4.3 The influence of microbial CUEC:N on soil POC and MAOC fractions
POC is a functional soil component for persistent soil organic carbon (Witzgall et al. , 2021). In contrast to POC, MAOC is more physically or chemically protected, which makes it less vulnerable to mineralization (Abramoff et al. , 2018). We found that high N application (210 kg N ha-1) increased POC and MAOC content under two tillage practices (Fig. 9), which is similar to the previous study (Ye et al. , 2018). The possible reason was that plant biomass (Stewartet al. , 2016; Thomas et al. , 2010; Wang et al. , 2018) and microbial residues (Chen et al. , 2020a) increased with increasing N application. However, some discrepant findings showed that N addition decreased (Ye et al. , 2018) or had no significant influence on MAOC (Yuan et al. , 2020). The main reason for the inconsistent results could be that microbial residues controlled the changes of soil MAOC pool under N addition and the microbial residues were different due to different N application rates among these studies (Averill & Waring, 2018; Chen et al. , 2020a; Su et al. , 2020; Yang et al. , 2020b).
We further found that the CUEC:Nwas significantly positively correlated with POC and MAOC and the increase rates were higher under NT than CT, which was influenced by nitrogen application (Fig. S2). Moreover, the POC also had a positive effect on MAOC (Fig. 9) because a portion of POC was degraded by microbes and then formed part of the MAOC (Su et al. , 2020). Therefore, these results highlight that nitrogen regulates the influence of microbial CUEC:N on soil organic carbon fractions under tillage practices.