Long-term straw returning favors soil structure and soil physicochemical properties (Biswas et al., 2017; Chivenge et al., 2011). Crop straw incorporation can loosen soil, reduce soil erosion caused by wind and rain (Liu et al., 2010; Oldfield et al., 2019; Zhu et al., 1983), reduce runoff, increase crop productivity (Liu et al., 2010; Mi et al., 2016), and even protect the soil surface from heavy raindrops (Anderson et al., 2015). Considerable straw resources in crop production are not fully and effectively utilized (Wang et al., 2010; Xie et al., 2011; Zhang et al., 2009). Straw returning provides an environmentally friendly insight into the long-term influences of soil aggregates on cropping patterns (Wang et al., 2010; Zhang et al., 2009). Many studies have investigated the influence of straw returning at regional and global scales (Bronick et al., 2005; Xie et al., 2011). Straw mulching (Li et al., 2012), straw stubble returning (Jat et al., 2009) and the combination of straw and plastic mulch (Wang et al., 2010; Wang et al., 2013) all were reported. Thus, the improved soil qualities and grain yields of optimized straw incorporation (low ratios of carbon and nitrogen) across crop growing seasons merit research.
Soil tillage and straw returning methods are the main external factors that significantly affect soil aggregation, soil aggregate stability, and the organic carbon fixation rate (Amirinejad et al., 2011; Li et al., 2012; Verzeaux et al., 2016; Xue et al., 2020). Straw residue increases aggregate (Six et al., 2000) and macro-and micro-aggregate formation (Bronick et al., 2005; Ye et al., 2018). Soil aggregates have brought significant influences on soil ecosystems (Amirinejad et al., 2011; Carter, 1992). Soil structure is the main driver for soil aggregate distribution, formation and turnover, and soil aggregates are the basic units of soil structure and are used as an index for evaluating soil quality (Amirinejad et al., 2011; Bronick and Lal, 2005; Zhu et al., 1983). Soil aggregate turnover is influenced by agricultural practices (Biswas et al., 2017; Carter, 1992) and their stability is often critical for carbon sequestration, soil degradation, and soil water movement (Xue et al., 2020; Ye et al., 2018). Winstone et al. (2019) reported that the composition and basic characteristics of soil aggregates determine the physical processes of soil erosion, compaction, and consolidation. Aggregate turnover (breakdown and formation) is largely responsible for soil organic matter accumulation and nutrient material formation (Six et al., 2000; Tisdall and Oades, 1982; Plante and McGill, 2002). Ludwig et al. (2011) stated that conservation management practices (no-tillage, mulching, cover crops, and chemical fertilizer and manure applications) can increase soil water-stable aggregates, which is significant for understanding the effects of no-tillage management for improving soils and preventing soil degradation over time (Blanco-Canqui and Lal, 2007). The current straw mulching practices can have undesirable side effects, such as lower soil temperature and more pests and diseases (Kienzler et al., 2012). Straw returned to the field can also promote soil hydraulic conductivity and improve soil water storage in semi-arid regions which have high evaporation and low rainfall (Tisdall and Oades, 1982; Zhang et al., 2009). Several field experiments have indicated that crop residues improve the soil hydrothermal regime (Liu et al., 2010), increase soil organic carbon communication (Amirinejad et al., 2011; Jiang et al., 2019; Kienzler et al., 2012; Verzeaux et al., 2016), and promote soil porosity (Cui et al., 2019; Jat et al., 2009).
There are motivations for decreasing the straw C:N ratio associated with promoting straw use efficiencies, such as higher straw decomposition and an increasing nutrient in soil (Sørensen et al., 1996; Zhao et al. 2014). Ammoniated straw (i.e., C: N ratio = 25:1) improved straw hemicellulose, and straw lignin characteristics and promoted crude protein content, thus increasing the release of nutrients from the straw (Angers et al., 1997; Biswas et al., 2017; Liu et al., 2010; Mao et al., 1999; Wang et al., 2013). Shorter straw (i.e., crop residues chopped finely) decompose faster than longer straw (Angers et al., 1997; Sørensen et al., 1996). Soil microbes compete with crops for nutrients during straw decomposition, which can cause crop nitrogen deficiency (Cabiles et al., 2008; Wang et al., 2012). More rapid straw decomposition improves microbial biomass stability and promotes soil quality (Jensen, 1994; Zhao et al., 2014). Studies of straw with a low C/N ratio returned to the farmland are unexplored, i.e., most studies have focused on the amount of straw returning and combinations of straw and fertilizer (Blanco-Canqui and Lal, 2007). However, it remains unclear whether long-term straw incorporation with a lower C/N ratio affects soil structure (soil physicochemical properties) and grain productivity.
We hypothesized that straw with a low C/N ratio (ammoniated maize straw) plowed into the soil would improve soil quality, promote soil structure, and enhance grain yield, relative to traditional straw plowed into the soil. We undertook a five-year field experiment in semi-arid areas of northwest China during winter wheat (Triticum aestivum L.) growing seasons to investigate the potential effects of ammoniated maize straw incorporation on soil physicochemical properties and winter wheat productivity. The study aimed to: (1) assess the variation in soil aggregates under optimized straw incorporation, (2) explore how the spatial variability of SOC is affected by aggregate size class, and (3) identify an optimal straw incorporation practice to promote winter wheat yield in semi-arid regions.