1. INTRODUCTION
Globally, approximately 1.1 × 109 ha of lands are affected by soil salinity (Hopmans et al., 2021), which is threatening agricultural production, soil health, and food security (Sahab et al., 2020). China, for instance, has approximately 3.6 × 107 ha of saline soils, which possess great potential for agricultural utilization after amelioration (Rezapour et al., 2017; Yang et al., 2022). Saline soil plays a vital part in global carbon (C) and nitrogen (N) cycling, especially under the conditions of human intervention (Setia et al., 2013). Additionally, saline soil has low soil C and N contents, which intensify the negative effects of soil salinization (Huo et al., 2017).
Straw returning is widely recommended to maintain soil C and N contents through efficient use of the crop straw resource (Xia et al., 2018). Straw deep returning as an interlayer is an effective farming strategy for saline soil amelioration (Zhao et al., 2013a), which mitigates soil salinity (Zhao et al., 2013b), improves soil structure (Cong et al., 2019), regulates soil microbial community (Li et al., 2016), and boosts crop yield (Zhao et al., 2016). Soil C and N in farmland was mainly derived from crop straw, which affects soil C and N availability after mineralization (Lian et al., 2015). However, contradictory results were observed with different amounts of straw returning (Jin et al., 2020), due to local climatic conditions and soil properties (Hao et al., 2022). Excessive straw returning usually resulted in an undesirable soil C:N ratio (Zhang et al., 2015), prevented straw mineralization (Shahbaz et al., 2017a), and reduced crop production (Islam et al., 2022). However, insufficient straw amendment weakened soil C and N sequestration (Zhang et al., 2016), and did not meet the balance of soil C and N pools (Andruschkewitsch et al., 2013). Thus, it is critical to quantify the impacts of various amounts of straw deep returning as interlayers on soil C and N, especially in saline soils.
Inability to incorporate straw into deep soil constrain the C and N sequestrations (Zhao et al., 2018), while the legacy effects of straw deep returning on soil C and N remain unclear in saline soils (Zhai et al., 2021). The SOC content in 20-40 cm soil depth was increased in the first two years, while decreased from the third year under the condition of straw deep returning with 12 Mg ha-1 (Cong et al., 2019). However, this trend was influenced by the amount of straw input (Zhang et al., 2021), and it needed further investigation for saline soil (He et al., 2022). Meanwhile, soil C and N to straw deep returning varied widely at different soil depths (Dikgwatlhe et al., 2014). Straw deep returning (20-40 cm) increased SOC content by 8% in topsoil (0-20 cm), and up to 27% in the subsoil (20-40 cm) in the third year (Zheng et al., 2021). These resulted in different soil stratification ratios, a soil quality indicator of nutrient distribution in soil profile (Melero et al., 2012). Similar or opposite responses of soil TN content to straw returning were detected, compared to SOC content (Pittelkow et al., 2015), which led to varied soil C:N ratio. Thus, it is necessary to determine the impacts of straw deep returning as interlayer on SOC, TN, C:N ratio, and stratification ratio, considering the amount of straw input (Cong et al., 2019), experimental year (Berhane et al., 2020), and soil depth (Liu et al., 2021).
Therefore, we conducted a four-year field experiment with four levels of straw interlayers in the Hetao Irrigation District. We hypothesized that 1) the SOC increased in the first several years after straw deep returning, while decreased from subsequent years, it ranged according to the amount of straw input (Cong et al., 2019) and soil depth (Latifmanesh et al., 2020); and 2) straw deep returning led to similar trend between TN and SOC, which resulted in similar soil C:N ratio, while remarkable lower soil stratification ratios (Liu et al., 2021). Thus, we aimed to 1) quantify the effects of straw interlayers with different input rates on SOC and TN within 0-80 cm soil depth in four years after straw deep returning; 2) explore the relationships between straw interlayer and C:N ratio / stratification ratios, aiming to higher soil quality and productivity in a semi-arid saline soil.