4.2 Rhizosphere secretions
Rhizosphere secretions are important in the organic and inorganic matter cycling and energy flow between plants and the environment. Organisms such as plants and microbes adapt to their habitats and influence their surroundings by releasing various high- and low-molecular-weight metabolites and ions into the environment, with up to 50% of photosynthetic products potentially released as rhizosphere secretions (Zhang, Li & Wang, 2007; van Dam & Bouwmeester, 2016). High-molecular-weight metabolites in secretions (carbohydrates, hydrocarbons, etc.) may adhere strongly to soil particles. Such adhesion promotes the formation of soil aggregates, changes the soil structure, and improves the physical and chemical properties of the soil. In addition, organic acid anions could increase availability of nutrients in the rhizosphere by their chelating capacity (Killham, 1994). The acid/ester compounds detected in the present study were mainly dibutyl phthalate (DBP), which is a potential allelopathic substance. Low concentration of DBP increased the content of chlorophyll in leaves and the activities of urease and catalase in soil, and enhanced the plant capacity to resist stress, but high concentration had the opposite effect (Keire et al., 2001; Deng et al., 2017).
Different secretions would have differential effects on the surroundings, including an effect on the formation of a specific microbial community in the rhizosphere soil (Sturz & Christie, 2003), and influencing not only the abundance and type of microorganisms, but also growth and metabolism as well as community composition of microorganisms (Haldar & Sengupta, 2015; Ankati & Podile, 2019; Vives-Peri, de Ollas, Gómez-Cadenas & Perez-Clemente, 2020). Obviously, the rhizosphere microbial community structure is the result of a series of complex interactions and feedbacks between the roots, the microbes and the physical and chemical environment of the soil. There are numerous studies demonstrating that plant development influences the composition and function of the soil microbiome (e.g. Rodriguez, Muñera & Peñuela, 2016; Zhalnina et al., 2018; Shao et al., 2020; Xu et al., 2020). The microorganisms that tend to gather near roots, such asGeobacter, Cupriavidus, Halorhodospira, Marichromatium, Rhodobacter, Rhodovulum, Rubrivivax, Sideroxydans ,Pseudacidovorax, Sinorhizobium, Leptothrix andMethylocystis, etc. in the present study (Figs 3, 4) influence the rhizosphere environment and may decompose and transform rhizosphere secretions.