4.2. Effects of straw interlayer on TN and C:N ratio in saline soil
Our study also indicated that the straw interlayers increased the TN content and storage, especially in the soil near the straw interlayers, resulting in the decrease in stratification ratio of TN. It is mostly associated with the improvement of soil aggregate structure, and the decrease of soil bulk density after the straw interlayer placement (Zhang et al., 2015). Meanwhile, the C-rich straw stimulates microorganism growth in the process of decomposition and promotes their fixation of soil N (Cao et al., 2020). Additionally, straw returning can promote transformation of mineral N to organic N and reduce N loss via leaching (Yang et al., 2015). Thus, straw interlayer has positive effects on TN content and storage in soil layers near interlayer, further reducing the stratification ratio of TN content.
In our study, the straw interlayers increased C:N ratio in 20-60 cm depth. This may be explained by the nutrient composition of maize straw which has a C:N ratio of ~65:1 (Cai et al., 2015). After being buried in soil, the contribution rate of straw interlayer to SOC is higher than that of TN (Liu et al., 2021). In addition, saline soils generally have lower C:N ratio, which may imply that mineralization and decomposition of SOC are reduced and SOC content is increased when large amounts of organic materials was added into soil (Tian et al., 2020). These can all contribute to increasing the soil C:N ratio.
Interestingly, we found that the improvement in soil C:N ratio declined with extension of straw interlayer burial time. As the time of straw burying developed, the difference in C:N ratio brought by different amounts of straw interlayer gradually weakened. In addition, as the burying period increases, the SOC gradually stabilized (Liu et al., 2014). However, straw returning with N fertilizer application enhanced N stabilization (Wang et al., 2021), which led to decrease of C:N ratio. Therefore, it indicated that soil C:N ratio increased at earlier period of straw interlayer burial and then stabilized.