Abstract: Litter decomposition is key to biogeochemical cycling, yet how mixing effects differ between intra-specific organs versus inter-specific leaves remains unclear. We conducted a two-year field litterbag experiment in a temperate grassland with three dominant species (Leymus chinensis (Trin. ex Bunge) Tzvelev, Vicia amoena Fisch. ex DC.‌ and Potentilla bifurca L.). We established six mixtures (three intra-specific leaf-culm and three inter-specific leaf-leaf) alongside their six single-component litter, all incubated under ambient and nitrogen-enriched conditions. In addition to mass loss, we measured a set of nutrient and carbon (C) traits, including the decomposition of nitrogen (N), phosphorus (P), cellulose, hemicellulose, and lignin. Nitrogen enrichment had no effect on any synergistic mixing effect. Averaged across all mixtures, we found positive synergistic effects on the loss of mass, N, P, cellulose, and hemicellulose, but not on lignin loss. While the magnitude of synergistic mass loss was similar between mixture types, the underlying biogeochemical pathways diverged: synergistic N and P release were stronger in leaf-leaf mixtures, whereas synergistic cellulose decomposition was more pronounced in leaf-culm mixtures. The stronger nutrient synergy in leaf-leaf mixtures was driven by a disparity in initial C concentration. Across all mixtures, Rao’s quadratic entropy positively predicted cellulose synergy but negatively predicted N synergy, revealing a general trade-off that operates independently of mixture type. Thus, inter-specific mixtures enhanced decomposition primarily through facilitating nutrient release, whereas intra-specific mixtures did so mainly by promoting cellulose breakdown. These findings reveal that plant species loss and altered organ allocation drive non-additive effects via distinct biogeochemical pathways.

Key words: Carbon fractions, Litter decomposition, Mixing effect, Nitrogen deposition, Temperate Steppe