3.3. C:N ratio
The relative changes of C:N ratio compared to CK were higher in the early stage of straw interlayer burial (2015-2016), especially in 20-40 cm and 40-60 cm depths, which increased by 9-12% and 19-26%, respectively. Subsequently, the relative changes of C:N ratio between straw interlayer treatments fluctuated slightly (Figure 3c).
The C:N ratio increased at the beginning while subsequently decreased with increasing soil depth (Figure 4c). In 2015, compared with CK, SL6, SL12 and SL18 increased C:N ratio by 12, 17 and 9% in 20-40 cm soil depth, and 36, 34 and 16% in 40-60 cm depth, respectively (p< 0.05). In 2016, compared with CK, straw interlayers increased C:N ratio by 6-15% in 20-40 cm depth, SL12 and SL18 increased C:N ratio by 17 and 21% in 40-60 cm (p < 0.05). In 2017, straw interlayers (6-18 Mg ha-1) boosted C:N ratio by 8-11% and 6-9% in 20-40 cm and 40-60 cm depth compared with CK, respectively. In 2018, C:N ratio increased with increasing amount of straw interlayer, but no significant difference with CK.
3.4. Stratification ratio s of SOC and TN
The stratification ratios of SOC and TN were influenced by straw interlayers (p < 0.001) and experimental year (p< 0.01) (Figure 6). Compared to CK, straw interlayers decreased the stratification ratios (0-20 cm: 20-60 cm) of SOC and TN by 5-15% (Fig. 6a) and 4-9% (Fig. 6b) (p < 0.05). In contrast, the stratification ratios of SOC and TN for 0-20 cm: 60-80 cm showed an opposite trend.