Litter mixture effects on nitrogen dynamics during decomposition predominantly vary among biomes but little with litter identity, diversity and soil fauna

Nitrogen (N) is essential for net primary production, with much of the required N in terrestrial ecosystems derived from recycling via litter decomposition. The diversity and identity of plant species and decomposer organisms affect N cycling during litter decomposition, yet the generality and magnitude of these effects remain uncertain. To fill this gap, a decomposition experiment with four leaf litter species that differed widely in initial litter quality was conducted including single species and all possible multispecies mixtures, with and without microarthropods access across a broad latitudinal gradient covering four major forest biomes of the Northern Hemisphere. The results showed that leaf litter N dynamics (both N loss and N immobilization) in single species treatments depended primarily on litter species identity and the local environmental context. We found strong mixture effects, that overall tended to increase N loss and to reduce 15N transfer. The relative mixture effects on N dynamics differed among forest biomes, but were little affected by the other factors we manipulated. The N loss of individual litter species in mixtures not only depended on litter identity and soil microarthropod access, but also on forest biomes; while 15N transfer depended strongly on litter mixing, independently of litter species richness or composition of the mixtures. Litter N dynamics were mainly driven by a small subset of litter traits, regardless of species richness and microarthropod access. Overall, our results highlight that litter mixture strongly affects N dynamics during decomposition, with the mixture effects predominantly varying among forest biomes but little with litter identity, diversity and microarthropod access. To improve predictions on how changes in tree species composition and diversity may impact nutrient dynamics in forest ecosystems in face of increasing N deposition, interactions between litter and soil but also within litter mixtures need closer attention.