3.1. Soil Organic Carbon (SOC)
The relative changes of SOC content compared to CK were influenced by straw interlayers, and experimental year × straw interlayers interaction (p< 0.05) (Figure 3a). The relative changes of SOC content were decreased with the experimental years for SL6 and SL12, while it was increased with the experimental years for SL18. The highest SOC changes were found in 20-40 cm and 40-60 cm depths with 14-32% and 11-57%, respectively. In the first two years of straw interlayer burying, the relative change of SOC of each soil layer was the largest in SL12, but in later years SL18 showed an advantage. In 2018, the relative change of SOC under SL18 was the largest among all treatments.
The SOC content gradually decreased with deeper soil layers and increased in the straw interlayers (p < 0.05) (Figure 4a). In 2015, the SOC content in SL12 with the greatest magnitude and increase by 19-57% (p < 0.05) compared with no straw interlayer. In 2016, the SOC content of straw interlayers was significantly increased by 24-31% and 32-35% (p < 0.05), respectively at the depths of 20-40 cm and 40-60 cm, compared with CK. Straw interlayer burial late stage (2017-2018), the SOC content of SL18 were higher than other treatments, and the largest increase was found in the depths of 20-40 cm and 40-60 cm.
Straw interlayers significantly affected SOC storage (p< 0.05) (Figure 5a). Compared with CK, SL6, SL12 and SL18 increased SOC storage by 8-22%, 14-34%, and 16-23% in 0-80 cm soil depth from 2015 to 2018, respectively. The SOC storage in SL12 with greatest magnitude among all straw treatments in 2015-2017.