2018a).
Moreover, most studies relate to SOC sequestration and grain yields affected by both straw return and tillage in the intensive wheat-maize system have focused on the period after maize harvest (Tian et al., 2016; Kan et al., 2020), but did not consider the effects of both straw return and tillage management practices after wheat harvest. Although long-term straw return is clearly an effective field management practice for sequestering C in soils, and a linear relationship exists between SOC sequestration and grain yields (Liu et al., 2014; Zhao et al., 2018a; Xu et al., 2019), there is a paucity of information on the effects of both no-tillage before maize planting and maize straw return on SOC sequestration and its relationship with grain yields and yield stability in the intensive wheat-maize system. Therefore, the present study utilized a long-term field experiment initiated in 2008 and aimed to examine the combined effects of no-tillage before maize planting and maize straw return on SOC sequestration, grain yields, yield stability and agricultural sustainability. The information obtained can be used to optimize both no-tillage and straw return management practices, enhance crop production, and improve agricultural sustainability for the wheat-maize rotation system in dry sub-humid areas of northwestern China and other areas with similar climates and cropping systems.