1 Introduction
Sugarcane has become the most important cash crop in southern China. During the lifetime of sugarcane crop, it suffers from various pests in different development stages, and requires a large amount of pesticides and fertilizers to sustain its growth and health (Franco et al., 2011; Cherry et al., 2017). However, the heavy use of pesticides and fertilizers in sugarcane cultivation causes serious impacts on the agricultural environment, including agricultural non-point source pollution, increasing cost efficiency, energy conservation and emission, and effects on beneficial invertebrates, as well as threats to food safety and quality (Bokhtiar & Sakurai 2005; Klaine et al., 2010; Velasco et al., 2012). In 2015, China’s Ministry of Agriculture introduced two actions that seek to achieve zero growth in the use of chemical fertilizers and pesticides by 2020 (Jin & Zhou, 2018).
Pesticide-fertilizer combinations or mixture products have been thriving and are increasingly applied in the farmland, as they serve a dual purpose with both nutritional and insecticidal characteristics, which could help achieve sugarcane pesticide-fertilizer integration and save labor and costs (Achorn & Wright, 1982; Xie et al., 2017 & 2018). Tiange, for instance, is a popular multifunctional pesticide-fertilizer product used in sugarcane cultivation in southern China, could effectively suppress pests including sugarcane borer, thrip, aphid, and cockchafer (Xie et al., 2017). Xie et al. (2019) found that the combination of 50% organic fertilizer replacement and 50% pesticide reduction can produce the optimal results for both mitigating nutrient loss and balancing tea yield and quality. Yein et al. (2013) reported that pesticides and fertilizers in combination were much more effective at controlling the root-knot nematode infesting mung than using only pesticide or fertilizer alone. However, the domestic industry of pesticide and fertilizer integration is in its initial stage, the combination products currently lack industry standards and evaluation systems, and the potential impact on rhizosphere microbiome is still underestimated.
Rhizosphere microflora has been shown to be a key component of agricultural ecosystems that not only plays a significant role in basic soil processes but is also actively involved in enhancing soil fertility and crop productivity (Haney et al., 2015; Huang et al., 2020; Korenblum et al., 2020). However, research on the effects of pesticide and fertilizer combinations (SPF) on the rhizosphere microbial diversity and function of sugarcane is still rare. Currently, the overuse of synthetic fertilizers and pesticides has been reported to cause adverse effects on the soil microflora, and can substantially influence plant health and productivity (Hartmann et al., 2015; Zhu et al., 2016). In particular, fertilizers and pesticides tend to have long persistence in the soil, which is bound to affect the soil microflora and consequently disturbing soil health (Prashar & Shah, 2016). Previous studies have found that chemical and bio-organic fertilizers have distinctive effects on the structure and function of the soil microbiome (Cai et al., 2017; Xiong et al., 2017). Compared with organic fertilizers, chemical fertilizers have many disadvantages, despite the ability to stimulate the growth of the fast-growing microbes by introducing more readily-available nutrients (Esperschütz et al., 2007). For instance, a high concentration of nitrogen fertilizer strongly modifies the composition of the soil microbiome, which could negatively impact soil carbon cycling and promote the abundance of fungal genera (Paungfoo-Lonhienne et al., 2015). On the contrary, bio-fertilizers can improve soil health by direct suppression of pathogens or via modification of the indigenous microbial community (Xiong et al., 2017). In addition, organic fertilizers introduce more organic carbon into the soil and hold a closer neutral pH value, which is considered a dominant factor in determining the composition of soil microbiomes (Pang et al., 2017).
Investigating shifts driven by SPF in the composition of the soil microbiome would be helpful for maintaining and/or enhancing soil fertility and productivity, as well as for protecting soil ecosystems against disturbances (Tian et al., 2015). In this study, we hypothesized that pesticide inputs supplemented with organic fertilizer could maintain the composition of the soil microbiome more sustainably than the conventional method based on applying the pesticide and fertilizer separately. Here we conducted a field experiment to reveal any shifts in the rhizosphere microbiome with different pesticide and/or fertilizer treatments, as well as to compare treatment effects on the structure and function of the soil microbiome. The objectives of this study were to: (1) explore the effects of a SPF (pesticide and fertilizer combinations) on the structure and diversity of sugarcane’s rhizosphere microbiome; and (2) reveal the potential interactions between the dual reductions in pesticide and fertilizer use and sugarcane quality.