Many mechanisms have been proposed for why leaf litter decaying in mixtures does not always decompose additively. For example, physical, chemical, and biotic factors could be driving changes in moisture, temperature, and nutrient retention that lead to acceleration or deceleration of the decomposition of one or more of the species within the mixture. I will present data from three litter decomposition experiments that, in combination, support the idea that multiple mechanisms are driving these interactions. First, using mixtures of litter labeled with 15N combined with litter at natural abundance nitrogen levels, we observed that in the mixtures, levels of 15N increased in the litter initially containing less N and decreased in the litter that initially contained more N, supporting the “nutrient transfer hypothesis”. We also examined the importance of biological changes in the mixtures by manipulating the presence of the fungal community, finding this to have a strong effect on nutrient transfer. Finally, physical changes may also play an important role, as demonstrated by a litterbag experiment in which litter surface area was manipulated. Together, these data highlight the importance of physical, chemical, and biotic changes in the litter mixtures. Improved understanding of the mechanisms behind changes in decomposition rates in diverse litter mixtures could improve predictions of carbon and nutrient dynamics in naturally diverse ecosystems.