“…δ 13 C of SOC generally increases with soil depth (e.g., Brunn, Spielvogel, Sauer, & Oelmann, 2014; Guillaume et al, 2015; Wang, Wei, et al, 2017). δ 13 C changes with soil depth are related to four processes: (a) physical mixing (Bird et al, 2004; Brunn et al, 2014), showing that it increased promoted homogenization of soil column based on one process‐based modeling, and in return decreased the δ 13 C gradients with depth (Acton et al, 2013); (b) microbial decomposition (Gautam, Lee, Song, & Bong, 2017; Wang, Wei, et al, 2017), this is one commonly held cognition that isotopic fractionation often occurs during microbial decomposition (Guinina & Kuzyakov, 2014). Microbes degrade 12 C more readily than 13 C during SOC decomposition (Lerch, Nunan, Dignac, Chenu, & Mariotti, 2011), and thus the residual SOC is enriched more 13 C components in deeper soil layers (Diochon & Kellman, 2008; Guinina & Kuzyakov, 2014).…”