4 Discussion
To estimate agrochemical compounds including pesticide and fertilizer
mixtures is one of the most important factors in evaluating the
toxicological or ecological impacts of these compounds (Schmitz et al.,
2014). In our study, SPF is an agrochemical compound widely used on
sugarcane seedlings in South China, which principal components including
pesticide (clothianidin, a chemical compound) and an organic fertilizer.
However, there is less researches about its impact on soil and water as
well as relevant insects and microbes. This study was conducted to
investigate the effects of SPF on the rhizosphere microbial community of
field-grown sugarcane. Here we conducted 16S rRNA gene amplicon
sequencing to detect the disturbance to the diversity and structure of
rhizosphere microbiota under different pesticide and/or fertilizer
treatments. We found that the bacterial and fungal compositions at the
phylum level affected by clothianidin in the SPF and SP groups were
different from the effects experienced in the other treatment groups.
Meanwhile, the structure of Proteobacteria, Actinobacteria, Firmicutes,
Acidobacteria, and Actinobacteria in these different groups was variable
with different relative abundances (Figure 3a). While, the abundances of
fungal phyla including Ascomycota and Basidiomycota in the SPF and SP
groups were almost the same, indicating that the dominant fungi would be
more affected by clothianidin (Figure 4a). As expected, the application
of a chemical compound clothianidin was a major factor shaping the
rhizosphere microbial community taxonomic structure (Zhu et al., 2016;
Diez et al., 2017). However, organic fertilizers addition was proved to
be able to enrich the microbial diversity (Cai et al., 2017) and could
counteract the effects caused by mixing with clothianidin in the SPF
treatment soil when compared with the SP group with only pesticide
addition. The effects of SPF with organic fertilizer addition on the
rhizosphere microbiome were also found not to be significant as revealed
by the diversity (Figure 1) and function (Figure 5) as the microbial
community of SPF compared to the SP group.
As is reported, it usually involves more than one microorganism when
pesticide degradation occurs in soil (Doolotkeldieva et al., 2018).
Comparing to the SK control treatment, the more abundant phyla present
in the SPF treatment, such as Bacteroidetes, Firmicutes, and
Acidobacteria, have been reported to be pesticide-tolerant bacteria and
could be responsible for the biodegradation of applied pesticides
(Hassen et al., 2018). The bacterial families Burkholderiaceae and
Streptomycetaceae with enriched abundances shown in the SPF and SP
groups (Figure 3b) are considered to be active microbiodegraders (Mamta
& Khursheed, 2015). Four bacterial genera including Dyella ,Sphingomonas , Catenulispora , Mucilaginibacter , andTumebacillus were found highly increased in the SPF and SP
groups, which could be able to degrade clothianidin (Polivkova et al.,
2018). In particular, Dyella ginsengisoli was found to be a
highly efficient biphenyl-degrading strain that could accelerate the
startup period of the biphenyl bioremediation process (Zhao et al.,
2010). If these clothianidin degrading genera found in this study could
be isolated and cultured, it would be helpful to improve soil quality
with external addition to clothianidin-contaminated farmlands. In
addition, some strains in Ascomycota of this study, the dominant fungal
phylum in all groups, have been reported to produce laccase with high
yield, and thus have potential applications in environmental protection,
e.g., decoloring the wastewater (Neoh et al., 2014), as well as biomass
conversion (Xie et al., 2014).
The alpha-diversity of the bacterial communities in the SA treatment
compared with the SPF and SF treatments were found not significantly
different but showed highly increased compared to SP (Figure 1). This
suggested that the common excipients added to the SPF would not cause
the significant disturbance of diversity in the soil microbial
community. For the functional prediction of the bacteria community, the
SPF treatment also showed more positive biological metabolism functions,
including pyruvate metabolism, glycolysis, carbon fixation, butanoate
metabolism, methane metabolism, and pantothenate biosynthesis, which
indicated it would not have severe negative impacts on the soil
environment. The functional comparison was also conducted between the
excipient treatment (SA) and the SK control treatment, showing lower
adverse effects on the rhizosphere bacteria community of sugarcane. On
the contrary, the function of the SA-treated bacterial community showed
more significantly enhanced (p < 0.05) in their
biochemical cycling, including oxidative phosphorylation, nitrogen
metabolism, sulfur metabolism, and glutathione metabolism, as compared
to the SK treatment (Figure 5a). This indicated the excipient mixture we
used in this study (added into the SPF, SP, and SF treatments) was not
too harsh on the bacterial diversity and function of soil. The fungal
functions revealed by FunGuild.mode also showed that there was much
lower proportion of the SPF-treated soil to become pathotroph type
compared to SA and SK (Figure 5b). In addition, clothianidin used in
this study was reported to be a decomposable pesticide taking about
20-25 days to decompose in the soil (Yang et al., 2018), which would
cause less stress on the soil environment. Moreover, the SPF treatment
with the organic fertilizer addition applied to filed-grown sugarcane
could release the strong rhizosphere effects caused by clothianidin,
which would improve the root biomass and health of sugarcane seedlings.
In this study, we investigated the effects of a pesticide-fertilizer
combination, as well as the mixture of single active compounds, on the
sugarcane rhizosphere soil microflora in an attempt to understand the
possible interaction effects. The results, based on field experiments,
revealed that using a pesticide-fertilizer mixture in the field-grown
sugarcane does not adversely affect the diversity and function of soil
microbiome. Moreover, appropriate organic fertilizer input and pesticide
reduction management practices are not only helpful for decreasing the
labor cost of sugarcane cultivation, but also beneficial to reducing the
potential risk of environmental pollution. Therefore, the interactive
effects of different fertilizer and pesticide managements on the
rhizosphere microbiome will be extremely helpful in determining a
reasonable application scheme for the fertilizers and pesticides.