In the new early view paper “Do physical plant litter traits explain non-additivity in litter mixtures? A test of the improved microenvironmental conditions theory”, Marika Makkonen and co-workers, present a new theory on decomposition rates. Here’s their own summary of the paper:
Terrestrial plant litter decomposition is a key component in carbon flux models. The models and thus the predictions they produce could be improved by ensuring the comprehensiveness of the variables included in the model and the close resemblances between nature and the input data. Usually the input data is derived from litter monoculture studies and this creates a crucial source of error, as monocultures do not present well the majority of land cover. Importantly decomposition rates usually differ between litter monocultures and mixtures. The causes for this non-additive effect are still debated and unclear. One plausible theory suggests that the non-additivity of litter mixtures derives partly from improved microclimatic conditions given by physically more diverse plant species in mixtures compared to monocultures. The physical characteristics of litter determine e.g. water acquisition and retention and thus alter the microenvironmental conditions determining the habitat and resource availability for decomposers.
We tested this theory in a dry subarctic birch forest in the Swedish Lapland in two contrasting moisture conditions. By testing some water holding capacity (WHC) traits, we found clear support for this theory and thus our results strongly encourage the inclusion of plant litter physical traits as the predictors of the decomposition rates. Yet the modeling will face more challenges as we found the direction of non-additivity (positive or negative deviation compared to monocultures) in litter mixtures to vary between climatic (moisture) conditions. Namely we found that the higher dissimilarity in WHC traits between the component litter species in a mixture increased synergistic effects in litter mixtures under limiting moisture conditions whereas, increased antagonistic effects were observed under improved moisture conditions. We also observed differences in non-additivity, its relation to WHC traits and their modifications by different climatic conditions between litter mixtures of varying decomposability further obscuring the process. Although the non-additivity of litter mixture remains complex, some major advances were made by this study.