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.