Litter heterogeneity modulates fungal activity, C mineralization and N retention in the boreal forest floor

The implications of forest floor heterogeneity for fungal activity, nutrient retention and carbon sequestration within the forest floor remain poorly documented. This information would be particularly relevant to situations where large changes in fresh litter occur such as following stand replacing disturbances. Numerous laboratory studies have documented mycelial translocation of simple forms of C and nutrients between contrasting substrates, allowing fungi to overcome local deficiencies. In slightly more complex but less controlled conditions, we assessed how factors contributing to forest floor heterogeneity-decay state, litter origin and tree species-individually affect fungal activity during decomposition. We also assessed how the juxtaposition of litters of contrasting nutrient and C status (decay state) alter fungal activity within individual substrates. We expected fungal biomass to be reallocated to C-rich litters while lignocellulolytic activity would increase in all juxtaposed litters. A microcosm experiment was conducted in which wood and leaf litters of one softwood species (Pinus banksiana) and one hardwood species (Populus tremuloïdes) were incubated alone or in combination with litters of contrasting decay states. Litter mass loss, change in N content, C mineralization, fungal biomass, specific respiration rate and lignocellulolytic activity were measured after 15 and 30 weeks. The decay state of litter had the most pronounced and consistent effects on fungal activity, with higher fungal biomass and lignocellulolytic enzyme activity in well decomposed litters and higher mass loss, C mineralization and specific respiration rate in fresh litters. In juxtaposed litters, fungal biomass was initially reallocated to fresh litters when incubated with well decomposed litters. Cellulolytic activity also increased by 30% in juxtaposed fresh litters while Mn-peroxidase activity increased by 42% in both fresh and well decomposed litters. With the longer incubation period, C mineralization and specific respiration of fungal biomass increased in juxtaposed well decomposed wood, indicating an increase in overflow metabolism presumably in response to an increase in labile C. Fresh litters that were juxtaposed increased their N content while that of single litters decreased. A better understanding of fine scale mechanisms affecting litter decomposition could improve our ability to forecast ecosystem response to disturbance.