Soil salinization is a critical environmental issue restricting agricultural production. Straw deep returning as interlayer (40 cm) has been a popularized practice to alleviate salt stress. However, the legacy effects of straw interlayer associated with the straw input amount on soil organic carbon (SOC) and total nitrogen (TN) in saline soil remain unclear. Therefore, a four-year (2015-2018) field experiment was conducted with four levels (i.e., 0, 6, 12 and 18 Mg ha -1) of straw returning as interlayer. Compared with no straw interlayer (CK), straw interlayers increased SOC content by 14-32% and 11-57% in 20-40 cm and 40-60 cm, respectively. Lower increases were for soil TN content (8-22% in 20-40 cm and 6-34% in 40-60 cm) than SOC content, which led to increase soil C:N ratio in the 20-60 cm soil depth. Compared with CK, remarkable increases of SOC and soil TN contents in 20-60 cm led to the decrease of stratification ratios (0-20: 20-60 cm), which promoted uniform distributions of SOC and TN in soil profiles. Even though soil parameters ranged widely according to the straw input, straw interlayer with 12 Mg ha -1 had higher SOC, TN, C:N ratio, and lower soil stratification ratio in 2015-2017, which contributed to salt leaching, water retention, and yield increment. These results highlighted the legacy effects of straw interlayers maintained more than four years, which led to an underestimation for previous short-term experiments, and demonstrated a great potential for subsoil fertility and salt-affected soil amelioration.

The combination of plastic film mulching and subsurface organic amendment is a novel strategy for saline soil amelioration and utilization in China. However, how the strategy affect soil organic carbon (SOC) contents directly and indirectly (physical protection and microbiological regulation) were still not-documented. Therefore, four treatments, i.e., no amendment with and without plastic film mulching, subsurface (10-30 cm soil depth) organic amendment with and without plastic film mulching, were arranged and sampled after three-year filed experiment. Compared with no amendment with and without plastic film mulching, subsurface organic amendment increased the SOC content in the 0-40 cm soil depth by 70% and 90%, respectively. Plastic film mulching decreased SOC by 16% without organic amendment. Subsurface organic amendment transformed the dominant aggregation particles from <0.053 mm to 0.25-2 mm, indicating that both direct carbon input and indirect physical protection contributed to SOC increment. Conversely, SOC decreased with plastic film mulching due to the 14% lower fungal diversity compared with soil without plastic film mulching, was supported by the positive path coefficient from fungal diversity to SOC. Therefore, the combination of plastic film mulching and subsurface organic amendment increased SOC by 61% by direct carbon input and indirect physical protection and microbial regulation. In conclusion, subsurface organic amendment with plastic film mulching reinforced soil organic carbon increment through altering saline soil aggregate structure and regulating fungal community, and confirmed it is a feasible way to increase SOC for saline soil amelioration.