5. CONCLUSIONS
A 4-year field experiment demonstrated that straw interlayers (40 cm soil depth) increased SOC content by 16-24% compared to no straw interlayer (CK), ranged widely according to the straw input. Compared with CK, the increase of SOC content was decreased for straw interlayers with 6 and 12 Mg ha-1, while it was increased for 18 Mg ha-1 with the experimental years. The SOC content of straw interlayer with 12 Mg ha-1 was highest in 2015-2017, while it was highest with 18 Mg ha-1 in 2018. The TN contents of straw interlayers were increased by 9-13% compared to CK, while the increment was lower than SOC content. Remarkable increases were obtained in the soil layers above (20-40 cm) and below (40-60 cm) the straw interlayers. Therefore, the C:N ratios of straw interlayers increased in 20-60 cm layers. Compared with CK, straw interlayers decreased the stratification ratios (0-20: 20-60 cm ) of SOC and TN contents by 5-15% and 4-9%, respectively, which promoted a uniform distribution of SOC and TN contents in 0-60 cm soil layer. Overall, 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 salts leaching, water retention, and yield increments. Our results highlighted the legacy effects of straw interlayers on soil fertility and productivity maintained more than four years associated with the amount of straw input, which led to the underestimation for previous short-term experiments. In conclusion, straw deep returning as interlayer was an effective strategy to increase the subsoil nutrients, reduce soil stratification ratio, and promote saline soil utilization.
Conflict of interest
We declare no competing interest.