4. Discussion
4.1 Soil microbial diversity and community structure
Soil microbial communities are essential to maintaining soil ecosystem function and can be affected by tillage and N application (Bärlocher & Boddy, 2016; Keszthelyi et al. , 2008). We found that NT treatment increased bacterial and fungal diversity in 0-10 cm layer compared to CT treatment (Tables S6 and S7). The difference between CT and NT could be due to the reduction of soil physical disturbance and protection from fungal hyphae and their mycelial network under the no-tillage system (Ceja-Navarro et al. , 2010; Verbruggen & Toby Kiers, 2010; Wanget al. , 2017).
Furthermore, soil fungal and bacterial diversity decreased with increasing N application rates in the 0–10 cm layer and was higher under NT treatment than under CT (Figs. 5 and 9). One possible reason is that the straw in no-tillage has a wide C/N ratio (Thierfelder et al. , 2018), which leads to an N limitation under this tillage system because microbe needs more N under this condition. A previous meta-analysis showed that appropriate N addition (<100 kg N ha−1year−1) is essential to stimulate microbial growth in no-tillage systems because it regulates soil C/N (Thierfelder et al. , 2018; Zhou et al. , 2017). However, excessive N fertilization suppresses the diversity of soil microbes because of the toxic effect of urea (Omar & Ismail, 1999; Wang et al. , 2018). In this study, high N application rate (210 kg N ha-1) could induce toxicity, resulting in lesser microbial diversity. In addition, CT had lower soil SOC (Liet al. , 2010; Liu et al. , 2021) and C/N ratio compared with NT (Fiorini et al. , 2020), which leads to carbon limiting for microorganisms. Hence, the effect of N application had a smaller effect on microbial diversity under CT than NT. Previous studies showed that low N application (35–140 kg N ha-1­­) decreased soil bacterial diversity (Wanget al. , 2015) and our study highlighted that there was the same conclusion under high N application rate (105–210 kg N ha-1), which extends our knowledge of the effect of N application on microbial diversity. In addition, increasing N application rates had a negative effect on some dominant flora such as Chloroflexi (Fig. 4), which also degrades SOM because Chloroflexi plays an important role in the decomposition of refractory C compounds (Li et al. , 2019b; Piazzaet al. , 2019). These results further indicate that N application needs to be considered when studying the effect of tillage management on microbial properties.
Tillage management could also influence the vertical distribution of soil microbial communities (Nunes et al. , 2020). We found no difference in enzyme activities, total PLFAs, and bacterial and fungal diversity among soil layers under CT treatment (Figs. 1, 2, 5, and 6). The main reason was that soil microbial communities in different soil layers would be similar to each other after homogenization induced by plowing under CT (Sun et al. , 2018). However, fungal and bacterial diversity decreased as soil depth increased under NT(Figs. 5 and 6), which was supported by the previous study (Jumpponen et al. , 2010). This was likely because no-tillage creates heterogeneous soil (Sun et al. , 2018). Moreover, the decrease rate of fungal and bacterial diversity with increasing soil depth was higher under N1 than N2 and N3 for NT treatment, indicating that a low N rate can enhance top soil bacterial and fungal diversity under NT. Hence, it is not sufficient to only consider the surface layer when investigating bacterial and fungal diversity response to N application rates in no-tillage systems.