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.