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.