Effects of intercropping on composition and molecular diversity of soil dissolved organic matter in apple orchards: Different roles of bacteria and fungi

Abstract

Dissolved organic matter (DOM) plays a crucial role in soil biogeochemical processes. However, the dynamic changes in DOM composition under intercropping systems of different ages, as well as the underlying mechanisms through which microbes influence DOM formation, remain unclear. In this study, we analysed soil samples collected at varying depths from intercropping systems with white clover of different ages to determine DOM dynamics and how DOM is regulated by bacteria and fungi. The results showed that the DOM content increased by 16 and 61 % at depths of 0–40 cm in 8-year and 16-year intercropping systems, respectively, compared with clean tillage (CT). The 8-year soil profiles showed an increase in the relative abundance of biologically refractory DOM compounds (tannin- and humic-like), leading to enhanced chemodiversity. In contrast, the DOM composition in the 16-year soil profiles was similar to that in the CT. Significant changes in the bacterial community were observed in the 8-year soil profiles, characterised by increased relative abundances of dominant Proteobacteria, Nitrospirae, Cytophagia, and Chitinophagia, along with enhanced bacterial alpha diversity. The correlation results indicated that bacterial taxa exhibited strong and interconnected positive correlations with the DOM diversity index, highlighting the bacterial role in the transformation of DOM compounds and the increase in DOM chemodiversity. Conversely, the fungi appeared to selectively degrade lignin to form biologically refractory compounds because they exhibited negative correlations with numerous lignin-like compounds and showed positive correlations with humic-like molecules. In conclusion, this study suggests that bacteria and fungi regulate DOM diversity and the formation of biologically refractory compounds via distinct pathways, providing important insights into the roles of microbes in mediating soil DOM formation and transformation.