Nonadditive effects of mixing cottonwood genotypes on litter decomposition and nutrient dynamics

Plant species litter mixtures often result in nonadditive differences in ecosystem processes when compared to the average of their individual components. However, these studies are just beginning to be extended to the genotype level and to our knowledge have not incorporated the effects of herbivory or genotype-by-herbivore interactions. With a two-year field study, using genotypes that differed by as few as three restriction length polymorphism (RFLP) molecular markers, we found three major patterns when we mixed leaf litters from different genotypes both with and without previous herbivory. First, leaf litter genotype mixtures, regardless of herbivory, demonstrated nonadditive rates of decomposition and nutrient flux. Second, mixed genotype litter without herbivory decomposed faster than the same genotypes with herbivory. Third, in genotype mixtures, with and without herbivory, we found that net rates of immobilization of both nitrogen and phosphorus can differ from expected values (based on genotype means) by as much as 57%. These results show that mixing litter genotypes can alter rates of decay and nutrient flux and that the effects are reduced with herbivory. Nonadditive effects at the genotype level that we report here are nearly as large as what has been recorded for plant species mixtures and may have important, though under-appreciated, roles in ecosystems. These data further suggest that genetic diversity and genotype-by-herbivore interactions can affect fundamental ecosystem processes such as litter decomposition and nutrient flux.