Oomycete pathogens suppress litter decomposition in alpine meadow soils

Litter decomposition is of critical importance for nutrient cycling and carbon fluxes and is strongly influenced by litter quality and soil microbial community composition. Fungal and oomycete pathogens are prevalent in global grasslands and, due to differences in their physiological characteristics, host ranges, functions, and life cycles, they may have distinct effects on litter decomposition. However, how fungal and oomycete pathogens regulate litter decomposition via alterations to litter quality and microbial community remains unclear. To address this question, we conducted a year-long field experiment to measure litter decomposition on the mineral topsoil of an alpine meadow, employing fungicide and oomyceticide treatments. We found that fungicide application did not significantly affect litter mass loss. In contrast, in situ application of oomyceticide significantly increased litter mass loss. Mechanistically, suppression of oomycete pathogens significantly increased the relative abundance of species within the phylum Mortierellomycota in the soil, thereby accelerating litter decomposition. Our results indicated the critical role of oomycete pathogens in mediating the effects of organisms belonging to higher trophic levels on litter decomposition, revealing complex interactions between higher trophic levels and their potential impacts on terrestrial ecosystem functioning under ongoing anthropogenic disturbance. • Fungicide application did not affect litter decomposition, whereas oomyceticide significantly increased it; • Litter quality was not significantly affected by oomycete pathogen suppression; • Suppressing oomycetes elevated soil Mortierellomycota abundance, which accelerated decomposition; • Oomycetes mediate higher trophic-level influences on decomposition, influencing ecosystem functioning under disturbance.