4.2 Relationship of soil microbial characteristic and microbial CUEC:N
Soil microbial CUE can affect soil C cycling (Spohn et al. , 2016). We found that NT increased the soil microbial CUEC:N on average compared with CT (Fig. 2) because no-tillage can decrease soil temperature by surface mulching and further increase microbial CUE (Apple et al. , 2006; Wetterstedt & Agren, 2011). In addition, higher residue production under NT is constantly supplying fresh and labile organic substrates for microbial activity and biomass thus explaining the greater CUE observed under NT compared with CT (Álvaro-Fuentes et al. , 2013). Microbial CUEC:Nincreased with increasing N application under both tillage treatments (Fig. 2). The reason is that N addition can reduce microbial respiration metabolism (Liuet al. , 2018; Spohn et al. , 2016; Thiet et al. , 2006) and increase microbial biomass (Jha et al. , 2020), resulting in higher microbial CUEC:N.
Furthermore, although a recent study showed that microbial diversity drives CUE in artificial soil (Domeignoz-Horta et al. , 2020), to the best of our knowledge, few experimental studies have directly demonstrated the interaction effect of tillage management and N application on microbial CUE in a field experiment. In this study, the PLS-PM showed that tillage and nitrogen influenced microbial CUEC:N through the microbial diversity and community structure (Fig. 9). We also found that the bacterial and fungal diversity had different influences on microbial CUEC:N(Fig. 9) under two tillage and these relationships were regulated by N application (Fig. S1) under no-tillage. Bacterial diversity positively influenced microbial CUEC:N, whereas fungal diversity had an adverse impact on microbial CUEC:N (Fig. 9). The difference points to the importance of studying the diversity of fungal and bacterial communities separately for predicting soil C cycling. In addition, microbial network complexity driving carbon cycling with direct feedback effects on multiple ecosystem functions (Morriën et al. , 2017; Wagg et al. , 2019; Zhou et al. , 2010), which could also influence microbial CUE. Further research should be undertaken to explore the effect of bacterial and fungal networks on microbial CUEC:N.