4.1. Effect of straw interlayer on SOC in saline soil
Straw interlayers significantly increased SOC contents in the present study, mainly due to exogenous organic material inputs with straw applications (Cong et al., 2019). Several studies demonstrated that straw returning to surface soils at rates greater than 9 Mg ha-1 could not increase SOC (Zhou et al., 2018), while straw deep returning as interlayers at rates of 12-18 Mg ha-1 increased SOC content in the third and fourth year after the treatments in this study. The contrasting results are probably because of the differing straw placement in these studies. Subsurface soil has lower microbial activity, decomposition and transformation rates than the surface soil (Latifmanesh et al., 2020), and can thus help retain C released from the straw. Our results confirm that straw interlayers have greater potential for SOC sequestration (Zou et al. 2016; Huo et al., 2017).
The SOC content of straw interlayer with 12 Mg ha-1was highest in the first three years after straw deep returning. Overall, SOC content increased with straw C input when the added straw amount was smaller than the optimal value that allowed good straw and soil contact, but it was maintained or decreased when straw C input exceeded the optimal amount (Zhu et al., 2015). In the early period of burying a large amount of straw (i.e., 18 Mg ha-1 in this study), the straw was excessive and condensed in restricted soil space, it meant that the straw was in a reduced environment (Wang et al., 2015). This indicated that the straw interlayer was difficult to degrade and that its poor contact with the soil led to low decomposition rate and less SOC accumulation (Andruschkewitsch et al., 2013). Meanwhile, the C:N ratio in C-rich maize straw is generally higher than in the soil (Cai et al., 2015), and after the addition of 18 Mg straw ha-1, microbial activities are restricted by the N availability in the early stage of straw decomposition (Laird and Chang 2013), which prompted microbial competition with crops for mineral N from the soil, affecting the composition and diversity of soil microorganisms, and thereby reducing straw decomposition rate (Yang et al., 2015). Whereas, SOC content significantly increased for the straw rates of 18 Mg ha-1 than 12 Mg ha-1in the fourth year after straw deep returning. This is because that the limitation of soil space was weakened by the increase of straw burial years (Zhang et al., 2020).
Straw deep returning as interlayers increased SOC content and storage in soil layers above and below the straw interlayer. This is probably because the straw interlayers improved soil water-holding potential and reduced soil salt (Zhao et al., 2016), which promoted root growth and increased root C (Zhang et al., 2019) and inputs of exudates (Haichar et al., 2014). Meanwhile, the favorable soil porosity under straw interlayer promoted soluble organic C leaching to soil layer below straw interlayer (Huo et al., 2017). Additionally, the higher SOC content in 20-60 cm depth under straw interlayers resulted in lower stratification ratio (0-20 cm: 20-60 cm) compared with no straw interlayer. These data indicated that the straw interlayers were beneficial to solve the problem of nutrient accumulation caused by conventional straw (e.g., straw mulching or topsoil mixing) (Zou et al., 2016). This suggested that straw interlayer buried at 40 cm depth significantly increased SOC content and storage in 20-60 cm depth, and furthermore improved homogenous distribution of SOC content.