Optimized straw interlayer improves organic carbon and total nitrogen in soil profile: A four-year experiment in a saline soil

Abstract

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 ) 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 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.