Climate and vegetation jointly regulate the effects of fungal diversity on belowground litter decomposition in the Otindag Sandy Land

Litter decomposition drives carbon and nutrient cycling in complex sandy grassland ecosystems of arid and semiarid regions. However, the interactive effects of grassland types, climate, soil properties, microbial communities and plant diversity are still unclear. We conducted a one-year field experiment in the Otindag Sandy Land using 272 tea bags (136 green tea and 136 red tea), buried across four grassland types: desert, sandy, typical and sparse forest grasslands (temperate savanna). After sequencing soil bacterial and fungal communities, we coupled these data with measurements of climate, soil and vegetation to quantify the respective and interactive roles of biotic and abiotic drivers in tea-bag mass loss. Litter decomposition rates (measured via green and red tea bags) showed no significant differences among the four grassland types, despite a numerical trend where mass loss was highest in the temperate savanna, followed by typical, desert, and sandy grasslands. Green tea loss was primarily driven by biotic factors, whereas red tea decay was affected by both biotic and environmental variables. Structural-equation modeling revealed that climate (temperature, precipitation) and plant diversity indirectly influenced decomposition by modulating microbial diversity, particularly fungal dominance. Notably, the temperate savanna contained a high relative abundance of lichenized fungi, dominated by an unclassified Arthopyreniaceae taxon, which was positively correlated with higher decomposition rates. Our results highlight the synergistic roles of climate, vegetation and grassland-specific microbial assemblages in regulating belowground litter decomposition in arid and semiarid sandy grasslands, and underscore the disproportionate influence of lichenized fungi in temperate savanna systems. • A standardized decomposition experiment across multiple grassland types. • Drivers were substrate-dependent. • Climate and plant diversity indirectly shaped decomposition via microbial diversity. • High abundance of lichenized fungi was positively linked to faster decomposition.