Plant functional groups shape microbial colonization and decomposition dynamics in grassland soils

Litter decomposition is a key ecosystem process that governs nutrient release and organic matter turnover in terrestrial ecosystems. While plants are known to influence rhizosphere microbiome, their role in shaping microbial colonization of litter, and further regulating decomposition remains less understood. Here, we employed a field-based Tea Bag Index (TBI) experiment to investigate how living plant functional groups (PFGs), including C3, C4, forb, and N₂-fixing legumes affect decomposition of standardized tea substrates (Green tea = labile; Rooibos tea = recalcitrant) and the associated microbial communities. Our results demonstrate that PFG type exerted a stronger influence on decomposition rate than species richness. The PFG impacts on decomposition were linked directly with shifts in substrate-colonizing communities, and indirectly with higher soil nitrate, N mineralization, and favourable moisture conditions. Microbial assemblages on Green vs Rooibos tea were distinct, indicating strong substrate filtering with PFG-mediated selection of decomposer communities. Across both substrates, PFGs and soil properties jointly explained most of the variance in decomposition rate, with additional, context-dependent contributions from bacterial and faunal (protist and metazoan) diversity reflecting their functional roles in litter breakdown. These findings underscore the central role of PFGs in structuring decomposer communities and regulating key soil processes. Preserving plant functional diversity is therefore essential for preserving microbial-mediated soil processes and ensuring grassland ecosystem resilience.