Does initial litter chemistry explain litter mixture effects on decomposition?

We tested the hypothesis that interactions in litter mixtures (expressed as the difference between observed and expected decomposition rates) are greater when the component species differ more in their initial litter chemistry. Thereto, we collected freshly senesced leaf litter from a wide range of species from an old field and woodland vegetation, and a fen ecosystem in The Netherlands. Litterbags with either mono-specific litter (20 and 15 species), or litter mixtures (50 and 42 species pairs) of randomly drawn combinations of two representatives from different plant functional types were incubated for 20, 35 and 54 weeks in a purpose-built decomposition bed (woodland/old field) or in the field (fen). Species showed a wide range of decomposition rates. For the woodland/old field species, initial litter C and P concentrations were significantly correlated with litter decomposition rate. For the fen species, litter phenolics concentration was correlated with decomposition rate. If the Sphagnum species were left out of the analyses, initial litter P and phenolics concentration both showed a significant relationship, albeit only with the remaining mass after 1 year. Differences between observed and expected decomposition were often considerable in individual litter mixtures. Regression analysis showed that such differences were not related to the differences in litter chemistry of the component species. Furthermore, litter mixtures containing species with very different initial litter chemistry did not show stronger interaction when tested against litter mixtures containing chemically similar litter types. From these observations we conclude that the difference in initial single litter chemistry parameters of the component is not a useful concept to explain interactions in litter mixtures.